7 Muscle Compositionbundles of muscle fibres, connective tissue and nervesfibres are made of long cylindrical cellscells contain contractile elements (myofibrils)both sensory and motor nervesmotor nerves control contractions of groups of fibres (motor unit)
8 Muscle ContractionConcentric: (also called isotonic) muscle contracts and shortensEccentric: muscle contracts and lengthens (overload)Isometric: muscle contracts and stays the same length
9 Muscle Strength proportional to muscle cross-section usually measured as torqueforce applied against a moment arm (bone) to an axis of rotation (joint)Static strength: measured during isometric contractionDynamic strength: measured during movement
10 Basic BiomechanicsStatics model (åF=0, å Moments=0), isometric contractionForce at the point of application of the loadWeight of the limb is also a force at the center of gravity of the limbåF can be calculated
11 Problem in TextPerson holding a 20kg weight in both hands. What are the force and moment at the elbow?Given:Mass =20kgForce of segment = 16NLength of segment = .36mAssume:COG of segment is at the midpoint!20kg
12 Problem in Text 1. Convert mass to Force 20kg*9.8 m/s2 = 196 N 2. Divide by # of hands.196N/2 hands = 98N/hand98 N
13 Problem in Text åF=0 1. Convert mass to Force Felbow 20kg*9.8 m/s2 = 196 N2. Divide by # of hands.196N/2 hands = 98N/hand3. Calculate F elbow.åF=0Felbow – 16N – 98N = 0Felbow= 114N [up]Felbow16 N98 N
14 Problem in Text Felbow 16 N 98 N 1. Convert mass to Force 20kg*9.8 m/s2 = 196 N2. Divide by # of hands.196N/2 hands = 98N/hand3. Calculate F elbow.åF=0Felbow – 16N – 98N = 0Felbow= 114N [up]4. Calculate M elbow.åM=0Melbow-16N*.18m +(-98N)*.36m=0Melbow=38.16N*mFelbow.36m.18m16 N98 N
15 Multi-segment modelsRepeat for each segment, working the forces and moments backHow would you work out the Force and Moment in the shoulder?What information would you need?
16 Lower Back Pain estimated at 1/3 of worker’s compensation payments may affect 50-70% of the population in generalBoth in high lifting jobs and jobs with prolonged sitting
17 Biomechanics of Lower Back Pain Calculation in text 300N load to 5458N back compressive forceBack must support many times the lifted load, largely due to the moment arms involvedCalculation of compressive forces vs. muscle strength can identify problems
18 NIOSH Lifting GuideSets numbers that are associated with risk of back injuryTwo limits (for simple lifts)Action limit (AL): small proportion of the population may experience increased risk of injuryMaximum permissible limit (MPL): Most people would experience a high risk of injury. 3xALWeightInjuries rareInjuries inevitableALMPL
19 NIOSH Lifting GuideRecommended 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 painCovers more complex liftsBiomechanical criteria 3.4kN at L5/S1Epidemiological criteria show damage at 4.4kNPhysiological criteria to set repetition rate at kcal.min
20 Lifting Equation RWL=LC*HM*VM*DM*AM*FM*CM General form RWL = max possible load * modifiersModifiers reduce the RWL so thatRWL<=LC(all modifiers <1)
21 The ModifiersLC: load constant, maximum recommended weight for a simple liftHM: horizontal multiplier, decreases weight with distance from spineVM: vertical multiplier, lifting from near floor harderDM: distance multiplier, accommodates for vertical distance that must be liftedAM: assymetric multiplier, reductions for torso twistingCM: coupling modifier, depends on whether loads have handles for liftingFM: frequency modifier, how frequently is the load lifted
23 Lifting EquationMultipliers can all be obtained from tables (11.1, 11.2, 11.3)Multipliers are unitlessMultipliers are always less than or equal to 1 …why?
24 Example in the TextA 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.
25 Example in the TextFMA 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.WeightdurationCMVMHMDMAM
28 Calculation of RWL RWL=LCxHMxVMxDMxAMxFMxCM RWL=51lbx.625x.895x.92x.744x.55x1RWL= 10.74lbsThe load is greater than the RWL so there is a risk of back injuryLifting Index = RWL/LoadIF >1 then the load is too highLI= 10.74/15 = 1.4
29 Designing to avoid back pain More importantly, NIOSH equation gives ways to reduce injuryreduce horizontal distancekeep load at waist heightreduce distance to be travelledreduce twistingadd handlesreduce frequency of lifts