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Biomechanics of Work

Biomechanics of Work

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Biomechanics of Work

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  1. Biomechanics of Work Chapter 11

  2. NIOSH Report & Others • 500,000 workers suffer overexertion injuries each year • 60 % involve lifting and lower back. Compensation & indirect costs total $27 – $56 Billion (1991). • 10 % involve upper extremities (fingers, hand, wrists, arms, & shoulders) due to cumulative trauma disorders (1987)

  3. Musculoskeletal System • Bones (206) & connective tissues • Tendons – fibrous connective tissues connecting muscles to bones • Ligaments – fibrous tissues that keep articulate joints in place • Cartilage – translucent elastic tissue • Fascia cover body structures separating one from another • Muscles (400) – composed of bunches of muscle fibers, connective tissue, & nerves. • Only body tissue that can expand or contract when fired by a nerve impulse. • Long cylindrical cells. • Force is dependent on cross section of bundle

  4. Biomechanical Models • Fundamental Basis (Newton’s laws) • Mass remains in uniform motion or at rest until acted on by an unbalanced force. • Force is proportional to the acceleration of a mass • Any action is opposed by reaction of equal magnitude • Static equilibrium • Sum of all external forces on object equal zero • Sum of all external moments must equal zero

  5. Single-Segment Planer, Static Model

  6. Single-Segment Planer, Static Model (cont.) W = mg W is weight in newtons m is mass in kilograms g is gravitational constant (9.8 m/s2) W = 20kg X 9.8 m/s2 = 196 N Wload on each hand = 98 N W forearm & hand = 16 N S(forces at elbow = 0) = - 16 N – 98 N + R elbow = 0 R elbow = 114 N S(moments at elbow = 0) = (- 16N) (.18 m) + (- 98N) (.36m) + M elbow = 0 M elbow = 38.16

  7. Two-Segment Planer, Static Model

  8. Low Back Biomechanics of Lifting

  9. Low Back Biomechanics of Lifting (cont.) M load & torso= Wloadx h + Wtorso x b Where: h– horizontal distance from load to L5/S1 disk b – horizontal distance from center of mass of the torso to the L5/S1 disk M back-muscle = F back-muscle x 5(N–cm) S(momentsat L5/S1 disk = 0) Fback-muscle x 5 = W loadx h + W torso x b Fback-muscle = (Wloadx h + W torso x b)/5 Assume h = 40 cm & b = 20 cm then F back-muscle = 8Wload+ 4Wtorso Assume W load= 300 N or 30kg (75lb) & Wtorso= 350 N (80lb) then Fback-muscle = 3800 N or 388kg (855lb)

  10. NIOSH Lifting Guide RWL = LC x HM x VM x DM x AM x FM x Cm

  11. NIOSH Lifting Guide (cont.) RWL = LC x HM x VM x DM x AM x FM x Cm

  12. NIOSH Lifting Guide Example

  13. NIOSH Lifting Guide Example LI = Lifting Index

  14. Manual Materials Handling • Material Handling Devices (MHDs) • Located as close as possible to body • Located about thigh or waist high • Don’t locate large packages close to the floor (30 inches optimum) • Minimize torso twist • Minimize frequent lifting per work period

  15. Reducing Asymmetric Multiplier

  16. Reducing Vertical & Horizontal Multipliers

  17. Seated Work Chair Design

  18. Disk Pressure Measurements in Standing & Unsupported Sitting

  19. Common Forms of Cumulative Trauma Disorders (CTDs) • Tendon-related – in repetitive work muscles steel blood from tendons & inflammation results • Neuritis – repetitive work in awkward positions irritate & damage nerves • Ischemia – tingling/numbness caused by lack of blood flow • Bursitis – inflammation of a bursa (sac containing synovial or viscous fluid)

  20. CTDs of Certain Joints of the Extremities & Remedies • Finger – vibration-induced white fingers (Raynaud’s syndrome) • Hand & Wrist – Carpal tunnel syndrome • Elbow – tennis elbow (lateral epicondylitis), golfer’s elbow (medial epicondylitis) & telephone operator’s elbow • Shoulder – Tasks requiring hands & arms above the shoulder cause rotator-cuff irritation, swimmer’s shoulder, or pitchers arm and can result in injury • Evaluate & redesign tasks that cause CTDs. Understand that certain worker populations are more predisposed to these injuries

  21. Wrist Bending Implications

  22. Hand Tool Design • Do not bend the wrist • Shape tool handles to assist grip • Provide adequate grip span • Accommodate sex differences • Provide finger & glove clearances • Accommodate sex differences

  23. Hand Tool Design

  24. Grip Strength Male/Female