This document discusses human physiology as it relates to work. It describes how tasks must match human capabilities to avoid underloading or overloading workers. It examines factors like oxygen uptake, heart rate, fatigue, and the need for work/rest cycles to prevent worker breakdown or damage. Work is classified from light to extremely heavy based on energy expenditure and heart rate. The ability to tolerate physical work depends on cardiovascular and respiratory systems.

1. WORK PHYSIOLOGY
People perform widely different tasks in
daily work situations. These tasks must be
matched with human capabilities to avoid
“underloading”, in which human capacities are
not utilized properly, as well as “overloading”,
which may cause the employee to break down
and suffer reduced performance capability or
even permanent damage.

2. Work physiologists study the chanced
appearing in the functions of the main organs
and systems (cardiovascular and respiratory) of
workers at work and evaluate the capacities and
limitations of the worker for performing physical
work; they also determine human tolerance to
stresses produced by the physical environment.

3. Capacity for physical work
An individual’s physical tolerance
to physical work is usually determined
by the capacity of his or her
cardiovascular and respiratory systems
to deliver oxygen to the working
muscles and to metabolize chemically
stored energy.

4. Maximum oxygen uptake is often
used to describe the upper limit of
this capacity.
If a person is pushed beyond this
limit in an emergency situation,
anaerobic processes provide the
additional energy required.

5. The biochemical processes that
transform foodstuffs into energy available for
work are quite complex; they involve series
of aerobic and anaerobic steps.
Nevertheless, measurement of the
volume oxygen consumed provides a
relatively simple overall index of energy
consumption and hence of the energy
demands of work.
Utilization of 1 liter of oxygen yields
approximately 5 kilocalories (kcal).

6. To put oxygen consumption and
energy demands into proper perspective,
consider the abilities of trained athletes
who may reach maximal oxygen uptake
capacity of up to 6 L/min.
Aside from a person’s physique, age
and gender influence the oxygen uptake
capacity.

7. Men who are 20 years of age have an
average maximal capacity of 3-3.5
L/min; women of the same age have an
average capacity of 2.3-2.8 l/min.
At age of 60 the capacity is
diminished to about 2.2-2.5 L/min. for
men and 1.8-2.0 L/min for women.

8. The ability to move oxygen from the
air to the active muscle can be improved
through physical training by up to 20 %.
Of course, an individual’s capacity
also depends on other “central” functions
(e.g. of the circulatory of the cardiac
system) and on “local” capacities (e.g. of
muscles).

9. To determine the individual’s
tolerance to physical work the
following lung volumes and
capacities can be measured:
- tidal volume (TV) – the volume
of gas inspired or expired during
each respiratory cycle;

10. - vital capacity (VC) – the maximum
amount of air that can be exhaled from the
lungs after a maximum inspiration. It is a very
sensitive index for estimation of work capacity
- forced expiratory volume in one
second (FEV1) – the volume of air which can
be forcibly expelled during the first second of
expiration. Its reduction indicates reduction of
airflow rates and prolongation of expiration.

11. Energy cost of work
Totally, the heaviest work that a young, fit
man can sustain for prolonged periods is
about 500 kcal/hr. Among the general
population, this figure is somewhat lower:
400-425 kcal/hr.

12. Industrial jobs seldom demand
such a high-energy expenditure
over the course of a workday.
Rest pauses, providing tools, and
receiving instruction all tend to
reduce the average energy
expenditure considerably.

13. When intermittent tasks are performed, the
average expenditure may be calculated using
the following formula:
M = (M1t1 + M2t2 + ………Mntn)t-1
where M is the total energy cost; M1, M2 and
so on are the energy costs of individual tasks;
t1, t2 and so on indicate the duration of the
individual task; and t is the total elapsed time.

14. Heart rate at work
There is a close interaction between
the human circulatory and metabolic
systems.
Nutrients and oxygen must be
brought to the working muscles and
metabolic by-products removed from
them to ensure proper functioning.

15. Therefore, heart rate (which is a
primary indicator of circulatory
functions) and oxygen
consumption (representing the
metabolic processes taking place in
the body) have a linear and
reliable relationship in the range
between light and heavy work.

16. When very light work loads or very
heavy ones are being handled, the
relationship may not be reliable.
It is also not reliable under severe
environment conditions or when
workers are under mental stress.

17. Given such a linear relationship,
one can often simply substitute
heart rate measurements for a
measurement of a metabolic
process such as oxygen
consumption.
This is a very attractive shortcut,
since heart rate measurements
can be performed rather easily.

18. The simplest technique for heart rate
assessment is to palpate an artery, often
in the wrist, and to count the number of
heartbeats over a period of time such as
15 seconds and calculate the average
heart beat per minute.
More refined methods utilize various
plethysmographic techniques.

19. More expensive techniques rely on
electric signals generated by the
nervous system that control heart
rate.
When using this technique, electrodes
are usually placed on the patient’s
chest. This allows recording of heart
rate when the worker is performing his
typical working operations –
“heart rate at work”.

20. Some of these instruments are computerized
and have memory enough to record the heart
rate up to 16 hours and to perform various
analyses by software.
It is quite important to record the basic, at
rest, values of heart rate, to have in mind the
individual (or due to other, even pathological
conditions in the body) variability of heart
rate, and to follow up its dynamical changes
during the workday.

21. This is useful to provide a
proper work/rest regimen.

22. Use of heart rate measurements has
one major advantage over oxygen
measurement as an indicator of
metabolic processes:
heart rate reacts faster to work
demands and therefore more easily
indicates quick changes in body
functions due to changes in work
requirements.

