Dept. of Biomedical, Industrial, & Human Factors Engineering 1 Muscular-Skeletal System Physiological Characteristics Irritability (excitability): react to stimuli (electrical stimulation) Chemical reaction creates muscle contraction Contractility: increase tension Shorter and thicker Extensibility: stretched beyond resting length Requires antagonist or gravity force Elasticity: return to resting length

Dept. of Biomedical, Industrial, & Human Factors Engineering 2 Muscular-Skeletal System Classification of muscle contraction Isometric: no change in muscle length No physical work performed Tension usually constant Concentric: decreasing muscle length Positive work Acceleration of limb during movement Tension decreases Eccentric: increasing muscle length Negative work Deceleration of limb Tension increases Isotonic: applied force is constant Rare in practice

Dept. of Biomedical, Industrial, & Human Factors Engineering 3 Muscular Tension Length of muscle Maximum tension occurs at resting length (or slightly longer) All active myosin sites lined up with actin attachment sites Joint angle changes length % max tension % resting length

Dept. of Biomedical, Industrial, & Human Factors Engineering 4 Muscles Velocity of Contraction Maximum velocity at zero tension Maximum force at zero velocity Cross-Sectional Area Max force ( N/mm 2 ) Only gender difference is cross-sectional area Women narrower muscle Women 2/3 force of men Electrical Process of Muscle Resting potential of muscle fiber 90 mV with inside negatively charge relative to exterior Due to imbalance of ions Action Potential is reversal of resting potential Positive charge applied (depolarization) Lasts 2-4 msec, speed 5 m/s Refractory period is where muscle has decreased ion permeability 1-3 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/s 2 1 N = 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 Celsius The Calorie which is used for energy content of food is actually a kilocalorie 1 kcal = 1000 cal = 1 Cal (food) 1 kJ = 1000 J 1 kcal = kJ 1 kcal = ft lbs Power = Work per unit time Measures Watt (W) = 1 J/s Horsepower (hp) = 736 W

Dept. of Biomedical, Industrial, & Human Factors Engineering 6 Muscular Activity Metabolism 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 Energy Metabolism - Sources of energy (see Figure 8-2 Sanders & McCormick (7 th ed) First 3-5 secs adenosine 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 Regenerated To continue muscular activity, ATP must be regenerated creatine phosphate + ADP  creatine + ATP creatine phosphate is high energy existing in small amounts in muscles Depletion of creatine phosphate occurs in about 15 sec Blood 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 Energy Metabolism – further breakdown may be Anaerobic work – if O 2 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 O 2 is supplied, pyruvic acid is broken down into water and carbon dioxide, releasing large amounts of ATP glucose + 38 phosphate + 38 ADP + 6 O 2  6 CO H 2 O + 38 ATP Oxidation of pyruvic acid in aerobic work involves enzymes, co-enzymes, and fatty acids (Krebs cycle, figure 3.4 – Pulat) O 2 is key to efficient work. Its supply requires more blood be pumped to muscle per unit time as well as heavier breathing to oxygenate blood Kilocalorie (kcal) – most common measure of energy requirement for physical activity Resting energy  0.3 kcal per minute for man of about 154 lbs Resting male (laying down and no digestive activity)  1700 kcal/day Resting female (laying down and no digestive activity)  1400 kcal/day