Dept. of Biomedical, Industrial, & Human Factors Engineering 1

Muscular ActivityMetabolism

Supplies the energy needed to slide the actin filaments over the myosin filaments. It is a chemical process of converting food into mechanical work and heat.

Some mechanical work is consumed by the body while other is consumed by physical activity

Basic source of energy for contraction of the muscle is glycogen or glucose which is abundant in the blood

Sources of energy (next time)

Dept. of Biomedical, Industrial, & Human Factors Engineering 2

EnergyMetabolism - Sources of energy (see Figure 8-2 Sanders & McCormick (7th ed)

First 3-5 secsadenosine triphosphate (ATP)-a high energy phosphate compound is mobilized. It breaks down to adenosine diphosphate (ADP) which releases energy.

ATP ADP + P (phosphate radical) + free energy

ATP RegeneratedTo continue muscular activity, ATP must be regenerated

creatine phosphate + ADP creatine + ATPcreatine phosphate is high energy existing in small amounts in muscles

Depletion of creatine phosphate occurs in about 15 secBlood glucose or glycogen is mobilized. Glucose is a blood sugar which is converted by various stages first into pyruvic acid.

Dept. of Biomedical, Industrial, & Human Factors Engineering 3

EnergyMetabolism – further breakdown may be

Anaerobic work – if O2 is not supplied to the muscle, pyruvic acid is converted into lactic acid while ATP is regenerated. Lactic acid accumulation causes muscle fatigue and pain

glucose + 2 phosphate + 2 ADP 2 lactate + 2 ATP

Aerobic work – if O2 is supplied, pyruvic acid is broken down into water and carbon dioxide, releasing large amounts of ATP

glucose + 38 phosphate + 38 ADP + 6 O2 6 CO2 + 44 H2O + 38 ATPOxidation of pyruvic acid in aerobic work involves enzymes, co-enzymes, and fatty acids (Krebs cycle, figure 3.4 – Pulat)

O2 is key to efficient work. Its supply requires more blood be pumped to muscle per unit time as well as heavier breathing to oxygenate bloodKilocalorie (kcal) – most common measure of energy requirement for physical activity

Resting energy 0.3 kcal per minute for man of about 154 lbsResting male (laying down and no digestive activity) 1700 kcal/dayResting female (laying down and no digestive activity) 1400 kcal/day

Dept. of Biomedical, Industrial, & Human Factors Engineering 4

Energy

MetabolismBasal metabolic rate (BMR) – amount of energy needed per unit of time to sustain life

Total metabolism equals sum of:Basal metabolism

Activity metabolism

Digestive metabolism (10 % sum of basal + activity)

Dept. of Biomedical, Industrial, & Human Factors Engineering 5

Supporting Systems

Respiratory ResponseNose and mouth, pharynx, larynx, trachea, lungs (consisting of bronchi and bronchioles, alveolar ducts, alveolar sacs, and alveoli)

Primary function is to expose a large volume of water-saturated air to a large volume of blood to facilitate exchange of gases between air and inhaled air

Gas exchange 70-90 m2 for average adult (surface area)

At rest, 250 ml of O2 is absorbed and 200 ml of CO2 expelled

Lung capacity7-8 L for tall athletic young male

Women – 10% less

Untrained – 60-80 % of volume

Dept. of Biomedical, Industrial, & Human Factors Engineering 6

Supporting SystemsCirculation

Heart acts as double pump: blood vessels outside the heart can be categorized into 2 classes

Pulmonary vessels – transport blood from the right ventricle of the heart through the lungs and back to the left atrium of the heartSystemic vessels – transport blood from the left ventricle of the heart to all the other parts of the body including the head and lower body back to the right atriumPulmonary and systemic peripheral circulationKidneys – remove waste (toxic) from the bloodLiver, lungs, skin & intestines also eliminate wasteArteries – carry blood away from the heartVeins – carry blood to the heart

NutritionIngestion, digestion, absorption, and utilization of nutrient substancesProvides necessary fuel and chemicals for physical activity

