Introduction

Work-related Musculoskeletal Disorders (WMSD) of the neck and upper limb continue to be of interest to individuals, organisations and researchers. This is due to the significant disability, time lost from work, increasing worker's compensation and increasing number of cases coming before the courts that can be associated with them (Ong, 1992; Barker, 1995; Stock et al, 1996). Terminology differs around the world. WMSD is the most currently accepted term and is used commonly in Europe and increasingly around the world. WMSD is also referred to as Repetitive Strain Injury (RSI) in New Zealand, Australia and the United Kingdom; Cumulative Trauma Disorders (CTD) in North America; Occupational Cervicobrachial Disorders (OCD) in Scandinavia and Japan, Repetitive Motion Trauma (RMT) in North America and Occupational Overuse Syndrome (OOS) in New Zealand. A WMSD may be characterised by diffuse upper limb pain that may limit function and can be associated with tenderness, stiffness, fatigue and sensory disturbance (Pheasant, 1991; Kuorinka, 1992; Ong, 1992; Mackinnon and Novak, 1994; Greening and Lynn, 1998). Visual Display Units (VDU) are now common place in offices, industry and at home and as a result WMSD among VDU users has become a widespread problem (Hagberg and Sundelin, 1986; Ong, 1992; Hales et al, 1994). Most of the literature recognises this problem as being multifactorial and recommends prevention through ergonomic improvement of work stations, physical activation of workers and improvement in work organisation (Kuorinka, 1992; Stock et al, 1996), for example, job redesign and breaks. This paper is concerned with rest breaks and is a review of the literature, both past and present regarding rest breaks and pauses during VDU use.

Causation

The causes of VDU-related WMSD continue to be debated (Kuorinka, 1992; Hales et al, 1994). Studies have indicated that VDU operators have more health complaints than conventional office workers do (WHO, 1986; cited in Ong, 1992). Diffuse muscle tension unrelated to postural demands was showed to be present in subjects performing VDU tasks involving mental load and sustained visual attention by Westgaard and Bjørklund (1987; cited in Pheasant, 1991). Hales et al (1994) found that WMSD in VDU users in a telecommunications company were associated with non-white race, thyroid conditions, use of bifocals, and psychosocial work factors including increasing work pressure, surges in work load and routine work lacking decision-making opportunities. A large longitudinal study in Sweden (Bergqvist et al, 1995a; Bergqvist et al, 1995b) identified some groups of VDU users with increased risk of WMSD. The results showed the individual factors that were important in relation to musculoskeletal problems were age, women with children at home, and stomach related stress reactions. Ergonomic factors that showed the most association with neck and shoulder problems were only sitting or only standing, high keyboards and high VDUs. Arm and hand problems were associated with hand and keyboard position and non-use of lower arm support. Organisational factors showed that limited or extensive peer contacts were associated with WMSD and limited rest break opportunity appeared to be a major factor for several muscular problems. Time pressures at work and the work-rest schedules were also shown to be associated with WMSD in a Brazilian study of a group involved in telephone-computer tasks (Ferreira et al, 1997).

Muscular Endurance, Fatigue and Recovery

It is generally assumed that rest breaks are advocated during continuous VDU work in an attempt to reduce fatigue and enhance endurance. Early studies regarding fatigue and recovery support this assumption. Chaffin (1973) described a neuromuscular theory for fatigue mechanisms: a sustained contraction of a muscle can result in the fibres losing their tension producing capacity, additional motor units are then needed to maintain the total tension state of the muscle. An increased number of active motor units is achieved by increased tension in the muscle spindle sensory systems, thus facilitating central nervous system commands to the motor units that produce movement and tension. He hypothesised that the metabolites produced through muscle contraction are unable to be removed through normal capillary exchange because of the reduction in blood flow due to the muscle contraction. The accumulation of these metabolites stimulates nociceptor endings in the muscle and is interpreted as muscle pain. This is the theory that has now developed into the 'muscle tension theory' or 'pain-spasm cycle' associated with diffuse WMSD (Pheasant, 1991). It has been demonstrated with electromyography (EMG) studies that people with a one year or more history of diffuse shoulder-neck pain have shorter muscle endurance times with faster fatigue on their more painful side (Hagberg and Kvarnström, 1984).

Rohmert's (1973a) studies on fatigue and recovery found that no reduction in maximum strength occurs if the holding force is limited to 15% of Maximum Voluntary Contraction (MVC). This assumption was, however, based on an investigation period of only 10-25 minutes (Hagberg, 1981; Pheasant, 1991). Other studies have indicated that when the holding time is more than an hour, the endurance limit may only be 8% of MVC ( Jonsson, 1988). Sjøgaard et al (1986; cited in Jonsson, 1988) showed that muscular fatigue appears at 5% MVC sustained for one hour and others have suggested that forces greater than 10% MVC cannot be sustained for greater than 10 to 15 minutes without perception of fatigue (Swanson et al, 1989). Many work situations are characterised by static loads that are around 5% MVC (Jonsson, 1988) and sustained forces reaching between 6% and 20% of MVC have been reported in keyboard tasks (Swanson et al, 1989).

