Determination of physical stress in agricultural work

International Journal of Industrial Ergonomics, 10 (1992) 275-284 Elsevier

275

Determination of physical stress in agricultural work Veli-Matti Tuure Department of Agricultural Engineering and Household Technology, Helsinki Unit'ersity, Viikki F, SF-O0710 Helsinki, Finland (Received February 12, 1992; accepted in revised form June 6, 1992)

Abstract The aim of this study was to find a method or combination of methods appropriate for defining the physical stress imposed by jobs carried out in the farm environment. Besides charting research methods, various other methods were applied to three sample jobs (making a load of bales, lowering of tomatoes and mechanised loading and transportation of timber). The sample jobs were selected to represent dynamic loading, static stress and work posture stress. The methods chosen were: oxygen consumption, pulse-blood pressure measurement, electromyography, the OWAS work posture observation method, and the Borg RPE scale for recording the test person's subjective stress estimate. The experiments were conducted using four test persons in real working environments. According to this study different methods for determining the physical stress of work are needed. The stress of dynamic work can best be investigated by means of oxygen consumption or heartbeat frequency. Furthermore, with these methods it is possible to classify jobs. The static stress can he determinated by means of the stress felt by the test person and electromyographic recordings of changes in muscle tension, although these are quite person-sensitive methods. Static stress also affects heartbeat frequency and blood pressure. The OWAS method is probably the best method available when studying the work postures in agricultural work - at least when involved in whole body work requiring moving about on the part of the person doing the job.

Relevance to industry This study can help a researcher to select appropriate method(s) for determining physical stress in farm work and thus to develop work ergonomically.

Keywords Physical stress; investigation methods; agriculture; work load.

Introduction In i n d u s t r y a n d f o r e s t r y e s p e c i a l l y , t h e s t u d y o f t h e p h y s i c a l stress o f w o r k h a s a l o n g t r a d i t i o n . I n agriculture, however, where, regardless of mechanisation, jobs imposing considerable physical stress o n t h e w o r k e r c o n t i n u e to b e d o n e , s t u d i e s d e a l i n g w i t h w o r k stress a r e r e l a t i v e l y f e w as yet.

Correspondence to: Veli-Matti Tuure, Department of Agricultural Engineering and Household Technology, Helsinki University, Viikki F, SF-00710 Helsinki, Finland

T h e i d e n t i f i c a t i o n o f t h e e x t e n t o f p h y s i c a l stress and the ergonomic development of the various j o b s in a g r i c u l t u r e w o u l d , n e v e r t h e l e s s , b e g r e a t l y a s s i s t e d by a s c e r t a i n i n g t h e p h y s i c a l stress o f t h e w o r k . I n a g r i c u l t u r e , t h e t r e n d has b e e n f o r b a d w o r k p o s t u r e s a n d static s t r e s s i n g o f m u s c l e s to g a i n i n c r e a s i n g p r o m i n e n c e as h e a v y j o b s a r e g r a d u a l l y r e l e g a t e d to m a c h i n e s . T h e stress i m p o s e d o n t h e h u m a n b o d y by h e a v y w o r k is r e f l e c t e d in t h e f o r m o f r e s p o n s e s o f v a r y i n g d e g r e e , t h e s e b e i n g d e p e n d e n t on t h e person's individual characteristics. The physical s t r a i n is n o t n e c e s s a r i l y always d e t r i m e n t a l - at-

0169-1936/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

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V.-M. Tuure / Physical stress in agricultural work

tention should also be paid to avoiding the lack of stress. The stressfulness of work is determined, on the one hand, by the stress factors of the work in question and, on the other hand, by the characteristics of the worker. Stress determinations are generally done by measuring the strain level of the person at work instead of measuring the stress factors of the work (Salminen, 1981). Because the effects of detrimental physical stress make themselves apparent at several levels, there are also several methods for determining them. Charting methods provide the user with a rough picture of the shortcomings of the job. Methods that actually determine the physical stress are then used to conduct a closer study of the problems previously identified (Herranen et al., 1985). These methods may be grouped as follows: questionnaire and interview methods for charting the feelings arising in the person doing a job, physiological methods for measuring organic changes in the person doing a job, and methods based on observation (Salminen, 1981). The purpose of the study is of primary significance in the selection of the method to be used. In the case of studies dealing with the stress imposed by agricultural jobs, the energetic load imposed by the job has been the most common object of study (Nag and Dutt, 1980; Zegers and van Dieen, 1987; Malgeryd, 1988; Castr6n and Peltola, 1989). In the main, this has been done by measuring heartbeat frequency. Attention has also been paid to work postures, lifting work and work conditions. The estimation of the stress imposed by work is a major problem in ergonomics. In the case of heavy physical work, energy consumption is commonly used as the basis for classifying jobs in terms of their stressfulness (Rodahl, 1978). Calculatory energy consumption is generally determined on the basis of oxygen consumption or heartbeat frequency during the job. In addition, there are various estimation methods based on observations. Despite the availability of numerous methods based on observations of work postures, the area of locomotor system stress estimation still lacks international classifications resembling stress classifications based on energy consumption (Nyg~rd, 1983; van Dieen, 1989). In addition to work postures, internationally approved classifications are also as yet unavailable for static stress and the use of force. These can,