Dept. of Biomedical, Industrial, & Human Factors Engineering 7

Muscular Activity

Dynamic activity – characterized by rhythemic contraction and relaxation of the muscles involved

Example: turning a handwheel to open a valve

Alternating tension and relaxation allows more blood to circulate

Static activity – characterized by a prolonged state of contraction, which restricts blood flow to muscles

Example: holding a box in static posture

Since no glucose or oxygen are being received – activity won’t last long

Design Element – Dynamic vs. StaticCompared to dynamic effort, static effort will require longer rest periods

Static effort could result in employee complaints and turnover

Designers should minimize job elements in a process with static loading

Dept. of Biomedical, Industrial, & Human Factors Engineering 8

Energy Cost of WorkEnergy demands increase with the onset of physical work. Increase depends on:

Physical conditioningIntensity of activityGenderBody weight

Cardiovascular responseAt rest

Heart rate (HR) – 60-85 beats per minuteNormal resting blood leaves lungs 97 % saturated – breathing harder will not increase O2 in muscles alone

At workIncrease cardiac output – to increase O2 to muscles, more blood must flow thereby increasing cardiac outputHeart rate increases - # pumping actionsStroke volume – volume per beat (rest 5 L/min; hard work – 25 L/min or more)

At 40% of a person’s max capacity stroke volume stabilizes while HR increases

Dept. of Biomedical, Industrial, & Human Factors Engineering 9

Energy Cost of WorkCardiovascular response

Oxygen lagFig 3.5, p. 34, Pulat

O2 and heart rate are linear under submaximal work (fig 3.6, 3.7, & 3.8 – Pulat)

Violated when:Arms over headHot environments

Increased blood pressure (BP) – consequence of increased cardiac output

Adrenal glands release catecholamines (hormones) that strength HR and increase BPBP is necessary to fill heart (note: @ 120 beats/min, heart has ½ sec to fill between strokes)

Dept. of Biomedical, Industrial, & Human Factors Engineering 10

Energy Cost of Work

Redistribution of blood (Sanders & McCormick, p. 231)

Blood Flow Distribution (%)

Part of body Resting Heavy Work

Muscles 15-20 70-75

Skin 5 10

Brain 15 3-4

Bones 3-5 0.5-1

Kidneys 20 2-4

Digestive System 20-25 3-5

Heart muscle 4-5 4-5

Dept. of Biomedical, Industrial, & Human Factors Engineering 11

Energy Cost of WorkPhysical work capacity – worker’s capacity for energy output function of:

Food, oxygen, sum of energy provided by aerobic and anaerobic processWorking at 30-40% of one’s maximum aerobic power in 8 hour shift causes notable muscular fatigue

Energy cost of specific activitiesO2 energy consumption have good relationship

Indirect measure of energy consumption

1 L of O2 4.8 kcalTable 3.2 – different activities kcal

FactorsAs rate of activity increases, energy cost increasesBody weight increase energy costMen 3000-3500 kcal/dayWomen 2500-3000 kcal/dayAt age 65 75 % calories expended compared to those at age 25

Dept. of Biomedical, Industrial, & Human Factors Engineering 12

Energy Cost of WorkKeeping energy cost of work at acceptable limits

For menMaximum time-weighted average of 5 kcal/min due to activity energy cost of workMaximum time-weighted HR average of 100 beats/min

For women4 kcal/min90 beat/min

Table 3.3 is a classification of workFactors affecting energy consumption

Method of workWork postureWork rate or paceTool design

Dept. of Biomedical, Industrial, & Human Factors Engineering 13

Energy Cost of WorkFatigue

Manifested in slight tiredness to complete exhaustionProduce lactate - which can be a good predictor of fatigueOne subjective measure is the Borg-RPE (rating of perceived exertion) scale – similar to scale being used in MP 1

Scale of 6-20 are linearly related to the HR expected for that level of exertionIntended to rate exertion during dynamic work

Motivation – highly motivated underrate their level

Work-rest cycleIf workload cannot be maintained within recommended limits (work standards) there must be restSome charts have been developed to determine amount of rest required

