QUANTIFICATION OF MOTOR UNIT ACTIVITY AND STEADINESS IN A

FUNCTIONAL TASK

by

Kayla Margaret Dawn Cornett

B.H.K, The University of British Columbia, 2011

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQIREMENTS

FOR THE DEGREE OF

MASTER OF SCIENCE

in

THE COLLEGE OF GRADUATE STUDIES

(Interdisciplinary Studies)

[Health and Exercise Sciences]

THE UNIVERSITY OF BRITISH COLUMBIA

(Okanagan)

October 2013

©Kayla Margaret Dawn Cornett, 2013

ii

Abstract

The ability to control functional movement is essential for daily life. Extensive

literature exists on lower motor unit (MU) recruitment thresholds and greater discharge

rates in anisometric compared with isometric contractions. This difference is in-part

related to task specificity. To-date the spinal control of functional movement has not been

evaluated relative to the performance of functional movement. The purpose of this thesis

was to quantify and evaluate MU activity and steadiness in a functional task compared to

anisometric and isometric contractions.

Thirteen female subjects (22.5 ± 2.9 years) were recruited. Surface and

intramuscular electromyography (EMG) were recorded from the elbow flexor muscles.

Subjects performed 4 experimental contractions; 1. a waterbottle drinking task where

subjects lifted a waterbottle, took a sip of water and lowered the bottle (functional task);

2. a waterbottle lifting task performed identical to the previous but without drinking; 3. an

anisometric contraction matched for load, range of motion, and acceleration; 4. a load

matched isometric contraction. Repeated measures ANOVAs were used to assess EMG,

MU recruitment and discharge rates and steadiness between the contractions.

Surface EMG was not different between the three movement tasks but lower

during the isometric contraction. The waterbottle drinking task had the highest discharge

rate, as well as discharge rate variability and was the least steady. There were no

differences in recruitment between the 4 contraction types. This was the first study to

evaluate MU activity in functional tasks. It is clear that functional tasks require unique

activation strategies which occur primarily through alterations in MU discharge rate.

Thus, spinal control is not only task specific, but also related to goal-directed outcomes of

the movement.

iii

Preface

Ethics approval for this research was granted by the University of British

Columbia’s Clinical Research Ethics Board on November 22, 2011. The ethics approval

certificate number for the current study is H11-01931. To date, the research included in

this thesis has not been published in full. Preliminary results were published and

presented in abstract form at national and international conferences.

iv

Table of Contents

Abstract ................................................................................................................................ ii

Preface................................................................................................................................. iii

Table of Contents ................................................................................................................ iv

List of Tables ...................................................................................................................... vi

List of Figures .................................................................................................................... vii

List of Abbreviations ........................................................................................................ viii

Glossary .............................................................................................................................. ix

Acknowledgements ............................................................................................................ xii

Dedication ......................................................................................................................... xiv

Chapter 1: Introduction ........................................................................................................ 1

1.1 Voluntary Movement and the Neuromuscular System .............................................. 1

1.2 Motor Unit Characteristics ......................................................................................... 6

1.3 Task Specificity .......................................................................................................... 8

1.4 Control of Muscle Contractions ............................................................................... 12

1.6 Summary of Literature ............................................................................................. 15

Chapter 2: Aims and Hypotheses ....................................................................................... 17

2.1 Purpose ..................................................................................................................... 17

2.2 Aims ......................................................................................................................... 17

2.3 Hypotheses ............................................................................................................... 17

Chapter 3: Methods ............................................................................................................ 19

3.1 Participants ............................................................................................................... 19

3.2 Experimental Set-Up ................................................................................................ 19

3.3 Experimental Protocol .............................................................................................. 23

3.4 Data Analysis ........................................................................................................... 25

3.5 Statistical Analysis ................................................................................................... 30

Chapter 4: Results .............................................................................................................. 31

4.1 Subjects .................................................................................................................... 31

4.2 Surface EMG ............................................................................................................ 31

4.3 Steadiness ................................................................................................................. 33

4.4 MU Characteristics ................................................................................................... 34

v

Chapter 5: Discussion ........................................................................................................ 38

5.1 Limitations ............................................................................................................... 42

5.2 Future Research ........................................................................................................ 43

Chapter 6: Conclusion........................................................................................................ 45

References .......................................................................................................................... 46

Appendices ......................................................................................................................... 52

Appendix A: Copyright Approval for Figure 4 .............................................................. 52

Appendix B: Ethics Approval ........................................................................................ 53

Appendix C: Pre-Study Questionnaire ........................................................................... 54

Appendix D: Subject Characteristic Data ...................................................................... 56

vi

List of Tables

Table 1: Generalization of differences in MU activity between isometric and

anisometric contractions ........................................................................................ 9

Table 2: Maximal voluntary contraction strength .............................................................. 31

vii

List of Figures

Figure 1: Voluntary control of muscle contractions ............................................................ 4

Figure 2: Motor unit activity in an anisometric contraction (A) and functional task

(B). ..................................................................................................................... 12

Figure 3: Positional differences in force steadiness of the elbow flexors.......................... 13

Figure 4: Sex differences in elbow flexor force steadiness across submaximal force

levels. ................................................................................................................. 14

Figure 5: Experimental set-up ............................................................................................ 21

Figure 6: EMG and MU set-up .......................................................................................... 22

Figure 7: Representative visual feedback. ......................................................................... 26

Figure 8: Representative surface EMG data analysis. ....................................................... 28

Figure 9: Representative motor unit analysis..................................................................... 29

Figure 10: EMG activity of the long (A) and short (B) head of the biceps brachii

during the 4 different contractions. ................................................................... 33

Figure 11: EMG activity of the brachioradialis during the four different contractions. .... 34

Figure 12: Steadiness during the 5 phases of the dynamic contractions. ........................... 35

Figure 13: Motor unit discharge rate of the long (A) and short (B) head of the biceps

brachii during the 4 different contractions ........................................................ 36

Figure 14: Motor unit discharge rate variability in the 4 different contractions. ............... 37

Figure 15: Motor unit recruitment time between the 4 contraction types.......................... 38

viii

List of Abbreviations

CV: Coefficient of Variation

EMG: Electromyography

LH: Long Head

MU: Motor Unit

MVC: Maximal Voluntary Contraction

SD: Standard Deviation

SEM: Standard Error of the Mean

SH: Short Head

ix

Glossary

1a afferents: A primary sensory afferent fibre which wraps around the nuclear chain and

bag fibres of the equatorial region of the muscle spindle. These afferents detect changes in

muscle length and rate of change in length, thus they provide information about the

velocity and direction of the muscle stretch.

Acceleration: The rate of change of velocity.

Action Potential: A change in electrical potential across a cell membrane. For the

purpose of this thesis, the primary cells under consideration are nerve and muscle cells.

Anisometric Contraction: Dynamic muscle contraction in which the muscle changes

length through movement of the joints range of motion.

Coefficient of Variation: Ratio of the standard deviation over the average. Represents

the variability around the mean of the population.

Concentric Contraction: Muscle shortening in the production of force.

Corticospinal tract: A group of nerve fibres that carries motor commands from the brain

to the spinal cord.

Discharge Rate: The firing frequency of action potentials.

Discharge Rate Variability: A measure of inconsistency of action potentials from the

mean discharge rate of the population.

Eccentric Contraction: Muscle lengthening in the production of force.

x

Electromyography: Electrical recording of global muscle activity.

Heteronymous Muscle: A different muscle.

Homonymous Muscle: The same muscle.

Indwelling EMG: Electrical recording of a single muscle fibres activity through the

placement of wires within a muscle.

Isometric Contraction: A muscle contraction in which length and range of motion of the

joint remains isolated to a singular position.

Motor Neuron: Neurons that send motor command signals from the spinal cord to

muscles or other effector organs.

Motor Unit: A motor neuron and all of the muscle fibres it innervates.

Muscle Spindle: A type of proprioceptor that is responsible for detecting change in

muscle length. Located within the extrafusal muscle fibres and composed of intrafusal

muscle fibres.

Nuclear Bag Fibres: A type of dynamic intrafusal muscle fibre located in the muscle

spindle.

Nuclear Chain Fibres: A type of static intrafusal muscle fibre located in the muscle

spindle.

Proprioceptors: Sensory receptors that provide information regarding the position of the

body, particularly the musculoskeletal system, in space.

xi

Recruitment Threshold: The force, or for the purpose of this thesis the time at which a

motor unit is first activated.

Steadiness: The ability to maintain a consistent, constant movement without wavering.

xii

Acknowledgements

First, and foremost I would like to acknowledge my supervisor, Dr Jennifer

Jakobi. Jenn, you are a true inspiration. Your passion for research, dedication to your

students and love of your family all gets balanced in an unbelievable way. Thank you for

sharing your knowledge with me and helping me grow as a researcher. Thank you for all

of your patience, support and understanding throughout these past years. None of this

would have been possible without you and I am truly grateful.

