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

Muscular-Skeletal System

Physiological CharacteristicsIrritability (excitability): react to stimuli (electrical stimulation)

Chemical reaction creates muscle contraction

Contractility: increase tensionShorter and thicker

Extensibility: stretched beyond resting lengthRequires antagonist or gravity force

Elasticity: return to resting length

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

Muscular-Skeletal System

Classification of muscle contractionIsometric: no change in muscle length

No physical work performed

Tension usually constant

Concentric: decreasing muscle lengthPositive work

Acceleration of limb during movement

Tension decreases

Eccentric: increasing muscle lengthNegative work

Deceleration of limb

Tension increases

Isotonic: applied force is constantRare in practice

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

Muscular Tension

Length of muscleMaximum tension occurs at resting length (or slightly longer)

All active myosin sites lined up with actin attachment sites

Joint angle changes length

60 100 180

100

50% max tension

% resting length

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

MusclesVelocity of Contraction

Maximum velocity at zero tensionMaximum force at zero velocity

Cross-Sectional AreaMax force (0.3-0.4 N/mm2)Only gender difference is cross-sectional area

Women narrower muscleWomen 2/3 force of men

Electrical Process of MuscleResting potential of muscle fiber

90 mV with inside negatively charge relative to exteriorDue to imbalance of ions

Action Potential is reversal of resting potentialPositive charge applied (depolarization)Lasts 2-4 msec, speed 5 m/s

Refractory period is where muscle has decreased ion permeability1-3 msec after action potential

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

Physiological Strain-Basic Concepts

Force – a unit of force is a newton (N) = 1 kg-m/s2

1 N = 0.225 lbf (pounds force)

Work or Energy – Work is done or energy is consumed when a force is applied over a distance

Measures:1 N x 1 m = 1 J (joule)Kilocalorie (kcal) = amount of heat required to raise the temperature of 1 kg of water from 15 degrees Celsius to 16 degrees CelsiusThe Calorie which is used for energy content of food is actually a kilocalorie

1 kcal = 1000 cal = 1 Cal (food)1 kJ = 1000 J1 kcal = 4.1868 kJ1 kcal = 3087.4 ft lbs

Power = Work per unit timeMeasures

Watt (W) = 1 J/sHorsepower (hp) = 736 W

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

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

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

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 8

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