Biomechanics • Biomechanics 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: • Ergonomics • Orthopedics • Sports science
Occupational Biomechanics • Occupational Biomechanicsis 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 demands • Costs due to overexertion injuries - LIFTING • Large variations in population strength • Basis for understanding and preventing overexertion injuries
Free-Body Diagrams • Free-body diagrams are schematic representations of a system identifying all forces and all moments acting on the components of the system.
2-D Model of the Elbow: Unknown Elbow force and moment 17.0 cm 10 N 35.0 cm 180 N From Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.2
2-D Model of the Elbow From Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.7
Unknown 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 Biomechanics Example ELBOW COM HAND
General Approach 1. Establish coordinate system (sign convention) 2. Draw Free Body Diagram, including known and unknown forces/moments 3. Solve for external moment(s) at joint 4. 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)
Example : Solution _ _ SME = 0 = ME + ME -> ME = -ME ME = MLA + MH = (WLA x maLA) + (FH x maH) ME = (-10 x 0.17) + (-180 x 0.35) = -1.7 - 63 ME = -64.7 Nm (or 64.4 Nm CW) ME = -ME -> ME = 64.7 ME = (FJT x maJT) + (FB x maB) = FB x 0.05 FB = 1294 N (up) External moment is due to external forces _ _ _ Internal moment is due to internal forces
Example 1: Solution _ _ SFE = 0 = FE + FE -> FE = -FE FE = WLA + FH = -10 + (-180) FE = -190 N (or 190 N down) FE = - FE -> FE = 190 FE = FJT + FB FJT = 190 - 1294 = -1104 N (down) _ _ _ • Thus, an 18 kg mass (~40#) requires 1300N (~290#) of muscle force and causes 1100N (250#) of joint contact force.
Assumptions Made in 2-D Static Analysis • Joints are frictionless • No motion • No out-of-plane forces (Flatland) • Known anthropometry (segment sizes and weights) • Known forces and directions • Known postures • 1 muscle • Known muscle geometry • No muscle antagonism (e.g. triceps) • Others
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.
From Biomechanics to Task Evaluation • Biomechanical analysis yields external moments at selected joints • Compare external moments with joint strength (maximum internal moment) • Typically use static data, since dynamic strength data are limited • Use appropriate strength data (i.e. same posture) • Two Options: • Compare moments with an individuals joint strength • Compare moments with population distributions to obtain percentiles (more common)
If ME = 15.4 Nm, what % of the population has sufficient strength to perform the task (at least for a short time)? Example use of z-score m = 40 Nm; s = 15 Nm (from strength table) z = (15.4 - 40)/15 = -1.64 (std dev below the mean) From table, the area A corresponding to z = -1.64 is 0.95 Thus, 95% of the population has strength ≥ 15.4 Nm
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 D Forces: masses, accelerations (increase or decrease, depending on the specific task) Moment arms: distances, postures, work layout Increase C Design task to avoid loading of relatively weak joints Maximize joint strength (typically in middle of ROM) Use only strong workers Task Evaluation and Ergonomic Controls
Aerobic vs. Anaerobic Metabolism • Aerobic • Use of O2, efficient, high capacity • Anaerobic • No O2, inefficient, low capacity • Aerobic used during normal work (exercise) levels, anaerobic added during extreme demands • Anaerobic metabolism -> lactic acid (pain, cramps, tremors) • D < C (energy demands < energy generation capacity)
Oxygen Uptake and Energy Production Respiratory Circulatory Atmosphere Muscle System System Blood Capillary Oxygen Tidal Volume System Available Heart Rate Respiratory Rate Stroke Volume Energy Production (E) Oxygen Uptake (VO2)
Changes with Endurance Training • Low force, high repetition training • increased SVmax => increased COmax • incr. efficiency of gas exchange in lungs (more O2) • incr. in O2 carrying molecule (hemoglobin) • increase in #capillaries in muscle
Problems with Excessive Work Load • Elevated HR • cannot maintain energy equilibrium • insufficient blood supply to heart may increase risk of heart attack in at-risk individuals • Elevated Respiratory Rate • chest pain in at-risk individuals • loss of fine control • General and Localized Muscle Fatigue • insufficient oxygen -> anaerobic metabolism -> lactic acid -> pain, cramping • A fatigued worker is less satisfied, less productive, less efficient, and more prone to errors
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 Values • Subjective Evaluation • Estimate from HR • Job Task Analysis More Complex More Accurate