UNH ECE896: Human Factors - Chapter 8


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  • Image provided by Jemma Taipan
  • Image provided by Jemma Taipan.
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  • http://s4.hubimg.com/u/1795743_f520.jpg
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  • Problems with measuring heart rate:
    Linear relationship vary between people
    Can only use this when moderate/heavy work is preferred
    Lots of other factors such as emotional stress
  • http://www.nasa.gov/images/content/233871main_VO2max1.jpg
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    1.4.1. Using the RWL and LI to Guide Ergonomic Design
    The recommended weight limit (RWL) and lifting index (LI) can be used to guide ergonomic design in several ways: (1) The individual multipliers can be used to identify specific job-related problems. The relative magnitude of each multiplier indicates the relative contribution of each task factor (e.g., horizontal, vertical, frequency, etc.)
    (2) The RWL can be used to guide the redesign of existing manual lifting jobs or to design new manual lifting jobs. For example, if the task variables are fixed, then the maximum weight of the load could be selected so as not to exceed the RWL; if the weight is fixed, then the task variables could be optimized so as not to exceed the RWL.
    (3) The LI can be used to estimate the relative magnitude of physical stress for a task or job. The greater the LI, the smaller the fraction of workers capable of safely sustaining the level of activity. Thus, two or more job designs could be compared.
    (4) The LI can be used to prioritize ergonomic redesign. For example, a series of suspected hazardous jobs could be rank ordered according to the LI and a control strategy could be developed according to the rank ordering (i.e., jobs with lifting indices above 1.0 or higher would benefit the most from redesign).

    Restrictions to NIOSH Lifting Equation
    In summary, the Revised NIOSH Lifting Equation does not apply if any of the following occur:
    Lifting/lowering with one hand
    Lifting/lowering for over 8 hours
    Lifting/lowering while seated or kneeling
    Lifting/lowering in a restricted work space
    Lifting/lowering unstable objects
    Lifting/lowering while carrying, pushing or pulling
    Lifting/lowering with wheelbarrows or shovels
    Lifting/lowering with high speed motion (faster than about 30 inches/second)
    Lifting/lowering with unreasonable foot/floor coupling (< 0.4 coefficient of friction between the sole and the floor)
    Lifting/lowering in an unfavorable environment (i.e., temperature significantly outside 66-79 degrees F (19-26 degrees C) range; relative humidity outside 35-50% range)

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  • UNH ECE896: Human Factors - Chapter 8

    1. 1. Chapter 8: Physical Work and Manual Materials Handlings Human Factors in Engineering and Design Mark S. Sanders and Ernest J. McCormick Seventh Edition, 1993 ECE896: Human Factors Dr. William H. Lenharth Presented by Mark Taipan December 2nd, 2010
    2. 2. • National Safety Council – 25% of industrial injuries related to MMH – $1 billion in compensation costs for 12 million workdays • 7 million people annually will suffer back injury (Caillet 1981) – Almost half related to lifting objects • Projected it won’t change regardless of improved medical care, automation in industry and preemployment exams (Ayoub and Mital 1989) HEALTH RISKS
    3. 3. • Nature of Muscles • Contractibility of Muscles • Muscle Metabolism
    4. 4. • Focus on skeletal muscles • Fibers that are connected with nerve-filled tissue • Can only contract and shorten its length • myosin motor proteins and actin filaments NATURE of MUSCLES Muscle Physiology: Nature of Muscles
    5. 5. Muscle Physiology: Contractibility of Muscles
    6. 6. MUSCLE METABOLISM Muscle Physiology: Muscle Metabolism Avoid build-up of lactic acid. Too much causes muscle fatigue and pain!
