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Problems in determining rest allowances
Applied Ergonomics 1973, 4.2, 91-95
Problems in determining rest allowances Part 1: Use of modern methods to evaluate stress and strain in static muscular work Walter Rohmert Institut fdr Arbeitswissenschaft, University of Technology, Darmstadt, German Federal Republic After mentioning today's practice in handling the problem of setting rest allowances, definitions are given for the terms of fatigue, recovery and degree of fatigue. Discriminating between stress and strain leads to the possibilities in determining both. Measurable effects of fatigue in static muscular work are discussed. The procedure for determining optimal working rhythms by measuring stress and strain in static muscular work is explained.
Today, the routine work of establishing the values of rest allowances in any practical work situation is being delegated to less skilled work study personnel. This seems to be true all over the world. Analysing this fact leads us to the conclusion that industrial managers are adopting the view that problems exist in determining rest allowances, but cannot be solved under practical requirements. Therefore less skilled personnel are suitable for handling this problem. In the American journal Industrial Engineering, G. Cornman (1970) uses the pithy headline "Don't try to measure fatigue", because "any allowance must satisfy three basic conditions: It must be as uniform as possible; It must be understood by all concerned; It must be verifiable. Hardly any of the above can be satisfied". The same arguments will be stated also in West German industry (in a survey by Schmiktke and Mallach, 1969): "The offered techniques of stress analysis are too unaccustomed for operation; too complicated in application; limited to isolated stressors, which do not occur in this modality in industry; not relevant for routine industrial cycles of stressors, etc." There are typical answers to the question, how less skilled personnel tackle the problem of rest allowances, as for example: •
We set tiring tasks outside the bonus wage plan.
•
We compensate fatigue by conceding "aggravating" vacation privilege.
•
We have no rest allowances but concede in cases of heavy work a higher percentage of personal time.
•
We allow short rest periods (for the total line or section) after significant work periods.
•
We believe that unavoidable time delays are sufficient for recovery.
•
We think that rest allowances will be covered by job classification and compensating bonus or will be "taken into account" by the rating.
• We calculate all standard times generously.
• We don't specify rest allowances; the worker knows nothing about it. • We like to give more emphasis to money than to standard time. • If people are content, rest allowances are all right. • If anybody is fatigued we replace him by another with a higher individual capacity. • Rest allowances are taboo for us. • We have no work for which rest allowances could be necessary.
Fatigue and recovery These statements are mainly made by engineers and technicians who have learned a lot about fatigue of material but nothing about fatigue of man. But there are indeed differences between these phenomena. Fatigue and recovery are periodic processes in every living organism. Organic fatigue is in itself neither a morbid nor a harmful process for the organism. In this, organic fatigue differs from the material fatigue known from the inorganic field of the theory of metallic materials. The biological phenomena of fatigue are reversible, recovery is possible; phenomena of inorganic fatigue, on the other hand, are irreversible. By material fatigue is meant the phenomenon that with prolonged and in particular periodic mechanical stress, some materials deteriorate in their resistance to this stress. These fatigue phenomena are due to the structural changes in the metals, which, once existing, cannot be eliminated. It is less easy and clear to describe what we wish to understand by man's fatigue. Everyone knows fatigue from everyday experience. Fatigue can be described as a state which is characterised by a feeling of tiredness combined with a reduction or undesired variation in the performance of the activity. Not all the functions of the human organism become tired as a result of use. Even when asleep, for example, we fill and empty our lungs with air;
Applied Ergonomics
June 1973
91
our heart continues its pumping activity without pause; we maintain our body temperature unchanged at about 37°C. Obviously, the basic functions of breathing and heart activity are possible throughout life without fatigue and without pauses for recovery. The same is also true of some of the various activities of our occupation and the daily round. On the other hand, we Fred after fairly prolonged heavy work that there is a reduction in capacity which we call fatigue. This does not apply to muscular activity alone. The sensory organs, nerve centres, the cells of the sweat glands also become tired. It is, however, the aim of every cell to balance out the capacity lost by its activity, a process which we call recovery. If, therefore, the terms fatigue and recovery appear defined with respect to one another, in ordinary language differences of meaning exist, leading constantly to misunderstandings. For example, we conceive of fatigue and recovery as processes in time, to which we allocate a speed. In everyday language we speak of rapid or slow tiring, of rapid or delayed recovery. But we also conceive of fatigue and recovery as states which exist at any point of time; in everyday speech we say, for example, that an organism is very tired or is well recovered. In Fig 1, the two possible meanings are separated, the one concerned with the progress within a given time and the other which considers the state at a particular time. This means:
Fatigue Reduction of the functional capacity of an organ or of the organism as a result of action; fatigue is eliminated by recovery, fatigue and recovery being understood as time processes. The state caused by tiring can be measured as a degree of fatigue. Fatigue increases the degree of fatigue, recovery reduces it.