Next I would like to acknowledge Noelannah and Kaitlyn. Noelannah, thank you

for being my second hand throughout all of my data collection. I really appreciate all of

the long hours your put in with me. Thank you for all of the good memories in the lab and

for being a true friend. I couldn’t have asked for a better person to share this experience

with. Kaitlyn, thank you for introducing me to so many aspects of research and helping

me learn my way in the field. You were a true mentor to me and I will always look up to

you as a researcher.

Thank you to my committee, Bruce, Gareth and Paul for supporting me

throughout this journey and showing an interest in the research I conducted.

To all of my fellow research students, especially the lab next door, thank you for

all of the potlucks, weekend trips and outings, laughs and memories that I will always

treasure. You made this experience so enjoyable and I can’t imagine any of it without

you.

To my family, thank you for all of your love and support throughout these past

two years. Thank you for always being there for me, helping whenever I needed,

listening, and providing some much needed relief during stressful moments of research. I

xiii

love you all. Finally, to Hugh. Thank you for all of your love and support, no matter what

I choose to do. You are my rock in so many ways – the person I lean on when things get

tough, the person who can make me smile, and the person I can count on. The things you

do for me do not go unnoticed; I truly appreciate everything you do.

xiv

Dedication

For Great-Grandma. Forever in my heart.

1

Chapter 1: Introduction

The ability to perform purposeful tasks such as drinking or pouring water, brushing

your teeth and washing your face requires muscle control. This control occurs at many

levels of the central and peripheral nervous system with the final point of organizational

output being the motor unit (MU). A MU is composed of a single motor neuron and all of

the muscle fibres it innervates. Extensive literature exists on MU activity and force

control for isometric contractions, but there are minimal scientific reports for MU activity

during anisometric contractions and no information on functional movement. Isometric

contractions are muscle contractions in which no change in joint angle or muscle length

occurs, whereas anisometric contractions involve the joint moving through a range of

motion during the muscle contraction. Functional movement also involves the joint

moving through a range of motion, but there is a pre-planned objective of executing a task

for a goal directed outcome. Each of these types of movements require specific motor

programming that is typically initiated from higher order centres in the brain where neural

impulses are transmitted down the central nervous system through to the peripheral

nervous system where ultimately skeletal muscle generates the programmed action.

However, as the action of the task increases in complexity the programming and

coordination of the muscular system becomes more involved (Verstynen, Diedrichsen,

Albert, Aparicio, & Ivry, 2005).

1.1 Voluntary Movement and the Neuromuscular System

The generation of voluntary movement requires processing of information from the

brain throughout the nervous system and down to the level of the muscle. The primary

motor cortex of the brain sends signals via the corticospinal tract through the spinal cord.

The corticospinal tract is one of the largest in the spinal cord and two thirds of its axons

2

originate in the motor cortex (McArdle, Katch, & Katch, 2010). The corticospinal and

rubrospinal tracts compose the pyramidal tract which activates skeletal muscles for

voluntary control. The extrapyramidal tract, on the other hand, contains axons that

originate in the brainstem and control posture and muscle tone. The signals from the

primary motor cortex, both excitatory and inhibitory, travel down, for example, the

corticospinal tract and converge on the cell bodies of the alpha motor neurons located in

the grey matter of the anterior horn of the spinal cord (Seeley VanPutte, Regen, & Russo,

2011). If the nerve cell membrane reaches the threshold potential for excitation of

approximately -55mV, an action potential occurs and travels along the axon of the alpha

motor neuron to the muscle fibres it innervates (Figure 1). Motor neurons receive inputs

from both ionotropic and neuromodulatory inputs (Heckman, Gorassini, & Bennet, 2005).

The descending tracts as well as sensory afferent fibres are involved in transmission of

ionotropic inputs. These result in neurotransmitters being released and binding to ligand-

gated channels to generate excitatory postsynaptic potentials or inhibitory postsynaptic

potentials (Heckman, Johnson, Mottram, & Schuster, 2008). Neuromodulatory inputs, on

the other hand, involve G protein receptors which activate signalling cascades to alter

electric properties of the cell. Both of these types of inputs together determine the

outcome of the motor neurons activity.

The area connecting the motor nerve and the muscle fibre is known as the

neuromuscular junction. The neuromuscular junction consists of the plasma membrane of

the motor nerve, the synaptic cleft or space between the nerve and muscle, and the motor

end plate which is located on the sarcolemma of the muscle fibre. The action potential

must be transmitted across this junction to successfully reach the skeletal muscle fibres.

The action potential in the alpha motor neuron causes acetylcholine which is packaged in

vesicles to migrate to the pre-synaptic membrane, dock and fuse to the membrane for

3

release into the synaptic cleft (Seeley et al, 2011). The electrical impulse has now been

converted to a chemical stimulus. The acetylcholine crosses the synaptic cleft and

attaches to the post-synaptic membrane (located on the muscle fibre). This binding causes

the sodium potassium channel to open. Sodium then moves in and potassium slowly

moves out. This causes a charge difference across the membrane. When the threshold (-

55mV) is reached an action potential occurs and travels through the transverse tubule

system. This excitation changes the permeability of the skeletal muscle membrane

activating voltage-gated calcium channels. Calcium is released from the sarcoplasmic

reticulum and binds to troponin-tropomyosin complex located on the actin filaments of

the muscle fibre. The binding of calcium causes the shape of troponin to change removing

tropomyosin from the binding sites on the actin filament. This movement opens the

binding sites between myosin and actin and initiates cross-bridge formation and cycling

between actin and myosin. This cycle of binding and a powerful stroke between actin and

myosin with the subsequent detachment of these proteins due to the phosphorylation of

adenosine triphosphate is the basis of a cross bridge cycle and generates muscle

contraction (McArdle et al, 2010).

4

Figure 1: Voluntary control of muscle contractions

The primary motor cortex of the brain sends signals via the lateral

corticospinal tract through the spinal cord. The signals cross sides in the medulla

oblongata through the pyramidal decussation and travel the opposite side of the

spinal cord than the side of the brain where the original signal was initiated. The

signals from the primary motor cortex, both excitatory and inhibitory, converge on

the cell bodies of the alpha motor neurons, and interneurons (not shown), located

in the grey matter of the spinal cord. If threshold is reached, an action potential

occurs and travels along the axon of the alpha motor neuron to the muscle fibres it

innervates. The area between the end of the motor neuron and the start of the

muscle fibre is known as the neuromuscular junction. The electrical impulse is

converted to a chemical stimulus which generates muscle contraction.

Two important sensory organs, the muscle spindle and the golgi tendon organ, play

an additional and necessary role in the control of voluntary movements. These organs

monitor movement and modify subsequent behaviour of the muscles (MacIntosh,

5

Gardiner, & McComas, 2006). The intrafusal fibres of the muscle spindle are important

for coordination of planned and learned movements as they provide feedback of changes

in muscle length. The muscle spindle is located within the muscle and is composed of

intrafusal fibres that are surrounded by the extrafusal muscle fibres. The nuclear bag and

nuclear chain intrafusal fibres are proprioceptors that are responsible for revealing

information about the static length and rate of change (velocity) of muscle length,

respectively. These intrafusal fibres which comprise the muscle spindles are innervated by

gamma motor nerves which increase the sensitivity of the muscle spindle to stretch. The

necessary output information detected by the muscle spindle travels via the 1a afferent to

the spinal cord and synapses onto tracts ascending to the brain, to other motor neurons

innervating the homonymous muscle as well as interneurons which synapse with cell

bodies of both homonymous as well as heteronymous muscle. These homonymous and

heteronymous inputs are organized through polysynaptic interneurons which interact with

motor neurons to initiate inhibition and/or excitation of agonist and antagonist muscles on

both sides of the body (MacIntosh et al, 2006). The strongest of these connections are

made to motor neurons which supply contractile tissue in the same area as the spindle

itself. This arrangement allows for functional compartmentalization and may allow

certain MUs to be preferentially activated for given tasks. Golgi tendon organs provide

further information on muscle forces during normal activities. These proprioceptors are

located in the musculotendinous junctions rather than the muscle belly. The golgi tendon

organ is most sensitive to changes in contractile force. Thus, both the muscle spindle and

golgi tendon organs provide proprioceptive information via reflex feedback to the motor

neuron pool to assist in regulating voluntary movement (MacIntosh et al, 2006).