Dept. of Biomedical, Industrial, & Human Factors Engineering 14

Rest Allowance(Kodak, 1986)

Rest Allowance

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Example:Someone working 10 minutes @VH level get 80% rest or 8 minutes

Dept. of Biomedical, Industrial, & Human Factors Engineering 15

Rest AllowanceMurrell (1965)

Solve Problem 3 in PulatUsing Murrell and method in text book

general)in -1.5 males,-1.7 females,-(1.4 metabolism Basal

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Dept. of Biomedical, Industrial, & Human Factors Engineering 16

Strength and EnduranceStrength – the maximum force that one can exert voluntarily (kilograms using dynamometer)

Static – measured standing or sitting postureExample: Pushing a button on a cell phoneDesign so that 95% of population can exceed these forces

Dynamic – measured during work using dynamic or isokinetic strength testing devices that control for speed of movement

Speed of movement affects lifting ability

Characteristics of strengthStrength peak by late 20s, gradual decline to 75% at age 65Women on average have 2/3 strength of menExercise can increase by as much as 50%Maximum grip @ 135 degrees of elbow flex

Endurance – ability to maintain activity over timeResearch says that people can only maintain maximum effort only briefly

20% of peak static over time30% of peak dynamic work

Dept. of Biomedical, Industrial, & Human Factors Engineering 17

Nervous SystemCoordinates and regulates body activities Responsible for the initiation and control of muscular activity The basic element of the nervous system is the neuron. Neuron is composed of a nerve cell along with a minimum of two nerve fibers.

Dendrite brings messages to the nerve cell; multiple dendrites may be associated with a cell therefore multiple paths may send signals Axon takes messages to another cell; a neuron may not have more than one axon. Myelin coats the nerve fibers which serves to protect one nerve fiber's messages from another; short-circuiting is prevented Synapse is the gap between the dendrite of one neuron and the axon of another neuron. Chemical/electrical process allow an impulse of one neuron to be passed to a succeeding on unaltered in 1/1000 of a second.

Dept. of Biomedical, Industrial, & Human Factors Engineering 18

Nervous SystemThree major parts to the nervous system

Central Nervous system which includes the brain and spinal cord Peripheral Nervous system which controls the voluntary activity Automatic system which covers the balance of the nervous system

Automatic Nervous System Controls involuntary body activity including function of glands, smooth muscle tissue, and the heart. Includes the:

Sympathetic or thoracolumbar division stimulating these fibers creates small amounts of thick saliva, depression of gastrointestinal activity, and increased heart rate and occur in emergencies such as freight and consume energy. parasympathetic or craniosacral division

For example when people are in a health threat, parasympathetic affect blood pressure, constricting of the pupils, etc.

An axon in hand controls 3 -6 muscle fibers; An axon in leg controls 100 -200 muscle fibers

Dept. of Biomedical, Industrial, & Human Factors Engineering 19

Nervous SystemPeripheral Nervous System

Controls the voluntary activity of the body 2 major elements are the sensory and motor systems

Sensory system is responsible for conveying information from your senses to the central mechanisms; sensory neurons have one axon and one dendrite eachOnce a message is received and processed through the Central nervous system, signals are passed down the pathways to the appropriate muscles. Motor neurons lie in the central mechanisms, and their axons travel in groups called motor nerves

Speed of transmission Motor fibers 70-120 m/s Others 12-70 m/s Considering the distance between the sensory organs and motor organs to the central mechanism, it is not uncommon to have reaction times in the 300-500 millisecond range

Dept. of Biomedical, Industrial, & Human Factors Engineering 20

Nervous SystemCentral Nervous System

Two parts: spinal cord and brain They are connected by the brain stem Spinal cord is gray matter encased in a bony column called the spinal column.

contain the reflex neurons contains the neurons of the sensory system

The brain medulla - connects the spinal cord with the higher centers of the brain; nuclei for the neurons of the automatic nervous system cerebral cortex is the part of the brain where information is processed; where information is stored; most important with respect to perception and processing of information Thalamus - relay station for the brain; sorts out messages and directs them to correct motor channels