Degree of fatigue The state of functional capacity of an organ or of an organism reached through fatigue and recovery as a result of strain. Recovery Increase of the functional capacity of an organ or organism, of which the functional capacity was reduced as a result of fatigue; recovery occurs by ending, reducing or changing the action.
Measuring the effects of fatigue In industrial activity, we are mostly interested in comprehending fatigue during work and determining c
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~-E c
o
E
o-~1
"~_t.. O •~ O '~-
I
o~o: oo
i
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~
C7~
E
t-
II
~o
~o
,4
./41 Cumulative time
Fig 1 Degree off fatigue, fatigue and recovery 92
Applied Ergonomics
June 1973
u~.= 6 0
Holding strength os pecentoge of maximum strength =400/0 =300/0 = 200/0
Cl.-~
o -6~
40
Eo
•~
32
E
.o =_ g
S
20
~u t_ t_
I ///
~
~
~
~ S O / o ,
0
,
,oo,o 14°/° ~
2
,
;
,
4
,
;
6
Duration of holding in minutes Fig 2 Decrease in maximum strength in static muscular work
recovery after completion of the work. In order to do this, we must measure by some scale the degree of fatigue at given intervals of time. The most immediate and impressive result of fatigue is the reduction in the maximum working capacity. In the example of static muscular work, the reduction in the maximum working capacity (= maximum strength) which is equivalent to the muscular fatigue, can be measured directly (Rohmert, 1962). After static holding tasks of varying heaviness and duration, the maximum strength still available was compared with the maximum strength measured before the test. The following results were found (see Fig 2): 1. No reduction in maximum strength occurs, if the holding force is limited to 15% of maximum strength. Tiring static muscular work starts with holding forces > 15% of the maximum; therefore this very important limit is called Endurance Limit (EL). 2. The longer the tiring static muscular work (with > 15% of maximum strength) lasts, the greater is the reduction in the available maximum strength. 3. With the same holding time, the reduction in the available maximum strength is greater, the heavier the previous tiring holding work. 4. There are no differences in the results as regards groups of muscles used, or from worker to worker, if the holding forces are related to the individual maximum forces (or to the individual Endurance Limit). 5. Recovery (regeneration of decreased maximal strength) is a function of the degree of fatigue. Compensating the same decrease in maximal strength ( i e, 10%, 20%) takes the same amount for recovery (i e, 0.72 rain, 2.20 min). See Fig 3.
Stress and strain From Fig 3, it is clear that a particular stress causes equal fatigue, leads to an equal degree of fatigue, and claims for equal recovery. In different subjects the same objective stress can lead to different strains due to the different individual working capacities (in the example of static muscular work this means the different individual maximal strength). Fig 4 illustrates these relationships schematically.
Stress This is meant as the sum of all parameters of work influencing man at work, which are perceived or sensed mainly over the receptor system or which put demands to the effector system.
