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Shoulder-arm muscular activity and reproducibility in carpenters' work
Clinical Biomechanics 1990; 5: 81-87
Shoulder-arm muscular activity and reproducibility in carpenters’ work E HammarskjGld RPT K Harms-Ringdahl RPT J Ekholm MD PhD
Dr MHI
SC
Kinesiology Research Group, Department of Rehabilitation and Physical Medicine and Department of Anatomy, Karolinska Institute, Stockholm, Sweden
Summary
The purpose of the present study was to analyse the levels of muscle activity that experienced carpenters reach in some arm-shoulder muscles when performing familiar tasks, and to find whether there are intra- and inter-individual similarities in muscle activity. Ten healthy carpenters performed twice on the same day: their tasks were, manual screwdriving at eye level, manual sawing on a saw horse, and nailing upward into the ceiling. Two weeks later the experiment was repeated. Normalized electromyography was used. The median activity levels during the period analysed did not exceed 15% of maximum electromyographic activity for any of the muscle groups studied. For some muscles and activities there were wide ranges of median activity levels. The most activated muscles in all three tasks were the trapezius pars descendens, infraspinatus and anterior deltoid. There was an intra-individual similarity between the median values from each trial. Relevance
The prevalence of arm-shoulder-neck disorders has become nearly as great as that of low back disorders. Muscular overload has been discussed as one etiological factor. No studies have been found analysing the extent to which the ordinary working tasks of carpenters load their muscles or the degree of regularity in muscle activation. Key words: EMG, shoulder and arm muscles, intra-individual, tools, sawing, screwing, nailing
inter-individual,
Introduction
During the last few decades, working conditions in the construction trade have changed due to mechanization and automation. For many construction workers this change has lead to a greater proportion of installation work, which implies that the workers more or less act as human stands for their own hand-held tools and machines. This creates monotonous work, One negative result is localized muscle fatigue in certain arm and
Received: 21 July 1989 Accepted: 27 December 1989 Correspondence and reprint requests to:
Eva HammarskjBld, Department of Rehabilitation and Physical Medicine. Karolinska Institute. P.O. Box 60 500, S-104 01 Stockholm, Sweden 0 1990 Butterwotth-Heinemann 0268~0033/90/020081-07
Ltd
reproducibility,
carpenters, hand-
shoulder muscles’32. The prevalence of arm-shoulderneck disorders has become nearly as great as that of low back disorders for certain groups3. Each year, approximately two-thirds of electricians report pain from the shoulder region and one third from the elbow4S5. Elbow pain is often believed to depend on monotonous screwdriving. Nailing during carpentry work represents a similar case. Electromyography (EMG) has often been used to analyse muscular work load, and several researchers have used EMG to analyse dynamic activities”‘-“. Calculating loading moments for certain arm-shoulder muscles (i.e. trapezius, erector spinae, infraspinatus) is extremely complicated. However, EMG permits the study of activity levels as an indirect measure of load. The reliability of EMG recordings using surface electrodes is acceptable for their use in long-term studies, repeated at intervals of several days”.
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C/in. Biomech. 1990; 5: No 2
Our earlier study of repeated work movements with different hand tools, in which ten experienced carpenters repeated the same tasks over two separate days, showed intra-individual reproducibility with respect to certain manual precision’“. An intra-individual similarity was also visible in the EMG recordings, suggesting that workers reproduce their personal level of muscle activity as they repeat ordinary work. In gait analysis (sometimes described by EMG14*i5), individuals are expected to repeat their own movement patterns and the same leg muscle activation during repeated walking at the same rate. Studies of hand-tremor have shown higher intra-individual reproducibility than inter-individual resemblance under constant conditions’6.‘7. The amplitude of tremor was concluded to be a personal characteristic. In EMG studies of vocational tasks or experimental movements, the results are usually presented as mean or median values of data collected from all the participating subjects. No studies were found analysing the extent to which ordinary working tasks give the same muscle activity in different individuals or in the same individual during repetitions under the same conditions. The purpose of the present study was to analyse the amplitude levels of muscle activity in some arm-shoulder muscles in experienced carpenters during sawing, screwing and nailing. The following specific questions were analysed. 1. At what levels of activity are some neck, shoulder and arm muscles used during three carpentry tasks using ordinary hand tools? 2. Do different carpenters (inter-individually) activate their muscles in the same way during performance of the same tasks? 3. How regular (intra-individually) is muscle activity when a carpenter repeats nailing, sawing and screwing movements?
Materials and methods
a
b
Figure 1. Tasks performed: (c) sawing.
C (a) nailing, (b) screwing,
ordinary work. After each task, perceived exertion was rated according to the Borg scale from 6 to 20”. Screwing: the subjects were instructed to screw five countersunk wood screws (38x4 mm) into predrilled holes (3 mm) in vertically-mounted chipboard. They used a manual screwdriver, and worked at their own comfortable pace. The holes were at eye level for each individual subject. Sawing: the subjects sawed five pieces from a pine joist (50~200 mm) at pre-marked positions, again at their own comfortable pace. The joist was placed on a saw horse. Nailing: in the third task the subjects were instructed to hammer in 10 nails (50 mmx2 mm), at their own pace, upwards into horizontally mounted chipboard on the ceiling (2-05 m height). The subjects all had good experience of working with the selected tools. Before the experimental sessions started, they practised by screwing in one screw, cutting one joist and hammering in one nail. The three tasks were then performed in random order twice during one day, with a rest of 20-30 min in between, and repeated twice in random order 14 days later. Therefore. each task was performed on four occasions.