    7. 7. • Respiratory Response • Cardiovascular Response
    8. 8. Response to muscle work: 1) increase rate of breathing and volume of air 2) oxygen debt 3) aerobic glycolsis RESPIRATORY RESPONSE Work Physiology: Respiratory Response Rest: 0.5 Liters of oxygen per minute Heavy Work: 5 Liters of oxygen per minute
    9. 9. • Increased Cardiac Output – > Increased blood flow • Increased Blood Pressure – > Strained heart • Redistribution of Blood Flow – > Blood flow focused at muscles CARDIOVASCULAR RESPONSE Work Physiology: Cardiovascular Response
    10. 10. • Maximum Aerobic Power • Heart Rate • Measures of Local Muscle Activity
    11. 11. • Approximately 21% of air is oxygen • Oxygen consumption linearly increases with rate of work • Oxygen uptake eventually levels off – This is a person’s Maximum Aerobic Power (MAP) • Age affects MAP 𝑶 𝟐 & MAXIMUM AEROBIC POWER Measures of Physiological Strain: O2 and Maximum Aerobic Power
    12. 12. Oxygen consumption hard to assess at the job; easier to measure heart rate Linear relationship between oxygen consumption and heart rate Different relationship between different people Problem! There are factors that affect heart rate other than oxygen… HEART RATE Measures of Physiological Strain: Heart Rate
    13. 13. • Measure physiological strain of individual muscles or muscle groups • Electromyography (EMG) Electrical activity from muscle contractions MEASURE of LOCAL MUSCLE ACTIVITY Measures of Physiological Strain: Measure of Local Muscle Activity
    14. 14. • Work Efficiency • Grades of Work • Factors Affecting Energy Consumption
    15. 15. • 70% of energy -> heat • 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 % = 𝑤𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 ∗ 100 • Tools, posture, and activities affect work efficiency WORK EFFICIENCY Physical Workload: Work Efficiency
    16. 16. Physical Workload: Work Efficiency
    17. 17. GRADES of WORK Physical Workload: Grades of Work
    18. 18. Physical Workload: Factors Affecting Energy Consumption FACTORS AFFECTING ENERGY CONSUMPTION: Methods of Work
    19. 19. Physical Workload: Factors Affecting Energy Consumption FACTORS AFFECTING ENERGY CONSUMPTION: Work Posture
    20. 20. Physical Workload: Factors Affecting Energy Consumption FACTORS AFFECTING ENERGY CONSUMPTION: Work Rate
    21. 21. Physical Workload: Factors Affecting Energy Consumption FACTORS AFFECTING ENERGY CONSUMPTION: Tool Design
    22. 22. • Recommended Limits • Work-Rest Cycles
    23. 23. • Various upper limits proposed < 35% of MAP (Michael, Hutton and Horvath [1961], Blink [1962]) < 5.0kcal/min for men, 3.35kcal/min for women (Ayoub and Mital [1989]) RECOMMENDED LIMITS Keeping Energy Expenditure Within Bounds: Recommended Limits
    24. 24. • Rest compensate for excess requirements • Murrell’s Equation: • 𝑅𝑒𝑠𝑡 𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑚𝑖𝑛 = 𝑇𝑜𝑡𝑎𝑙 𝑊𝑜𝑟𝑘 𝑇𝑖𝑚𝑒(𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝐸𝑛𝑒𝑟𝑔𝑦 𝐶𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 −𝑅𝑒𝑐𝑜𝑚𝑚𝑒𝑛𝑑𝑒𝑑 𝑎𝑣𝑒𝑟𝑎𝑔𝑒) 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝐸𝑛𝑒𝑟𝑔𝑦 𝐶𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛−1.5 • As amount of work increases, more rest required • Exercise helps WORK-REST CYCLE Keeping Energy Expenditure Within Bounds: Work-Rest Cycle
    25. 25. • Measurement of Strength • Personal Factors Affecting Strength • Endurance
    26. 26. • Measure groups of muscles • Static strength – Exert force on an immovable object – Angle of joints, motivation, manner in force, posture affect strength • Dynamic strength – Acceleration and joint angles make it difficult to measure – Speed is a factor (slower yield higher levels of measured strength) MEASUREMENT of STRENGTH Strength and Endurance: Definition of Strength
    27. 27. • Gender and Strength • Age and Strength PERSONAL FACTORS AFFECTING STRENGTH Strength and Endurance: Personal Factors Affecting Strength
    28. 28. • Force and frequency of repetition • If maintaining static force, the force required should be well below each individual’s own static force capacity ENDURANCE Strength and Endurance: Endurance