- ~ ~
Individual I ~ I Strain>endurance limit: J capacities !"1 o~ro., 1"1 non-suitable strain i t J I. leads to fatigue
Fig 4 Relationship between stress and strain
Strain This is explained as the sum of all different effects of stress in man conditional to different individual conditions. The heaviness of strain depends therefore equally on the heaviness of stress as on the quantity of individual conditions:
Static muscular work
Strain = f] (Stress, Individual Capacities) From this theoretical function emerge two different methodological ways for analysing strain: 1. Stress is developed from measures of stress (= stressors) and by linking with suitable measures for evaluating individual capacities; strain will be developed after the formal function analytically. 2. Analysing integrating measures of strain (= parameters of strain) allow the deduction of strain. In analysing one has to consider that all measures appear as time dependant values corresponding to alternating work content. Therefore we have to distinguish between a workcontent-related dimension and a time related quality (Rohmert, 1960): Stress = t"2 (Intensity of Work, Duration of Work)
Determination of optimal working rhythms by measuring stress and strain in static muscular work In many cases of dynamic muscular work, an optima] working rhythm is given already by natural alternation of muscle contractions and relaxations. The natural alternation between tiring contractions and recovering relaxations must only be interrupted by additional rest pauses in those cases in which: intensity of each contraction is too high (above the endurance limit),
I0 r
~
t
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"-- ...
~
OfO:,mo,imum,orc
-I~
E
Holding force
_..~r]l/I~ -o-2 maximum
/
--!--
072
--I
e~
.~8~ a
,el v i.._0.45 = ! :
0"72 = I Time of experiment
Fig 3 Change in available maximum force during and after tiring static work
i l I working I rest allowance I working I rest allowance period period Dynamic muscular work
I
rlclrklrklrk Working period
14rklrklrl4~kl
J
Restallowance Working Restellowanozl per od /
Fig 5 Changeof the degree of fatigue in static and dynamic muscular work alternating with rest pauses (schematically)
speed is not optimal, relation between time for relaxation to contraction is too small, static muscular components with > 15% maximal strength are included. The lower diagram in Fig 5 illustrates this for the example of cycling. In static muscular work, each working period consists of one sustained contraction. Static contractions of more than 15% of the maximum strength enforce frequent interruptions, the position and length of which are related to the intensity and duration of contraction. The upper diagram in Fig 5 illustrates this. Static muscular work requires a very exact working rhythm to ensure that muscular function will not be overloaded. Static work is a non-physiological type of muscular work. Heart rate is a sensitive and fine discriminating measure for evaluating strain in muscular work. In addition to this, heart rate can be measured and analysed easily in practice without any disturbance of the worker by using radiotelemetric equipment. Therefore heart rate is taken as an evaluating measure for setting the rest allowances which compensate for the fatiguing effects of physical strain. An additional measure is the working capacity of the muscle (maximal endurance or number of working periods up to exhaustion respectively). By definition, the quantitative figures of rest allowances based on physiological experiments are likewise a quantitative measure for evaluating physical strain. We have distinguished rest allowances for both types of muscular work, illustrated in Fig 5 (Rohmert, 1962). In the experiments, we considered the necessary duration of the resting period in relation to the duration of a separate working period (= rest allowance), which is necessary to remove any remainder of fatigue. Fig 6 gives the results for static muscular work where different working periods of
Applied Ergonomics
June 1973
93
3-0
60
Holding time expressed as a percentage of the maximum holding time • 75°/0 2-5 o 50°/o
5O
33 °/o A
25°1o 15O/o force required for static effort = 5 0 % of max. force x
o4o
2.0 2. E ~,£
"6~,
=b 8 I.O ,_ = <
0'5
A I
0
IO0
I
I
200
300
I
400
500
0
tOO
200
300
400
500
Rest allowance in °/o of holding time
Fig 6 Inttuence of rest allowances in static work (holding a torque with the lower arm)
holding 50% of the maximal torque (f/Fmax) of the lower ann alternated with different resting periods. Holding 50% maximum torque is possible for about 1-1 minutes (Tmax) up to exhaustion (Monod and Scherrer, 1957; Rohmert, 1960, 1968; and Caldwell, 1963). From 15% up to 75% (t/Tmax) of this exhaustion time was given in the experiments• Doubling the holding time from 25% to 50% of the maximal time requires the prolonging of rest allowances from 150% to 400%; tripling the duration of working period to 75% necessitated a resting period, after each working period, seven times longer than the working period• A similar exponential influence was found in increasing the intensity of work.