Subjects Ten volunteer carpenters participated in the experiments. Their average age was 36 years (range 22-48 years). They had worked as carpenters for an average of 18 years (range 5-25 years). Mean height and weight were l-78 m and 79 kg (range 164-l-89 m, 67-94 kg). One subject was left-handed. Tasks The hand-tools chosen were a manual screwdriver (handle length 100 mm, diameter 26 mm), a hand-saw (length 550 mm, seven teeth per 25 mm) and a carpenter’s hammer (weight 450 g). The subjects were instructed to perform three standardized tasks (Figure 1)) with a 20-30 minute rest in between to avoid fatigue. They were asked to work at the same rate as in their
Muscular activity Electromyographic signals were recorded during the whole experiment simultaneously from seven muscles or muscle groups of the dominant arm and shoulder. The muscles were chosen to represent activity during gripping and supination, arm flexion and extension as well as elevation-abduction and external rotation. Flexible and disposable Ag-AgCl surface electrodes (Medicotest, Glstykke, Denmark) were attached to the skin with, approximately, a 30 mm centre electrode distance. The electrode sites, as shown in Figure 2, were as follows. The trapezius pars descendens, at the anterolateral margin. The trapezius pars ascendens, following the fibres just medial to the medial edge of the scapula.
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cdrded in parallel on a UV recorder (Honeywell, Visicorder 1508, Skarholmen, Sweden). The signals were simultaneously visualized on an oscilloscope in parallel, used especially when testing electrode function. A normalization was performed to allow comparison of the levels of activity in different recordings and between different individuals. For this, a reference level was recorded for each pair of electrodes at the beginning and at the end of the experiments. The highest reference level obtained was chosen to represent the highest activity level of the muscle (100% EMG). The levels were recorded during standardized, maximum voluntary isometric test contractions against resistance, with the trunk stabilized in a sitting position. This technique has been described in detail by Schtildt and HarmsRingdahl “. For comparison, EMG amplitudes recorded from the middle parts of the screwing, sawing and nailing tasks were chosen as representative sequences for analysis. These sequences were as follows: 20 seconds during the middle part of screwing in the third of five screws; 20 seconds during the middle part of sawing the third of five sections of the joist; and the time needed to hammer in the fourth, fifth, sixth and seventh of ten nails (range 13-40 seconds). EMG recordings from the representative sequences were plotted with a digitizer (GTCO Corporation 1117A, Rockville, MD 20850, USA), recording 200 values per cycle. A computer program (Medakt, developed at the department of Physical Medicine, Karolinska Institute, Stockholm) calculated values as percentages of the reference EMG
Figure 2. Surface electrode locations
The thoracic erector spinae, covered by the rhomboids under the triangular aponeurosis of the trapezius; the upper electrode at the level of vertebrae C-T,. The anterior part of the deltoid. The infraspinatus, attached with the arm raised. The belly of the brachial biceps. The mid-point of the two radial extensor carpi. The signals were recorded as full-wave rectified lowpass filtered and time-averaged EMG, using a time constant of O-1 s (Devices AC 8 and Neurolog 104, 703, ULAB, Stockholm, Sweden). To check the effects of disturbances, unfiltered direct EMG signals were re-
Table 1. Median activity levels of normalized EMG, from four trials per subject, for seven muscles or muscle groups while screwing, sawing and nailing
1
2
3
4
5
6
7
8
9
10
Median of medians
Screwing Trap desc Trap asc Er spin/rh Ant delt Infraspin. Biceps Ext carpi
19 1 3 14 8 5 15
14 2 0 16 13 26 3
18 6 2 15 5 9 1
13 3 0 20 18 8 2
29 5 2 14 11 5 13
15 6 7 21 19 9 9
12 7 4 4 10 6 0
30 1 5 13 21 10 19
14 2 1 4 14 15 0
8 7 0 12 11 12 1
15 4 2 14 12 9 2
Sawing Trap desc Trap asc Er spin/rh Ant delt lnfraspin Biceps Ext carpi
6 4 8 7 8 0 4
5 3 : 4 2 2
17 4 10 17 14 3 3
7 1 8 11 17 5 1
19 5 6 15 19 2 4
22 8 11 21 30 7 6
12 0 11 6 13 2 0
12 5 19 15 13 1 5
8 3 10 24 15 4 5
10 5 7 10 14 1
4
11 4 9 13 14 2 4
16 5 6 14 15 1 14
13 3 1 11 9 1 1
44 11 9 21 9 1 1
18 1 2 14 25 2 5
26 6 4 14 17 1 6
15 1 2 8 11 0 0
11 1 8 4 10 0 0
44 1 6 8 23
17 15 8 7 19 3 3
7 9 1 12 15 1 1
16 4 5 12 15 1 2
Subject no.
Nailing Trap desc Trap asc Er spin/rh Ant delt Biceps Ext carpi
84
Clin. Biomech.
1990; 5: No 2 Screwing
and values as medians (showing the level at which half the time the activity was higher and half the time it was lower). levels,
Statistics
To study the intra-individual variability of muscle activity between tests performed on the same day and on different days, the representative parts of all the four trials were studied for the 10 subjects. The median activity levels of each trial and muscle group were intraindividually tested using analysis of variance (ANOVA).
