PWC and Evaluation

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Physical work capacity and evaluation

INTRODUCTION

Physical work capacity refers to a worker’s capacity for energy output . This will

depend primarily on the energy available to the worker in the form of food and oxygen and the

sum of the energy provided by oxygen-dependent and oxygen independent processes . The rate of

energy consumption during physical work is the sum of the basal energy consumption and the

metabolic cost of the work in terms of energy consumption. The basal metabolic rate (BMR) is the

rate of energy consumption necessary to maintain life.

For continuous work at moderate intensities, oxygen-dependent processes

usually make the major contribution to energy output. For each litre of oxygen consumed, about 20

kilojoules of energy is released. Work capacity depends on the ability to take up oxygen and deliver

it to the cells for use in the oxidation of foodstuffs. The Ability to work at a high rate is associated

with a high oxygen uptake. Exercise physiologists and sports scientists have used the term ‘VO2 max’ to

describe an individual’s capacity to utilise oxygen (aerobic capacity). VO2 max has traditionally

been estimated by having subjects run on a treadmill or pedal a bicycle ergometer while their

oxygen uptake is measured. Some factors that influence a person’s capacity to carry out physical

work are:

Age:Age has a significant effect on work capacity. Vo2 max declines gradually after

the age of 20. A 60-year-old has an aerobic capacity of about 70% of a 25-year-old. This is due to a

reduction in cardiac output. Current thinking stresses that the fundamental ageing phenomenon is

due to a loss of muscle function. Since the heart is essentially a muscle, this explains the loss of

aerobic capacity with age.

Body weight:Body weight (particularly the percentage of body tissue that is composed of fat)

influences all activities in which the worker has to move his own body. In exercise physiology and

sports science, it is usually more meaningful to express Vo2 max in relative terms by expressing aperson’s oxygen consumption in terms of their body weight (litres of oxygen per minute per

kilogram of body weight). This takes into account the fact that the leaner runner will be at an

advantage over his plumper rival, all other things being equal. In this sense, it is possible to increase

one’s relative Vo2 max by shedding excess kilograms of fat.

Sex:Women have a lower Vo2 max than men and usually have a higher percentage

of body fat. They also have less haemoglobin than men. The cardiac output per litre of oxygen

uptake is higher in women than in men . For a woman, the heart must therefore pump more

oxygenated blood than for a man in order to deliver one litre of oxygen to the tissues. Apart fromthose that are obvious, few of the differences between men and women have any ergonomic



implications. Most of the difference in work capacity is really due to differences in body size. In

general, women do appear to have lower upper body strength, controlling for body size.

Tobacco smoking:

Tobacco smoke contains about 4% by volume carbon monoxide (CO). CO has an

affinity for haemoglobin (combining to form carboxyhaemoglobin) 200 times as powerful as that of

oxygen. Smoking therefore reduces work capacity by reducing the oxygen carrying capacity of the

blood. It also causes chronic damage to the respiratory system, which impairs the ventilation of the

lungs and the transfer of oxygen from the air to the blood.

Alcohol:Alcohol may increase cardiac output in sub maximal work, thereby reducing

cardiac efficiency. It also affects liver function and can cause a predisposition to hypoglycaemia.

Nutritional status and general health:

A balanced diet is important to ensure adequate amounts of necessary

foodstuffs and to minimise the accumulation of excess body fat. Excess body fat lowers a person’s

relative Vo2 max as was described above. In developed countries, many people eat a diet high in

saturated fats. Such a diet causes raised plasma concentrations of cholesterol. Tiny crystals of

cholesterol are deposited on the inside walls of the arteries and this eventually leads to a disease of

the arteries known as atherosclerosis. The continued accumulation of cholesterol forms deposits,

called plaques that eventually reduce the cross-sectional area of the artery and thus impede blood

flow. Additionally, the arteries lose flexibility (atherosclerosis is sometimes called ‘hardening of the

arteries’ for this reason). These changes in the structure of arteries can impede the flow of blood tothe muscles and to the heart itself, resulting in decreased performance

Training:Work capacity can be enhanced by physical training (to increase a worker’s VO2

max) and job training in more efficient work methods (to obtain more output per litre of oxygen

consumed by the worker or to enable the worker to safely exert larger forces by using better

techniques). Specific training regimes can be developed to strengthen particular parts of the

musculoskeletal system with the goal of improving performance or preventing injury. Strength

training requires that the body part in question be exercised at near-maximal levels. Over a period of

several months, the muscle fibres increase in size owing to an increase in the number of myofibrils,

and an increase in strength is observed.

