Evaluation of Shoulder Muscular Fatigue Induced During VDT Tasks

INTERNATIONAL JOURNAL OF HUMAN COMPUTER INTERACTION, 15(3), 407 417 Copyright 2003, Lawrence Erlbaum Associates, Inc. Evaluation of Shoulder Muscular Fatigue Induced During VDT Tasks Atsuo Murata Department of Computer Sciences Faculty of Information Sciences, Hiroshima City University Atsushi Uetake Syuichiro Matsumoto Yosuke Takasawa Housing Facilities Research Laboratory Tsukuba Research Institute, Technology & Development Headquarters Sekisui Chemical Co. Ltd. This study was designed to evaluate localized muscular fatigue induced during visual display terminal (VDT) tasks. In the experimental paradigm used, electromyography (EMG) signals were not recorded during the VDT task but during isometric contractions in which the load imposed on the shoulder muscle was kept constant. The change in mean power frequency (MPF) and the root mean square values of EMG signals with time were explored. The correspondence between these measures and the psychological rating of localized muscular fatigue also were examined. The effectiveness of the experimental paradigm and of the measures used for evaluation of localized muscular fatigue are discussed. MPF measured during isometric contraction was found to be a sensitive measure of localized muscular fatigue. 1. INTRODUCTION Electromyography (EMG) is a record over time of electrical potentials originating in muscles. EMG is best suited for examining the development of tension within a muscle and the firing rates of particular motor units in relation to the recruitment of others, as well as for revealing muscle activity too small to produce visible movement. The term muscular fatigue is, in general, interpreted as a subjective sensation such as increased effort to maintain force or discomfort and pain associated with Requests for reprints should be sent to Atsuo Murata, Hiroshima City University, Department of Computer Sciences, 3-4-1, Ozukahigashi, Asaminami-ku, Hiroshima 731-3194 Japan. E-mail: murata@cs.hiroshima-cu.ac

408 Murata et al. muscular activity. Muscular fatigue might be more scientifically defined as impaired motor performance, increased EMG activity for a given performance, a shift in the EMG power spectrum toward low frequencies, or impaired force generation (Hagberg, 1981; Kumar, 1999; Merletti, Knaflis, & DeLuca, 1990; Öberg, Sandsjo, & Kadefors, 1990, 1991, 1994; Öberg, Sandsjo, Kadefors, & Larsson, 1992; Sadoyama, Masuda, & Miyano, 1983). Knowledge of muscular fatigue can be used in human factors or ergonomics to improve working methods or design workplaces. Most office tasks require the use of a visual display terminal (VDT), and workers spend most of their hours using a keyboard or a mouse. Localized (local) muscular fatigue is crucial in many VDT tasks (Conway, 1999). The constrained working posture during keyboard or mouse operation can cause discomfort in the shoulders, elbows, and wrists (Bendix & Jessen, 1986; Hunting, Laubli, & Grandjean, 1981; Karlqvist, Hagberg, & Selin, 1994). Twisting of the hands toward the ulnar side, for example, is related to an increase in localized muscular fatigue of the arms. Many studies have found a high prevalence of musculoskeletal discomfort for VDT users (Balliet, Dainoff, & Mark, 1996; Cook & Kothiyal, 1998; Horgen, Aarås, Fagerthun, & Larsen, 1995; Pan & Schleifer, 1996). Aarås (1994) studied the relation between trapezius load and the development of musculoskeletal discomfort in the upper body and found that an increase in the number of micropauses (periods below 1% maximum voluntary contraction [MVC]) seemed to reduce the incidence of musculoskeletal discomfort. Jonsson (1982) attempted to evaluate local muscular fatigue in the shoulder during the constrained task. Öberg et al. (1994) made subjective and objective evaluations of shoulder muscle fatigue by using the mean power frequency (MPF) of the EMG power spectrum. They pointed out that MPF and the subjective (psychological) rating of fatigue did not correspond with each other, especially at a low load level. Under the dynamic developmental process of muscular fatigue, the organism swings between a stage of fatigue and that of recovery and rest. The feeling of muscular fatigue is a warning signal. In ordinary working activities, the muscular load level varies within short time periods. Most VDT tasks are prolonged at low load levels, a situation that can be harmful. When one is evaluating localized muscular fatigue, it is important to relate the physiological indexes to the psychologically experienced feeling of muscular fatigue. The complex interactions between load, exposure time, exposure pattern, pause length, recovery and rest, and other factors also must be considered in the evaluation of muscular fatigue. In human computer interaction, it is essential to develop an effective evaluation method of localized muscular fatigue induced during VDT tasks. Many attempts have been made to evaluate localized muscular fatigue by using EMG recordings of body segments such as the shoulder and forearm while participants perform VDT tasks. Evaluating localized muscular fatigue during VDT task is difficult because such tasks involve many movements that require the activation of different muscles and motor units. Although it is reasonable to evaluate localized muscular fatigue on the basis of the change in EMG in the forearm and shoulder muscles, the use of EMG signals recorded during the task cannot be recommended because EMG signals are subject to interference and distortion when movement