z
o-+&+__l Trap
desc Trap
asc Er spun Ant rhomb
Results
delt
lnfraspin
Biceps
Ext carpi
lnfraspin
Biceps
Ext carpi
Sawing
Levels of activity and inter-individual differences
Median EMG activity levels for all subjects during the representative parts of the performances are presented in Table 1. The medians of median activity levels during the period analysed did not exceed 16% of maximum EMG for any of the muscle groups studied. However, for some muscles and activities, there were wide ranges of median activity level (Figure 3). The most activated muscles in all the three tasks were the trapezius pars descendens, infraspinatus and anterior deltoid. These were the only muscles to show peak values of around 100%. They were also the only muscles exposed to ‘static load’, i.e. for 90% of the time the activity levels were at or above lo-15% EMG”. This was the case for the trapezius in two subjects during screwing and in three subjects during nailing (32% EMG or more for one subject). For the infraspinatus, static load occurred in one person during screwing and sawing, and for the anterior deltoid it occurred once during screwing and once during nailing. These values, indicating static load, concerned five of the subjects. The remaining four muscle groups (the trapezius pars ascendens, erector spinae/rhomboid, biceps and extensor carpii) were all activated only minimally. The two subjects using the lowest activity levels for all the muscles tested were numbers seven and ten. These were the only individuals who had no median activity level higher than 15% of maximal EMG (Table 1). The subjects with the highest activity levels were three and eight. They had median values (five and six respectively) exceeding 15% of maximum EMG. For subject three, the most highly activated muscle was the trapezius pars descendens in all three tasks, and the anterior deltoid during sawing and nailing. For subject eight, it was the trapezius pars descendens and infraspinatus during screwing and nailing, the erector spinae/rhomboid during sawing, and the extensor carpi radialis during screwing. This individual was left-handed. To hammer in 10 nails took from 30-90 seconds. The median value of rated perceived exertion for all 10 subjects was 10.5 (‘fairly light’), (range 7-17). For sawing, the time varied between two and three minutes, and the median rating was 13 (‘somewhat hard’), (range 11-16). For screwing, between three and four minutes were
Trap
desc Trap
asc Er spin rhomb
Ant delt
Nailing E ; .;i
40-.
2 55 Y w D .-2 E ;; 2
30-.
20-, I
I T
lo-. 1 0, Trap
I .
desc Trap
T
i ,
I
asc Er spin rhomb
I .
, Ant
delt
lnfraspin
Biceps
. Ext carp!
I
Figure 3. Median values (squares) and ranges (vertical bars) of EMG amplitude while screwing, sawing and nailing (y axis; n = 10). Recorded muscles are indicated below the horizontal axis.
needed and the median rating was 17 (‘very hard’). (range 13-18). Further details of the perceived exertion are described elsewhere13. Reliability and intra-individual regularity
There was an intra-individual similarity between the median values from each trial. The results of analyses of variance (ANOVA), as seen in Table 2. showed very few significant differences. The inter- and intra-individual reproducibility of muscle activity is illustrated in Figure 4, where the EMG recordings while hammering in the fifth nail are presented for the three subjects that showed the most differing inter-individual muscle activities (subjects three, eight and ten).
Hammarskjijld Table 2. Results from analyses of variance (ANOVA) performed to test differences between median activities of EMG recordings during screwing, sawing and nailing. Analysis A compares values of first and second day, and B first and second trial within each day. Analysis C compares all the four trials. Seven muscle groups were tested. (* = significant difference at 5% level; - = non-significant difference, for intra-individual comparison between the ten subjects) Muscle
Screwing A B C
A
Trapdesc
---
-*
Trapasc
___
_
*_
Erspin/Rh Antdelt
___
-_-
-
-
Sawing B C
Nailing ABC
-
---
-
-_-
___ -
-
Infraspin.
-
-
-
-
-
-
-
*
-
Biceps Extcarpi
_ _
_ _
_ _
-
* _
_
_
-
_ _
Discussion
chosen muscles represented gripping, supination, forward-backward movements of the arm and external rotation, i.e. those muscles expected to be active during
The
% EMC
Trap
desc
Trap
asc
1
Subject
3
I
% EMC
et al.: Shoulder-arm
muscular activity in carpenters’ work
eye-level screwing, sawing and upward nailing. The EMG amplitudes from isometric, dynamic, or mixed isometric-dynamic activities are all related to the maximum voluntary isometric contraction (test contraction) from the position where the activity was shown to be the highest for each muscle group. Compared to isometric work, dynamic concentric work (with a faster rate of contraction) increases EMG amplitudes2’. Ideally, test contractions would have been obtained for each muscle in all the ranges of motion and at the same speeds as during the different activities. However, isometric strength is a reasonable predictor of dynamic strength22. Without more knowledge of the correlation between isometric and dynamic EMG, it is difficult to make proper estimates of the amplitude levels. It is known that the strength-endurance relationship for the neck muscles varies in many ways”. The mechanism for this may be linked to muscle fibre composition, the intra-muscular pressure, the blood flow and so on. A commonly cited limit for avoiding muscular fatigue is 10-U% MVC (maximum voluntary capacity) for mixed isometric-dynamic work23724.Although not directly comparable with EMG levels, these values are of considerable interest. Subject
8
% EMC
Subject
Er spin/ rhomb
Ant
85
delt
lnfraspin
Biceps
Ext carpi
Trial
number
Figure 4. EMG recordings (seven muscle groups, % normalized EMG, y axis) from subjects 3, 8 and 10 while nailing the fifth of 10 nails during four trials (x axis, normalized working cycles).
10
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The median values for the three tasks are presented in Table 1. These medians of median levels exceeded 15%
maximum EMG only for the trapezius pars descendens during nailing (16%) and this was the task that was performed the quickest. Only the trapezius, infraspinatus and deltoid reached total medians of 10-U% EMG. The subjects were all healthy and very familiar with the tasks having had an average of 18 years work experience. Most of them could be expected to work economically, yet the results revealed individual EMG patterns. Five subjects, at least during one task and for at least one of the muscles, were exposed to high static load. Some of these individuals might benefit from learning how to decrease the activity in these particular muscles. During nailing, two subjects activated the trapezius pars descendens to a median level of 44% of their maximum EMG, while one used only 7% (Table 1). None of these subjects was the shortest or the tallest of the ten. During sawing, one individual activated the trapezius pars descendens to 22% and infraspinatus to 30% of maximum EMG, while another showed only 5% and 4%, respectively. Screwdriving required 19% of maximum EMG from the infraspinatus and 21% from the anterior portion of the deltoid in one subject, while another used only 10% and 4% of the same muscles’ recorded maxima. Extensor carpi radialis longus and brevis are often considered to be highly activated during screwdriving and hammering. However, the present study did not support this belief. Only three subjects reached median activities over 10% EMG for the two radial extensor muscles during screwing, and the left-handed person reached the highest median value (19% EMG). Six subjects showed median values O-3% EMG. The supination movement during screwing activated the biceps slightly more: three subjects showed median values higher than 10% EMG, while the lowest value was 5% (for two subjects). Obviously, performance of the same tasks entails different muscle activity patterns in different individuals. Perhaps some workers always activate their muscles inappropriately after having once settled into an unfavourable motor programme for certain activities when first learned. If this could be verified by EMG recordings, EMG biofeedback might be used for retraining in order to learn how to relax overloaded muscles25-28. The intra-individual reliability of median activity levels from four trials during the two days was high. The differences that sometimes occurred may be due to the fact that the subjects actually changed their muscle pattern for the current task. Conclusions Some experienced, healthy carpenters seem to activate their arm and shoulder muscles to a lesser extent than others during manual screwdriving, sawing and nailing. There are considerable inter-individual differences in
muscle activation, whereas the intra-individual ity and reproducibility is high.