function linked by multiplication according to the formula (for graphical evaluation see Fig 7): R.A.= 18. (~)1"4. (F~. 0.15)0.5. 100 pe r cent, iff/F <0.15 With R.A. = Rest Allowance in percentage of t t = duration of contraction (working period) in minutes. The formulae of Rohmert (1962) describing the attitude of heart rate in muscular work represent solutions of the mathematical function: Fatigue = f3 (Strain (t), Individual Endurance Limit)
A sufficient rest allowance (R.A.) in static muscular work depends on force (f/Fmax) and duration (t/Tmax) of muscular contraction in the manner of an exponential
Different expressions are given for recognised types of muscular work:
Cl (Peff) x, Working Heart Rate = f4 (Strain) =
PEL
C2 (f-~L)x2
yl
• (t)
[ Pelf > PEL
• (t)y2 [
f> f
static EL
fEL = 0" 15 Fma x
With: Ci PefF f EL 94
dynamic
muscular work
= constants
t
= effective capacity = Endurance Limit
xi, Yi = exponents Working Heart Rate = Heart Rate above resting level
Applied Ergonomics
June 1973
= duration of work
IO
/
9
8
/
/
IO /
/
/
/
/
/
/
/
t--
/
~
J
/
/ /
j
/
J
/
J
/
E
/
.,.a
"ID
/
O e-
4
J J
E r-
6
J
/
r-
t-
/
/
/
/
0.5 I / 1400
o
O
O'1
O.2
0-3 0'4 0.5 0.6 Force developed, in fractions of maximum
0-7 force
0.8
0-9
1.0
Fig 7 Percentage rest allowances for various combinations of holding forces and time
The examples studied and the constants and exponents determined are valid for laboratory conditions with time constant intensity of work. In most industrial work situations, however, intensity is alternating during the shift. Also the type of strain is often alternating in time series. This alternation of work must be considered as well as additional physical and social environmental factors. This will be done in the second part of this article. References
Caldwell, L.S. 1963 Journal o f Engineering Psychology, 2, 155-161, Relative muscle loading and endurance.
Cornman, G. 1970 lndustrialEngineering, April, 2, 10-16, Fagigue Allowances - a systematic method.
Monod, H., and Scherrer, J. 1957 Capacitd de travail statique d'un groupe musculaire
synergique chez l'Homme. C. R. Soc. Biol., Paris 151, 1358-1362.
Rohmert, W. 1960 lnt Z angew Physiol einschl Arbeits-physioL 18, 123-164. Ermittlung yon Erholungspausen far statische Arbeit des Menschen.
Rohmert, W. 1962 Untersuchungen fiber MuskelermUdung und Arbeitsgestahung. Beuth-Vertrieb, Berlin, Kt~ln, Frankfurt.
Rohmert, W. 1968 Die Beziehung zwischen Kraft und Ausdauer bei statischer Muskelarbeit. In: "Muskelarbeit und Muskeltraining" A.W. Gentner Verlag, Stuttgart, pp 118-136.
Schmidtke, H., and Mallaeh, A. 1969 REFA - Nachrichten 22, 77- 81. Erholungszuschl/~ge vei leistungskontrollierter lndustriearbeit.
