ARM AND SHOULDER MUSCLE LOAD IN VARIOUS KEYBOARD OPERATING JOBS OF OMEN

Transcription

1 J. Human Ergol.,11: 89-97,1982 ARM AND SHOULDER MUSCLE LOAD IN VARIOUS KEYBOARD OPERATING JOBS OF OMEN Noriaki ONIsHI, Kazuhiro SAKAI, and azutaka KoGI Division of Work Physiology and Psychology, Institute for Science of Labour, Sugao, Miyanzae-ku, Kawasaki, Japan Loading of upper limb muscles of 250 female office workers were studied. They were using English language typewriters, Japanese language typewriters, telex machines, telephone exchangers, visual display terminals, and insurance calculating machines. Their mean body height was cm, but their keyboard height was cm except for telex machines, the seat height being mostly cm. More than 60 % of the office machine operators were complaining of pains and stiffness of shoulder and arm muscles. Their feelings of eyestrain and shoulder fatigue increased during the day's work more markedly than those of general office workers. During operations, moderate static contractions were seen in the forearm extensor and the trapezius. These static components were remarkable during speedy operations which were common to typewriting and telex work, often reaching % or more of the maximum contraction level. The mean tenderness threshold of the trapezius region decreased from morning till the end of work and from day to day, decreasing by about 250 g/cm2 a week. It is, therefore, essential for these office machine operators to have ergonomic keyboard layout, frequent breaks, and spontaneous rests. Mechanization in factories and offices have changed various manipulative tasks to physically light work. But many of these tasks are often exceedingly repetitive. They require high density work of repeating similar motions in a constrained posture. In addition to the problems of monotony, localized muscle fatigue in the neck-shoulder-arm region and in the back has come to be prevalent (NISHIYAMA et a!.,1973; ONISHI et al.,1973; OHARA et al., 1976; HUNTING et al.,1980). Especially in offices, the majority of workers engaged in repetitive operations are female. These operations demand simple but repetitive use of the upper limb muscles, whereas women may have weaker musculature compared with men. We have found that repetitive upper limb motions tend to induce continuous contractions of arm or shoulder muscles to raise the hands, and that individual differ- 89

2 90 N. ONISHI, K. SAKAI, and K. KOGI ences in electromyographic amplitude are large (ONISHI et al., 1977). This may be connected with the postural load of these operators resulting from poor machine design and layout (HUNTING et al.,1980). Efforts have been made to reduce local muscle load by changing work surface height and inserting frequent breaks (OHARA et al., 1976; SAITo et al., 1980). But these efforts have been restricted only to a few jobs of women such as data entry and cash register operations. Workers themselves are apt to make light of these localized loading as the tasks seem simple. We recently examined a series of office machine operators. We have found that the fatigue by their upper limb motions is very closely associated with the static nature of the muscle load. We could also confirm that the static load was related to speedy keyboard operations with scarcely any time for spontaneous rests and to ergonomically wrong keyboard height. Measured myoelectric activities of the shoulder muscles have proved large enough to induce tenderness sensitivity changes of cumulative nature. METHODS Two hundred and fifty female office workers of age were examined. They were mostly below 30 years old, the average age being 25.5 years. They consisted of office machine operators and general office workers. The operators were dealing with English language typewriters, Japanese language typewriters, telex machines, telephone exchangers, visual display terminals, and insurance calculating machines. They worked 33 hr 45 min a week, daily from 09:15 until 17:00 with a one-hour lunch recess from 1200 to Measurements on their physique and muscle strength were done. Before, during, and after work, they filled in two kinds of questionnaire sheets, one asking about their fatigue feelings and the other asking about the bodily parts in which they felt fatigue. For 41 workers, the surface electromyograms of the upper limb muscles were recorded during work on a data recorder. They were later reproduced and integrated to produce pulses whose number corresponded to the myoelectric activity. The compression tenderness threshold of these workers was measured in the trapezius region by means of a strain-gauge pressure sensor. Its round and fiat top of 6 mm in diameter was compressed upon the skin over the middle part of the trapezius until the subject felt a distinct pain. The threshold pressure was immediately printed on a digital printer. This measurement was repeated four times a day for 5 days a week. RESULTS As Table 1 shows, the keyboard height and the seat height were notably high for the female office workers studied by comparison to their physique. Their mean body height was cm, a little taller than the average Japanse woman.

