Work System Design
Measuring Work by Physiological
• Understand how work is measured
• Understand the limits of human physiology
• Understand the changes in Oxygen
consumption, heart rate, pulmonary ventilation,
body temperature and lactic acid concentration
in the blood when physical work is done
• Central to the drive for productivity improvement is the
challenge of finding an acceptable work rate for a given job.
• Work rate can set either:
– Subjectively (industrial engineers)
• In principle, any increase in oxygen uptake over and above
that required for basal metabolism can be used as an index of
physiological cost to an individual.
Measurement of the physiological work
• The classical method of determining energy expenditure at
work involves the measurement of oxygen uptake using the
How does a Douglas bag work?
• Hundred liters of exhaled air will be collected in a
• From the person doing some task
• Air in the bag is then analyzed
• To find the amount of oxygen metabolized
• Rate of energy expenditure can be calculated
Indirect measures of energy expenditure
• Heart rate increases as a function of workload and oxygen
• Because it is more easily measured than oxygen uptake, heart
rate is often used as an indirect measurement of energy
• Heart rate can be likened to a signal that integrates the total
stress on the body.
• Heart rate measurement can therefore be used as an index of
the physiological workload.
Portable Heart Rate Recorder
Curves showing relationship between energy expenditure in Cal/min,
speed of walking in miles/hr, body weight in pounds
Energy Cost of Work and Workload
• The increase of metabolism from resting to working
is called working metabolism or metabolic cost of
• The metabolic or energy expenditure is the sum of the
basal metabolism rate and working metabolism rate.
Energy Expenditure = Basal metabolism +
working metabolism --- unit is (kcal/min)
Energy Expenditure Rates for Various Activities
Estimates of energy
Source; Based on Durnin & Passmore, 1967, Woodson 1981
Applied Physiology at the Workplace
• Rate for energy expenditure of a work is linearly related to the
amount of oxygen consumed by the body and heart rate.
• Therefore, oxygen consumption rate and heart rate are often
used to quantify the workload of physical work.
• There is a linear relationship between oxygen consumption and
• For every liter of oxygen consumed, an average of about 4.8
kcal of energy is released. Thus, the amount of aerobic
metabolism or energy expenditure of work can be determined
by multiplying the oxygen consumption (liter/min) by 4.8
Applied Physiology at the Workplace
• Heart rate is the number of heart beats per minutes.
• Heart rate increase as work load and energy demands are
• Heart rate is linearly related to oxygen consumption.
• Heart rate can be influenced by emotional stress, drinking
coffee or tea, working static or awkward posture, or working
in hot environments
Subjective measures of physical effort
• The most common method of obtaining subjective
estimates of physical effort is by use of the Borg Rating of
Perceived Exertion (RPE) scale (Borg, 1982).
• Workers rate their perceived level of exertion during or after
performing the task on a scale from 6 to 20, corresponding to
heart rates of 60 to 200 beats per minute.
• High positive correlations between heart rate and the RPE are
usually found .
• The Borg scale is normally used with other measures, typically
heart rate and oxygen consumption
Calculation of Rest Period in Manual
w = length of the working period
b = oxygen uptake
s = ‘standard’ uptake for continuous work
Heart rate and VO2 Max
• VO2Max is the max amount of oxygen that can be
consumed by a person.
• It was found that VO2Max is related to heart rate.
• So if heart rate can be measured, then VO2Max can be
found as well.
• It is generally believed that individuals can work
continuously over an 8-hour shift at a rate of 30–50% of
their maximum capacity.
• %MHR = 0.64 × %VO2max + 37
Oxygen consumption and heart rate are objective measures of workload.
Despite the global trend to replace manual work with mechanized and automated
systems, physiological aspects of work design are of great importance in view of the
modern interest in eliminating avoidable ill-health.
In low-technology jobs, the problems of avoiding excessive physical stress remain.
In high-technology jobs, the problem is to avoid mental overstress and find ways of
increasing the physical activity of the, predominantly, sedentary workers.
This is particularly relevant when it is remembered that post-industrialised countries
have an ageing population.