Motivation:Motivation is an extremely important determinant of work capacity. For present

purposes, it may be noted that a worker’s level of motivation may be affected by intrinsic factors

such as personality, personal and career goals, need for achievement at work, and so on, and

extrinsic factors such as work organisation, method of remuneration and the availability of

alternative forms of employment. Piece rate systems (where the worker is paid according to how

much is produced) may motivate workers to work at an increased rate but have been associated

with increased risk of accidents and of developing musculoskeletal ailments. Motivation is discussed

in more detail in other chapters of this book.



Environmental factors influencing work capacity:

Air pollution:Air pollution may increase the resistance to air flow of the respiratory airways

and, in the long term, cause damage to the lungs, permanently reducing the capabilities. If the

source of the pollution involves the combustion of organic compounds, carbon monoxide may be

one of the by-products and decrements in work capacity may result. When the concentration of

carbon monoxide exceeds 6.5 parts per million it begins to accumulate in the blood during sub

maximal exercise. To put this into perspective, carbon monoxide concentrations from 37 to 54 parts

per million occur in urban traffic. The work capacity of people doing heavy manual work in urban

areas may well be degraded because of this.

Climatic factors:

Extreme environments can have significant effects on work capacity.

Noise:Noise is a stressor that can elevate the heart rate and reduce cardiac efficiency,

thus decreasing the work capacity.

Altitude:The capacity for sustained work is reduced at high altitude because the partial

pressure of oxygen is less (the air becomes ‘thinner’ with increasing altitude). Less oxygen is

available per unit volume of air and thus the functional VO2 max is reduced. Some adaptation to

work at altitude can take place after continual exposure for several weeks. Short term, high-intensity

activities (such as lifting a heavy weight) are not influenced by altitude because their performance

does not involve oxygen-dependent processes.

TOOLS AND TECHNIQUES TO MEASURE PWC

Douglas bag:

The classical method of determining energy expenditure at work involves the

measurement of oxygen uptake using the Douglas bag. The subject inhales air from the atmosphere

and exhales it through a mask connected by tubing to a large bag. As the subject carries out the task,

the initially empty bag is filled with expired air. After about 50 litres of air has been collected, the

task is terminated or the expired air is diverted to a second empty bag. The volume of air in the filled

bag is calculated and its gaseous composition is analysed using electronic gas analysers. The oxygen

content of the air in the bag can be compared with that of the atmosphere to determine the amount

of oxygen metabolised by the subject. If the time taken for the subject to fill the bag is known, the

rate of oxygen uptake can be calculated. From this, the rate of energy expenditure can be calculated.

The Douglas bag method is well established but can be inconvenient and interfere with performance



of the task Owing to the bulky nature of the equipment required. More compact instruments have

been developed to facilitate the measurement of oxygen consumption of moving subjects.

Oxylog:The Oxylog measures the volume of inspired air as it passes through a turbine

flow meter in a mask placed over the subject’s nose and face. Some of the expired air passesthrough the Oxylog itself, where its oxygen content is measured. Harrison (1982) reports that the

Oxylog is sufficiently accurate to yield reliable estimates of oxygen uptake in the working

environment.

Pack test

The pack test, also known more officially as the Work Capacity Test (WCT), is

a U.S. Forest Service physical test for wild land fire-fighters. The pack test is intentionally stressful as

it tests the capacity of muscular strength and aerobic endurance of the fire-fighter. There are three

tests known as arduous, moderate, and light. The pack test may be given as part of the S-130/S-

190 basic wild land fire-fighter course.

The pack test replaced an earlier physical fitness test called the step test, which

measured physical fitness based on beginning and ending heart rate after a short workout on a set

of stairs. It was believed that the pack test more accurately measures the ability to perform arduous

labour for a sustained period of time (e.g. 45 minutes) than the step test. These tests give only a

rough range of physical work capacity (PWC).