6

1.2 Motor Unit Characteristics

A MU is defined as the alpha motor neuron and all of the muscle fibres it

innervates. The MU controls muscle contractions through the number of fibres activated

(recruitment) and the speed at which they discharge (discharge rate). The cell body of the

motoneuron within the spinal cord determines the order in which a MU is activated. MU

recruitment is orderly and occurs from the smallest cell body to the largest (Henneman,

1957). Discharge rate also increases from the lowest level which occurs at recruitment

until force is maximal. The earliest recruited units typically have the highest discharge

rates, as defined through the onion skin phenomenon (De Luca and Erim, 1994). MUs are

organized in the spinal cord according to the muscle they innervate and potentially via

their contribution to various tasks (ter haar Romeny, Denier van der Gon, & Gielen,

1984). The term motor neuron pool describes the group of MUs that innervate a muscle

(McArdle et al, 2010). MUs consisting of only a small number of muscle fibres tend to be

involved in precise movements, whereas MUs with motor neurons that innervate a large

number of fibres are typically involved in more powerful, gross movements. Force control

between precision and forceful contractions are generally controlled via recruitment of

MUs or rate coding strategies of the recruited MUs. Seki and Narusawa (1996)

demonstrated that during precise movements rate coding is predominantly used to control

force; however, during the more powerful movements recruitment of MUs is the primary

means to regulate force. Functional tasks require more precise movements to complete a

task accurately and it is plausible that rate coding strategies might be the primary means

for force control. Whereas the more gross, simple anisometric and isometric contractions

may not require the same degree of rate coding strategies. Thus, MU discharge rates are

likely to occupy a more prominent role in the control of functional tasks.

7

The type of fibre each motor neuron innervates is determined by the classification

of the MU. MUs are differentiated through three distinct types; fast fatigable, fast fatigue-

resistant and slow. Fast fatigable MUs produce high amounts of force at a high

contraction speed; however, they fatigue quickly. The motor neurons of these MUs

innervate fast glycolytic muscle fibres. Fast-fatigue resistant MUs produce moderate

amounts of force at a high contraction speed and do not fatigue as quickly. The motor

neurons of these MUs innervate the fast oxidative glycolytic muscle fibres. Finally, the

slow MUs produce low amounts of force at slow contraction speeds and do not fatigue

easily. Slow oxidative muscle fibres are included in these MUs. The slow twitch units are

generally recruited first as they have the lowest voltage threshold for activation as

described by Henneman’s size principal (1957). Voltage thresholds determine when a

MU is activated. Smaller MUs, which include the slow-twitch units, require less

depolarization to reach threshold. As the contraction continues the fast-fatigue resistant

and fast-fatigable MUs become active which have a higher threshold for activation. This

strategy is beneficial for the production and control of movement as the more fatigue

resistant fibres are activated first and for the longest period of time. Further it ensures that

a smooth force increment occurs because at low efforts smaller units are recruited, thus

the change in force occurs in small increments with the primary means of force control

being MU discharge rate. As the level of voluntary drive increases, the threshold for

activation is achieved and larger MUs becoming engaged. Recruitment of each additional

MU results in greater increments in force (Henneman & Olson, 1965; Zajac & Faden,

1985).

Although the size principle is widely accepted by scientists, it has typically been

described for slow ramp forces. Current literature indicates that MUs can also be

selectively recruited for particular movements. In rapid, powerful movements, fast-twitch

8

fatigue resistant fibres are recruited first instead of slow oxidative fibres and then fast-

twitch fatigable fibres follow (McArdle et al, 2010). This differential activation relative to

particular movements is further explained by the concept of task specificity.

1.3 Task Specificity

A concept known as task specificity suggests that descending drive and the

subsequent MU activity vary depending on the task being performed. Both the intent to

perform a specific task and the need for more careful control to properly execute the task

may cause neuromuscular control processes to differ between tasks. An example of this

can be shown through studies which examined both force and position tasks. In these

studies a force task was one in which a subject pulled up against resistance with the arm

fixed into a traditional isometric set-up with the joint angle fixed. In the position task, a

load was hung from the participants arm and the joint was not fixed, thus the objective of

the task was to maintain the initial arm position with a similar load to the force task. The

force tasks were found to have significantly higher MU discharge rates than the position

task and the force tasks were also steadiest (Hunter, Ryan, Ortega, & Enoka, 2002;

Mottram, Jakobi, Semmler, & Enoka, 2005). This suggests that there are different control

strategies based on the demand of the task and these strategies in neural activity influence

the movement outcome. Supporting evidence for this level of task specificity is also

demonstrated through studies which examined isometric and anisometric contractions.

Anisometric contractions are more difficult to execute than simple isometric contractions

and force control is lower during the anisometric contractions (Burnett, Laidlaw, &

Enoka, 2000; Graves, Kornatz, & Enoka, 2000).

Measurement of arm muscles (biceps brachii, brachioradialis, and anterior deltoid)

in loaded anisometric contractions (concentric and eccentric phases) and isometric

9

contractions suggested task specific differences of greater MU activity in anisometric

contractions than isometric contractions (Andrew, 1985; Linnamo, Moritani, Nicol, &

Komi, 2002; Moritani, Muramatsu, & Muro, 1988). This difference in MU activity is

also evident at the level of the whole muscle. For example, electromyography (EMG) was

greater in the anisometric contraction and the recruitment threshold was lower compared

to the isometric contractions (Theeuwen, Gielen, & Miller, 1994). MU discharge rate also

differed between these two types of contractions. The anisometric contractions have

higher discharge rates than the isometric contractions (Harwood, Davidson, & Rice, 2010;

Tax, Denier van der Gon, Gielen, & van den Temple, 1989). Finally, recruitment

threshold is lower in anisometric contractions than isometric contractions (Table 1). This

suggests that the net synaptic input onto the motor neuron pool differs between isometric

and anisometric contractions and offers further evidence for the concept of task specificity

(Enoka, 1995).

Table 1: Generalization of differences in MU activity between isometric and

anisometric contractions

Isometric Anisometric

MU Recruitment

Threshold

Higher Lower

MU Discharge

Rate

Lower Higher

Many functional tasks necessary for daily living (drinking from a cup, pouring a

glass of water from a jug, etc) contain both the eccentric and concentric phases of

anisometric muscle contractions. Thus, further to the differences between isometric and

anisometric contractions there may be additional differences within anisometric

contractions between the two phases of the movement. Two reviews have extensively

evaluated the control strategies for eccentric and concentric contractions (Duchateau &

10

Baudry, 2013; Enoka, 1996). It remains unclear whether the MU recruitment threshold

differs between eccentric and concentric contractions as studies evaluating differences at

high contraction velocities reported that high-threshold units were preferentially activated

in eccentric contractions compared with concentric contractions (Nardone, Romano, &

Schieppati, 1989). However, more recent studies have not reported these same differences

in recruitment during slower contractions (Bawa & Jones, 1999; Laidlaw, Bilodeau, &

Enoka, 2000; Sogaard, Christensen, Jensen, Finsen, & Sjogaard, 1996; Stotz & Bawa,

2001) and instead recruitment occurs similar to concentric contractions. MU discharge

rate, on the other hand, is consistently reported as differing between eccentric and

concentric contractions. Discharge rate is lower during eccentric contractions compared to

concentric contractions (Del Valle & Thomas, 2006; Laidlaw et al, 2000; Semmler,

Kornatz, Dinenno, Zhou, & Enoka, 2002; Sogaard et al, 1996; Stotz & Bawa, 2001; Tax

et al, 1989). Duchateau & Baudry (2013) state that these results suggest rate coding is

controlled differently between these two contraction types. Fang, Siemionow, Sahgal,

Xiong & Yue (2001) showed that this unique strategy occurs because the brain plans and

processes eccentric contractions in a unique manner to concentric contractions. EEG

derived movement-related cortical potentials (both positive and negative) were higher in

eccentric contractions than concentric contractions. The higher negative potential suggests

that more cortical planning is required for eccentric contractions and the higher positive

potential indicates that sensory feedback is used to a greater degree for eccentric

contractions (Fang et al, 2001). Overall, if differences occur within anisometric

contractions between eccentric and concentric aspects it is likely that unique activation

strategies will also exist between functional tasks and anisometric contractions. As most

functional tasks encompass both the eccentric and concentric aspects of muscle

contraction, MU activity during all phases of a functional task must be considered.

11

No published studies to date have examined MU activity during a functional task.

During a pilot MU experiment when a subject drank from a cup MU patterns were

observed to differ between this movement and a controlled shortening contraction (Figure

2). Within the functional task MUs were recruited earlier and both the discharge rate and

discharge rate variability were higher relative to the control shortening task. This pilot

work, offers further evidence of task specific activation patterns. There is also evidence to

suggest that with a muscle there are selective compartments activated independently of

other regions of the muscle in response to a given task (Brown, Edwards, & Jakobi, 2010;

Harwood, Edwards, & Jakobi, 2008). For example, ter Haar Romeny and colleagues

(1984) found that motor units in different areas of the LH of the biceps brachii were

selectively activated depending on the task that was being performed (lateral MUs for

flexion but medial MUs for supination). Furthermore, these groups of MU may have

specific central connections and recruitment patterns specialized for goal-directed tasks

(Hoffer et al, 1987). Thus, it is hypothesized that MU activity will be greater (earlier MU

recruitment and higher discharge rate) in functional tasks than simple, anisometric

contractions. Furthermore, the task with the highest MU discharge rates will be most

steady.