however, be used in making subjective estimates of the stress level and to compare jobs, stages of work and work methods. Various reference values are also available. The aim of this study was to find a method or combination of methods appropriate for defining the physical stress imposed by jobs carried out specifically in the farm environment. The aim also was to find out which method should be used in various stress situations and what the methods indicate of the stressfulness of the work in question.

Methods

Research methods of various types were selected for the experimental part of this study and they were applied to three sample jobs. The methods chosen were: oxygen consumption measurement, pulse-blood pressure measurement, electromyography, the OWAS work posture observation method (Anon., 1979), and the Borg RPE scale (Borg, 1978) for recording the test person's subjective stress estimate. The three sample jobs were selected to represent dynamic loading (making a load of bales, figure 1), static stress (lowering of tomatoes, figure 2) and work posture stress (mechanised loading and transportation of timber, figure 3). All the selected study methods were applied simultaneously. The duration of the measurements was mainly between 15 and 30 minutes. The experiments were conducted using four male test persons called A, B, C and D (table 1). Each of them performed three replicates of each sample job in real working environment. All the test persons were mainly sedentary people. They had taken part in agricultural work before but their experiences in agricultural work varied from occasional to nearly daily. Oxygen consumption was measured during work with a portable Morgan Oxylog measuring instrument and the consumption was read once a minute. The reading accuracy was 0.01 l / m i n and the range 0.25-3 1/min. Heart rate and blood pressure were measured with a portable Oxford Medilog ABP instrument. The measurements were done automatically every second minute. The RPP value - the main

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Fig. 1. Dynamic loading: making a load of bales. Test person took 80 bales (~ 15.7 kg) from the ground and carried them 3.0 m to a trailer (lifting height 150 cm). Photo: Vesa Majuri.

in preliminary tests when making a load of bales the change in muscle tension was the strongest in the trapezius. Before measurements, test contractions were done against 25 kg static load at a defined posture (Attebrant Eriksson, 1986). The Muscle Tester measured the muscle tension ten times in a second in two muscles. In this study the average values of one second were saved. The OWAS working posture analysing system was used - with the help of video and computer

variable of heart rate and blood pressure measurements - was the product of systolic blood pressure and heart rate divided by one hundred. Muscle Tester M E 3000 and surface electrodes were used in measuring the E M G signal from the upper part of the trapezius muscle left and right (Anon., 1988). The trapezius was selected because pains in the shoulder region are quite common in tractor work, lowering tomatoes is apparently stressful for arms and shoulder and because

Table 1 Age, height, weight and some other characteristic values of the test persons, during rest and maximum performance. The oxygen uptake (VO 2) and the muscle condition were determined by a training centre. Test person

Age (years) Height (cm) Weight (kg) HR(rest) ( b e a t s / m i n ) VO2(rest) ( L / m i n ) Syst(rest) (mmHg) HR(max) a (beats/rain) VO2(max) (L/rain) Muscle condition b

A

B

C

D

29 183 77 71 0.29 114 187 1.9 3

26 176 72 61 0.32 108 189 3.0 4

25 175 68 79 0.30 117 190 3.6 2

27 189 70 81 0.35 117 188 3.5 3

a H R ( m a x ) = 210-0.8 × (age) (Nyg~rd, 1988) b 1 very poor; 2 poor; 3 moderate; 4 good; 5 very good.

x

s

27 181 72 72 0.32 114 189 3.0 3

1.7 6.6 3.9 9.1 0.03 4.2 1.3 0.8 0.8

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techniques - for mapping and classifying working postures. The method is based on observations and it contains estimations - action categories -

for all the typical working postures. The working postures were judged by calculating the average of action categories in each experiment. The ob-

Fig. 2. Static stress: lowering tomatoes. Tomatoes were lowered in standing position on a car which could be driven by pressing a button on a standing level (height 125 cm). Photo: Veli-Matti Tuure.

