1) Manual material handling (MMH) tasks like lifting pose health risks like back injuries, costing over $1 billion annually in compensation costs. 2) Maximum recommended limits for energy expenditure during physical work are less than 35% of maximum aerobic power and 5.0 kcal/min for men or 3.35 kcal/min for women. 3) Job design approaches to reducing risks of MMH overexertion include decreasing weight of objects handled, using multiple workers for heavy loads, pushing rather than carrying, and minimizing lifting heights and distances.

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. • 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. • Nature of Muscles
• Contractibility of Muscles
• Muscle Metabolism

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. Muscle Physiology: Contractibility of Muscles

6. MUSCLE METABOLISM
Muscle Physiology: Muscle Metabolism
Avoid build-up of lactic acid. Too much causes muscle fatigue and pain!

7. • Respiratory Response
• Cardiovascular Response

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. • 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. • Maximum Aerobic Power
• Heart Rate
• Measures of Local Muscle Activity

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. 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. • 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. • Work Efficiency
• Grades of Work
• Factors Affecting Energy
Consumption

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. Physical Workload: Work Efficiency

17. GRADES of WORK
Physical Workload: Grades of Work

18. Physical Workload: Factors Affecting Energy Consumption
FACTORS AFFECTING ENERGY CONSUMPTION:
Methods of Work

19. Physical Workload: Factors Affecting Energy Consumption
FACTORS AFFECTING ENERGY CONSUMPTION:
Work Posture

20. Physical Workload: Factors Affecting Energy Consumption
FACTORS AFFECTING ENERGY CONSUMPTION:
Work Rate

21. Physical Workload: Factors Affecting Energy Consumption
FACTORS AFFECTING ENERGY CONSUMPTION:
Tool Design

22. • Recommended Limits
• Work-Rest Cycles

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. • 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. • Measurement of Strength
• Personal Factors Affecting Strength
• Endurance

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. • Gender and
Strength
• Age and
Strength
PERSONAL FACTORS AFFECTING STRENGTH
Strength and Endurance: Personal Factors Affecting Strength

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. • Approaches to Assessing MMH Capabilities
• Lifting Tasks
• Carrying Tasks
• Pushing Tasks

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. 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. 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. 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. • 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. HORIZONTAL POSITION of LOAD
Manual Materials Handling: Lifting Tasks

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. 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. FREQUENCY of LIFTING
Manual Materials Handling: Lifting Tasks
• Endurance a factor: occasional lifting is better

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. CARRYING TASKS
Manual Materials Handling: Carrying Tasks
• Limit weight, frequency and distance when
carrying objects

41. PUSHING TASKS
Manual Materials Handling: Pushing Tasks

42. • Job Design
• Worker Selection & Training

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. • 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. “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