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TI 2111 Work System Design and Ergonomics 11. Occupational Biomechanics & Physiology

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TI 2111 Work System Design and Ergonomics 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

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TI 2111 Work System Design and Ergonomics 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 demands Costs due to overexertion injuries - LIFTING Large variations in population strength Basis for understanding and preventing overexertion injuries

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TI 2111 Work System Design and Ergonomics Problems (example)

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TI 2111 Work System Design and Ergonomics 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.

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TI 2111 Work System Design and Ergonomics 2-D Model of the Elbow: From Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig cm 35.0 cm 180 N 10 N Unknown Elbow force and moment

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TI 2111 Work System Design and Ergonomics 2-D Model of the Elbow From Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.7

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TI 2111 Work System Design and Ergonomics Biomechanics Example Unknown values: Biceps and external elbow force (F B and F E ), and any joint contact force between upper and lower arms (F JT ) External elbow moment (M E ) Lower arm selected as free body HANDCOMELBOW

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TI 2111 Work System Design and Ergonomics 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)

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TI 2111 Work System Design and Ergonomics Example : Solution M E = 0 = M E + M E -> M E = -M E M E = M LA + M H = (W LA x ma LA ) + (F H x ma H ) M E = (-10 x 0.17) + (-180 x 0.35) = M E = Nm (or 64.4 Nm CW) M E = -M E -> M E = 64.7 M E = (F JT x ma JT ) + (F B x ma B ) = F B x 0.05 F B = 1294 N (up) __ _ _ External moment is due to external forces Internal moment is due to internal forces _

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TI 2111 Work System Design and Ergonomics Example 1: Solution F E = 0 = F E + F E -> F E = -F E F E = W LA + F H = (-180) F E = -190 N (or 190 N down) F E = - F E -> F E = 190 F E = F JT + F B F JT = = N (down) __ _ _ _ Thus, an 18 kg mass (~40#) requires 1300N (~290#) of muscle force and causes 1100N (250#) of joint contact force.

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TI 2111 Work System Design and Ergonomics 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

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TI 2111 Work System Design and Ergonomics 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.

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TI 2111 Work System Design and Ergonomics 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)

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TI 2111 Work System Design and Ergonomics Example use of z-score If M E = 15.4 Nm, what % of the population has sufficient strength to perform the task (at least for a short time)? = 40 Nm; = 15 Nm (from strength table) z = ( )/15 = (std dev below the mean) From table, the area A corresponding to z = is 0.95 Thus, 95% of the population has strength ≥ 15.4 Nm

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TI 2111 Work System Design and Ergonomics 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 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

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TI 2111 Work System Design and Ergonomics UM 2-D Static Strength Model

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TI 2111 Work System Design and Ergonomics Work Physiology

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TI 2111 Work System Design and Ergonomics Aerobic vs. Anaerobic Metabolism Aerobic Use of O 2, efficient, high capacity Anaerobic No O 2, 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)

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TI 2111 Work System Design and Ergonomics Oxygen Consumption and Exercise

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TI 2111 Work System Design and Ergonomics Oxygen Uptake and Energy Production Respiratory System Circulatory System Muscle Oxygen Available Tidal Volume Respiratory Rate Blood Stroke Volume Heart Rate Capillary System Atmosphere Oxygen Uptake (VO 2 ) Energy Production (E)

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TI 2111 Work System Design and Ergonomics Changes with Endurance Training Low force, high repetition training increased SVmax => increased COmax incr. efficiency of gas exchange in lungs (more O 2 ) incr. in O 2 carrying molecule (hemoglobin) increase in #capillaries in muscle

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TI 2111 Work System Design and Ergonomics 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

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TI 2111 Work System Design and Ergonomics 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

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