3 ARM AND SHOULDER MUSCLE LOAD 91 Table 1. Keyboard height and seat height in different office jobs examined. Mean body height of the 250 office workers studied was }3.3 cm; mean body height 51.8 ±2.7 kg; mean grip strength of the right hand 28.9 }2.2 kg; mean back muscle strength 66.6 } 5.6 kg. Fig. 1. Percentages of workers who had pains or stiffness in the upper limb regions due to work. Black areas show those who had absented themselves on account of the strong pains or stiffness. But they were weaker with respect to grip strength and back muscle strength, with mean strength of 28.9 kg and 66.6 kg, respectively. The median keyboard height was cm, for typewriters and visual display terminals, while it was relatively lower at 71 cm only for telex machines. The first and third quartile values were 75 and 78 cm for English language typewriters and 87 and 88 cm for Japanese typewriters. Accordingly, the median seat height was usually high, mostly between

4 92 N. ONISHI, K. SAKAI, and K. KOGI cm. This high seat level was common to most operators of these machines. In the case of typewriters and terminal units, the high keyboard position was simply because they were placed on the desk top with little arrangement to reduce the keyboard height. The high keyboard surfaces were apparently a cause of particular upper limb load of these operators. In various kinds of office work, those workers who were complaining of pains or stiffness of the upper limb muscles accounted for more than 60 %, as shown in Fig, 1. The percentages were higher for telephone, telex, and visual display terminal operators. About 15 % of the workers had strong pains or stiffness while not absenting themselves from work, the rate being 8-23 % in different jobs as shown by dotted areas. There were six workers, or 2.8 %, who had absented themselves from offices due to strong pains or stiffness of the upper limb muscles, as shown by the black areas. Thus most of these operators were suspected of having chronic muscle fatigue in the upper limb region. When the percentages of those who had fatigue feelings before work, before lunch recess, and after work were compared, there was a remarkable contrast between general office workers and office machine operators (Fig. 2). The general office workers had significantly higher rates than the operators in general tiredness or drowsiness which increased as the work continued. The operators, on the other hand, had higher rates as to eyestrain and shoulder stiffness, which also increased with time. At the end of work, 64% of the operators felt eyestrain, and 38 shoulder stiffness. Fig. 2. Changes in percentages of workers who felt corresponding fatigue feelings before work (0915), before lunch recess (1200), and after work (1700). Results from 157 office machine operators and 25 general office workers.

5 ARM AND SHOULDER MUSCLE LOAD 93 Fig. 3. Percentages of workers who have fatigue feeling in various parts of the body before, during and after work. Fig. 4. An example of electromyographic activities of arm, shoulder, and back muscles during keyboard and related operations at an English language typewriter. In the first half, the operator was using the keyboard, the paper sheet was exchanged when the deltoid muscle activity increased.

6 94 N. ONISHI, K. SAKAI, and K. KGGI When the workers were asked to indicate parts of the body where they felt fatigue, using a sheet with a front and a back view drawings of the human body, fatigue was found to be concentrated in the shoulder, eyes, forearms, and back (Fig. 3). The rate of those who had fatigue in the shoulders was seen to increase already during the morning period. It was 40% at 1200,44% at 1430, and 42% at The rate of those who had eyestrain gradually increased for these operators, 23 % at 1200, 30% at 1430, and 38 % at Fatigue also became more frequent, though at a lesser degree, at the forearms and the back. Typical electromyographic activities during work are shown in Fig. 4 for an English language typist. Continued static contractions were seen in the forearm extensor and the right trapezius. They were seen not only during keyboard operations but also during other kinds of work, such as exchanging sheets, correcting misprints, and writing. They were as large as mv. Figure 5 compares changes in the integrated values of EMG during operation of the forearm extensor and the right and left trapezius muscles for different oper- Fig. 5. Temporal changes in the integrated electromyographic activity of the forearm extensor and the right and left trapezius muscles during operating various office machines.

7 ARM AND SHOULDER MUSCLE LOAD 95 ators. Sustained contractions of these muscles were common to all these jobs. When dynamic motions increased the integrated values, a certain level of static contractions was maintained almost all throughout the operating time. These static components were more notable for the right trapezius than for the left trapezius. For some operators, such as English language typists, the integrated electromyogram of the right trapezius increased with time of operation in every work session despite a slight fall in the frequency of key touches. In order to know the extent of static components during keyboard operations, the electromyographic activities of the forearm extensor and the trapezius are shown in Fig. 5 in terms of percent maximum voluntary contraction. The range and the median vaule for key operating work are shown for each job. Black circles indicate cases in which the contraction was primarily static. The activities of the forearm extensor ranged between % and those of the trapezius approximately between %. The static type of contractions were seen in many operators for both of these muscles; of 38 cases examined, 18 cases had contractions of a static nature in the case of the forearm extensor, and 16 cases in the case of the right trapezius. Eight cases showed static contractions also for the left trapezius. While the static components of the forearm extensor were found among typists and telex operators, the static trapezius activities were found in all the jobs, more frequently in typing, telex operation and insurance calculating machine operation. The operating speed was actually very high for typists and telex operators. It should be further noted that the trapezius tended to be statically contracting if the operator had no static load for the forearm extensor. Fig. 6. Ranges and median levels of the electromyographic activity in percent maximum contraction during operations of various office machines. A, English language typewriter; B, Japanese language typewriter; C, telex machine; D, visual display terminal; E, telephone exchanger; F, insurance calculating machine. Each number below A, B, etc. gives the number of subjects examined.

