unh ece896: human factors - chapter 8

Chapter 8: Physical Work and Manual Materials Handlings

Human Factors in Engineering and DesignMark S. Sanders and Ernest J. McCormick

Seventh Edition, 1993

ECE896: Human FactorsDr. William H. Lenharth

Presented by Mark TaipanDecember 2nd, 2010

• 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

• Nature of Muscles• Contractibility of Muscles• Muscle Metabolism

Muscle Physiology

• 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

Muscle Physiology: Contractibility of Muscles

MUSCLE METABOLISM

Muscle Physiology: Muscle Metabolism

Avoid build-up of lactic acid. Too much causes muscle fatigue and pain!

• Respiratory Response• Cardiovascular Response

Work Physiology

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 minuteHeavy Work: 5 Liters of oxygen per minute

• 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

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

Measure of Physiological Strain

• 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

& MAXIMUM AEROBIC POWER

Measures of Physiological Strain: O2 and Maximum Aerobic Power

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

• 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

• Work Efficiency• Grades of Work• Factors Affecting Energy

Consumption

Physical Workload

• 70% of energy -> heat

• Tools, posture, and activities affect work efficiency

WORK EFFICIENCY

Physical Workload: Work Efficiency

Physical Workload: Work Efficiency

GRADES of WORK

Physical Workload: Grades of Work

Physical Workload: Factors Affecting Energy Consumption

FACTORS AFFECTING ENERGY CONSUMPTION: Methods of Work


FACTORS AFFECTING ENERGY CONSUMPTION: Work Posture


FACTORS AFFECTING ENERGY CONSUMPTION: Work Rate


FACTORS AFFECTING ENERGY CONSUMPTION: Tool Design

• Recommended Limits• Work-Rest Cycles

Keeping Energy Expenditure Within Bounds

• 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

• Rest compensate for excess requirements

• Murrell’s Equation:

• As amount of work increases, more rest required

• Exercise helps

WORK-REST CYCLE

Keeping Energy Expenditure Within Bounds: Work-Rest Cycle

• Measurement of Strength• Personal Factors Affecting Strength• Endurance

Strength and Endurance

• 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

• Gender and Strength

• Age and Strength

PERSONAL FACTORS AFFECTING STRENGTH

Strength and Endurance: Personal Factors Affecting Strength

• 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

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

Manual Materials Handling

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

PHYSIOLOGICAL APPROACH


• 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

PSYCHOPHYSICAL APPROACH


• 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

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

• 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


HORIZONTAL POSITION of LOAD


HEIGHT and RANGE of LIFT


• Categories:– Floor to knuckle– Knuckle to shoulder– Shoulder to reach

• Davies (1972) states that the efficient lift range is between 40 and 60 inches

METHOD of LIFTING FROM THE FLOOR


• 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

FREQUENCY of LIFTING


• Endurance a factor: occasional lifting is better

OBJECT CHARACTERISTICS


• 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

CARRYING TASKS

Manual Materials Handling: Carrying Tasks

• Limit weight, frequency and distance when carrying objects

PUSHING TASKS

Manual Materials Handling: Pushing Tasks

• Job Design• Worker Selection & Training

Reducing the Risk of MMH Overexertion

• 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

• 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

“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

unh ece896: human factors - chapter 8

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