Biomechanics of WorkChapter 11

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)

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

Biomechanical Models Fundamental Basis (Newton’s laws)

1. Mass remains in uniform motion or at rest until acted on by an unbalanced force.

2. Force is proportional to the acceleration of a mass

3. 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

Single-Segment Planer, Static Model

Single-Segment Planer, Static Model (cont.)

W = mgW is weight in newtonsm is mass in kilogramsg is gravitational constant (9.8 m/s2)W = 20kg X 9.8 m/s2 = 196 NW load on each hand = 98 N W forearm & hand = 16 NS(forces at elbow = 0) = - 16 N – 98 N + R elbow = 0R elbow = 114 NS(moments at elbow = 0) = (- 16N) (.18 m) + (- 98N) (.36m) + M

elbow = 0M elbow = 38.16

Two-Segment Planer, Static Model

Low Back Biomechanics of Lifting

Low Back Biomechanics of Lifting (cont.)M load & torso = W load x h + W torso x b

Where:h – horizontal distance from load to L5/S1 diskb – horizontal distance from center of mass of the torso to the L5/S1

diskM back-muscle = F back-muscle x 5(N–cm)S(moments at L5/S1 disk = 0)F back-muscle x 5 = W load x h + W torso x bF back-muscle = (W load x h + W torso x b)/5Assume h = 40 cm & b = 20 cm thenF back-muscle = 8W load + 4W torso Assume W load = 300 N or 30kg (75lb) & W torso = 350 N (80lb)

thenF back-muscle = 3800 N or 388kg (855lb)

NIOSH Lifting Guide

RWL = LC x HM x VM x DM x AM x FM x Cm

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

Cm

NIOSH Lifting Guide Example

NIOSH Lifting Guide Example

LI = Lifting Index

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

Reducing Asymmetric Multiplier

Reducing Vertical & Horizontal Multipliers

Seated Work Chair Design

Disk Pressure Measurements in Standing & Unsupported Sitting

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)

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

Wrist Bending Implications

Hand Tool Design

1. Do not bend the wrist2. Shape tool handles to assist grip3. Provide adequate grip span

– Accommodate sex differences 4. Provide finger & glove clearances

– Accommodate sex differences

Hand Tool Design

Grip Strength Male/Female