23. Work classification
The work demands are usually rated
from light to extremely heavy in terms
of
energy expenditure per minute, and
the relative heart rate in beats peer
minute is also given.

24. Classification of Light to Heavy work according to
Energy expenditure and Heart rate
Classification
Total Energy expenditure Heart Rate
(kcal/min) (beats/min)
Light work 2.5 – 3 90 or less
Medium work 3–5 90 – 100
Heavy work 5 – 7.5 100 – 120
Very heavy work 7.5 – 10 120 – 140
Extremely heavy work 10 – 15 140 or more

25. When the work is light, the energy
needs of the working muscles are
supplied by oxygen available in the
blood and by glycogen in the muscles.
There is no building of lactic acid or
other metabolic by-products that
would limit a person’s ability to
continue such work.

26. In heavy work, during which the heart
rate is about 120 beats/min, the oxygen
required is still supplied if the person is
physically capable to do such work and
specifically trained in this job.
However, the lactic acid concentration
produced during the initial phase of work
is not reduced but remains high until the
end of the work period and returns to
normal level after cessation of the work.

27. In course of light, medium, and
even heavy (if the person is trained
and capable) work, the metabolic
and other physiological functions
can attain a steady-state
condition during the work period.
No steady-state exists in the
course of very heavy work.

28. In this case, the oxygen deficit incurred
during the early phase of work increases
throughout the duration of the effort and
metabolic by-products accumulate,
making intermittent rest periods
necessary or even forcing the person to
stop this effort completely.
That is why it is not advisable to perform
work tasks requiring such efforts,
excluding very short periods of time.

29. Matching people and their work
Obviously, it is important to match human
capabilities with the related requirement
of a given job. If the job demands equal the
worker’s capabilities or if they exceed them,
the person will be under much strain and may
be not able to perform task.
Hence, various functional stress tests, which
are administered by a physician, have been
developed to assess an individual’s capability
to perform physically demanding work.

30. Bicycle ergometers, treadmills, or steps
are used to simulate stressful demands.
The reactions of individual in terms of
oxygen consumption, heart rate, or blood
pressure are used to assess that person’s
ability to withstand such demands.
It is of big importance that the loading
dosage is exact considering the age, body
weight and gender.

31. Example for such stress test is the
Master’s step test.
The initial values of the blood
pressure and heart rate beats are
measured before loading and each
minute after loading till the
recovery to the initial values.

32. The worker’s reaction is estimated
as good when the increasing of
heart rate is accompanied with an
increasing of the maximal blood
pressure with unchanged or slightly
increased values of the minimal
blood pressure.

33. The reaction is astenic when the
heart rate is increased, the
heartbeats are weak, and the
minimal and maximal blood
pressure - slightly increased.
Such reaction indicates insufficient
heart function.

34. In case of disturbed elasticity of
the blood vessels the reaction is
hypertonic – the maximal blood
pressure increases suddenly, with
the same tend for the minimal one;
heart rate is also increased.

35. In case of disturbed balance of
the autonomic nervous system
and the tonus of the peripheral
blood vessels is decreased, the
reaction is dystonic - the heart rate
is increased and the minimal blood
pressure suddenly decreases.

36. Fatigue
Fatigue is an overexertion phenomenon
that leads to a temporary decrease in
physical performance. It is often
associated with a buildup of lactic acid in
the body.
Subjective feeling of fatigue is feeling
tired and deteriorated mental and
physical activities may result from it.

37. The sensation of fatigue has a
protective function similar to
hunger; feeling fatigue forces one
to avoid further stress and allows
recovery to take place.
Fatigue-induced low
performance can be completely
restored to its full level by rest.

38. Many different factors can produce
fatigue and the most important are:
- physical work intensity (static and
dynamic work);
- lack of rest (sleep), and poor eating
habits; and
- psychological factors – worry,
conflict,
- monotony – a sensation associated
with the lack of stimuli.

39. Severe, continuous daily fatigue
eventually leads to chronic
fatigue. Not only is the feeling of
tiredness intensified and continuous
after work, but occasionally a
person feels tired before beginning
work.
The following signs signal chronic
fatigue:

40. - Increased irritability – intolerance,
antisocial behavior
- Tendency to depression –unmotivated
worries
- General weakness and a dislike for
work
- Nonspecific physical complaints –
headaches, giddiness, palpitations, rapid
breathing, loss of appetite, indigestion, or
insomnia.

41. When physical complaints of this
kind appear, the condition can be
called ‘clinical fatigue”.
As a result of this condition
absence from work increase in
frequency and duration, workers
need longer rest periods, and they
may show increased susceptibility
to illness.

42. It is often difficult to separate mental
or emotional events from physical
causes of fatigue. In clinical fatigue
one is hardly to distinguish between
cause and effect.
A negative attitude toward one’s
work, superiors, or place of work can
just as well be a cause of clinical
fatigue as the result of it.

43. Work/rest cycles
In order to avoid the fatigue and
especially the chronic fatigue, rest
pauses must be taken.
Frequent short rest periods
reduce cumulative fatigue better
than a few long breaks.
The worst procedure is to let the
worker go home early, exhausted.

44. A general principle for the
schedule work/rest cycles is to
break up excessively hard work
into bouts of work that are as short
as is practical for the task at hand,
or intermingling light tasks with
heavy ones.

45. The light, secondary work task thus
actually constitutes rest time from
the heavy, primary task.
Sharpening tools or walking to get
material and other interruptions can
provide productive respites from
heavy work.