Applied Ergonomics
June 1973
95
Problems in determining rest allowances Part 1: Use of modern methods to evaluate stress and strain in static muscular work Walter Rohmert Institut fdr Arbeitswissenschaft, University of Technology, Darmstadt, German Federal Republic After mentioning today's practice in handling the problem of setting rest allowances, definitions are given for the terms of fatigue, recovery and degree of fatigue. Discriminating between stress and strain leads to the possibilities in determining both. Measurable effects of fatigue in static muscular work are discussed. The procedure for determining optimal working rhythms by measuring stress and strain in static muscular work is explained.
Today, the routine work of establishing the values of rest allowances in any practical work situation is being delegated to less skilled work study personnel. This seems to be true all over the world. Analysing this fact leads us to the conclusion that industrial managers are adopting the view that problems exist in determining rest allowances, but cannot be solved under practical requirements. Therefore less skilled personnel are suitable for handling this problem. In the American journal Industrial Engineering, G. Cornman (1970) uses the pithy headline "Don't try to measure fatigue", because "any allowance must satisfy three basic conditions: It must be as uniform as possible; It must be understood by all concerned; It must be verifiable. Hardly any of the above can be satisfied". The same arguments will be stated also in West German industry (in a survey by Schmiktke and Mallach, 1969): "The offered techniques of stress analysis are too unaccustomed for operation; too complicated in application; limited to isolated stressors, which do not occur in this modality in industry; not relevant for routine industrial cycles of stressors, etc." There are typical answers to the question, how less skilled personnel tackle the problem of rest allowances, as for example: •
We set tiring tasks outside the bonus wage plan.
•
We compensate fatigue by conceding "aggravating" vacation privilege.
•
We have no rest allowances but concede in cases of heavy work a higher percentage of personal time.
•
We allow short rest periods (for the total line or section) after significant work periods.
•
We believe that unavoidable time delays are sufficient for recovery.
•
We think that rest allowances will be covered by job classification and compensating bonus or will be "taken into account" by the rating.
• We calculate all standard times generously.
• We don't specify rest allowances; the worker knows nothing about it. • We like to give more emphasis to money than to standard time. • If people are content, rest allowances are all right. • If anybody is fatigued we replace him by another with a higher individual capacity. • Rest allowances are taboo for us. • We have no work for which rest allowances could be necessary.
Fatigue and recovery These statements are mainly made by engineers and technicians who have learned a lot about fatigue of material but nothing about fatigue of man. But there are indeed differences between these phenomena. Fatigue and recovery are periodic processes in every living organism. Organic fatigue is in itself neither a morbid nor a harmful process for the organism. In this, organic fatigue differs from the material fatigue known from the inorganic field of the theory of metallic materials. The biological phenomena of fatigue are reversible, recovery is possible; phenomena of inorganic fatigue, on the other hand, are irreversible. By material fatigue is meant the phenomenon that with prolonged and in particular periodic mechanical stress, some materials deteriorate in their resistance to this stress. These fatigue phenomena are due to the structural changes in the metals, which, once existing, cannot be eliminated. It is less easy and clear to describe what we wish to understand by man's fatigue. Everyone knows fatigue from everyday experience. Fatigue can be described as a state which is characterised by a feeling of tiredness combined with a reduction or undesired variation in the performance of the activity. Not all the functions of the human organism become tired as a result of use. Even when asleep, for example, we fill and empty our lungs with air;
Applied Ergonomics
June 1973
91
our heart continues its pumping activity without pause; we maintain our body temperature unchanged at about 37°C. Obviously, the basic functions of breathing and heart activity are possible throughout life without fatigue and without pauses for recovery. The same is also true of some of the various activities of our occupation and the daily round. On the other hand, we Fred after fairly prolonged heavy work that there is a reduction in capacity which we call fatigue. This does not apply to muscular activity alone. The sensory organs, nerve centres, the cells of the sweat glands also become tired. It is, however, the aim of every cell to balance out the capacity lost by its activity, a process which we call recovery. If, therefore, the terms fatigue and recovery appear defined with respect to one another, in ordinary language differences of meaning exist, leading constantly to misunderstandings. For example, we conceive of fatigue and recovery as processes in time, to which we allocate a speed. In everyday language we speak of rapid or slow tiring, of rapid or delayed recovery. But we also conceive of fatigue and recovery as states which exist at any point of time; in everyday speech we say, for example, that an organism is very tired or is well recovered. In Fig 1, the two possible meanings are separated, the one concerned with the progress within a given time and the other which considers the state at a particular time. This means:
Fatigue Reduction of the functional capacity of an organ or of the organism as a result of action; fatigue is eliminated by recovery, fatigue and recovery being understood as time processes. The state caused by tiring can be measured as a degree of fatigue. Fatigue increases the degree of fatigue, recovery reduces it.