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279

Fig. 3. Work posture stress: mechanised loading and transportation of timber. The loading included timber lifting with a loader fixed to a farm tractor and transportation driving in a forest (205 m) and along a road through a forest (151 m). Photo: Pekka Viitaniemi.

servation interval was 4 s (making a load of bales) or 10 s (lowering of tomatoes and loading and transportation of timber). The test person's subjective stress estimate (RPE value) was asked three times during each experiment: the first time 5 minutes after beginning, in the middle and just after ending.

Results Concordance analysis indicated that the main variables measured the same property very significantly (the whole material, p < 0.001). The values were at their maximum in the job of making a load of bales which clearly represented the high-

Table 2 Oxygen consumption (VO2), RPP index, OWAS action category, the EMG values of the trapezius (left: EMGVSUHT, right: E M G O S U H T ) proportioned to the standard load and the RPE class averages in the sample jobs. Standard deviations of the averages in the repetitions (n = 12) are mentioned in brackets. VO z

RPP

OWAS

EMGV SUHT

EMGO SUHT

RPE

Lowering of tomatoes

0.672 (0.05)

127.7 (17.8)

1.064 (0.05)

21.0 (9.2)

30.6 (14.1)

12.1 (0.7)

Making a load of bales

2.030 (0.19)

196.4 (32.0)

1.201 (0.06)

39.5 (11.1)

49.3 (17.7)

13.5 (1.3)

Loading and transportation of timber

0.543 (0.10)

126.6 (32.5)

1.006 (0.01)

18.9 (4.8)

15.4 (9.0)

9.9 (1.1)

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OXYGEN CONSUMPTION (I/rain) 2,5

2 1,5

1 0,5

A

B

C

D

A

Lowering of

B

C

D

A

B

C

D

Making a load

Mechanised loading

qf h~lr, q

and tranqpr, rtatinn

ot timber OWAS ACTION CATEGORY 1,4 1,2 1 0,8 0,6 0,4 0,2 0 B

C

D

A

B

C

D

Making a load of bales

Lowering of tomatoes

A

B

C

D

Mechanised loading and transportation

RPE CLASS 16 14 12 10 8 6 4 2 0 A

B

C

Lowering of tomatoes

A

B

C

Making a load of bales

D

A

B

C

D

Mechanised loading and transportation ot tlmDer

Fig. 4. The average values and standard deviations in each sample job by the test persons A, B, C and D.

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281

RPP INDEX

A

B

C

D

A

Lowering of tomatoes

B

C

D

Making a load of bales

EMG PROPORTIONED

A

0

C

D

Mechanised loading and transportation of timber

TO THE STANDARD LOAD, LEFT (%)

.

60

__

----c2___

SD -

40 30 20 IO 0 A

B

C

D

A

Lowering of tomatoes

B

C

D

0

C

D

Mechanised loading and transoortation of timber

Making a load of bales

EMG PROPORTIONED

A

TO THE STANDARD

LOAD, RIGHT (%)

100 ____ ________

90

l

A

0

C

Lowering of tOmatoP

D

A

0

C

Making a load of bales

Fig. 4. (continued),

D

-

A

B

C

D

Mechanised loading and transoottation ot timber

V.-M. Tuure / Physical stress in agricultural work

282

est energy consumption level and at their minimum in tractor cab work which represented the lowest energy consumption (table 2). When looked at job-specifically, in which the method's internal variation was low, the main variables were statistically significantly uniform ( p < 0.01)

only in the case of lowering of tomatoes - the stress factors of work do not appear in the same ratio for the different jobs. The individual data in all the sample jobs are described in the bar diagrams in figure 4. In addition to energy consumption, variation

Table 3 Sorted varimax rotated factor loadings. Loadings less than 0.2500 have been replaced by zero. h 2 = communality. Factor

VO 2 / V O 2 (rest) Work H VO 2 VO 2 / V O 2 (max) H R / H R (rest) H R / H R (max) HR OWAS R P P / R P P (rest) RPP RH RPE EMGVSUHT Mean M e a n / m e a n (rest) Diast Diast/diast (rest) Syst/syst (rest) Syst Test person C OWAS arms Work T OWAS back Work O Temperature Test person D EMGO Test person A EMGOSUHT EMGV Test person B

h2

1

2

3

4

0.983 0.968 0.965 0.944 0.928 (/.871 0.869 0.867 0.860 0.821 - 0.762 0.725 0.664 0 0 0 0 0 0 0 0 - 0.453 0 - 0.516 0.410 0 0.288 0 0.577 0.432 0

0 0 0 11 0 0 0 0 0.258 0 0 0 0 0.946 0.939 0.879 0.721 0.653 0.620 - 0.508 0 0 0 0 0 0 0 0.300 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0.516 0 0 0 0 0 0 0 0 0.950 0.849 - 0.841 - 0.821 0.663 0 0 0 0.278 0 0

