# BIOMECHANICS OF WORK Chapter 11 in your text.

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BIOMECHANICS OF WORK Chapter 11 in your text

The Musculoskeletal System
Bones, muscle and connective tissue supports and protects body parts maintains posture allows movement generates heat and maintains body temperature

Bones 206 bones Body “framework” Protective: rib cage and skull
Provide for action: arms, legs linked at joints by tendons and ligaments Tendons: connect bone to muscle Ligaments: connect bone to bone

Joints Connection of two or more bones Movement
no mobility joints (e.g. in skull) hinge joints (elbow) pivot joints (wrist) ball and socket joints (hip and shoulder) 3DOF

Muscles 400 muscles 40-50% of your body weight
half of your body’s energy needs

Muscles

Muscle Composition bundles of muscle fibres, connective tissue and nerves fibres are made of long cylindrical cells cells contain contractile elements (myofibrils) both sensory and motor nerves motor nerves control contractions of groups of fibres (motor unit)

Muscle Contraction Concentric: (also called isotonic) muscle contracts and shortens Eccentric: muscle contracts and lengthens (overload) Isometric: muscle contracts and stays the same length

Muscle Strength proportional to muscle cross-section
usually measured as torque force applied against a moment arm (bone) to an axis of rotation (joint) Static strength: measured during isometric contraction Dynamic strength: measured during movement

Basic Biomechanics Statics model (åF=0, å Moments=0), isometric contraction Force at the point of application of the load Weight of the limb is also a force at the center of gravity of the limb åF can be calculated

Problem in Text Person holding a 20kg weight in both hands. What are the force and moment at the elbow? Given: Mass =20kg Force of segment = 16N Length of segment = .36m Assume: COG of segment is at the midpoint! 20kg

Problem in Text 1. Convert mass to Force 20kg*9.8 m/s2 = 196 N
2. Divide by # of hands. 196N/2 hands = 98N/hand 98 N

Problem in Text åF=0 1. Convert mass to Force Felbow
20kg*9.8 m/s2 = 196 N 2. Divide by # of hands. 196N/2 hands = 98N/hand 3. Calculate F elbow. åF=0 Felbow – 16N – 98N = 0 Felbow= 114N [up] Felbow 16 N 98 N

Problem in Text Felbow 16 N 98 N 1. Convert mass to Force
20kg*9.8 m/s2 = 196 N 2. Divide by # of hands. 196N/2 hands = 98N/hand 3. Calculate F elbow. åF=0 Felbow – 16N – 98N = 0 Felbow= 114N [up] 4. Calculate M elbow. åM=0 Melbow-16N*.18m +(-98N)*.36m=0 Melbow=38.16N*m Felbow .36m .18m 16 N 98 N

Multi-segment models Repeat for each segment, working the forces and moments back How would you work out the Force and Moment in the shoulder? What information would you need?

Lower Back Pain estimated at 1/3 of worker’s compensation payments
may affect 50-70% of the population in general Both in high lifting jobs and jobs with prolonged sitting

Biomechanics of Lower Back Pain
Calculation in text 300N load to 5458N back compressive force Back must support many times the lifted load, largely due to the moment arms involved Calculation of compressive forces vs. muscle strength can identify problems

NIOSH Lifting Guide Sets numbers that are associated with risk of back injury Two limits (for simple lifts) Action limit (AL): small proportion of the population may experience increased risk of injury Maximum permissible limit (MPL): Most people would experience a high risk of injury. 3xAL Weight Injuries rare Injuries inevitable AL MPL

NIOSH Lifting Guide Recommended Weight Limit (RWL): a load value that most healthy people could lift for a substantial period of time without an increased risk of low back pain Covers more complex lifts Biomechanical criteria 3.4kN at L5/S1 Epidemiological criteria show damage at 4.4kN Physiological criteria to set repetition rate at kcal.min

Lifting Equation RWL=LC*HM*VM*DM*AM*FM*CM General form
RWL = max possible load * modifiers Modifiers reduce the RWL so that RWL<=LC (all modifiers <1)

The Modifiers LC: load constant, maximum recommended weight for a simple lift HM: horizontal multiplier, decreases weight with distance from spine VM: vertical multiplier, lifting from near floor harder DM: distance multiplier, accommodates for vertical distance that must be lifted AM: assymetric multiplier, reductions for torso twisting CM: coupling modifier, depends on whether loads have handles for lifting FM: frequency modifier, how frequently is the load lifted

Modifiers (diagrammatically)
HM VM Originating height DM AM FM CM

Lifting Equation Multipliers can all be obtained from tables (11.1, 11.2, 11.3) Multipliers are unitless Multipliers are always less than or equal to 1 …why?

Example in the Text A worker must move boxes from 1 conveyor to another at a rate of 3 boxes/minute. Each box weighs 15lbs and the worker works for 8 hours a day. The box can be grasped quite comfortably. The horizontal distance is 16 inches, the vertical is 44 inches to start and 62 inches to finish. The worker must twist at the torso 80 degrees.

Example in the Text FM A worker must move boxes from 1 conveyor to another at a rate of 3 boxes/minute. Each box weighs 15lbs and the worker works for 8 hours a day. The box can be grasped quite comfortably. The horizontal distance is 16 inches, the vertical is 44 inches to start and 62 inches to finish. The worker must twist at the torso 80 degrees. Weight duration CM VM HM DM AM

Information h=16” v=44” d=18” A=80degrees F=3 lifts/minute C=good
job duration = 8 hours/day weight = 15lbs

Multipliers HM (T11.1): 10/h=10/16=.625
VM (T11.1):( |v-30|)=.895 DM (T11.1): ( /d)=0.82+1/8/18=.92 AM (T11.1): a= x80=.744 FM(T11.2): 0.55 (v<75, work 8hrs, 3lifts) CM (T11.3): 1 (good, v<75cm)

Calculation of RWL RWL=LCxHMxVMxDMxAMxFMxCM
RWL=51lbx.625x.895x.92x.744x.55x1 RWL= 10.74lbs The load is greater than the RWL so there is a risk of back injury Lifting Index = RWL/Load IF >1 then the load is too high LI= 10.74/15 = 1.4

Designing to avoid back pain
More importantly, NIOSH equation gives ways to reduce injury reduce horizontal distance keep load at waist height reduce distance to be travelled reduce twisting add handles reduce frequency of lifts