Shoulder Muscular Fatigue During VDT Tasks 409 occurs. In isometric contractions, that is, when there is no movement, there is a close correspondence between the surface EMG summed over time and the tension developed in the muscle. However, the relation does not hold for isotonic measurement (when movement occurs). It is expected that the characteristics of localized muscular fatigue explained by a shift in the EMG power spectrum toward low frequencies and an increase in the EMG amplitude or the percentage MVC cannot be used effectively if the isometric contraction is not kept constant. Aarås and Ro (1997) and Aarås, Ro, and Thoresen (1999) evaluated a newly developed mouse that gives the operator a more neutral forearm position. They used the percentage MVC obtained based on the root mean square (RMS) value of the EMG signal and the force relation. They indicated that the newly developed mouse reduces muscular workload on the trapezius, extensor carpi ulnaris, and extensor digitorum communis. However, they did not evaluate localized muscular fatigue on the basis of the change in MPF with time. They rather evaluated the workload induced on the shoulder muscle. In the evaluation of localized muscular fatigue in VDT tasks, it is necessary to evaluate accumulated muscular fatigue by using an experimental paradigm that consists of repeated measurements of EMG signals under isometric contraction conditions for a comparatively long period. In other words, the change in EMG RMS and MPF over time must be explored. Using an experimental paradigm where EMG signals were not recorded during the VDT task but during isometric contractions in which a constant load on the shoulder muscle was maintained, the authors attempted to evaluate localized muscular fatigue of the shoulder muscle during a VDT task. With such an experimental paradigm, the changes in MPF and RMS values of EMG signals with time were explored. The correspondence between these measures and the psychological rating of fatigue also was examined. Based on the results, the effectiveness of the experimental paradigm and the measures used to evaluate localized muscular fatigue is discussed. Some implications for the evaluation of localized shoulder muscular fatigue also are given. 2. METHODS 2.1. Participants Participants were five male undergraduate students, ages 19 to 23. They were all accustomed to keyboard operations. All had experience operating the word processor used in this experiment. 2.2. Apparatus The EMG signal was amplified with a polygraph (Nihonkoden, Telemeter System) and directly sent to a signal processor (NEC Medical System, Tokyo, Japan, DP110).

410 Murata et al. 2.3. Experimental Task The experimental task was word processing. The participant entered a document by using word processing software. The participant was required to perform the word processing task as accurately and quickly as possible. The task consisted of four blocks, each corresponding to a 30-min period during which the document was entered. 2.4. Procedure The EMG signal was recorded from the upper part of the trapezius muscle. The surface EMG was recorded with 5-mm bipolar surface electrodes (Ag AgCl) placed over the right trapezius muscle, midway between the C7 spinous process and the acromion, with an interelectrode distance of 5 mm. The interelectrode distance was chosen even though a 2-cm distance is more common because the latter distance requires the measurement of many motor units, which leads to the failure of highly accurate measurements. A surface reference was placed over the spinous process of the C7 vertebra. The electrode location was longitudinal to the direction of the muscle fiber. The skin was gently abraded with sandpaper and cleaned with alcohol before application of the surface electrodes. Each electrode was attached so that interelectrode resistance was below 5 kω and impedance ranged mostly between 1 and3kω. The EMG signals were sent directly to the signal processor with a sampling frequency of 1 khz. The EMG signal was passed through a low-pass (fourth-order Butterworth) filter with a cutoff frequency of 800 Hz. RMS and MPF of the EMG signal were used to evaluate localized muscular fatigue. The fast Fourier transform (FFT) was used to calculate MPF. The EMG was recorded for about 1 min at every measurement epoch. Each 1-min recording was divided into 60 segments. The first and last segments were excluded from the analysis to eliminate start and stop transitions in the participant and the instrument. For each segment, the mean value for 1,000 samples was calculated. When the mean value was added to the 1,000 data points, the total number of data points was 1,024. With these data, FFT was performed to calculate power spectrums. The power spectrums of 58 segments were averaged to calculate MPF for one measurement. The RMS values of 58 segments also were averaged for one measurement. The mean values of MPF and EMG RMS before the VDT task were used as reference values for normalization. All subsequent MPF and RMS values were divided by this reference value, thus giving an initial value of 1.0 for the normalized data. By this normalization procedure, interindividual variation was eliminated and all normalized variables became centered around 1.0. Each participant was asked to report sensations of localized muscular fatigue in the neck or shoulder separately. The perceived muscle fatigue was quantified by using a numerical, 7-point scale with verbal anchors. A rating of one meant that localized muscle fatigue in the neck or shoulder was not perceived at all. A rating of seven indicated that the participant felt severe localized fatigue in the neck or shoulder. The psychological scores were recorded every 30 min, once before each