12

Figure 2: Motor unit activity in an anisometric contraction (A) and functional task

(B).

The functional task has earlier recruitment of MUs, more MUs overall are

recruited, and the discharge rate is more variable compared to the anisometric

contraction. This provides a clear visual indication that MU activity is different in

functional tasks than anisometric contractions. sec, seconds; SH, short head of the

biceps brachii; mV, millivolts.

1.4 Control of Muscle Contractions

Steadiness is the ability to maintain a muscle contraction at a given target or force

and offers a quantitative measure of functional ability. Steadiness varies with many

different conditions including position (Figure 3), age, intensity of contraction and sex

(Figure 4) (Brown et al, 2010; Harwood et al, 2008; Tracey & Enoka, 2002). When the

forearm is in the pronated position, steadiness is significantly less than when the forearm

is in the neutral or supinated position (Figure 3). Further, older adults are significantly

less steady than younger adults (Harwood et al, 2008) and males are steadier than females

(Brown et al, 2010) (Figure 4). The underlying cause for force steadiness differing across

13

positions, between sexes and with age remains uncertain. Muscle strength contributes to

these position, sex- and age-related differences in steadiness (Brown et al, 2010);

however, MU activity and MU discharge rate variability may also play a role (Graves et

al, 2000; Marmon, Pascoe, Schwartz, & Enoka , 2011). In addition, steadiness may vary

depending on the task being performed.

(Unpublished data from Neuromuscular lab, UBCO)

Figure 3: Positional differences in force steadiness of the elbow flexors.

In the pronated forearm position subjects are less steady compared to the

neutral forearm position across different submaximal force levels for isometric

contractions. Subjects were assessed at 2.5, 10, 17.5 and 25% of MVC. CV,

coefficient of variation which is inversely related to force steadiness (the higher

the number the less steady the contraction); MVC, maximal voluntary contraction.

*Pronated significantly less steady than neutral position (p<0.001)

14

(Obtained from Brown et al, 2010) (Appendix A)

Figure 4: Sex differences in elbow flexor force steadiness across submaximal

force levels.

Force steadiness is represented as the coefficient of variation which is

inversely related to force steadiness. Men were significantly steadier than women

across all submaximal force levels. CV, coefficient of variation; MVC, maximal

voluntary contraction. *Women significantly less steady than men (P<0.05)

Steadiness is a greater predictor of functional performance than strength (Marmon

et al, 2011; Seynnes et al, 2005); however, no studies to date have examined MU activity

and steadiness in functional tasks relative to anisometric or isometric contractions. In

anisometric and isometric contractions steadiness declines with advancing age and this

effect is greater in females relative to males (Brown et al, 2010, Harwood et al. 2010).

Greater age-related declines in functional ability in females relative to males is well

established and is often attributed to alterations within the musculoskeletal system relative

to muscle size and quality (Ganesh, Fried, Taylor, Pieper, & Hoenig, 2011; Harwood et

al. 2010); yet, the contribution of the nervous system remains less clear. The

neuromuscular system and MU activity is traditionally studied less frequently in females

than males, yet they experience greater and earlier decline of functional ability. It is

15

important to determine whether differences in MU activity are evident between functional

tasks and lab-based anisometric contractions in this population to better understand the

earlier functional decline females’ experience. Teasing out the differences between

functional tasks and anisometric contractions will help in understanding the underlying

cause of functional change, and facilitate the design of training/rehabilitation programs to

help females maintain functional independence.

1.6 Summary of Literature

To date, no studies have examined MU activity in purposeful, functional tasks. It is

well established that recruitment of MUs is lower in anisometric contractions compared

with isometric contractions (Harwood et al, 2010; Ivanova, Garland, & Miller, 1997;

Linnamo et al, 2002; Tax et al, 1989; Theeuwen et al, 1994). The lower recruitment

threshold suggests that MUs are activated at a lower force level in anisometric

contractions compared with isometric. Recruitment threshold of MUs is generally

believed to follow Henneman’s size principal (1957); however, this fundamental tenet is

now seen as a more integrative approach to neuromuscular activation. Hodson-Tole and

Wakeling (2008) suggest that mechanics, sensory feedback (Basmajian, 1963) and

descending drive all influence the recruitment of MUs and thus differences would occur

between functional tasks and anisometric contractions as a consequence of these

contributing variables that differ between conditions.

It is also well established that MU discharge rates are higher in anisometric

contractions compared with isometric contractions. (Harwood et al, 2010; Ivanova et al,

1997; Linnamo et al, 2002; Tax et al, 1989; Theeuwen et al, 1994), as discharge rate

increases linearly with both velocity and intensity of contractions (Harwood et al, 2010).

Further, it has recently become clear that discharge rate does differ between eccentric and

16

concentric phases of anisometric contractions. Discharge rate is lower during the eccentric

phase than the concentric phase (Duchateau and Baudry, 2013). Both MU recruitment and

discharge rate are likely to further differ in functional movement as the complexity of

contraction continues to increase from isometric to anisometric to functional tasks.

Because steadiness is a good predictor of functional performance (Marmon et al,

2011) it is likely that steadiness will also differ between these three contraction types. As

functional tasks require the greatest amount of control to properly execute it is plausible

to assume that the functional tasks will produce the steadiest contractions. Females are

less steady than males and experience functional decline at an earlier age. Thus, it is

important to understand the spinal control of functional tasks, particularly in females, as

performance of functional tasks is essential to daily living. Differences in spinal control

during functional movement will highlight that these contractions are mechanistically

different from laboratory based anisometric and isometric conditions. Increased motor

unit activity during these functional movements will indicate that spinal control is not

only task specific, but sensitive to goal directed outcomes that involve higher level of

processing.

17

Chapter 2: Aims and Hypotheses

2.1 Purpose

The purpose of this study was to evaluate MU activity and steadiness between

controlled lab-based anisometric contractions and functional tasks in young women.

Further, specific differences in the eccentric and concentric aspects of the contractions

were examined.

2.2 Aims

a. To determine whether the addition of a specific purpose, namely lifting a

waterbottle and drinking, will lower MU recruitment thresholds.

b. To determine whether the addition of a task specific purpose will change MU

discharge rate

c. To compare the concentric and eccentric phases of the contractions and determine

whether the MU discharge rates which are reported to be lower in the eccentric

phase remains consistent for anisometric and functional tasks.

d. To evaluate steadiness between the contraction types.

2.3 Hypotheses

a. MU recruitment threshold will be lower in the water drinking functional task

compared with anisometric contractions which will be lower compared with

isometric contractions.

18

b. MU discharge rate will be higher in the water drinking functional task compared

with anisometric contractions which will be higher compared with isometric

contractions.

c. The eccentric (lowering) phase of all contractions will maintain a lower discharge

rate regardless of the addition of a specific purpose.

d. The water drinking functional task will produce the steadiest contraction.

19

Chapter 3: Methods

3.1 Participants

Thirteen healthy female subjects aged 18 – 30 years were recruited for this study.

Subjects were excluded it they had current or previous neuromuscular disease, were male,

were under the age of 18 or over the age of 30 years, or participated in high level of

athletic training (ie elite athletes or varsity athletes). Further, participants were screened

for skilled task training in activities that require fine motor control such as musicians and

artists. Persons practiced in fine motor control were excluded, as training alters MU

activation strategies (Semmler & Nordstrom, 1998). All subjects were tested during the

follicular phase (days 1-13) of their menstrual cycle and were currently taking oral

contraceptives to control for hormone levels.

Subjects were asked to refrain from caffeine 12 hours prior to testing and from any

physical activity on the day of testing. Informed and written consent was obtained from

all participants prior to participation in the study. All procedures were approved according

to the Clinical Research Ethics Board at the University of British Columbia and

conformed to the Declaration of Helsinki (Appendix B).

3.2 Experimental Set-Up

Muscle activity was recorded from the SH and LH of the biceps brachii, triceps

brachii, and brachioradialis. Intramuscular EMG was recorded from the SH and LH of the

biceps brachii to assess MU recruitment and discharge rates. Acceleration was recorded at

the subject’s wrist.

Subjects were seated in a custom-made chair and visual feedback was provided on

19-in computer screen monitor (1280 x 1024 resolution) located 1 m in front of

20

The screen was located slightly off centre, in front of the subject’s right arm, to

that both the screen and the waterbottle were in the line of vision for all phases of

for lifting and lowering the waterbottle as well as drinking through the straw. The

distance between the subject’s eyes and the screen was measured as was the angle

between the subject’s eyes and the centre of the computer monitor to ensure that

subjects were receiving the same amount of visual feedback. The range of motion

required to move the waterbottle from the starting position to place the straw in

subject’s mouth was measured to ensure all contractions were performed over the

distance (

Figure 5).