reliabil-
Acknowledgements
This study was supported by the Karolinska Institute, the Research Foundation for Occupational Safety and Health in the Swedish Construction Industry (Bygghtilsan) and the Swedish Work Environment Fund (project 89-0411‘).
References 1 Malmqvist R, Ekholm I, Lindstrbm L,PetersCn 1, &tengren R. Measurements of localized muscle fatigue in building work. Ergonomics 1981; 24: 695-709 2 Herberts P, Kadefors R, Hiigfors C, Sigholm S. Shoulder pain and heavy manual labour. Clin Orthop 1984; 191: 168-78 3 Hagberg M. Occupational musculoskeletal disorders-a new epidemiological challenge. In: Hogstedt C. Reuterwall C, eds. Progress in Occupational Epidemiology. Amsterdam: Elsevier Science Publishers, 1988: 15-26 4 Bjireborn B-M. Prevalence of elbow disorders in electrical installation work. Course project, The Swedish National Board of Occupational Safety and Health, 1984 (in Swedish) 5 Lindstrand 0. Discomfort in the upper extremities with regard to the use of powered screwdrivers in electrical installation work. Course project, The Swedish National Board of Occupational Safety and Health, 1988 (in Swedish) 6 Kadefors R, Peters& I, Herberts P. Muscular reaction to welding work: An electromyographic investigation. Ergonomics 1976; 19: 543-58 7 Hagberg M. On Evaluation of Local Muscular Load and Fatigue by Electromyography. The Swedish National Board of Occupational Safety and Health. Arbete och H&Isa, 1981: 24 8 Jonsson B, Ericson BE, Hagberg M. Electromyographic methods for analysis of the load on individual muscles and signs of muscular fatigue during long-term work. The Swedish National Board of Occupational Safety and Health. Undersiikningsrapport, 1981: 9 (in Swedish) 9 Harms-Ringdahl K, Ekholm J, Arborelius UP, Nemeth G, Schiildt K. Load moment, muscle strength and level of muscular activity during internal rotation training of the shoulder. Stand J Rehab Med 1983; Suppl9: 125-35 10 BjiirkstCn M, Itani T, Jonsson B, Yoshizawa M. Evaluation of muscular load in shoulder and forearm muscles among medical secretaries during occupational typing and some non-occupational activities. In: Jonsson B, ed. Biomechanics X-A. Illinois: Human Kinetics Publishers 1987; 6A: 35-9 11 Ericson MO. Muscular function during ergometer cycling. Stand J Rehab Med 1988; 20: 35-41 12 Komi PV, Buskirk ER. Reproducibility of electromyographic measurements with inserted wire electrodes and surface electrodes. Electromyography 1970; 4: 357-67 13 Hammarskjald E, Ekholm J, Harms-Ringdahl K. Reproducibility of work movements with carpenters’ handtools. Ergonomics 1989; 32: 1005-18 14 Carlsiiij S. How man moves. London: Heinemann, 1972 15 Basmajian J, de Luca C. Muscles alive. Their functions revealed by electromyography. Baltimore: Williams & Wilkins, 1985; 370-84 16 Sslzer M. Untersuchung iiber die Reproduzierbarkeit von Hand-Tremormessungen. Eur J Appi Physiol 1973; 32: 71-80
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et al.: Shoulder-arm
17 Salle H, Verberk M, Kemper C. Tremor, effect of physical
18 19
20
21 22
23
load and reproducibility. Am Ind Hyg Assc J 1986; 47: 533-66 Borg G. Perceived exertion as an indicator of somatic stress. Stand J Rehab Med 1970; 2-3: 92-8 Schtildt K, Harms-Ringdahl K. Activity levels during isometric test contractions of neck and shoulder muscles. Stand J Rehab Med 1988; 20: 117-27 Jonsson B. Quantitative electromyographic evaluation of muscular load during work. Stand J Rehab Med 1978; Suppl6: 69-74 Winter DA. Biomechanics of Human Movement. New York: John Wiley, 1979 Lambert 0. The relationship between maximum isometric strength at different speeds. Intern Fed Phys Educ Bull 1965; 35: 13-21 Petrofsky JS, Phillips CA. The strength-endurance relationship in skeletal muscle: its application to helmet
24
25
26
27
28
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design. Aviation Space and Environmental Medicine 1982; 3: 65-69 Jonsson B. Measurements and evaluation of local muscular strain in the shoulder during constrained work. J Human Ergo1 1982; 11: 73-88 Ekiund J, Esping C, Tuner E, Esping B, Jonsson B, Vaba J. EMG-biofeedback in occupational health. ASF report, Swedish Work Environment Fund Stockholm, Sweden, 1979: 140 Kadefors R, Petersen I. Occupational training: welding. Project Lindholmen Industrial Development Centre, Gothenburg, 1987 (in Swedish) Johansson S. Biofeedback training of forestry machine drivers. Course project at National Institute of Occupational Health, Lycksele, Sweden, 1988 (in Swedish) Parenmark G, Engvall B, Malmkvist A-K. Ergonomic on-the-job training of assembly workers. ApplErgonomics 1988; 19: 143-6
Shoulder-arm muscular activity and reproducibility in carpenters’ work E HammarskjGld RPT K Harms-Ringdahl RPT J Ekholm MD PhD
Dr MHI
SC
Kinesiology Research Group, Department of Rehabilitation and Physical Medicine and Department of Anatomy, Karolinska Institute, Stockholm, Sweden
Summary
The purpose of the present study was to analyse the levels of muscle activity that experienced carpenters reach in some arm-shoulder muscles when performing familiar tasks, and to find whether there are intra- and inter-individual similarities in muscle activity. Ten healthy carpenters performed twice on the same day: their tasks were, manual screwdriving at eye level, manual sawing on a saw horse, and nailing upward into the ceiling. Two weeks later the experiment was repeated. Normalized electromyography was used. The median activity levels during the period analysed did not exceed 15% of maximum electromyographic activity for any of the muscle groups studied. For some muscles and activities there were wide ranges of median activity levels. The most activated muscles in all three tasks were the trapezius pars descendens, infraspinatus and anterior deltoid. There was an intra-individual similarity between the median values from each trial. Relevance