8 96 N. ONISHI, K. SAKAI, and K. KOGI Fig. 7. Daily changes of the mean tenderness threshold in the middle part of the right trapezius. Each value is shown as a deviation from the overall average. 1, before work (0915) ; 2, before lunch recess (1200) ; 3, after lunch recess (1300) ; 4, after work (1700). Considering the level of sustained muscle contractions below which no fatigue would appear should be about 10 % or even less (K0GI and HAKAMADA, 1962; BJORKSTEN and J NS5 N, 1977), the observed static level seemed high enough to induce muscle fatigue. Our findings clearly account for the high rates of shoulder fatigue among these office machine operators. The results of compression tenderness threshold measurements in the trapezius region support this, too. They are given in Fig. 7 for 41 operators of office machines for five days a week. The measurements were done four times a day, at around 0915, 1200, 1300, and On Monday, the median tenderness threshold was 752 g/cm2 for the trapezius before work, nine out of 41 operators having a threshold of more than 1,500 g/cm2. The mean threshold decreased gradually as work proceeded, to reach a level 160 g/cm2 lower at the end of Monday's work. The pre-work level decreased from day to day, by 170 g/cm2 from Monday to Friday. The range of recovery from the evening till the morning of the following day also diminished in the latter half of the week. Thus, at the end of the week, the mean threshold was 250 g/cm2 smaller than at the beginning of the week. As muscle fatigue could have a long-lasting after-effect (PIEHL, 1974) and shoulder muscles are known to be readily affected by accumulated fatigue (HAGBERG,1981), these threshold changes would represent chronically developing fatigue of the shoulder muscles. CONCLUSION The office machine operators were frequently complaining of shoulder fatigue. This was accounted for by static loading of the trapezius. The static loading was individually different, but if it was present, it reached the 20-30% level of the maximum contraction. This was apparently due to the high keyboard surface for these operators and to the high operating speed. Similar static components were also seen in the forearm extensors which also worked to elevate the hands for keyboard operations. The resulting fatigue in the shoulder muscles was so remarkable as to signif-

9 ARM AND SHOULDER MUSCLE LOAD 97 icantly lower tenderness threshold during a day's work and from day to day This was in accordance with the increase with time of subjective shoulder fatigue t seems urgent to organize appropriate work-rest schedules for such operators uitable work space design alone would not be enough to prevent the shoulder.. I. S fatigue. While the keyboard height should be substantially lowered by providi special work desks to accommodate typewriters other desk-top office machines, visual display terminals, and, frequent insertion of short breaks must be considered. We would suggest inserting a break each time the duration of con tinuously operating period reaches min (K00I the nature of work might have an effect similar to that of a break, 1981). Changing wholly ng. In any types of office machine work in which static contractions of shoulder muscles are suspect such insertion of short breaks would be indispensable. This should accompany more suitable arrangement of the work space which allows the shoulder muscles t o h ave spontaneous rests as part of the work rhythm. ed, - REFERENCES BJORKSTEN, A. and JONSSON, B. (1977) Endurance limit of force in long-term intermittent static contractions. Scand. J. Work Environ. Health, 3: HAGBERG, M. (1981) Work load and fatigue in repetitive arm elevations. Ergonomics, 24: HUNTING, W., GRANDJEAN, E., and MAEDA, K. (1980) Constrained postures in accounting machine operators. Appl. Ergon., 11: KOGI, K. (1981) Finding appropriate work-rest rhythm for occupational strain on the basis of electromyographic and behavioural changes. 10th Int. Congr. EEG Clinical Neurophysiol.,K yoto. Kooi, K. and HAKAMADA, T (1962) Slowing of surface electromyogram and muscle strength in muscle fatigue. Rep. Inst. Sci. Labour, No. 60, NISHIYAMA, K., NAKASEKO, M., and HOSOKAWA, M. (1973) Cash register operators' work and its hygienical problems in supermarket. Jpn. J. Ind. Health, 15: OHARA, H., AOYAMA, H., and ITANI, T. (1976) Health hazard among cash register operations and the effects of improved working conditions. J. Human Ergol., 5: ONIsHI, N., NOMURA, H., and SAKAI, K. (1973) Fatigue and strength of upper limb muscles of flight reservation system operators. J. Human Ergol., 2: ONISHI, N., SAKAI, K., ITANI, T., and SHINDO, H. (1977) Muscle load and fatigue of film rolling workers. J. Human Ergol., 6: PIEHL, K. (1974) Time course for refilling glycogen stores in human muscle fibres foll owing exercise induced glycogen depletion. Acta Physiol. Scand., 90: SAITO, H., Kooi, K., SAKAI, K., and UENO, Y. (1980) Work-rest schedules and related problems of key operators dealing with computer input data: Results of a survey on 310 establishments J. Sci. Labour, 56(3): Part U,