Bipolar surface electrodes (4mm Ag/AgCl) were placed over the muscle belly of the

SH, LH, brachioradialis and wrist flexors with an inter-electrode distance of

approximately 1 centimetre. Reference electrodes for the SH and LH of the biceps brachii

were placed over the acromion process of the scapula. The lateral epicondyle was used as

the bony prominence for reference of the brachioradialis and the medial epicondyle for

the wrist flexors (Figure 6). The skin of the arm and forearm was exfoliated with low

friction cleansing pads and 70% isopropyl alcohol swabs to allow better conductivity for

the electrodes. The signal from the surface EMG was sampled at 1024 Hz (1401, CED,

Cambridge, England) amplified (x 500) with an isolated bioamplifier (Coulbourn

Electronic, Allentown, Pennsylvania), band-pass filtered (8Hz – 500Hz) (Coulbourn,

Allentown, Pennsylvania) and converted from analog to digital format (CED, Cambridge,

England). An accelerometer (V94-41 Acceleration 10G, Coulbourn, Allentown,

Pennsylvania) was taped to the subject’s wrist. The recording from this device was

displayed in real-time on the computer screen monitor to provide the subject with visual

21

feedback of the movements. Acceleration data was also recorded to allow calculation of

steadiness during the contractions.

22

Figure 5: Experimental set-up

Representative diagram of the experimental set-up. Subjects sat in a

custom-made chair so that their hips and knees were at 90°. A stool was provided

for their feet to rest on. A table was located above their legs and the waterbottle

rested on this prior to the contraction. Visual feedback was displayed on a

computer monitor that was 1m in front of the subject. The distance between each

subjects eyes and the angle from the centre of the computer screen to the eyes was

measured to ensure all subjects were receiving similar visual feedback. The

distance between the initial position of the waterbottle on the table and the position

where the straw reached the subjects mouth (showing in the dotted lines) was

measured with a goniometer to ensure all contractions were performed through the

same range of motion.

23

Figure 6: EMG and MU set-up

Representative picture of the EMG and MU set-up. Not all surface

electrodes and fine wires are visible. The surface electrodes recording the LH of the

biceps brachii are visible. All other electrode pairs were attached in a similar

fashion. Part of the electrodes on the SH of the biceps brachii are evident on the

medial aspect of the arm. The triceps brachii electrodes are located on the posterior

aspect of the arm and cannot be seen in this picture. The electrodes located under

the mesh netting, inferior to the elbow joint are recording brachioradialis EMG.

The mesh netting was used for additional securement of the wires during the

movement which assisted in reducing movement artefact in the signals. Ground

electrodes are apparent on the elbow, shoulder, and clavicle. The two clips located

to the right of the preamplifier (black box) were used to hook the fine wire

electrodes into the preamplifier. The fine wire is inserted into the LH of the biceps

brachii between the two surface electrodes. A similar set-up was used with clips and

amplifiers for the fine wires of the SH.

Custom-made fine wire electrodes were used to record single MU activity. Three

wires (25-50 µm diameter, California Fine Wire, Grover Beach, California) of

approximately 25 cm in length were aligned and the ends were glued together. These fine

24

wire units were threaded through a one-inch long 25 gauge hypodermic needle. The wires

and hypodermic needles were then autoclaved for sterilization. One hypodermic needle

was inserted into the muscle belly of the SH of the biceps brachii and a second one into

the LH of the biceps brachii (Figure 6). The wire was placed approximately 2.5

centimetres into the muscle; pending the size of each individual muscle. Immediately

after insertion the hypodermic needle was withdrawn and the wires remained in-place for

the duration of the experiment. The ends of the wires not inserted into the muscle were

separated. The coating of the ends of the wires was burned off and scraped with

sandpaper to enhance contact and reduce impedance. These were attached to a custom-

made pre-amplifier that amplified the signal 10 times (University of Windsor). If a clear

signal was not obtained the remaining third wire was used instead of one of the original

two wires. A ground surface electrode was placed over the medial end of the clavicle for

the SH, and on the lateral end of the clavicle for the LH. The MU signal was sampled at a

rate of 16 666 Hz, amplified (100-1000 times) and band-pass filtered (8Hz – 1kHz)

(Coulbourn, Allentown, Pennsylvania). Intramuscular EMG was converted from analog

to digital format by a 16-bit A/D converter (1401 plus, CED, Cambridge, England).

3.3 Experimental Protocol

Participants attended the lab on one occasion for this study. Subjects completed

the Edinburgh handedness questionnaire (Oldfield, 1971) as well as a background

information questionnaire regarding hobbies, physical activities and previous employment

(Appendix C). Subjects performed three isometric maximal voluntary contractions

(MVCs) of each of the biceps brachii, triceps brachii and brachioradialis so that all force

data could be normalized. MVCs were held for 5-seconds each and approximately 2

minutes of rest was given between each trial. If the MVCs were not consistent additional

25

attempts were performed to ensure subjects reached their true maximal effort. Strong

verbal encouragement was given during the MVC contractions. The highest MVC was

used as the subject’s maximal effort. Subjects then performed the 4 experimental

contractions in a randomized order with approximately 2 minutes of rest between each

contraction. Three trials of each condition were conducted resulting in a total of 12

experimental contractions. The functional task was a waterbottle drinking task. The 3

other tasks (waterbottle lift, anisometric and isometric contractions) were matched to this

contraction. All tasks were performed with the wrist in the neutral forearm position and

were matched for shape, weight, velocity, acceleration, and range of motion.

The waterbottle drinking task consisted of lifting a waterbottle and the attached

straw (800 grams) off of a table and holding this initial position for 5 seconds. The bottle

was then slowly raised towards the subjects’ mouth over ten seconds. A second hold was

performed in which the subject held the bottle in this lifted position for 5 seconds. In this

task the subject took a sip of water through the straw. The bottle was then lowered back

towards the table over 10 seconds and then a final 5 second hold was executed just above

the table surface. The entire contraction took 35 seconds and subjects were provided with

visual feedback of acceleration throughout the entire task (Figure 7).

The visual feedback displayed real-time tracings of the acceleration output.

Horizontal lines were set to display the maximum and minimum acceleration and subjects

were instructed to keep acceleration within this limited range. Vertical cursors were used

to display when a movement started or changed. Six vertical cursors were evident on the

screen to indicate the individual phases of the task: pick up the bottle and hold, lifting,

holding near the face, lowering, holding just above the table, and release of the water

bottle (Figure 7). Verbal feedback was also provided to indicate the initiation and

cessation of the task.

26

The second task was a waterbottle lift task. In this task an identical movement to

the waterbottle drink contraction was performed; however, during the holding phase near

the face no sip of water was taken. Third, a simple, anisometric control contraction was

also performed. For this contraction a waterbottle without a straw was weighted with lead

sinkers to equal the waterbottle drink and lift task (800 grams). The weights were equally

distributed and fastened to the inside of the bottle. Hand position, timing of the

movement and range of motion were matched to the drinking and lifting tasks, but the

subjects were not required to drink. Instead a 5 second hold was performed at the end of

the same range of motion of the prior two (waterbottle drink and waterbottle lift) tasks.

The last of the four contractions executed was a basic isometric contraction in

which the subject pulled up against a resistance while their arm was immobile. The target

force was matched to the other tasks and displayed on the computer monitor. Visual

feedback of force output was displayed. This contraction was held for a 35 second

duration to match the time of the other tasks. Isometric contractions were performed as a

second control contraction in order to confirm the results currently reported in the

literature between isometric and anisometric contractions and offer a further comparative

for the functional tasks.

3.4 Data Analysis

Data analysis was performed using custom-scripts for Spike 2 (CED, Cambridge,

England). Surface EMG signals were rectified and integrated (Figure 8). Integrated

surface EMG is calculated as the area under the rectified EMG signal. Averages for 0.5

second windows of the integrated EMG signal were obtained for each phase of the

contraction (1st hold, lift, 2nd hold, lower, 3rd hold). The 3 hold phases (1st hold, 2

nd

hold, 3rd

hold) were each 5 seconds in duration. The first two seconds and the last second

27

Figure 7: Representative visual feedback.

The tracing in the middle of the figure is the „live‟ feedback representation

of acceleration. The two horizontal lines are the maximum and minimum

acceptable accelerations with the target being zero acceleration achieved by

keeping the live tracing flat and in the middle of the two lines. The vertical lines

indicate when a movement starts or changes. At the first line which is labelled

“HOLD” the subject held the object off the table. At the LIFT line the subject

slowly raised the bottle to the mouth. At the 2nd HOLD line the subject held the

position at the top of the range of motion. The LOWER line indicated the object

was to be lowered. The final HOLD line indicates the last phase just above the

table and the RELEASE line is when the contraction ends. The horizontal axis

displays the time and the vertical axis the acceleration. This feedback was given

for the waterbottle functional task as well as the anisometric matched contraction.

were excluded to account for movement into and out of the phase to ensure that the

subject was holding the contraction, thus the analysis was done for 2 seconds at a stable

joint angle. The two movement phases (lift, lower) were 10 seconds in duration. The first

second was excluded as subjects were accelerating to begin the movement. The next 4

seconds were analyzed while the subject was moving at a constant velocity with no

acceleration. The final 5 seconds were excluded as some subjects had a smaller range of

motion and reached their mouth in less than 10 seconds. This ensured that the part of the

28

integrated EMG signal that was analyzed represented the movement phase. Surface EMG

from each phase of all contractions was divided by the maximal EMG obtained during the

MVC contractions. This allowed relative comparisons to be made between isometric,

anisometric and functional contractions and between the 4 muscle groups.

