Lecture 5Lecture 5Dimitar Stefanov

Definitions:•Purpose of the muscles is to produce tension•Metabolic efficiency – the measure of a muscle’s ability to convert metabolic energy to muscle tension (Example, cerebral palsy patient)•Mechanical efficiency – the ability of the central nervous system to control the tension patterns (the ability of the muscle to perform mechanical work)•Overall muscle efficiency – mechanical efficiency + metabolic efficiency •There are two types of work done by muscles:

Internal work – the work done by the muscles to move the limb segments through some desired patterns (Example: walking and running)External work – the work done by the muscles against forces and moments, which are external for the body (Examples: weight lifting, pushing a car)

•Net mechanical work – internal work + external work.

Mechanical Work, Energy and Power

Positive and negative work of musclesPositive and negative work of muscles

Positive work is done when the muscle moment acts in the same direction as the angular velocity of the joint.

FLEXOR

EXTENSOR

Net work done by the muscles = integral of the power of the muscle over the time of the muscle contraction.

Negative work is done when the muscle moment acts in the opposite direction to the movement of the joint.

The external force Fext acts on the segment and produces a joint moment greater than the muscle moment.

Muscle mechanical powerMuscle mechanical powerAt a given joint, muscle power is a product of the net muscle moment and angular velocity of the of the joint.

The sign of the muscle power changes during the movement performance.

Example:

The work done by a muscle during a period t1 to t2 is:

Pm – muscle power

2

1

t

t

mm dtPW [J]

Example.

Causes of insufficient movement•Co-contractions•Isometric contractions against gravity•Jerky movements•Generation of energy at one joint and absorption at another

Energy flows

Maintenance heat – amount of metabolic energy to keep the muscles alive

Forms of energy storage

(1) Potential energy, P.E.Energy due to gravity. It increases with the height of the body above ground.

(2) Kinetic energy, K.E.Translational K.E due to the translational velocity and rotational K.E due to the rotational velocity.

(3) Total energy and exchange within a segment, Es

Energy exchange between segments

Example:

Plot of vertical displacement and horizontal velocity of head-arms-trunk (H.A.T.)

Double support phase

Mid-stance phase

Total energy of a multi-segment systemTotal energy of a multi-segment system

n

iib EE

1

where:Ei – the total energy of the i-th segmentn is the number of the segmentsEb is the total body energy.

Positive and negative work of the total bodyPositive and negative work of the total body

Example of a pendulum

100% conservative system!

Pendulum system with muscles

•The total body energy increases when muscles do positive work.•The total body energy decreases when muscles do negative work.

Muscle is not contractedMuscle is contracted

Overall efficiency of human movementOverall efficiency of human movement

The major problem is to calculate the internal mechanical work.The major problem is to calculate the internal mechanical work.

MUSCLE MECHANICSMUSCLE MECHANICS

Motor unit

The smallest subunit that can be controlled is called motor subunit because it is separately innervated by a motor axon.

The motor unit consists of:1. Synaptic junction in the ventral root of the spinal cord2. Motor axon3. Motor end plate in the muscle fibers.

• The number of the muscle fibers that are under control of 1 motor unit varies from 3 to 20,000 depending on the fineness of the control required.•A muscle fiber is about 100 m in diameter consisting of fibrils about 1 in diameter.•Fibrils consist of interacting action and myosin filaments.

Muscle fiberMuscle fiber

SarcoplasmSarcolemma – the plasma membraneSarcomere – repeating functional unit of microfilaments (length 2.6 m)

Basic structure of of the Basic structure of of the muscle contractive muscle contractive elementelement

MyofibrilSarcomere

Many filaments are in parallel and many sarcomere elements are in series to make up a single Many filaments are in parallel and many sarcomere elements are in series to make up a single contractive element.contractive element.

RecruitmentRecruitment of motor units of motor unitsExcitation of the motor unit all-or-nothing event

Two indications of the activation of the motor unit:• Motor unit action potential (electrical indication)• Twitch of tension (mechanical indication).

•All muscle fibers in a single motor unit contact at the same time;•Muscle fibers in the same muscle but belonging to different motor units may contract at different times.

EMG signal from indwelling electrode in a muscle

Size principle of recruitment of motor unitsSize principle of recruitment of motor units

• How the motor units are recruited?• Which motor units are recruited first?• Are the motor units always recruited in the same order?

Hinneman – The size of the newly recruited motor unit increases with the tension level at which it is recruited.

•The smallest unit is recruited first and the largest unit last. Because of the smallest units the tension can be changed in finely graded steps.•Movements requiring high forces but not needing fine control are accomplished by recruiting the larger motor units.

M.U. = motor unit;M.U.1 – smallest M.U.M.U.3 – largest M.U.

The muscle action potential (m.a.p.) increases with the size of the motor unit with which it is associated.

WHY?

If the motor unit is larger then:1. The motoneuron (that innervates it) is larger2. The depolarization potentials associated with the motor

end plate is greater.

Can we predict the size of a motor unit from the amplitude of the recorded signal? – No, it isn’t possible. Why?

Two types of motor units (M.U.):Type I – It considers the smaller units which produce low tension and have low time to peak (60 to 120 msec). (tonic M.U.)Type II – It includes the larger, fast twitch motor units (phasyc M.U.)

M.U. controlled by any motoneuron pool form a spectrum of sizes and excitations.

Force-length curve of a contractile element

4 m

Influence of parallel connective tissueThe connective tissue surround the contractive elements. The connective tissue is called the parallel elastic component.

Ft – tendon force;Fc – force of the contractive elementFp – force of the parallel elastic component.

Ft = Fc + Fp

Series elastic tissue Series elastic tissue Series elastic element – all connective tissue in series with the contractile component, including the tendon.

Isometric contractions