2 BiomechanicsBiomechanics uses the laws of physics and engineering mechanics to describe the motions of various body segments (kinematics) and understand the effects of forces and moments acting on the body (kinetics).Application:ErgonomicsOrthopedicsSports science
3 Occupational Biomechanics Occupational Biomechanics is a sub-discipline within the general field of biomechanics which studies the physical interaction of workers with their tools, machines and materials so as to enhance the workers performance while minimizing the risk of musculoskeletal injury.Motivation:About 1/3 of U.S. workers perform tasks that require high strength demandsCosts due to overexertion injuries - LIFTINGLarge variations in population strengthBasis for understanding and preventing overexertion injuries
5 Free-Body DiagramsFree-body diagrams are schematic representations of a system identifying all forces and all moments acting on the components of the system.
6 2-D Model of the Elbow: Unknown Elbow force and moment 17.0 cm10 N35.0 cm180 NFrom Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.2
7 2-D Model of the ElbowFrom Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.7
8 Biomechanics Example Unknown values: Lower arm selected as free body ELBOWCOMHANDUnknown values:Biceps and external elbow force (FB and FE), and any joint contact force between upper and lower arms (FJT)External elbow moment (ME)Lower arm selected as free body
9 General Approach 1. Establish coordinate system (sign convention) 2. Draw Free Body Diagram, including known and unknown forces/moments3. Solve for external moment(s) at joint4. Determine net internal moment(s), and solve for unknown internal force(s)5. Solve for external force(s) at joint [can also be done earlier]6. Determine net internal force(s), and solve for remaining unknown internal force(s)
10 Example : Solution SME = 0 = ME + ME -> ME = -ME __SME = 0 = ME + ME -> ME = -MEME = MLA + MH = (WLA x maLA) + (FH x maH)ME = (-10 x 0.17) + (-180 x 0.35) =ME = Nm (or 64.4 Nm CW)ME = -ME -> ME = 64.7ME = (FJT x maJT) + (FB x maB) = FB x 0.05FB = 1294 N (up)External moment is due to external forces___Internal moment is due to internal forces
11 Example 1: Solution SFE = 0 = FE + FE -> FE = -FE __SFE = 0 = FE + FE -> FE = -FEFE = WLA + FH = (-180)FE = -190 N (or 190 N down)FE = - FE -> FE = 190FE = FJT + FBFJT = = N (down)___Thus, an 18 kg mass (~40#) requires 1300N (~290#) of muscle force and causes 1100N (250#) of joint contact force.
12 Assumptions Made in 2-D Static Analysis Joints are frictionlessNo motionNo out-of-plane forces (Flatland)Known anthropometry (segment sizes and weights)Known forces and directionsKnown postures1 muscleKnown muscle geometryNo muscle antagonism (e.g. triceps)Others
13 3-D Biomechanical Models These models are difficult to build due to the increased complexity of calculations and difficulties posed by muscle geometry and indeterminacy.Additional problems introduced by indeterminacy; there are fewer equations (of equilibrium) than unknowns (muscle forces)While 3-D models are difficult to construct and validate, 3-D components of lifting, especially lateral bending, appear to significantly increase risk of injury.
14 From Biomechanics to Task Evaluation Biomechanical analysis yields external moments at selected jointsCompare external moments with joint strength (maximum internal moment)Typically use static data, since dynamic strength data are limitedUse appropriate strength data (i.e. same posture)Two Options:Compare moments with an individuals joint strengthCompare moments with population distributions to obtain percentiles (more common)
15 Example use of z-scoreIf ME = 15.4 Nm, what % of the population has sufficient strength to perform the task (at least for a short time)?m = 40 Nm; s = 15 Nm (from strength table)z = ( )/15 = (std dev below the mean)From table, the area A corresponding to z = is 0.95Thus, 95% of the population has strength ≥ 15.4 Nm
16 Task Evaluation and Ergonomic Controls Demand (moments) < Capacity (strength)Are the demands excessive?Is the percentage capable too small?What is an appropriate percentage? [95% or 99% capable commonly used]Strategies to Improve the Task:Decrease DForces: masses, accelerations (increase or decrease, depending on the specific task)Moment arms: distances, postures, work layoutIncrease CDesign task to avoid loading of relatively weak jointsMaximize joint strength (typically in middle of ROM)Use only strong workers
19 Aerobic vs. Anaerobic Metabolism Use of O2, efficient, high capacityAnaerobicNo O2, inefficient, low capacityAerobic used during normal work (exercise) levels, anaerobic added during extreme demandsAnaerobic metabolism -> lactic acid (pain, cramps, tremors)D < C (energy demands < energy generation capacity)Metabolism is the process of releasing energy stored in chemical bonds
21 Oxygen Uptake and Energy Production RespiratoryCirculatoryAtmosphereMuscleSystemSystemOxygenTidal VolumeBloodCapillaryAvailableSystemHeart RateRespiratoryRateStrokeVolumeOxygen Uptake (VO2)Energy Production (E)
22 Changes with Endurance Training Low force, high repetition trainingincreased SVmax => increased COmaxincr. efficiency of gas exchange in lungs (more O2)incr. in O2 carrying molecule (hemoglobin)increase in #capillaries in muscle
23 Problems with Excessive Work Load Elevated HRcannot maintain energy equilibriuminsufficient blood supply to heart may increase risk of heart attack in at-risk individualsElevated Respiratory Ratechest pain in at-risk individualsloss of fine controlGeneral and Localized Muscle Fatigueinsufficient oxygen -> anaerobic metabolism -> lactic acid -> pain, crampingA fatigued worker is less satisfied, less productive, less efficient, and more prone to errors
24 Evaluating Task Demands: Task demands can be evaluated the same way that maximum aerobic capacity is evaluated – by direct measurement of the oxygen uptake of a person performing the task.Indirect methods for estimating task demands:Tabular ValuesSubjective EvaluationEstimate from HRJob Task AnalysisMore ComplexMore Accurate