Steadiness was calculated as the coefficient of variation of the absolute

acceleration. Coefficient of variation is inversely related to steadiness, therefore the

higher the coefficient of variation the lower the force steadiness. The absolute level of

acceleration was used because the target acceleration for these contractions was

established to be zero. Thus, averaging the deviations resulted in signal cancellation

between the negative and positive phases giving a falsely low variation of acceleration.

By taking the absolute values (positive number) the calculated deviation was more

representative of the variation in acceleration.

Indwelling EMG recordings were high-pass filtered and the slope calculated to

make tracings clear for MU analysis. MU recordings were analyzed using a template

matching algorithm which allowed MUs to be identified based on waveform shape and

size. Templates were created and overlaying of subsequent action potentials allowed

coding to be completed so that all action potentials belonging to the same MU train were

assessed within one group (Figure 9). Visual inspection was then conducted to ensure

every single MU action potential was properly coded. MUs were analyzed if there was a

minimum of 6 discharges. Importantly, MUs were tracked between all contraction types

(isometric, anisometric, waterbottle lift, and waterbottle drink). MUs present in all 4

contraction types were coded the same so that comparisons between contractions could be

made for specific MUs. However, fewer action potentials were evident in the isometric

contraction thus MUs were also tracked if they were only present in the anisometric and

functional contractions. Any MUs only appearing in one contraction were analyzed and

29

coded individually for separate analysis of additionally recruited MUs. Sixty-eight MUs

were analyzed and tracked throughout the contraction types in the SH of the biceps

brachii and 53 in the LH of the biceps brachii.

Figure 8: Representative surface EMG data analysis.

Panel a (bottom tracing) shows the interference pattern of the surface

EMG recording from the SH of the biceps brachii. Panel b (second tracing) is the

rectified EMG output for the same task for the entire contraction duration. Panel c

shows the integrated EMG output for the 5 phases of the contraction. All EMG

tracings are shown in millivolts. SH, short head of the biceps brachii; v, volts;

iEMG, integrated EMG; sec, seconds.

30

Figure 9: Representative motor unit analysis

Motor unit analysis was performed using scripts in Spike 2. An

example analysis is shown including the raw intramuscular EMG recording,

filtered intramuscular EMG and MUs analyzed from this recording. The bottom

tracing shows the raw SH intramuscular EMG recording in mV. This recording

was then high-pass filtered using an IIR filter in Spike 2. The slope of this tracing

was then obtained to make MUs as clear as possible for analysis. All tracings

were filtered in an identical way so that all MUs were still comparable between

the 4 contraction types. The filtered tracing is shown above the raw intramuscular

EMG tracing labelled as SH (IIR). The three tracings above in the main panel of

the figure show the three different MUs that were analyzed. MUs were identified

using a template matching alogrithm which is shown in insert A). The top half of

this box shows the MU that is currently being analyzed. There are 6 boxes in the

bottom half of this template matching alogrithm, three of which are filled with

three different MUs numbered 1, 2 and 3. These three templates were used to

identify the MUs shown in the top three tracings of the main panel labelled MU1,

MU2, and MU3. Visual inspection was then employed to ensure all MUs were

coded correctly. Panel B shows enhanced magnified version of the three different

MUs are identified and correspond in colour and number to panel B. SH, short

head; mV, millivolts; MU, motor Unit.

31

Discharge rate (Hz), discharge rate variability (SD of discharge rate), interspike

intervals and the SD of interspike intervals were calculated. Recruitment threshold was

calculated as the time of onset of each specific MU train as all contractions were time-

locked so recruitment could be analyzed at the time of first discharge.

3.5 Statistical Analysis

The normality of all data was evaluated. All normally distributed data is presented

in text as means ± SD; all non-normally distributed data is presented as median ±

interquartile range. If not specified the data is normally distributed and the mean ± SD is

reported. All data in graphs is presented as mean ± standard error of the mean (SEM).

One-sample t-tests were conducted to evaluate differences in subject characteristics.

A 4 (muscle group - SH and LH of the biceps brachii, triceps brachii and

brachioradialis) x 5 (phase of contraction - 1st hold, lift, 2nd hold, lower, 3rd hold)

repeated measures ANOVA was used to evaluate differences in integrated EMG between

the 4 contraction types (isometric, anisometric, waterbottle lift, waterbottle drink). A

repeated measures ANOVA was also used to evaluate differences in acceleration

steadiness across the 5 phases of the contraction between the three dynamic contraction

types (anisometric, waterbottle lift and waterbottle drink).

A 2 (muscle group - SH and LH of the biceps brachii) x 5 (phase of contraction)

repeated measures ANOVA was used to evaluated differences in MU discharge rate and

MU discharge rate variability between the 4 contraction types (isometric, anisometric,

waterbottle lift and waterbottle drink). A repeated measures ANOVA was used to

evaluate recruitment time of MUs between the SH and LH and the 4 different contraction

types. Tukey's post hoc analysis was performed for all significant interactions. All

32

statistical procedures were performed using SPSS (v. 19, Chicago, IL). An α level of

p≤0.05 was used for statistical significance.

Chapter 4: Results

4.1 Subjects

Thirteen female subjects (22.5 ± 2.9 years; 59.2 ± 7.0 kg; 166.5 ± 7.8 cm) were

recruited for this study. Measurements of the distance between the subject’s eye and the

visual feedback computer screen monitor were taken prior to testing. The subjects were

125.0 ± 2.1 cm away from the screen. The angle was also measured using a goniometer

between the subject’s eye and the centre of the computer monitor and all subjects looked

slightly downwards at the computer monitor (-3.6 ± 2.4°). The range of motion at the

subjects elbow was taken to determine the degree to which the subject moved the

waterbottle or weight (38.5 ± 4.3°). Maximal strength measurements were obtained for

elbow flexion and elbow extension. Subjects were significantly stronger for elbow flexion

compared with elbow extension (p=0.001) (Table 2).

Table 2: Maximal voluntary contraction strength

Subject

#

Elbow flexion

MVC (N)

Elbow

Extension

MVC (N)

Average 145.127* 107.394

SD 30.720 20.176

Subject results are presented as the highest value of three

trials. *, biceps brachii strength was significantly stronger

than triceps brachii strength (p=0.001). N, newtons; SD,

standard deviation.

4.2 Surface EMG

The repeated measures ANOVA of integrated surface EMG did not result in a 3

way contraction x contraction phase x muscle group interaction (p>0.05) but resulted in a

33

significant contraction x phase interaction (p<0.001) and a significant contraction x

muscle group interaction (p<0.001). In the LH of the biceps brachii (p=0.03) the isometric

contraction had significantly lower integrated EMG than the other three contractions

(anisometric, waterbottle lift, waterbottle drink) in the 1st hold (p<0.01), lift (p<0.001), 2

nd

hold (p<0.01) and lower (p<0.05) phases of the contractions. However there were no

significant differences in the 3rd

hold phase of the contractions (Figure 10). In the SH of

the biceps brachii the isometric contraction had significantly lower integrated EMG than

the other three contractions (anisometric, waterbottle lift, waterbottle drink) in the 1st hold

(p<0.01), lift (p<0.001), 2nd

hold (p<0.001), and lower (p<0.001) phases of the

contractions. In the 3rd

hold phase the isometric contraction was only significantly lower

than the waterbottle drink contraction (p<0.05). Additionally, during the lower phase the

drink contraction had significantly lower integrated EMG than the anisometric and

waterbottle lift contractions (p=0.009) (Figure 10).

There was no significant contraction x phase interaction in the triceps brachii or

brachioradialis muscles (p>0.05). No main effects were observed for the triceps brachii;

however, a main effect for contraction was seen in the brachioradialis (p<0.001). The

isometric contraction had significantly lower integrated EMG than the other three

contraction types (p<0.001). Additionally the anisometric contraction had significantly

lower integrated EMG than the waterbottle lift and waterbottle drink contractions

(p<0.01) (Figure 11).

34

Figure 10: EMG activity of the long (A) and short (B) head of the biceps brachii

during the 4 different contractions.