Degree of fatigue The state of functional capacity of an organ or of an organism reached through fatigue and recovery as a result of strain. Recovery Increase of the functional capacity of an organ or organism, of which the functional capacity was reduced as a result of fatigue; recovery occurs by ending, reducing or changing the action.
Measuring the effects of fatigue In industrial activity, we are mostly interested in comprehending fatigue during work and determining c
I
~-E c
o
E
o-~1
"~_t.. O •~ O '~-
I
o~o: oo
i
_Oo1 E
~
C7~
E
t-
II
~o
~o
,4
./41 Cumulative time
Fig 1 Degree off fatigue, fatigue and recovery 92
Applied Ergonomics
June 1973
u~.= 6 0
Holding strength os pecentoge of maximum strength =400/0 =300/0 = 200/0
Cl.-~
o -6~
40
Eo
•~
32
E
.o =_ g
S
20
~u t_ t_
I ///
~
~
~
~ S O / o ,
0
,
,oo,o 14°/° ~
2
,
;
,
4
,
;
6
Duration of holding in minutes Fig 2 Decrease in maximum strength in static muscular work
recovery after completion of the work. In order to do this, we must measure by some scale the degree of fatigue at given intervals of time. The most immediate and impressive result of fatigue is the reduction in the maximum working capacity. In the example of static muscular work, the reduction in the maximum working capacity (= maximum strength) which is equivalent to the muscular fatigue, can be measured directly (Rohmert, 1962). After static holding tasks of varying heaviness and duration, the maximum strength still available was compared with the maximum strength measured before the test. The following results were found (see Fig 2): 1. No reduction in maximum strength occurs, if the holding force is limited to 15% of maximum strength. Tiring static muscular work starts with holding forces > 15% of the maximum; therefore this very important limit is called Endurance Limit (EL). 2. The longer the tiring static muscular work (with > 15% of maximum strength) lasts, the greater is the reduction in the available maximum strength. 3. With the same holding time, the reduction in the available maximum strength is greater, the heavier the previous tiring holding work. 4. There are no differences in the results as regards groups of muscles used, or from worker to worker, if the holding forces are related to the individual maximum forces (or to the individual Endurance Limit). 5. Recovery (regeneration of decreased maximal strength) is a function of the degree of fatigue. Compensating the same decrease in maximal strength ( i e, 10%, 20%) takes the same amount for recovery (i e, 0.72 rain, 2.20 min). See Fig 3.
Stress and strain From Fig 3, it is clear that a particular stress causes equal fatigue, leads to an equal degree of fatigue, and claims for equal recovery. In different subjects the same objective stress can lead to different strains due to the different individual working capacities (in the example of static muscular work this means the different individual maximal strength). Fig 4 illustrates these relationships schematically.
Stress This is meant as the sum of all parameters of work influencing man at work, which are perceived or sensed mainly over the receptor system or which put demands to the effector system.
