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
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
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
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
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
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
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
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
Source: http://wonder.cdc.gov/wonder/prevguid/p0000427/p0000427.asp#head005003000000000
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)