The prevalence of arm-shoulder-neck disorders has become nearly as great as that of low back disorders. Muscular overload has been discussed as one etiological factor. No studies have been found analysing the extent to which the ordinary working tasks of carpenters load their muscles or the degree of regularity in muscle activation. Key words: EMG, shoulder and arm muscles, intra-individual, tools, sawing, screwing, nailing
inter-individual,
Introduction
During the last few decades, working conditions in the construction trade have changed due to mechanization and automation. For many construction workers this change has lead to a greater proportion of installation work, which implies that the workers more or less act as human stands for their own hand-held tools and machines. This creates monotonous work, One negative result is localized muscle fatigue in certain arm and
Received: 21 July 1989 Accepted: 27 December 1989 Correspondence and reprint requests to:
Eva HammarskjBld, Department of Rehabilitation and Physical Medicine. Karolinska Institute. P.O. Box 60 500, S-104 01 Stockholm, Sweden 0 1990 Butterwotth-Heinemann 0268~0033/90/020081-07
Ltd
reproducibility,
carpenters, hand-
shoulder muscles’32. The prevalence of arm-shoulderneck disorders has become nearly as great as that of low back disorders for certain groups3. Each year, approximately two-thirds of electricians report pain from the shoulder region and one third from the elbow4S5. Elbow pain is often believed to depend on monotonous screwdriving. Nailing during carpentry work represents a similar case. Electromyography (EMG) has often been used to analyse muscular work load, and several researchers have used EMG to analyse dynamic activities”‘-“. Calculating loading moments for certain arm-shoulder muscles (i.e. trapezius, erector spinae, infraspinatus) is extremely complicated. However, EMG permits the study of activity levels as an indirect measure of load. The reliability of EMG recordings using surface electrodes is acceptable for their use in long-term studies, repeated at intervals of several days”.
82
C/in. Biomech. 1990; 5: No 2
Our earlier study of repeated work movements with different hand tools, in which ten experienced carpenters repeated the same tasks over two separate days, showed intra-individual reproducibility with respect to certain manual precision’“. An intra-individual similarity was also visible in the EMG recordings, suggesting that workers reproduce their personal level of muscle activity as they repeat ordinary work. In gait analysis (sometimes described by EMG14*i5), individuals are expected to repeat their own movement patterns and the same leg muscle activation during repeated walking at the same rate. Studies of hand-tremor have shown higher intra-individual reproducibility than inter-individual resemblance under constant conditions’6.‘7. The amplitude of tremor was concluded to be a personal characteristic. In EMG studies of vocational tasks or experimental movements, the results are usually presented as mean or median values of data collected from all the participating subjects. No studies were found analysing the extent to which ordinary working tasks give the same muscle activity in different individuals or in the same individual during repetitions under the same conditions. The purpose of the present study was to analyse the amplitude levels of muscle activity in some arm-shoulder muscles in experienced carpenters during sawing, screwing and nailing. The following specific questions were analysed. 1. At what levels of activity are some neck, shoulder and arm muscles used during three carpentry tasks using ordinary hand tools? 2. Do different carpenters (inter-individually) activate their muscles in the same way during performance of the same tasks? 3. How regular (intra-individually) is muscle activity when a carpenter repeats nailing, sawing and screwing movements?
Materials and methods
a
b
Figure 1. Tasks performed: (c) sawing.
C (a) nailing, (b) screwing,
ordinary work. After each task, perceived exertion was rated according to the Borg scale from 6 to 20”. Screwing: the subjects were instructed to screw five countersunk wood screws (38x4 mm) into predrilled holes (3 mm) in vertically-mounted chipboard. They used a manual screwdriver, and worked at their own comfortable pace. The holes were at eye level for each individual subject. Sawing: the subjects sawed five pieces from a pine joist (50~200 mm) at pre-marked positions, again at their own comfortable pace. The joist was placed on a saw horse. Nailing: in the third task the subjects were instructed to hammer in 10 nails (50 mmx2 mm), at their own pace, upwards into horizontally mounted chipboard on the ceiling (2-05 m height). The subjects all had good experience of working with the selected tools. Before the experimental sessions started, they practised by screwing in one screw, cutting one joist and hammering in one nail. The three tasks were then performed in random order twice during one day, with a rest of 20-30 min in between, and repeated twice in random order 14 days later. Therefore. each task was performed on four occasions.