0 0 0 0 0 0.396 0.399 0 0 0.418 0 - 0.303 0 0 0 0 0 0 0 0 0 0 0 0 (I.282 0.888 0.809 -0.624 1/.590 0.565 -0.258

VP

11.187

explicableness % sum %

36.1 36.1

4.500 145 50.6

Abbreviations rest during rest max during maximum performance Work H making a load of bales Work T lowering the tomatoes Work O mechanised loading and transportation of timber HR heart rate RH relative humidity Syst blood pressure, systolic

4.050 13.1 63.7 Diast Mean OWAS arms OWAS back EMGV EMGO

0.966 0.937 (/.931 0.891 0.861 0.915 0.914 0.752 0.806 0.849 0.581 0.884 0.441 0.895 0.882 0.773 0.520 0.426 0.384 (I.258 0.903 0.926 0.707 0.940 0.687 0.789 0.737 0.479 0.758 0.506 0.067

3.530 11.4 75.1 blood pressure, diastolic blood pressure, mean one arm or both arms above the shoulder level back in bending o r / a n d twisting position electromyography in upper trapezius left in microvolts electromyograpby in upper trapezius right in microvolts

V.-M. Tuure / Physical stress in agricultural work

in the material was caused according to factor analysis by the personal characteristics of the test persons and the work postures, temperature, the experienced stress of work and muscle activeness connected to the job types (table 3). Of the variables determined, the most person-sensitive were blood pressure, heartbeat frequency and electromyography. In addition, RPE reflected the differences of the test persons. Measuring oxygen consumption seemed to be a reliable method to determine the dynamic loading in agricultural work. In any case it is quite a stressful method for the worker and not so easy to use as the recent instruments for measuring heartbeat frequency. The recording of blood pressure gets disturbed particularly in dynamic work. In addition it can be difficult to get reliable values during moving. With electromyography it is possible to recognize local static stress. The method is very versatile and not difficult to use although using requires great care because the measuring is sensitive to extraneous disturbances. The interpretation can be difficult as the values are usually dependent on the person and situation. Although bad work postures were most numerous in the job requiring high energy consumption, it was also noted that there were a lot of unergonomic postures of certain parts of the body. This being so, the determining of energy consumption cannot replace that work posture analysis. The OWAS method proved to be a good method for studying the work postures in agricultural work when involved in whole body work requiring moving. In the estimation of work postures in sitting work, however, determining the postures as a whole by means of the OWAS method would appear to result in quite rough estimates. Moreover, the method is not very suitable when taking into account the need for force. The stress felt by the person doing a job (RPE) provides a simple research method for estimating the overall stress of work. It reflects best of all the energetic stress and posture stress, which can also be local. According to this investigation, it is evident that different methods for determining the physical stress of work are needed: a method for dynamic loading, for static stress and for work posture stress. If only one method is to be used it

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should be chosen according to the principal stress factor. If it is difficult to recognize the principal stress factor some charting method can be the most useful in the beginning.

Discussion

The energetic load imposed by dynamic work can be determined reliably by means of oxygen consumption when there is no anaerobic energy production in the work. Determining the energetic stress with heartbeat frequency as the basis is, nevertheless, easier and causes less interference to the carrying out of the job being studied (Alexander, 1986; Galer, 1987). This does, however, require the relationship between heartbeat frequency and the stress imposed by the job to be defined beforehand (Rodahl, 1978). In addition, it should be noted that at low heartbeat frequency levels the other factors, too, and not only energetic stress, begin to have an effect on the heartbeat frequency (Nag and Dutt, 1980; Petsson and Kilbom, 1981; Alexander, 1986; Malgeryd, 1988). In any case, measuring heartbeat frequency seems to be the most useful method to determine the energetic load in agricultural work. In agriculture the OWAS method is probably the best method available when studying the work postures involved in whole body work requiring moving about on the part of the person doing the job (van der Schilden, 1989). OWAS provides the user with an overall estimate of the appropriateness/inappropriateness of the work postures used. The staticness of the postures can also be observed. When estimating the overall stress of work, static stress should be taken into account along with energetic stress and posture stress. Static stress can be recognized by means of observations (OWAS), the stress felt by the test person and locally by means of electromyographic recordings of changes in muscle tension. E M G recordings also enable the user to determine the magnitude of muscle tension (Hagberg, 1981). Static stress also affects heartbeat frequency (Nag and Dutt, 1980; Persson and Kilbom, 1981; Alexander, 1986; Malgeryd, 1988) and blood pressure (Persson and Kilbom, 1981). The recording of blood pressure while working is not useful in agricultural work.

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Moreover, heartbeat frequency and blood pressure are affected by other factors from among which it is difficult to distinguish that of the static factor.

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