Arduous pack test:

Arduous: The pack test is a job-related test of the capacity for arduous work. It consists of a 3-mile

hike with a 45-pound pack over level terrain. A time of 45 minutes, the passing score for the test,

approximates an aerobic fitness score of 45, the established standard for wild land fire-fighters. The

http://en.wikipedia.org/wiki/U.S._Forest_Service

http://en.wikipedia.org/wiki/Wildland_firefighter

http://en.wikipedia.org/wiki/Aerobic_exercise

http://en.wikipedia.org/wiki/S-130/S-190


http://en.wikipedia.org/wiki/Heart_rate




http://en.wikipedia.org/wiki/Aerobic_exercise

http://en.wikipedia.org/wiki/Wildland_firefighter

http://en.wikipedia.org/wiki/U.S._Forest_Service



energy cost of the test is similar to the energy cost demanded on the job. The test is correlated to

measures of performance in field tasks such as working with hand tools or carrying loads over rough

terrain and with measures of aerobic and muscular fitness. The test’s length ensures that successful

participants will have the capacity to perform prolonged arduous work under adverse conditions,

with a reserve to meet emergencies.

Requirements: 3-mile hike with 45 lb pack in 45 minutes. No jogging or running.

Moderate field test:

Moderate: The field test is a job-related test of work capacity designed for those with moderately

strenuous duties. It consists of a 2- mile hike with a 25-pound pack. A time of 30 minutes, the

passing score, approximates an aerobic fitness score of 40.

Requirements: 2-mile hike with 25 lb pack in 30 minutes. No jogging or running.

Light walk test:

Light: The walk test is designed to determine the ability to carry out light duties. It consists of a 1-

mile test with no load that approximates an aerobic fitness score of 35. A time of 16 minutes, the

passing score for the test, ensures the ability to meet emergencies and evacuate to a safety zone.

The instructions for the pack test also apply to the field and walk tests. Test requirements for a given

position may change.

Requirements: 1-mile hike with no pack in 16 minutes. No jogging or running.

Step test:

The step test measured physical fitness based on beginning and ending heart

rate after a short workout on a set of stairs. The relation between the Vo2 and heart rate (HR) is

given as:

Vo2 = HR*k + c

Where k and c are constant.

Bicycle ergometer:

A cycloergometer, cycle ergometer or bicycle ergometer is a stationary bicycle

with an ergometer to measure the work done by the exerciser. Through the physical work capacity

of the person can be measured.



http://en.wikipedia.org/wiki/Ergometer

http://en.wikipedia.org/wiki/Ergometer





Treadmill tester:

The patient is brought to the exercise laboratory where the heart rate and blood

pressure are recorded at rest. The treadmill is then started at a relatively slow "warm-up" speed. The

treadmill speed and its slope or inclination are increased every three minutes according to a pre-programmed protocol (Bruce is the commonest protocol in the USA, but several other protocols are

perfectly acceptable). The protocol dictates the precise speed and slope. Each three minute interval

is known as a Stage (Stage 1, Stage 2, Stage 3, etc. Thus a patient completing Stage 3 has exercised

for 3 x 3 = 9 minutes). The patient's blood pressure is usually recorded during the second minute of

each Stage. However, it may be recorded more frequently if the readings are too high or too low.



CASE STUDY

Comparison of oxygen uptake during a conventional treadmill test and the

shuttle walking test in chronic airflow limitation

The purpose of this study was to investigate the relationship between performance on the shuttle

walking test and maximal oxygen uptake (VO2max) during a conventional treadmill test in patients

with chronic airflow limitation. Two different techniques were used to measure oxygen

consumption, i.e. conventional Douglas bag techniques (treadmill test) and a portable oxygen

consumption meter (shuttle test). Initially, 19 patients performed a shuttle walking test (after one

practice walk) and a maximal treadmill walking test, in a randomized, balanced design.

Subsequently, 10 patients, (after one practice) completed an unencumbered shuttle walking test and

one supporting the portable oxygen consumption meter, in random order. The results of the first

experiment revealed a strong relationship between performance during the shuttle walking test and

VO2max during the treadmill walking test (r=0.88). The results of the second experiment

consistently demonstrated an incremental increase in oxygen consumption and ventilation in

response to the increasing intensity of the shuttle walking test. Again, a strong relationship between

VO2max and performance on the shuttle test was demonstrated (r=0.81). We concluded that the

shuttle walking test is a valid field exercise test of functional capacity. Performance on the test

relates strongly to VO2max, the traditional indicator of cardiorespiratory capacity.