Muscle activity is represented as the mean integrated EMG as a

percentage of the subject's maximal EMG. All bars represent the average of all

subjects. a, significantly different than all other contraction types. MVC, maximal

voluntary contraction; aniso, anisometric contraction; Wb Lift, waterbottle lift

contraction; Wb Drink, waterbottle drink contraction; Iso, isometric contraction.

4.3 Steadiness

The repeated measures ANOVA of the coefficient of variation of acceleration

(steadiness) resulted in a significant contraction type x phase of contraction interaction.

Pairwise comparisons showed that during the 1st, 2

nd and 3

rd hold phases of the

contractions (anisometric, waterbottle lift, waterbottle drink) there were no significant

differences. The lift phase of the anisometric contraction was significantly steadier than

the waterbottle drink contraction (p=0.02); however, there were no differences between

the anisometric and waterbottle lift contraction or the waterbottle lift and waterbottle

drink contraction. During the lower phase the anisometric contraction was significantly

steadier than the waterbottle lift (p=0.016) and the waterbottle drink (p<0.001)

35

contractions. The waterbottle lift contraction was significantly steadier than the

waterbottle drink contraction (p=0.03) (Figure 12).

Figure 11: EMG activity of the brachioradialis during the four different

contractions.

Muscle activity is represented as the mean integrated EMG as a

percentage of the subject's maximal EMG. All bars represent the average of all

subjects across all phases of the contraction type. a, significantly different than all

other contraction types. MVC, maximal voluntary contraction; aniso, anisometric

contraction; Wb Lift, waterbottle lift contraction; Wb Drink, waterbottle drink

contraction; Iso, isometric contraction.

4.4 MU Characteristics

The repeated measures ANOVA for discharge rate did not show a 3 way

contraction x contraction phase x muscle group interaction. There was a significant

contraction x contraction phase interaction (p<0.001) and a significant contraction x

muscle group interaction (p=0.001). In the LH of the biceps brachii, during the 1st hold

and lower phases there was no difference (p>0.05); however, all other phases differed

(p<0.05). This was caused by the waterbottle drink contraction having a significantly

higher discharge rate than the other three contractions (anisometric, waterbottle lift, and

36

Figure 12: Steadiness during the 5 phases of the dynamic contractions.

Steadiness is represented as the coefficient of variation (CV) of

acceleration which is inversely related to steadiness. Therefore the higher the CV

the less steady the contraction. Steadiness did not differ between the holding

phases across the three contractions. During the lift phase the waterbottle drink

contraction was significantly less steady than the anisometric contraction. In the

lowering phase the waterbottle drink contraction was the least steady followed by

the waterbottle lift. The anisometric contraction was the steadiest in this phase.

Because steadiness was calculated using the absolute values of acceleration this

could not be directly compared to the isometric force steadiness. Therefore the

isometric contraction is omitted from this figure. a, significantly different than all

other contraction types; c, significantly different than anisometric. CV, coefficient

of variation; Aniso, anisometric contraction; Wb lift, waterbottle lift contraction;

Wb drink, waterbottle drink contraction.

isometric) in the lift phase (p<0.01). Additionally the waterbottle lift contraction had a

significantly higher discharge rate than the isometric contraction (p<0.05). In the 2nd

hold

phase the waterbottle drink contraction had a significantly higher DR than the isometric

contraction (p<0.05). Finally, during the 3rd

hold phase the isometric had significantly

higher DR than the other three contraction types (p<0.05) (Figure 13).

In the SH of the biceps brachii the 1st hold phase of the isometric contraction had a

significantly lower discharge rate than the waterbottle lift and waterbottle drink

contraction (p<0.05). During the lift phase all contractions were significantly different

37

from one another (p<0.05) except the anisometric and waterbottle lift contraction which

were not significantly different (p>0.05). The waterbottle drink contraction had a higher

discharge rate than all other contractions and the isometric contraction had a lower

discharge rate than all other contractions (p<0.05). During the 2nd

hold and lower phase

the waterbottle drink contraction had a significantly higher discharge rat than the

anisometric and isometric contraction (p<0.05) (Figure 13).

Figure 13: Motor unit discharge rate of the long (A) and short (B) head of the

biceps brachii during the 4 different contractions

Motor units were tracked throughout the 5 phases of the contraction and

between the 3 different contractions (anisometric, waterbottle lift, waterbottle

drink, and isometric). The waterbottle drink contraction had a significantly higher

discharge rate than the other three contraction types in the lifting phase in both

the SH and LH of the biceps brachii. In addition the waterbottle drink contraction

had a significantly higher discharge rate than the isometric contraction in the 2nd

hold phase in the LH and in the 1st hold, 2

nd hold and lower phases in the SH of the

biceps brachii. The anisometric and waterbottle lift contraction were similar

across all phases and both heads. a, significantly different than all other

contraction types; b, significantly different than isometric; c, significantly

different than anisometric. Aniso, anisometric; WB lift, waterbottle lift; WB drink,

waterbottle drink; Iso, isometric; Hz, hertz.

38

The repeated measures ANOVA for discharge rate variability did not reveal any

interactions; however, there was a main effect for contraction (p<0.001). Post-hoc

analysis revealed that the waterbottle drink contraction had a significantly higher

discharge rate variability than the other three contraction types (anisometric, waterbottle

lift, isometric) (p<0.05). Additionally the isometric contraction had a significantly lower

discharge rate variability than the other three contraction types (p<0.001). There were no

significant differences between the anisometric and waterbottle lift contraction (p>0.05)

(Figure 14).

Figure 14: Motor unit discharge rate variability in the 4 different contractions.

Discharge rate variability was significantly higher in the waterbottle drink

contraction than the other three contraction types (anisometric, waterbottle lift

and isometric). The isometric contraction had a significantly lower discharge rate

than the other three contraction types. The anisometric and waterbottle lift

contraction were not significantly different. a, significantly different than all other

contraction types. Aniso, anisometric; WB lift, waterbottle lift; WB drink,

waterbottle drink; Iso, isometric; Hz, hertz.

39

The repeated measures ANOVA for recruitment time did not result in a significant

muscle x contraction interaction or a significant main effect for contraction (p>0.05)

(Figure 15).

Figure 15: Motor unit recruitment time between the 4 contraction types.

MU recruitment time was obtained for all tracked motor units.

Recruitment time was considered to be the point when the MU first turned on and

discharged consistently 6 or more times. The contraction started at 20.00 seconds

therefore the earliest possible recruitment time was 20.00 seconds. There were no

significant differences between any of the contraction types for recruitment time.

Aniso, anisometric; WB lift, waterbottle lift; WB drink, waterbottle drink; Iso,

isometric; sec, seconds.

Chapter 5: Discussion

This is the first study to measure MU discharge rates and recruitment in a

functional task and compare this activity to isometric and anisometric contractions in

order to understand the production of steady movements. The main findings from this

study were: 1) isometric contractions performed at the same workload require less muscle

activity (measured via integrated EMG) than anisometric contractions and functional

tasks; 2) during the movement phases of the contractions, the functional task was

40

significantly less steady than the anisometric contraction; 3) the waterbottle drink

contraction had a significantly higher discharge rate than the other contraction types

during the lift phases; 4) the waterbottle drink contraction had the highest discharge rate

variability and the isometric contraction had the lowest; 5) recruitment time did not differ

between the 4 contraction types. Overall, the intent to drink (functional task) influenced

spinal integration, as measured in MU output and this change in the peripheral nervous

system effected task control.

In this study surface EMG of the SH and LH of the biceps brachii, triceps brachii

and brachioradialis were measured. All contractions that were performed were completed

at the same workload, thus surface EMG was expected to be similar between the four

contractions. However, in the SH and LH of the biceps brachii the isometric contraction

had significantly lower integrated EMG activity than the other three contraction types

(Figure 10). This is due to the addition of movement in the other contraction types

(Theeuwen et al, 1994). Although the relative load was the same, in the movement tasks

the load had to be moved against gravity whereas in the isometric contraction the arm

remained stable. The three dynamic contractions were not significantly different in terms

of integrated EMG. This indicated that the net drive to the muscle was similar between

the contraction types.

This study highlighted that execution of a functional tasks requires unique spinal

control, measured through the evaluation of MU activity, specifically quantification of the

rate coding strategy. This study is strengthened by tracking MUs across all 4 contraction

types. Therefore, these results are a compilation of specific differences in the same MU

across the conditions. Discharge rate was highest in the waterbottle drink contraction

compared to the other three contraction types. The only difference between the

waterbottle lift and waterbottle drink contraction was the addition of a sip of water during

41

the 2nd

hold phase. Therefore the 1st hold and lift phases were identical yet the discharge

rate was significantly higher in the lift phase in the waterbottle drink contraction. On the

other hand, the waterbottle lift contraction and lab-based anisometric control contraction

were not significantly different in terms of discharge rate. Thus, taking a drink of water,

making the task a true functional task, was enough to influence how the task was

controlled.