- ~ ~
Individual I ~ I Strain>endurance limit: J capacities !"1 o~ro., 1"1 non-suitable strain i t J I. leads to fatigue
Fig 4 Relationship between stress and strain
Strain This is explained as the sum of all different effects of stress in man conditional to different individual conditions. The heaviness of strain depends therefore equally on the heaviness of stress as on the quantity of individual conditions:
Static muscular work
Strain = f] (Stress, Individual Capacities) From this theoretical function emerge two different methodological ways for analysing strain: 1. Stress is developed from measures of stress (= stressors) and by linking with suitable measures for evaluating individual capacities; strain will be developed after the formal function analytically. 2. Analysing integrating measures of strain (= parameters of strain) allow the deduction of strain. In analysing one has to consider that all measures appear as time dependant values corresponding to alternating work content. Therefore we have to distinguish between a workcontent-related dimension and a time related quality (Rohmert, 1960): Stress = t"2 (Intensity of Work, Duration of Work)
Determination of optimal working rhythms by measuring stress and strain in static muscular work In many cases of dynamic muscular work, an optima] working rhythm is given already by natural alternation of muscle contractions and relaxations. The natural alternation between tiring contractions and recovering relaxations must only be interrupted by additional rest pauses in those cases in which: intensity of each contraction is too high (above the endurance limit),
I0 r
~
t
1111111111I
o~~ljllrlllllllllllltl t= P.S5-
"-- ...
~
OfO:,mo,imum,orc
-I~
E
Holding force
_..~r]l/I~ -o-2 maximum
/
--!--
072
--I
e~
.~8~ a
,el v i.._0.45 = ! :
0"72 = I Time of experiment
Fig 3 Change in available maximum force during and after tiring static work
i l I working I rest allowance I working I rest allowance period period Dynamic muscular work
I
rlclrklrklrk Working period
14rklrklrl4~kl
J
Restallowance Working Restellowanozl per od /
Fig 5 Changeof the degree of fatigue in static and dynamic muscular work alternating with rest pauses (schematically)
speed is not optimal, relation between time for relaxation to contraction is too small, static muscular components with > 15% maximal strength are included. The lower diagram in Fig 5 illustrates this for the example of cycling. In static muscular work, each working period consists of one sustained contraction. Static contractions of more than 15% of the maximum strength enforce frequent interruptions, the position and length of which are related to the intensity and duration of contraction. The upper diagram in Fig 5 illustrates this. Static muscular work requires a very exact working rhythm to ensure that muscular function will not be overloaded. Static work is a non-physiological type of muscular work. Heart rate is a sensitive and fine discriminating measure for evaluating strain in muscular work. In addition to this, heart rate can be measured and analysed easily in practice without any disturbance of the worker by using radiotelemetric equipment. Therefore heart rate is taken as an evaluating measure for setting the rest allowances which compensate for the fatiguing effects of physical strain. An additional measure is the working capacity of the muscle (maximal endurance or number of working periods up to exhaustion respectively). By definition, the quantitative figures of rest allowances based on physiological experiments are likewise a quantitative measure for evaluating physical strain. We have distinguished rest allowances for both types of muscular work, illustrated in Fig 5 (Rohmert, 1962). In the experiments, we considered the necessary duration of the resting period in relation to the duration of a separate working period (= rest allowance), which is necessary to remove any remainder of fatigue. Fig 6 gives the results for static muscular work where different working periods of
Applied Ergonomics
June 1973
93
3-0
60
Holding time expressed as a percentage of the maximum holding time • 75°/0 2-5 o 50°/o
5O
33 °/o A
25°1o 15O/o force required for static effort = 5 0 % of max. force x
o4o
2.0 2. E ~,£
"6~,
=b 8 I.O ,_ = <
0'5
A I
0
IO0
I
I
200
300
I
400
500
0
tOO
200
300
400
500
Rest allowance in °/o of holding time
Fig 6 Inttuence of rest allowances in static work (holding a torque with the lower arm)
holding 50% of the maximal torque (f/Fmax) of the lower ann alternated with different resting periods. Holding 50% maximum torque is possible for about 1-1 minutes (Tmax) up to exhaustion (Monod and Scherrer, 1957; Rohmert, 1960, 1968; and Caldwell, 1963). From 15% up to 75% (t/Tmax) of this exhaustion time was given in the experiments• Doubling the holding time from 25% to 50% of the maximal time requires the prolonging of rest allowances from 150% to 400%; tripling the duration of working period to 75% necessitated a resting period, after each working period, seven times longer than the working period• A similar exponential influence was found in increasing the intensity of work.