Patients and method

Patients with stable chronic airflow limitation were recruited. Individuals presenting with any is

order that might influence exercise performance were excluded. Twenty two patients were

recruited, which consisted of two separate experiments. In the first, the relationship between

shuttle test performance and VO2max measured during treadmill walking (n=19) was examined.

In the second, examination of the patients' physiological responses during the shuttle walking test

(n=10) was done. Seven patients completed both experiments.

Study design

For each experiment, patients were required to make three visits to the hospital at intervals one

week, at the same time of day. The baseline measurements were the same for all three visits for

experiments and included spirometry (forced expiratory volume in one second (FEV1) and forced

vital capacity (FVC)), completion of the Chronic Respiratory Disease Questionnaire (CRDQ) and

measurement of height and weight. Briefly, the test requires patients to walk at increasing speeds

up and down a 10 m course. The speed of walking, increasing every minute, is controlled by audio

signals played from a tape cassette. During the shuttle walking test, heart rate was recorded using a

short range telemetry device and Borg breathlessness score was recorded before and after

completion of the test.



Relationship between performance on shuttle walking test and treadmill VO2max(Experiment 1)

19 patients were recruited (17 males and 2 females; mean (SD) age 61 (7) yrs; weight 72 (14) kg;

height 1.70 (0.06) m).

In addition to the practice shuttle walking test, on the first visit a treadmill test was employed which

used a constant speed with increases of the gradient from 0%, by 2.5%, every 2 min. The speed of

walking was gauged for each patient, aiming for each individual to reach a symptom-limited VO2max

in 6 –10 min. The second and third visits were presented to the patient in a randomized, balanced

design, i.e. the treadmill walk followed a week later by the second shuttle walking test or vice versa.

The treadmill visit required the patients to perform a symptom-limited maximal exercise test.

Expired air was collected in a series of Douglas bags during the second minute of each 2 min exercise

period and analysed for oxygen (O2) and carbon dioxide (CO2). The last collection of expired air was

made during the final minute of exercise, as indicated by the patient. During the second minute of each stage, heart rate was recorded from the monitor and the perceived breathlessness was

recorded. The results from the second shuttle walking test were used to evaluate the relationship

between the results from the treadmill test and performance on the field exercise test.

Measurement of VO2 during the shuttle walking test

(Experiment 2)

Ten patients (6 males and 4 females; aged 64 (7) yrs, weight 70 (15) kg; height 1.65 (0.07) m; FEV1

1.02 (0.38) l; FVC 2.06 (0.49) l) were recruited, and familiarized with the shuttle walking test and the

equipment . Oxygen uptake was determined during the shuttle walking test using a portable oxygen

consumption meter (Oxylog). This allowed the ambulatory measurement of ventilation (inspiratory

flow (VI)), percentage of O2 in expired air, and hence oxygen consumption. This data from the

Oxylog was recorded and stored. Prior to this study, the validity of the Oxylog measurements were

examined as described below. The reliability of the Oxylog had been confirmed previously in a small

group of healthy individuals.

Assessment of the validity of the Oxylog

Comparisons were made with Oxylog measurements (VO2 and ventilation, VI) and conventional

Douglas bag measurements during treadmill walking in 5 patients and in 10 healthy volunteers. A

consistent underestimation of ventilation (all volumes converted to VI at STDP) was identified with

the Oxylog system which was attributable in part to the poorly fitting face mask, a problem

previously reported. With the use of mouthpiece, there was no significant difference in the

measurement of VO2max between the Oxylog and Douglas bags, although a small but significant

underestimation in ventilation by the Oxylog persisted. The option of the Oxylog was, therefore,

favoured in preference to ambulatory Douglas bag collection, which is both cumbersome and

difficult to operate for repeated collections.

Statistical analysis

Analysis was performed using the CIA V1.1 software package and "Minitab V.8" statistical packagefor IBM compatible computers to examine the agreement between tests. The relationships between



performance variables were evaluated using standard parametric tests. Results of the CRDQ were

subject to appropriate nonparametric analysis. Values are presented as mean (SD) unless otherwise

stated, and a 5% level of significance was adopted throughout.

