Chapter 3:Physical Ergonomics

1. Chapter 3:Physical Ergonomics

2. Human Activity Continuum • As a result of activity, the body undergoes various changes: • Physiological • Psychological • Cognitive • Etc. • Stress and strain • Stress – some undesirable condition, circumstance, task, or other factor that impinges upon the individual (outside effector) • Examples – heavy work, immobilization, heat and cold, sleep loss, etc. • Strain – refers to the effects of stress on the individual • Examples – blood chemistry, oxygen consumption, electrical activity of the muscles or the brain, heart rate, work rate, errors, attitudes • People vary on what creates their stress and the level of stain associated with stress

3. Physical Psychomotor Cognitive Human Activity Continuum Dynamic activity Static loading Sensory functions Thinking calculating Strain measures HR O2 consumption HR recovery EMG EOG HRV EEG Dual-task Subjective ratings ex: NASA TLX Abbreviations: EMG - electromyogram EOG – electroculogram HRV – heart rate variability EEG - electroencephalogram

4. Stress and Performance • Stress does not always lead to ill effects • Sources of job stress • Workplace arrangement (dimensions, relative location, etc.) • Work content (speed and accuracy requirements, etc.) • Work organization (work-rest cycles, start-stop times, work control, etc.) • Work environment (co-workers, supervisor, heat, cold, fear of job loss, etc.) Peak Performance Performance Overload Boredom Stress

5. Muscular-Skeletal System • Movement Terminology • Flexion – decreasing joint angle • Extension – increasing joint angle • Abduction – away from the body • Adduction – towards the body • Pronation – inward rotation • Supination – outward rotation • Planes of Reference • Sagittal – vertical plane dividing left and right half of body • Lateral (frontal) – vertical plane dividing front and back of body • Horizontal (traverse) – horizontal plane dividing top and bottom of body Sagittal Lateral (frontal) Horizontal (traverse) YZ XZ XY

6. Muscular-Skeletal System • Skeletal System • 206 bones establish framework • Give body structure • Provide protection to vital organs (e.g., skull, ribs) • Basis for activity such as bones in arms, legs, etc.; Levers that produce movement • Bones are connected to each other via ligaments • Bones connected to muscles via tendons • Produce elements of blood within bone marrow • Store calcium and phosphorus

7. Muscular-Skeletal System • Bones - specialized form of connective tissue • Composition • Calcium carbonate/calcium phosphate (60-70%) • Stiffness (stress/strain) • Compressive strength • Collagen (protein) • Flexibility, tensile strength • Decrease with age • Water (25-30%) • Contributes to strength • Connective Tissue in Skeletal System • Cartilage – firm yet elastic and flexible, rapid growth and handles moderate stress • End of ribs (expansion) • Spinal disks (cushion) • Accounts for bone growth at ends • Hardens when fully developed

8. Muscular-Skeletal System • Bones connected to each other at joints • Hinge joints (wrist) • Pivot joints (elbow) • Ball and socket joints (shoulder, hip) • Joints • Fibrous – no movement (e.g., skull) • Cartilaginous – limited range of movement (e.g., spinal column) • Synovial – large range of movement • Fluid cavity to lubricate (e.g., knee)

9. Muscular-Skeletal System • Type of Movement (joints) • Gliding or angular (elbow, knee) • One dimensional • flexion/extension, abduction/adduction • Circumdunction (shoulder, hip, wrist) • Two dimensional, no rotation • Rotation (shoulder, hip, forearm) • Two dimensional about longitudinal axis • Limiting Factors for Range of Motion • Tension of ligaments – elbow or knee extension • Tension of antagonistic muscles – flex/stretch with extended knee • Contact of soft tissue – flex hip with knee relaxed, elbow or knee flex

10. Muscular-Skeletal System • Mobility study by Staff (1983) • 32 major body joints • Electric bubble goniometer used to measure range of motion • Females exhibited significantly greater mobility except in ankle flexion and wrist abduction • Bone development • Change shape, size, and structure depending on mechanical demands • Wolff’s law – bone deposited where needed and resorbed where not needed

11. Muscular-Skeletal System • Muscles – major prerequisite for human activity • 600 muscles in the body; 80 account for most vigorous activity • 3 types of muscles • Striated (skeletal) • Cardiac (heart) • Smooth (walls of blood vessels and internal organs) • Skeletal is the most concern to ergonomists • Composed of bundled muscle fibers (length range from 0.2 to 5.5 inches, but some are as large as 12 inches • Mechanical leverage – muscle applies forces on the bone(s) to which it is attached • Maximum force in extended state; little force in contracted state • Muscle generates mechanical work by converting chemical energy into mechanical energy

12. Muscular-Skeletal System • Muscles (con’t) • Three thin filaments • Actin (primary filament) • Troponin • Tropomyosin B proteins • Myosin (thick filament) slides over to bring about contraction • Figure 3.3 Pulat • Antogonist Pairs • Can only contract and pull, cannot push • For each motion, there is a muscle to perform opposite motion • Act as break to slow motion and prevent damage • Precise motor movement • Example: Bicep • flexion (bicep contract; tricep inhibit) • extension (tricep contract; bicep inhibit)

13. 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

14. 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

15. 100 50 % max tension 60 100 180 % resting length 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

16. Muscles • Velocity of Contraction • Maximum velocity at zero tension • Maximum force at zero velocity • Cross sectional area • Max force (0.3-0.4 N/mm2) • 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

17. Physiological Strain-Basic Physics 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 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 = 4.1868 kJ • 1 kcal = 3087.4 ft lbs • Power = work per unit time • Measures • Watt (W) = 1 J/s • Horsepower (hp) = 736 W

18. 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 (next time)