Shoulder Muscular Fatigue During VDT Tasks 411 30-min experimental block. The EMG signals also were recorded every 30 min. When EMG signals were recorded, the participant was required to maintain the same posture so that the recording could be conducted under the same isometric contraction. While holding a 1-kg dumbbell, the participant kept his right arm straight and elevated at 90º of abduction in the sagittal plane for 1 min. The psychological rating and the 1-min EMG recording were conducted before each experimental task was begun. Immediately after the four experimental blocks had been completed, another rating and EMG recording were carried out (Figure 1). As a performance measure during a VDT task, the number of characters entered per minute was recorded. To compare the effectiveness of MPF and EMG RMS between isometric contraction conditions and VDT task conditions, EMG signals also were measured during a VDT task for the middle 1 min of each experimental block. 3. RESULTS The normalized performance measure tended to be constant across the four blocks (Figure 2). Here, VDT1, VDT2, VDT3, and VDT4 denote the measurements for the first, second, third, and fourth 30-min experimental blocks, respectively. As a result of a one-way (block) analysis of variance (ANOVA) performed on the number of entered characters per minute, no significant main effect of time was detected. The psychological rating of localized fatigue in the neck and shoulder tended to increase with time (Figure 3). Here, BT, T30, T60, T90, and T120 denote the measurements before an experimental task and 30, 60, 90, and 120 min after the start of the task, respectively. The ratings for both neck and shoulder sensations were added. A Friedman nonparametric test conducted on the rating score revealed a FIGURE 1 Experimental procedure.

FIGURE 2 Number of entered characters per minute compared among four blocks. VDT = video display terminal. FIGURE 3 Rating score of localized fatigue from neck and shoulder compared among four blocks. 412

Shoulder Muscular Fatigue During VDT Tasks 413 significant main effect of block, χ 2 (4, N = 5) = 13.316, p <.01. The EMG RMS tended to increase with time (Figure 4). A similar ANOVA performed on EMG RMS revealed no significant main effect of block. The MPF decreased with time (Figure 5). As a result of one-way (block) ANOVA performed on MPF, a significant main effect of block was detected, F(4, 20) = 8.384, p <.01. As a result of a Student Newman Keuls post hoc test, the differences in the following pairs were significant: BT and T60 (p <.01), BT and T90 (p <.01), and BT and T120 (p <.01). For EMG RMS and MPF recorded during the VDT task, no significant main effect of block was detected (Figure 6). 4. DISCUSSION Localized muscular fatigue can be characterized by a feeling of tightening in the muscle, a sustained cramp with a deep and intermittent pain, and a continuous pain with a desire to cease the work or activity. The shoulder muscles are common sites of chronic and work-related disorders in VDT tasks. Localized muscular fatigue appeared in the shoulder and accumulated during a 2-hr experimental task (Figure 3). The accumulated localized muscular fatigue, however, did not affect work efficiency at all (Figure 2). The EMG RMS tended to increase with time, although a main effect of block (time) was not statistically significant. The EMG RMS tended to be higher during the experimental task than when there was no task FIGURE 4 Normalized electromyography (EMG) root mean square (rms) values measured under the same isometric contraction as a function of block.