The higher discharge rate in the waterbottle drink contraction must therefore be

occurring to assist in controlling the complexity of the task. It is well established that DR

is higher in anisometric contractions compared to isometric contractions (Altenberg,

Ruiter, Verdlijk, Mechelen, & de Haan, 2009; Kallio et al, 2013). These differences are

confirmed by this study as the isometric contraction had the lowest discharge rate. It

appears that these differences in rate coding strategy are further extended as task

complexity is augmented.

In addition to discharge rate, recruitment of motor units can also play a role in

controlling muscle contractions (Kukulka & Carmann, 1981). Recruitment time did not

differ between the contraction types in this study. This suggests that recruitment may

occur based on the load of the contraction whereas, rate coding likely involves a level of

higher order processing that accounts for complexity of the task. Recruitment time was

similar in the isometric contraction to the three dynamic contractions. This contradicts

previous findings that isometric contractions have a higher recruitment threshold than

anisometric contractions (Linnamo et al, 2002). However, it should be noted that in this

study in order to achieve the desired force level to match the dynamic contractions

subjects were given a 7 second ramp time to reach the force level. This occurred as in the

dynamic contractions; as soon as the waterbottle was lifted the full load was maintained in

a constant position. Thus, the full load of the water bottle was being used throughout the

42

isometric contraction, thus a slow ramp phase was used to ‘match’ conditions. This slow

ramp may have masked some of the differences. However, what is important to note is

that less MUs were activated during the isometric contraction compared to the other three

contraction types. Thus, recruitment was still lower during the isometric contraction

compared to the dynamic contractions, further indicating that spinal output to execute the

task is less in this condition.

Steadiness is associated with discharge rate variability. The more variable the

output of the spinal network the less steady the contraction. Typically, in isometric and

anisometric contractions, a higher discharge rate is associated with lower discharge rate

variability and this was observed in the water bottle lift. However, this was not evident in

the functional drinking tasks. In the waterbottle drink contraction the discharge rate was

higher as was the discharge rate variability resulting in a lower steadiness in this

contraction. Davids, Bennet, & Newell (2006) state that as learning occurs in a task and

experience in executing the movement increases the mechanical aspects of the system

become less rigid. Instead, as people perform tasks they learn which features are invariant

and which can vary and adapt to successfully perform the task in different environments

(Davids et al, 2006). This could explain why the waterbottle drinking task was least

steady. In order to successfully execute the task that subjects needed to direct the straw to

their mouths. However, how steady the movement was wouldn't change the outcome

(obtaining a sip of water) of the task. Meanwhile, during the anisometric contraction and

waterbottle lift contraction, removal of the drink made the task less familiar. In these

cases the subjects had to concentrate on carefully moving the waterbottle to the correct

position, and ignoring the innate drive to ‘drink’. Thus, the spinal output is heightened

and MU variability would be less in the lift relative to the drink contraction.

43

This is an important finding as some contractions of daily life require steadiness to

properly execute the task. For example, had the waterbottle been a mug of hot liquid it

would be important to maintain a steady contraction in order to prevent spilling. This

suggests that steadiness is not only influenced by the complexity of the task, but

familiarity with the functional movement. This has implications for people wanting to

maintain steadiness to complete tasks of daily living. Thus, it is important to determine

ways in which functional tasks become steadier. A recent study by Marmon, Gould, &

Enoka (2011) showed that practicing specific functional movements could improve

steadiness in older adults.

In addition to the differences observed between the 4 contraction types,

differences in the concentric (lift) and eccentric (lower) phases of the contractions were

also observed. It is well established that concentric contractions require unique control

strategies to eccentric contractions with concentric contractions having a higher discharge

rate (Duchateau & Baudry, 2013; Enoka, 1996). Across the three dynamic contractions

this held true in this study. The lift phase had a higher discharge rate than the lower phase

(Figure 13). This is important to note as adding the functional aspect to the task does not

influence the control strategies between concentric and eccentric phases.

Overall this study was the first to highlight that functional tasks require unique

control strategies to anisometric and isometric contractions. Recruitment does not control

the more complex functional tasks; rather rate coding is the key aspect of regulating

functional movement.

5.1 Limitations

This study evaluated an entirely novel concept of recording and quantifying MUs

during functional tasks. As this was a completely new experimental set-up and protocol

44

there were a few limitations that arose within this study that can be addressed by future

research. The equipment used for this set-up only allowed recordings from 6 muscle

groups (surface EMG of the SH and LH of the biceps brachii, triceps brachii and

brachioradialis and intramuscular EMG of the SH and LH of the biceps brachii). If the

subjects had pronated their forearm at all in order to place the straw into their mouth then

brachialis activity would also need to be considered.

Current motor unit recording techniques make it difficult to anlayze MUs at high

contraction velocities, particularly in this set up for functional movement. The movements

had to be performed at a slow pace in order to obtain clear MU recordings; this may not

have completely represented the movement time required to ‘drink’. However, even at

this slow pace, differences were still observed between the contraction types.

5.2 Future Research

This study highlights that with careful advancement of electrophysiology

recordings, single MU activity can be quantified in functional movement. Data here

underscores both the enormous potential, and the great need for extensive and future

research in understanding functional movement in humans. Motor unit populations

between muscles likely have unique and specific spinal control strategies that are directed

to the tasks undertaken. This specificity might also be openly affected by the complexity

of the task. The drinking task investigated involves a flexion and extension movement;

however, further differences may be present in contractions that involve more complex

movements such as forearm rotation and shoulder adduction. Finally, this study only

evaluated young female subjects. As MU activity and spinal control may differ between

males and females as well as between younger and older individuals it is important to

evaluate sex and age differences in future studies. This is important to understand spinal

45

integration in the functional control of movement between males and females and how

these control strategies change as we age. Differences with aging may be even greater

than those observed previously for isometric and anisometric contractions. Quantification

of the spinal network during functional movement will in turn, allow research to

determine ways to help older adults maintain daily abilities.

46

Chapter 6: Conclusion

The four contractions (waterbottle drink, waterbottle lift, anisometric and

isometric) were matched for load and duration. The isometric contraction required less

muscle activity (measured via surface EMG) to execute while the three dynamic

contractions (anisometric, waterbottle lift and waterbottle drink) all had similar surface

EMG. The waterbottle drink contraction had a higher discharge rate, higher discharge rate

variability, and lower steadiness than the other contraction types. Recruitment threshold

did not differ between the 4 contraction types. This highlights that that the functional task

required unique spinal control strategies to execute the movement. It is clear that the MU

recruitment in these tasks is based more on absolute load while rate coding is a key factor

responsible for controlling functional movement. This is the first study to evaluate motor

units in functional tasks and it is clear that these tasks cannot be assumed to have the

same mechanistic control as anisometric contractions.

47

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Appendices

Appendix A: Copyright Approval for Figure 4

54

Appendix B: Ethics Approval

55

Appendix C: Pre-Study Questionnaire

Date of Experiment: _________ Experimenter Name:_____________

Subject Name: _____________ Subject Code: ________________

Sex: _________ DOB (mm/dd/yyyy):____________

Weight (kg): _________ Height (cm): _________________

Dominant Hand: Right or Left

Mailing Address (For Experiment Findings Only): ________________________

_________________________________________________________

Phone Number: _________________

Are you a regular smoker? Yes No

If yes, how often?

_________________________________________________________

Have you had surgery in the past year? Yes No

If yes, what type? _____________________________________________

_________________________________________________________

Have you been diagnosed by a health professional as having any of the following?

(Check all that apply, and be specific where applicable)

Heart Trouble:_________ Arthritis:_________

High Blood Pressure:_________ High Cholesterol: _________

Cardiac Pacemaker:_________ Electronic Implant: _________

Stroke:_________ Back Problems: _________

Muscle problems:_________ Bone or Joint disorder: _________

56

Previous Injury:_________ Alcoholism: _________

Diabetes:_________ Depression: _________

Migraines: _________

Do you suffer from any allergies? (Include hay fever, sinus problems, and skin

sensitivities)

_________________________________________________________

_________________________________________________________

_________________________________________________________

Do you have difficulty hearing? ____________________________________

Do you have difficulty seeing? _____________________________________

Other health problems?

_________________________________________________________

_________________________________________________________

Are you currently using any medications?

_________________________________________________________

_________________________________________________________

Please include any other additional pertinent information that you may feel to be

beneficial to know while conducting this study. Thank you for you participation.

_________________________________________________________

_________________________________________________________

57

Appendix D: Subject Characteristic Data

Subject # Age

(years)

Weight

(kg)

Height

(cm)

1 27 64 173

2 20 63 172

3 23 50 163

4 29 58 160

5 23 57 163

6 20 70 179

7 20 65 165

8 21 59 152

9 24 56 163

10 22 55 167

11 24 52 158

12 20 71 176

13 20 50 173

Average 22.5 59.2 166.5

SD 2.9 7.0 7.8

Subject measurements taken prior to testing. kg, kilograms; cm, centimetres; SD,

standard deviation.