    29. 29. • Approaches to Assessing MMH Capabilities • Lifting Tasks • Carrying Tasks • Pushing Tasks
    30. 30. BIOMECHANICAL APPROACH Manual Materials Handling: Approaches to Assessing MMH Capabilities • Physics principles used for analyzing mechanical stresses and forces • Limited to analyzing infrequent MMH tasks
    31. 31. PHYSIOLOGICAL APPROACH Manual Materials Handling: Approaches to Assessing MMH Capabilities • Energy consumption and stresses acting on the cardiovascular system • Suited for MMH tasks done frequently over a duration of time • Models have been developed, each with their own constraints
    32. 32. PSYCHOPHYSICAL APPROACH Manual Materials Handling: Approaches to Assessing MMH Capabilities • People combine both biomechanical and physiological stresses to form their own opinion of perceived stress • Maximum Acceptable Weight of Load (MAWL) • Special controls are necessary to get valid data
    33. 33. LIFTING TASKS Manual Materials Handling: Lifting Tasks • Influence back injuries more than any other MMH tasks • Parameters – Horizontal Position of Load – Height and Range of Lift – Method of Lifting from the Floor – Frequency of Lifting – Object Characteristics
    34. 34. • 1994 NIOSH Lifting Equation (National Institute for Occupational Safety and Health) • Lifting Index = Load Weight / Recommended Weight Limit (Relative estimate of the physical stress associated with a manual lifting job) • Can only use in certain conditions NIOSH LIFTING EQUATION Manual Materials Handling: Lifting Tasks
    35. 35. HORIZONTAL POSITION of LOAD Manual Materials Handling: Lifting Tasks
    36. 36. HEIGHT and RANGE of LIFT Manual Materials Handling: Lifting Tasks • Categories: – Floor to knuckle – Knuckle to shoulder – Shoulder to reach • Davies (1972) states that the efficient lift range is between 40 and 60 inches
    37. 37. METHOD of LIFTING FROM THE FLOOR Manual Materials Handling: Lifting Tasks • Free-style (use thighs) – Least stressful – Requires least energy • Squat lift (lift with legs) – Results in lower biomechanical stresses on the lower back – Requires load to between the knees • Stoop lift (lift with back) – Toes should touch object and then lifted to minimize horizontal distance and compressive force
    38. 38. FREQUENCY of LIFTING Manual Materials Handling: Lifting Tasks • Endurance a factor: occasional lifting is better
    39. 39. OBJECT CHARACTERISTICS Manual Materials Handling: Lifting Tasks • Object size – Increase height, width, and then length – Keep center of gravity of load closer to body • Object shape – Collapsible objects (e.g. bags) yield higher MAWL • Load distribution and stability – Shifting center of gravity can reduce MAWLs by as much as 31% • Handles – Object with handles are safer and less stressful
    40. 40. CARRYING TASKS Manual Materials Handling: Carrying Tasks • Limit weight, frequency and distance when carrying objects
    41. 41. PUSHING TASKS Manual Materials Handling: Pushing Tasks
    42. 42. • Job Design • Worker Selection & Training
    43. 43. • Decrease the weight of the objects handled • Use two or more people to move heavy or large objects • Change the activity; for example, pull or, better yet, push rather than carry • Minimize horizontal distances between start and end of the lift • Stack materials no higher than shoulder height JOB DESIGN Reducing the Risk of MMH Overexertion: Job Design
    44. 44. • Keep heavy objects at knuckle height • Reduce frequency of lifting • Incorporate rest periods • Incorporate job rotation to less strenuous jobs • Design containers with handles that can be held close to the body JOB DESIGN (2) Reducing the Risk of MMH Overexertion: Job Design
    45. 45. “Enough evidence is available in support of training program effectiveness to warrant its further employment, provided those programs are conducted in conjunction with ergonomic job design and employee selection procedures.” WORKER SELECTION & TRAINING Reducing the Risk of MMH Overexertion: Worker Selection & Training