Subjects Ten volunteer carpenters participated in the experiments. Their average age was 36 years (range 22-48 years). They had worked as carpenters for an average of 18 years (range 5-25 years). Mean height and weight were l-78 m and 79 kg (range 164-l-89 m, 67-94 kg). One subject was left-handed. Tasks The hand-tools chosen were a manual screwdriver (handle length 100 mm, diameter 26 mm), a hand-saw (length 550 mm, seven teeth per 25 mm) and a carpenter’s hammer (weight 450 g). The subjects were instructed to perform three standardized tasks (Figure 1)) with a 20-30 minute rest in between to avoid fatigue. They were asked to work at the same rate as in their
Muscular activity Electromyographic signals were recorded during the whole experiment simultaneously from seven muscles or muscle groups of the dominant arm and shoulder. The muscles were chosen to represent activity during gripping and supination, arm flexion and extension as well as elevation-abduction and external rotation. Flexible and disposable Ag-AgCl surface electrodes (Medicotest, Glstykke, Denmark) were attached to the skin with, approximately, a 30 mm centre electrode distance. The electrode sites, as shown in Figure 2, were as follows. The trapezius pars descendens, at the anterolateral margin. The trapezius pars ascendens, following the fibres just medial to the medial edge of the scapula.
Hammarskjtid
et al.: Shoulder-arm
muscular
actMy in carpenters’ work
83
cdrded in parallel on a UV recorder (Honeywell, Visicorder 1508, Skarholmen, Sweden). The signals were simultaneously visualized on an oscilloscope in parallel, used especially when testing electrode function. A normalization was performed to allow comparison of the levels of activity in different recordings and between different individuals. For this, a reference level was recorded for each pair of electrodes at the beginning and at the end of the experiments. The highest reference level obtained was chosen to represent the highest activity level of the muscle (100% EMG). The levels were recorded during standardized, maximum voluntary isometric test contractions against resistance, with the trunk stabilized in a sitting position. This technique has been described in detail by Schtildt and HarmsRingdahl “. For comparison, EMG amplitudes recorded from the middle parts of the screwing, sawing and nailing tasks were chosen as representative sequences for analysis. These sequences were as follows: 20 seconds during the middle part of screwing in the third of five screws; 20 seconds during the middle part of sawing the third of five sections of the joist; and the time needed to hammer in the fourth, fifth, sixth and seventh of ten nails (range 13-40 seconds). EMG recordings from the representative sequences were plotted with a digitizer (GTCO Corporation 1117A, Rockville, MD 20850, USA), recording 200 values per cycle. A computer program (Medakt, developed at the department of Physical Medicine, Karolinska Institute, Stockholm) calculated values as percentages of the reference EMG
Figure 2. Surface electrode locations
The thoracic erector spinae, covered by the rhomboids under the triangular aponeurosis of the trapezius; the upper electrode at the level of vertebrae C-T,. The anterior part of the deltoid. The infraspinatus, attached with the arm raised. The belly of the brachial biceps. The mid-point of the two radial extensor carpi. The signals were recorded as full-wave rectified lowpass filtered and time-averaged EMG, using a time constant of O-1 s (Devices AC 8 and Neurolog 104, 703, ULAB, Stockholm, Sweden). To check the effects of disturbances, unfiltered direct EMG signals were re-
Table 1. Median activity levels of normalized EMG, from four trials per subject, for seven muscles or muscle groups while screwing, sawing and nailing
1
2
3
4
5
6
7
8
9
10
Median of medians
Screwing Trap desc Trap asc Er spin/rh Ant delt Infraspin. Biceps Ext carpi
19 1 3 14 8 5 15
14 2 0 16 13 26 3
18 6 2 15 5 9 1
13 3 0 20 18 8 2
29 5 2 14 11 5 13
15 6 7 21 19 9 9
12 7 4 4 10 6 0
30 1 5 13 21 10 19
14 2 1 4 14 15 0
8 7 0 12 11 12 1
15 4 2 14 12 9 2
Sawing Trap desc Trap asc Er spin/rh Ant delt lnfraspin Biceps Ext carpi
6 4 8 7 8 0 4
5 3 : 4 2 2
17 4 10 17 14 3 3
7 1 8 11 17 5 1
19 5 6 15 19 2 4
22 8 11 21 30 7 6
12 0 11 6 13 2 0
12 5 19 15 13 1 5
8 3 10 24 15 4 5
10 5 7 10 14 1
4
11 4 9 13 14 2 4
16 5 6 14 15 1 14
13 3 1 11 9 1 1
44 11 9 21 9 1 1
18 1 2 14 25 2 5
26 6 4 14 17 1 6
15 1 2 8 11 0 0
11 1 8 4 10 0 0
44 1 6 8 23
17 15 8 7 19 3 3
7 9 1 12 15 1 1
16 4 5 12 15 1 2
Subject no.
Nailing Trap desc Trap asc Er spin/rh Ant delt Biceps Ext carpi
84
Clin. Biomech.
1990; 5: No 2 Screwing
and values as medians (showing the level at which half the time the activity was higher and half the time it was lower). levels,
Statistics
To study the intra-individual variability of muscle activity between tests performed on the same day and on different days, the representative parts of all the four trials were studied for the 10 subjects. The median activity levels of each trial and muscle group were intraindividually tested using analysis of variance (ANOVA).
z
o-+&+__l Trap
desc Trap
asc Er spun Ant rhomb
Results
delt
lnfraspin
Biceps
Ext carpi
lnfraspin
Biceps
Ext carpi
Sawing
Levels of activity and inter-individual differences
Median EMG activity levels for all subjects during the representative parts of the performances are presented in Table 1. The medians of median activity levels during the period analysed did not exceed 16% of maximum EMG for any of the muscle groups studied. However, for some muscles and activities, there were wide ranges of median activity level (Figure 3). The most activated muscles in all the three tasks were the trapezius pars descendens, infraspinatus and anterior deltoid. These were the only muscles to show peak values of around 100%. They were also the only muscles exposed to ‘static load’, i.e. for 90% of the time the activity levels were at or above lo-15% EMG”. This was the case for the trapezius in two subjects during screwing and in three subjects during nailing (32% EMG or more for one subject). For the infraspinatus, static load occurred in one person during screwing and sawing, and for the anterior deltoid it occurred once during screwing and once during nailing. These values, indicating static load, concerned five of the subjects. The remaining four muscle groups (the trapezius pars ascendens, erector spinae/rhomboid, biceps and extensor carpii) were all activated only minimally. The two subjects using the lowest activity levels for all the muscles tested were numbers seven and ten. These were the only individuals who had no median activity level higher than 15% of maximal EMG (Table 1). The subjects with the highest activity levels were three and eight. They had median values (five and six respectively) exceeding 15% of maximum EMG. For subject three, the most highly activated muscle was the trapezius pars descendens in all three tasks, and the anterior deltoid during sawing and nailing. For subject eight, it was the trapezius pars descendens and infraspinatus during screwing and nailing, the erector spinae/rhomboid during sawing, and the extensor carpi radialis during screwing. This individual was left-handed. To hammer in 10 nails took from 30-90 seconds. The median value of rated perceived exertion for all 10 subjects was 10.5 (‘fairly light’), (range 7-17). For sawing, the time varied between two and three minutes, and the median rating was 13 (‘somewhat hard’), (range 11-16). For screwing, between three and four minutes were
Trap
desc Trap
asc Er spin rhomb
Ant delt
Nailing E ; .;i
40-.