414 Murata et al. FIGURE 5 Normalized mean power frequency (MPF) measured under the same isometric contraction as a function of block. loading (Figure 4). This result shows that the electrical activity of the trapezius muscle might increase with the accumulation of localized muscular fatigue, although a main effect of block (time) was not statistically significant. MPF also tended to decrease with the elapse of the experimental block (Figure 5), which indicates the decreased discharge of muscular centers. MPF was most sensitive to the elapse of time and the accumulation of psychological symptoms of localized muscular fatigue. The main effect of block (time) was statistically significant only in this case, indicating the MPF is especially useful for the evaluation of localized muscular fatigue. These results suggest that such measures can be reliably used to evaluate localized muscular fatigue if the isometric contraction is maintained. On the other hand, the EMG RMS and MPF obtained during the VDT task were not as sensitive to the psychological feeling of localized fatigue compared with those obtained under isometric contraction (Figure 6). As predicted, if EMG signals are measured during a VDT task, it is difficult to maintain the isometric contraction because movement frequently occurs during VDT tasks. The results clearly show that MPF measured under isometric contraction is promising for the evaluation of localized muscular fatigue during a VDT task. Document entry is frequently a part of VDT tasks. The relations between MPF and the psychological rating score for the neck and shoulder are shown in Figure 7. Only the correlation for Figure 7b was statistically significant (p <.01). The correlation for Figure 7b was by far higher than that for Figure 7a. When EMG signals were measured under isometric contraction conditions, there seemed to be a significant correlation between MPF and the perceived sensation of localized fatigue in

FIGURE 6 Normalized mean power frequency MPF measured during video display terminal (VDT) task as a function of block. FIGURE 7 Relation between rating score of localized muscular fatigue and normalized mean power frequency (MPF) (a) during video display terminal (VDT) task (contribution = 0.290) and (b) under isometric contraction (0.760). 415

416 Murata et al. the shoulder and neck. Symptoms related to localized muscular fatigue cannot be evaluated properly and reliably when the EMG is recorded during a VDT task. The difference in correlation between Figure 1, a and b, also validates the MPF obtained under isometric contractions. It is rational to assume that the central nervous system acts as a compensator during the early stage of fatigue. When a muscle is stimulated repeatedly, its electrical activity increases even when its isometric contractions remain at the same level. Therefore, it is appropriate to assume that localized muscular fatigue is still at the stage where a decrease in the frequency of discharge of muscular control centers (i.e., MPF) can compensate for the state of fatigue. The validity and reliability of EMG RMS and MPF seem to vary among different studies. In particular, many studies have used EMG recording when working with a VDT (Aarås & Ro, 1997; Aarås et al., 1999). If a recording procedure is adopted in which no VDT task is performed to maintain an isometric contraction, we can use these measures to assess localized muscular fatigue induced during a VDT task. Under isometric contraction conditions, the accumulated fatigue in the shoulder muscle seems especially to decrease the discharge of muscular centers. These phenomena must reflect the compensation mechanism of muscular control centers. In conclusion, the experimental paradigm and its results are applicable to the evaluation of localized muscular fatigue. Future research is needed on localized muscular fatigue in muscles other than the trapezius. The correspondence between MPF or EMG RMS and psychological feeling, as well as the validity of this study, also must be investigated in more detail by controlling the physical workload (demand) of the VDT task. REFERENCES Aarås, A. (1994). Relationship between trapezius load and the incidence of musculoskeletal illness in neck and shoulder. International Journal of Industrial Ergonomics, 14, 341 348. Aarås, A., & Ro, O. (1997). Workload when using a mouse as an input device. International Journal of Human Computer Interaction, 9, 105 118. Aarås, A., Ro, O., & Thoresen, M. (1999). Can a more neutral position of the forearm when operating a computer mouse reduce the pain level for visual display unit operators? A prospective epidemiological intervention study. International Journal of Human Computer Interaction, 11, 79 94. Balliet, J. A., Dainoff, M. J., & Mark, L. S. (1996). The effects of degree of upper arm flexion on shoulder neck discomfort at the VDT. International Journal of Human Computer Interaction, 8, 385 399. Bendix, T., & Jessen, F. (1986). Wrist support during typing Acontrolled, electromyographic study. Applied Ergonomics, 17, 162 168. Conway, F. T. (1999). Psychological mood state, psychological aspects of work, and musculoskeletal discomfort in intensive video display terminal (VDT) work. International Journal of Human Computer Interaction, 11, 95 107. Cook, C. J., & Kothiyal, K. (1998). Influence of mouse position on muscular activity in the neck, shoulder and arm in computer users. Applied Ergonomics, 29, 439 443. Hagberg, M. (1981). Electromyographic signs of shoulder muscular fatigue in two elevated arm positions. American Journal of Physical Medicine, 60, 111 121.

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