WORK PHYSIOLOGY People perform widely different tasks indaily work situations. These tasks must bematched with human capabilities to avoid“underloading”, in which human capacities arenot utilized properly, as well as “overloading”,which may cause the employee to break downand suffer reduced performance capability oreven permanent damage.
Work physiologists study the chancedappearing in the functions of the main organsand systems (cardiovascular and respiratory) ofworkers at work and evaluate the capacities andlimitations of the worker for performing physicalwork; they also determine human tolerance tostresses produced by the physical environment.
Capacity for physical work An individual’s physical toleranceto physical work is usually determinedby the capacity of his or hercardiovascular and respiratory systemsto deliver oxygen to the workingmuscles and to metabolize chemicallystored energy.
Maximum oxygen uptake is oftenused 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.
The biochemical processes thattransform 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).
To put oxygen consumption andenergy demands into proper perspective, consider the abilities of trained athleteswho may reach maximal oxygen uptake capacity of up to 6 L/min. Aside from a person’s physique, ageand gender influence the oxygen uptake capacity.
Men who are 20 years of age have an average maximal capacity of 3-3.5L/min; women of the same age have an average capacity of 2.3-2.8 l/min. At age of 60 the capacity isdiminished to about 2.2-2.5 L/min. for men and 1.8-2.0 L/min for women.
The ability to move oxygen from theair to the active muscle can be improvedthrough physical training by up to 20 %. Of course, an individual’s capacityalso depends on other “central” functions(e.g. of the circulatory of the cardiacsystem) and on “local” capacities (e.g. ofmuscles).
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;
- vital capacity (VC) – the maximumamount of air that can be exhaled from thelungs after a maximum inspiration. It is a verysensitive index for estimation of work capacity - forced expiratory volume in onesecond (FEV1) – the volume of air which canbe forcibly expelled during the first second ofexpiration. Its reduction indicates reduction ofairflow rates and prolongation of expiration.
Energy cost of workTotally, the heaviest work that a young, fit man can sustain for prolonged periods is about 500 kcal/hr. Among the generalpopulation, this figure is somewhat lower: 400-425 kcal/hr.
Industrial jobs seldom demandsuch 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.
When intermittent tasks are performed, theaverage expenditure may be calculated using the following formula: M = (M1t1 + M2t2 + ………Mntn)t-1 where M is the total energy cost; M1, M2 andso on are the energy costs of individual tasks; t1, t2 and so on indicate the duration of theindividual task; and t is the total elapsed time.
Heart rate at work There is a close interaction betweenthe human circulatory and metabolic systems. Nutrients and oxygen must be brought to the working muscles andmetabolic by-products removed from them to ensure proper functioning.
Therefore, heart rate (which is a primary indicator of circulatory functions) and oxygen consumption (representing themetabolic processes taking place in the body) have a linear andreliable relationship in the range between light and heavy work.
When very light work loads or veryheavy 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.
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.
The simplest technique for heart rateassessment is to palpate an artery, oftenin the wrist, and to count the number ofheartbeats over a period of time such as 15 seconds and calculate the average heart beat per minute. More refined methods utilize various plethysmographic techniques.
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 heartrate when the worker is performing his typical working operations – “heart rate at work”.
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, atrest, values of heart rate, to have in mind theindividual (or due to other, even pathological conditions in the body) variability of heart rate, and to follow up its dynamical changes during the workday.
This is useful to provide aproper work/rest regimen.
Use of heart rate measurements has one major advantage over oxygen measurement as an indicator of metabolic processes: heart rate reacts faster to workdemands and therefore more easily indicates quick changes in body functions due to changes in work requirements.
Work classification The work demands are usually ratedfrom light to extremely heavy in terms of energy expenditure per minute, and the relative heart rate in beats peer minute is also given.
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 lessMedium work 3–5 90 – 100Heavy work 5 – 7.5 100 – 120Very heavy work 7.5 – 10 120 – 140Extremely heavy work 10 – 15 140 or more
When the work is light, the energy needs of the working muscles are supplied by oxygen available in theblood 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.
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 concentrationproduced during the initial phase of work is not reduced but remains high until the end of the work period and returns tonormal level after cessation of the work.
In course of light, medium, andeven heavy (if the person is trained and capable) work, the metabolic and other physiological functions can attain a steady-statecondition during the work period. No steady-state exists in the course of very heavy work.
In this case, the oxygen deficit incurredduring the early phase of work increasesthroughout 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.
Matching people and their workObviously, 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 beendeveloped to assess an individual’s capability to perform physically demanding work.
Bicycle ergometers, treadmills, or steps are used to simulate stressful demands. The reactions of individual in terms ofoxygen 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 loadingdosage is exact considering the age, body weight and gender.
Example for such stress test is the Master’s step test. The initial values of the blood pressure and heart rate beats aremeasured before loading and each minute after loading till the recovery to the initial values.
The worker’s reaction is estimated as good when the increasing of heart rate is accompanied with an increasing of the maximal bloodpressure with unchanged or slightly increased values of the minimal blood pressure.
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.
In case of disturbed elasticity of the blood vessels the reaction is hypertonic – the maximal blood pressure increases suddenly, withthe same tend for the minimal one; heart rate is also increased.
In case of disturbed balance of the autonomic nervous system and the tonus of the peripheral blood vessels is decreased, thereaction is dystonic - the heart rateis increased and the minimal blood pressure suddenly decreases.
Fatigue Fatigue is an overexertion phenomenon that leads to a temporary decrease in physical performance. It is oftenassociated 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.
The sensation of fatigue has a protective function similar tohunger; feeling fatigue forces oneto avoid further stress and allows recovery to take place. Fatigue-induced lowperformance can be completelyrestored to its full level by rest.
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.
Severe, continuous daily fatigue eventually leads to chronic fatigue. Not only is the feeling oftiredness intensified and continuous after work, but occasionally a person feels tired before beginning work.The following signs signal chronic fatigue:
- Increased irritability – intolerance,antisocial behavior - Tendency to depression –unmotivatedworries- General weakness and a dislike forwork- Nonspecific physical complaints –headaches, giddiness, palpitations, rapidbreathing, loss of appetite, indigestion, orinsomnia.
When physical complaints of thiskind appear, the condition can be called ‘clinical fatigue”. As a result of this condition absence from work increase in frequency and duration, workersneed longer rest periods, and theymay show increased susceptibility to illness.
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’swork, superiors, or place of work can just as well be a cause of clinical fatigue as the result of it.
Work/rest cycles In order to avoid the fatigue andespecially the chronic fatigue, rest pauses must be taken. Frequent short rest periodsreduce cumulative fatigue better than a few long breaks. The worst procedure is to let theworker go home early, exhausted.
A general principle for the schedule work/rest cycles is to break up excessively hard workinto bouts of work that are as shortas is practical for the task at hand,or intermingling light tasks with heavy ones.
The light, secondary work task thusactually constitutes rest time from the heavy, primary task.Sharpening tools or walking to getmaterial and other interruptions can provide productive respites from heavy work.