2 55 Y w D .-2 E ;; 2
30-.
20-, I
I T
lo-. 1 0, Trap
I .
desc Trap
T
i ,
I
asc Er spin rhomb
I .
, Ant
delt
lnfraspin
Biceps
. Ext carp!
I
Figure 3. Median values (squares) and ranges (vertical bars) of EMG amplitude while screwing, sawing and nailing (y axis; n = 10). Recorded muscles are indicated below the horizontal axis.
needed and the median rating was 17 (‘very hard’). (range 13-18). Further details of the perceived exertion are described elsewhere13. Reliability and intra-individual regularity
There was an intra-individual similarity between the median values from each trial. The results of analyses of variance (ANOVA), as seen in Table 2. showed very few significant differences. The inter- and intra-individual reproducibility of muscle activity is illustrated in Figure 4, where the EMG recordings while hammering in the fifth nail are presented for the three subjects that showed the most differing inter-individual muscle activities (subjects three, eight and ten).
Hammarskjijld Table 2. Results from analyses of variance (ANOVA) performed to test differences between median activities of EMG recordings during screwing, sawing and nailing. Analysis A compares values of first and second day, and B first and second trial within each day. Analysis C compares all the four trials. Seven muscle groups were tested. (* = significant difference at 5% level; - = non-significant difference, for intra-individual comparison between the ten subjects) Muscle
Screwing A B C
A
Trapdesc
---
-*
Trapasc
___
_
*_
Erspin/Rh Antdelt
___
-_-
-
-
Sawing B C
Nailing ABC
-
---
-
-_-
___ -
-
Infraspin.
-
-
-
-
-
-
-
*
-
Biceps Extcarpi
_ _
_ _
_ _
-
* _
_
_
-
_ _
Discussion
chosen muscles represented gripping, supination, forward-backward movements of the arm and external rotation, i.e. those muscles expected to be active during
The
% EMC
Trap
desc
Trap
asc
1
Subject
3
I
% EMC
et al.: Shoulder-arm
muscular activity in carpenters’ work
eye-level screwing, sawing and upward nailing. The EMG amplitudes from isometric, dynamic, or mixed isometric-dynamic activities are all related to the maximum voluntary isometric contraction (test contraction) from the position where the activity was shown to be the highest for each muscle group. Compared to isometric work, dynamic concentric work (with a faster rate of contraction) increases EMG amplitudes2’. Ideally, test contractions would have been obtained for each muscle in all the ranges of motion and at the same speeds as during the different activities. However, isometric strength is a reasonable predictor of dynamic strength22. Without more knowledge of the correlation between isometric and dynamic EMG, it is difficult to make proper estimates of the amplitude levels. It is known that the strength-endurance relationship for the neck muscles varies in many ways”. The mechanism for this may be linked to muscle fibre composition, the intra-muscular pressure, the blood flow and so on. A commonly cited limit for avoiding muscular fatigue is 10-U% MVC (maximum voluntary capacity) for mixed isometric-dynamic work23724.Although not directly comparable with EMG levels, these values are of considerable interest. Subject
8
% EMC
Subject
Er spin/ rhomb
Ant
85
delt
lnfraspin
Biceps
Ext carpi
Trial
number
Figure 4. EMG recordings (seven muscle groups, % normalized EMG, y axis) from subjects 3, 8 and 10 while nailing the fifth of 10 nails during four trials (x axis, normalized working cycles).
10
86
C/in. Biomech.
1990; 5: No 2
The median values for the three tasks are presented in Table 1. These medians of median levels exceeded 15%
maximum EMG only for the trapezius pars descendens during nailing (16%) and this was the task that was performed the quickest. Only the trapezius, infraspinatus and deltoid reached total medians of 10-U% EMG. The subjects were all healthy and very familiar with the tasks having had an average of 18 years work experience. Most of them could be expected to work economically, yet the results revealed individual EMG patterns. Five subjects, at least during one task and for at least one of the muscles, were exposed to high static load. Some of these individuals might benefit from learning how to decrease the activity in these particular muscles. During nailing, two subjects activated the trapezius pars descendens to a median level of 44% of their maximum EMG, while one used only 7% (Table 1). None of these subjects was the shortest or the tallest of the ten. During sawing, one individual activated the trapezius pars descendens to 22% and infraspinatus to 30% of maximum EMG, while another showed only 5% and 4%, respectively. Screwdriving required 19% of maximum EMG from the infraspinatus and 21% from the anterior portion of the deltoid in one subject, while another used only 10% and 4% of the same muscles’ recorded maxima. Extensor carpi radialis longus and brevis are often considered to be highly activated during screwdriving and hammering. However, the present study did not support this belief. Only three subjects reached median activities over 10% EMG for the two radial extensor muscles during screwing, and the left-handed person reached the highest median value (19% EMG). Six subjects showed median values O-3% EMG. The supination movement during screwing activated the biceps slightly more: three subjects showed median values higher than 10% EMG, while the lowest value was 5% (for two subjects). Obviously, performance of the same tasks entails different muscle activity patterns in different individuals. Perhaps some workers always activate their muscles inappropriately after having once settled into an unfavourable motor programme for certain activities when first learned. If this could be verified by EMG recordings, EMG biofeedback might be used for retraining in order to learn how to relax overloaded muscles25-28. The intra-individual reliability of median activity levels from four trials during the two days was high. The differences that sometimes occurred may be due to the fact that the subjects actually changed their muscle pattern for the current task. Conclusions Some experienced, healthy carpenters seem to activate their arm and shoulder muscles to a lesser extent than others during manual screwdriving, sawing and nailing. There are considerable inter-individual differences in
muscle activation, whereas the intra-individual ity and reproducibility is high.