function linked by multiplication according to the formula (for graphical evaluation see Fig 7): R.A.= 18. (~)1"4. (F~. 0.15)0.5. 100 pe r cent, iff/F <0.15 With R.A. = Rest Allowance in percentage of t t = duration of contraction (working period) in minutes. The formulae of Rohmert (1962) describing the attitude of heart rate in muscular work represent solutions of the mathematical function: Fatigue = f3 (Strain (t), Individual Endurance Limit)
A sufficient rest allowance (R.A.) in static muscular work depends on force (f/Fmax) and duration (t/Tmax) of muscular contraction in the manner of an exponential
Different expressions are given for recognised types of muscular work:
Cl (Peff) x, Working Heart Rate = f4 (Strain) =
PEL
C2 (f-~L)x2
yl
• (t)
[ Pelf > PEL
• (t)y2 [
f> f
static EL
fEL = 0" 15 Fma x
With: Ci PefF f EL 94
dynamic
muscular work
= constants
t
= effective capacity = Endurance Limit
xi, Yi = exponents Working Heart Rate = Heart Rate above resting level
Applied Ergonomics
June 1973
= duration of work
IO
/
9
8
/
/
IO /
/
/
/
/
/
/
/
t--
/
~
J
/
/ /
j
/
J
/
J
/
E
/
.,.a
"ID
/
O e-
4
J J
E r-
6
J
/
r-
t-
/
/
/
/
0.5 I / 1400
o
O
O'1
O.2
0-3 0'4 0.5 0.6 Force developed, in fractions of maximum
0-7 force
0.8
0-9
1.0
Fig 7 Percentage rest allowances for various combinations of holding forces and time
The examples studied and the constants and exponents determined are valid for laboratory conditions with time constant intensity of work. In most industrial work situations, however, intensity is alternating during the shift. Also the type of strain is often alternating in time series. This alternation of work must be considered as well as additional physical and social environmental factors. This will be done in the second part of this article. References
Caldwell, L.S. 1963 Journal o f Engineering Psychology, 2, 155-161, Relative muscle loading and endurance.
Cornman, G. 1970 lndustrialEngineering, April, 2, 10-16, Fagigue Allowances - a systematic method.
Monod, H., and Scherrer, J. 1957 Capacitd de travail statique d'un groupe musculaire
synergique chez l'Homme. C. R. Soc. Biol., Paris 151, 1358-1362.
Rohmert, W. 1960 lnt Z angew Physiol einschl Arbeits-physioL 18, 123-164. Ermittlung yon Erholungspausen far statische Arbeit des Menschen.
Rohmert, W. 1962 Untersuchungen fiber MuskelermUdung und Arbeitsgestahung. Beuth-Vertrieb, Berlin, Kt~ln, Frankfurt.
Rohmert, W. 1968 Die Beziehung zwischen Kraft und Ausdauer bei statischer Muskelarbeit. In: "Muskelarbeit und Muskeltraining" A.W. Gentner Verlag, Stuttgart, pp 118-136.
Schmidtke, H., and Mallaeh, A. 1969 REFA - Nachrichten 22, 77- 81. Erholungszuschl/~ge vei leistungskontrollierter lndustriearbeit.
Applied Ergonomics
June 1973
95