Results

Relationship between performance on the shuttle walking

test and treadmill VO2max (Experiment 1)

Relationship between performance on the shuttle walking test and treadmill VO2max (Experiment 1)

The relationship between the distance walked on the shuttle walking test and the directly

determined VO2max was strong (r=0.88) (fig. 1). It was represented by the regression equation (with

95% confidence intervals):

VO2max=4.19 (1.12 –7.17) + 0.025 (0.018 –0.031)*distance

where VO2max is in ml·min-1·kg-1 and distance is in metres. The results of the treadmill test areshown in table 1. The mean walking speed was 2.52 mph and the mean duration of the test was 7.1

min. When the equation [(FEV1 × 18.9) + 19.7)] was used to predict maximal ventilation, this group

of patients attained a mean maximal ventilation representing 95% of predicted.

Nine patients exceeded 100% of their predicted maximal ventilation. The mean maximal heart rate

attained during the treadmill test represented 76% of predicted maximum heart rate. This rate was

significantly greater than that observed during the shuttle walking test (mean difference 17 (11)

beats·min-1).

Measurement of VO2 during the shuttle walking test

(Experiment 2)

The relationship between the peak VO2 recorded and the shuttle distance walked is shown in figure

2 (r=0.81). The mean distance walked was 375 (137) m for the control visit and 302 (133) m for the

Oxylog visit (p<0.05). The distance was shorter for all patients on the Oxylog visit but there was no

significant difference in the mean maximal heart rate (120 (14) and 118 (10) beats·min-1 for the

control and Oxylog visits, respectively) or post exercise Borg breathlessness score (5.5 and 5.1,

respectively). The data obtained from the Oxylog revealed that each patient demonstrated a gradual

increase in the level of ventilation and VO2 during the shuttle walking test. A linear increase in VO2

was demonstrated as the patient progressed through successive levels of the shuttle walkingtest (fig. 3). A corresponding linear increase in ventilation was demonstrated. The mean maximal VI

recorded was 24.9 (9.4) l·min-1. The mean VO2 attained was 0.74 (0.42) l·min-1 (or 10.1 ml·min-

1·kg-1). Seven patients participated in both the treadmill and the Oxylog experiments. The mean

VO2max measured in this subgroup was 12.9 (3.6) ml·min-1·kg-1 during the treadmill test and 11.1

(4.2) ml·min-1·kg-1 during the shuttle walking test. These two maximal values were strongly related

(r=0.86) and were not significantly different (mean difference, 1.8 ml·min-1·kg-1, 95% confidence

interval (95% CI) -0.9 –4.5).



Discussion

VO2max is considered to be the reference measurement of functional capacity. To ensure that

performance on or the shuttle walking test provides a valid measure of a patient's functional

capacity, the relationship between shuttle test performance and VO2max was examined.

The main finding was that the relationship with treadmill walking was strong, with 77% of the

variance common to both tests. Because of the linear relationship between walking speed andoxygen consumption, this is not surprising. The strong relationship between performance and



VO2max was repeated in the second experiment, which examined the physiological response to the

shuttle test. It is worth noting that the degree of underperformance appears to have been similar in

all patients, so that the relationship between performance and VO2max was maintained. The

strength of these relationships allows the prediction of a patient's VO2max from performance on the

shuttle walking test (fig. 1) with a degree of confidence that has not previously been possible using

other field exercise tests.

The heart rate response for both exercise tests (treadmill and shuttle walking test) produced the

same pattern, i.e. an incremental increase with the increase in exercise intensity. There was,

however, some disparity observed in the maximal heart rate recorded after the treadmill and shuttle

tests. The treadmill test provoked a higher maximal heart rate than the shuttle walking test. At a

comparable submaximal walking speed, the heart rate was again higher on the treadmill than on the

shuttle walking test. This may be accounted for by the laboratory environment and the equipment

that patients were required to carry during the treadmill test. Alternatively, the treadmill test may

genuinely have been a more stressful exercise test, because of protocol variation, although this is

not supported by the patients' subjective ratings of the two tests. The limited comparative VO2 datadetermined during treadmill walking and during the shuttle walking test reveals that the latter does