reliabil-
Acknowledgements
This study was supported by the Karolinska Institute, the Research Foundation for Occupational Safety and Health in the Swedish Construction Industry (Bygghtilsan) and the Swedish Work Environment Fund (project 89-0411‘).
References 1 Malmqvist R, Ekholm I, Lindstrbm L,PetersCn 1, &tengren R. Measurements of localized muscle fatigue in building work. Ergonomics 1981; 24: 695-709 2 Herberts P, Kadefors R, Hiigfors C, Sigholm S. Shoulder pain and heavy manual labour. Clin Orthop 1984; 191: 168-78 3 Hagberg M. Occupational musculoskeletal disorders-a new epidemiological challenge. In: Hogstedt C. Reuterwall C, eds. Progress in Occupational Epidemiology. Amsterdam: Elsevier Science Publishers, 1988: 15-26 4 Bjireborn B-M. Prevalence of elbow disorders in electrical installation work. Course project, The Swedish National Board of Occupational Safety and Health, 1984 (in Swedish) 5 Lindstrand 0. Discomfort in the upper extremities with regard to the use of powered screwdrivers in electrical installation work. Course project, The Swedish National Board of Occupational Safety and Health, 1988 (in Swedish) 6 Kadefors R, Peters& I, Herberts P. Muscular reaction to welding work: An electromyographic investigation. Ergonomics 1976; 19: 543-58 7 Hagberg M. On Evaluation of Local Muscular Load and Fatigue by Electromyography. The Swedish National Board of Occupational Safety and Health. Arbete och H&Isa, 1981: 24 8 Jonsson B, Ericson BE, Hagberg M. Electromyographic methods for analysis of the load on individual muscles and signs of muscular fatigue during long-term work. The Swedish National Board of Occupational Safety and Health. Undersiikningsrapport, 1981: 9 (in Swedish) 9 Harms-Ringdahl K, Ekholm J, Arborelius UP, Nemeth G, Schiildt K. Load moment, muscle strength and level of muscular activity during internal rotation training of the shoulder. Stand J Rehab Med 1983; Suppl9: 125-35 10 BjiirkstCn M, Itani T, Jonsson B, Yoshizawa M. Evaluation of muscular load in shoulder and forearm muscles among medical secretaries during occupational typing and some non-occupational activities. In: Jonsson B, ed. Biomechanics X-A. Illinois: Human Kinetics Publishers 1987; 6A: 35-9 11 Ericson MO. Muscular function during ergometer cycling. Stand J Rehab Med 1988; 20: 35-41 12 Komi PV, Buskirk ER. Reproducibility of electromyographic measurements with inserted wire electrodes and surface electrodes. Electromyography 1970; 4: 357-67 13 Hammarskjald E, Ekholm J, Harms-Ringdahl K. Reproducibility of work movements with carpenters’ handtools. Ergonomics 1989; 32: 1005-18 14 Carlsiiij S. How man moves. London: Heinemann, 1972 15 Basmajian J, de Luca C. Muscles alive. Their functions revealed by electromyography. Baltimore: Williams & Wilkins, 1985; 370-84 16 Sslzer M. Untersuchung iiber die Reproduzierbarkeit von Hand-Tremormessungen. Eur J Appi Physiol 1973; 32: 71-80
Hammarskjijld
et al.: Shoulder-arm
17 Salle H, Verberk M, Kemper C. Tremor, effect of physical
18 19
20
21 22
23
load and reproducibility. Am Ind Hyg Assc J 1986; 47: 533-66 Borg G. Perceived exertion as an indicator of somatic stress. Stand J Rehab Med 1970; 2-3: 92-8 Schtildt K, Harms-Ringdahl K. Activity levels during isometric test contractions of neck and shoulder muscles. Stand J Rehab Med 1988; 20: 117-27 Jonsson B. Quantitative electromyographic evaluation of muscular load during work. Stand J Rehab Med 1978; Suppl6: 69-74 Winter DA. Biomechanics of Human Movement. New York: John Wiley, 1979 Lambert 0. The relationship between maximum isometric strength at different speeds. Intern Fed Phys Educ Bull 1965; 35: 13-21 Petrofsky JS, Phillips CA. The strength-endurance relationship in skeletal muscle: its application to helmet
24
25
26
27
28
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design. Aviation Space and Environmental Medicine 1982; 3: 65-69 Jonsson B. Measurements and evaluation of local muscular strain in the shoulder during constrained work. J Human Ergo1 1982; 11: 73-88 Ekiund J, Esping C, Tuner E, Esping B, Jonsson B, Vaba J. EMG-biofeedback in occupational health. ASF report, Swedish Work Environment Fund Stockholm, Sweden, 1979: 140 Kadefors R, Petersen I. Occupational training: welding. Project Lindholmen Industrial Development Centre, Gothenburg, 1987 (in Swedish) Johansson S. Biofeedback training of forestry machine drivers. Course project at National Institute of Occupational Health, Lycksele, Sweden, 1988 (in Swedish) Parenmark G, Engvall B, Malmkvist A-K. Ergonomic on-the-job training of assembly workers. ApplErgonomics 1988; 19: 143-6