Biomechanics of Work
Chapter 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 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
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 bWhere:h – horizontal distance from load to L5/S1 diskb – 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(moments at L5/S1 disk = 0)
F back-muscle x 5 = W load x h + W torso x b
F back-muscle = (W load x h + W torso x b)/5
Assume 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) then
F 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
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