Work System Design

Measuring Work by Physiological Methods

Objectives• Understand how work is measured

physiologically• 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

Work Physiology• 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)– Objectively

• 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 Douglas bag

How does a Douglas bag work?

• Hundred liters of exhaled air will be collected in a bag

• 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 uptake.

• Because it is more easily measured than oxygen uptake, heart rate is often used as an indirect measurement of energy expenditure

• 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 Assessment

• The increase of metabolism from resting to working is called working metabolism or metabolic cost of work.

• 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 Activity Estimates of energy

expenditure rates(cal/min)

Sleeping 1.3

Sitting 1.6

Standing 2.3

Walking 3km/hr 2.8

Walking 6km/hr 5.2

Carpenter-assembling 3.9

Woodwork-packaging 4.1

Stockroom work 4.2

Welding 3.4

Sawing wood 6.8

chopping wood 8.0

Athletic activity 10.0

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 energy expenditure

• 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 kcal/liter.

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

increased.• 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

Borg Rating of Perceived Exertion (RPE) scale

Calculation of Rest Period in Manual Work

• Where

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

% of VO2Max and Time

Summary• 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.

Thank You