provide an adequate stimulus to elicit maximal values. The VO2max values from the seven patients

who participated in both the studies were not significantly different. This supports the hypothesis

that the shuttle walking test does provoke a comparable maximal exercise response to a

conventional treadmill test. The incremental increases in heart rate, VI and VO2 observed during the

shuttle walking test confirm that the shuttle walking test provokes a gradual physiological response

to exercise of increasing intensity, similar to that observed during a treadmill test. The gradual

increase in intensity allows the test to be conducted with a relatively low risk in any hospital corridor

or gym. In conclusion, performance on the shuttle walking test relates strongly to direct measures of

VO2max, allowing the prediction of VO2max. During shuttle walking, the cardiorespiratory response

to exercise develops in an incremental fashion.

Overall, the present study substantiates the proposal that the shuttle walking test is an incremental

maximal field exercise test of disability that provides an objective measure of a patient's

cardiorespiratory capacity.



FUTURE ASPECTS AND CURRENT RESEARCH

USE IN NURSING MANAGEMENT

Today, experimental investigations about physical work load in nursing are very few. Studies can be

conducted in the field survey, for example work load in nursing, fatigue in nursing, motivation for

nursing, a mental health in nursing and so on. A work environment or management in nursing as it

ought to be based on these studies has been proposed.

RESEARCH IN CANADA

Substantial numbers of Canadians with disabilities are willing and able to work – if not full-time, then

at least on an intermittent basis. Human Resources and Skills Development Canada commissioned

this research to better understand why some with disabilities who have intermittent work capacity

remain working, while others with similar disabilities become discouraged and drop out of the

labour force. The goal of the project was to identify the conditions, support services, and employer

practices that help people with disabilities and intermittent work capacity stay employed.

PHYSICAL WORK CAPACITY AND NUTRITIONAL STATUS IN MALE CHILDREN AND YOUNG ADULTS

Malnutrition is widespread in developing countries. Malnutrition results in deaths of several

children. Among those who survive, various degrees if stunted growth often persist throughout life.

The important factors in discussions on the long-range unfavourable effects of malnutrition are not

so much the deviations from normal of parameters but rather deviations of the functional

performance of the body. This again can be expressed in terms of capacity to perform physical work.

In Ethiopia physical work performance of males and young adults has been measured by making use

of modern examination techniques such as dynamometer and bicycle ergometer.

So far, no studies seem to have

So far, no studies seem to have been performed on malnourished subjects, nor indeed any study of

this kind in a developing country. So this direction of research can be followed in other developing

African and Asian countries

RESEARCHES IN PWC OF MINERS

The aerobic work capacity of 220 coal miners aged 22 to 63 years with a high physical work load and

78 industrial workers aged 19 to 58 years with a relatively light work load was measured to observe

if there was a relationship between the work load of these subjects and their aerobic work capacity.

All the subjects were subjected to a medical examination, spirometry, chest x Rays andanthropometric measurements. Aerobic work capacity was indirectly estimated extrapolating pulse

rates obtained al submaximal workloads in a bicycle ergometer to the calculated maximal cardiac

frequency for age.

APPLICATION IN ROWING SPORTS

Some researchers have used rowing ergometers with the aim of measuring mechanical parameters

of rowing and the work capacity of rowers of different skill levels. At the present time, a variety of

off-water devices are widely used by rowers in training and also for testing purposes.



GENERALISED FRAMEWORK FOR EVALUATING PWC

Evaluation of physical work capacity can be done by means of various methods described above. The

basic steps required in the evaluation process are:

First step involves selecting and grouping the people on the basis of sex, age and weight. Check if the subject suffers from chronic disease or any other ailments and take necessary

precautions accordingly

Next is determination of method by which the evaluation will take place. The various

methods are:

1. Douglas Bag

2. Oxylog

3. Pack test

4. Step Test

5. Treadmill

6. Bicycle Ergometer

The next step involves setup of the equipment for the test and familiarise the subject with

the procedure

Record the Heart Rate or the VO2 during the test as needed

Plot the HR vs VO2 curve and determine the VO2max and hence determine the PWC

The Physical Work Capacity thus calculated can be used to determine the relative work load of a job.

PWC and Evaluation

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