Hagberg (1981) showed that intermittent static contractions (2 seconds work, 2 seconds rest) meant that the MVC that could be maintained for one hour was three times greater than for a continuous static contraction. It was suggested that the rest periods even as short as 2 seconds facilitated blood flow removal of contraction-inhibiting metabolites and thus enhanced endurance. Byström et al (1991) studied the physiological response to a continuous and intermittent hand grip exercise to exhaustion at 25% MVC. The intermittent condition consisted of a 10-second pause every 3 minutes. They found that the maximal endurance time was 43% longer in the condition with pauses than the continuous exercise condition. The subjects also reported less fatigue with the intermittent exercise. Subjects in the pause group also returned more rapidly to their normal MVC. Blood flow was significantly higher during the rest pauses than during the contractions. However, the results did show that there were greater downward shifts in electromyography (EMG) frequency after intermittent exercise, indicating more motor units were being recruited to maintain the same level of tension. This change lasted 24 hours compared with 4 hours in the continuous exercise group. There was also a greater loss of potassium during exercise in the pause group; this can be explained by the longer exercise time. There was no physiological evidence to explain the differences in endurance times and the authors suggest that the longer times in the pauses group could be due to differences at a sensory level and also due to psychological factors. So, pauses could give an immediate sense of relief and therefore postpone the subjective threshold of fatigue. Due to this, they warned against using pauses to prolong muscle work exposure time and therefore risk musculoskeletal disorders. The study, however, was conducted at 25% MVC and to exhaustion so extrapolation of this risk into the VDU workplace is tenuous since the static load of keyboard tasks has been reported as between 6% and 20% MVC (Swanson et al, 1989). Also, exhaustion was defined as when the subject could no longer generate the target force, which is unrealistic for workplace VDU tasks.

Rohmert (1973a) also asserted that the longer the tiring, static work lasts the greater the reduction in available maximum strength and that recovery time is a function of the degree of fatigue. Compensating for the same decrease in maximal strength takes the same amount of recovery regardless of time held or percentage of maximum force used. For example, a 10% decrease in maximal strength takes 0.72min for recovery regardless of whether that decrease was due to a maximum holding force of 0.2 held for 1.55 minutes or a maximum holding force of 0.4 held for 0.45 minutes (Rohmert, 1973a). The duration of a resting period necessary to remove any remainder of fatigue in static muscle work was found to be dependent on force and duration of muscle contraction, in the manner of an exponential function linked by a multiplication formula. For example, doubling the hold time from 25% to 50% of maximum time (exhaustion time) requires the rest allowance be increased from 150% to 400% (Rohmert, 1973a). This may be an indication for short, frequent rest breaks.

Rest breaks and pauses

The question of rest breaks and the determination of their duration and scheduling has long been a focus of ergonomic studies in industry and more recently in the VDU workplace. Rohmert (1973b) suggested that with the exponential increase of fatigue during work the rule of thumb for rest breaks is 'little and often'. This ensures short working periods with a small average degree of fatigue as well as the frequent experience of a high rate of recovery at the beginning of a break (Rohmert, 1973b). Dul et al (1991) developed a model to find the optimum work-rest schedule for static work. The model predicted that for a given total work time and total rest time, many short work-rest periods are better than fewer long work-rest cycles and this model showed relatively good agreement with actual measurements. Kogi (1982) recommended that to decrease fatigue with repetitive work there must be brief intra-work pauses where the muscles are rested from static load and there must be a break after a period of continuous work.

Studies have been conducted to examine the optimal time period for breaks and the effects on productivity and musculoskeletal discomfort symptoms. Zwahlen et al (1984) conducted an experiment with VDU operators working on a data entry task and a task correcting errors on the screen. A 15-minute rest break was given in the morning and afternoon and there was a 45-minute lunch break. At the beginning and end of each work period a discomfort rating was completed on screen by the typists. The results showed that the rest breaks were beneficial in reducing musculoskeletal discomfort. The scores for visual discomfort were not as high as musculoskeletal scores and did not decrease as much with breaks. A considerable cumulative effect was seen over the course of the day for both musculoskeletal and visual discomfort. Keystroke rate was slightly higher after breaks. The authors suggested that 2 rest breaks in addition to lunch appeared not to be sufficient to adequately reduce musculoskeletal and visual stress in continuous VDU work, this is supported by Kogi (1982). Similarly, Sauter and Swanson (1991; cited in Swanson and Sauter, 1992) showed that though frequent rest breaks increased productivity and decreased psychological and musculoskeletal stress there was still increasing discomfort and declining productivity over the course of the day.

Koparadekar and Mital (1994) simulated a telephone directory assistant's data entry task to determine the preferable work-rest schedule. The schedules examined were :- 30 minutes followed by a 5 minute break; 60 minutes followed by a 10 minute break; 120 minutes without any break. Results showed that performance did deteriorate when the work duration was increased from 30 minute to 60 minutes with 11% more errors although this was not deemed statistically significant. If no rest is then provided performance deterioration became large with errors increasing by almost 80% during 120 minutes of continuous work. Similarly, the subjective responses did not change significantly between 30 and 60 minutes but did deteriorate significantly after 120 minutes without a break. The deterioration in performance was halted after a rest break.

Henning et al (1997) aimed to determine if frequent, short rest breaks had a positive influence on worker productivity and well-being during VDU work processing insurance claims at two sites. At the larger site three groups were studied: (i) neither breaks nor exercises (ii) breaks only and (iii) breaks and exercise. The smaller site was evaluated with a within-subjects design; that is, the same subjects were compared over different types of conditions. 3 weeks of no breaks or exercise was followed by 3 weeks of breaks only and ended with 3 weeks of breaks and exercises. The breaks were at 15-minute intervals for 30 seconds for the first three breaks and 3 minutes for the last break in each hour, in addition to regular breaks. During the breaks only condition, workers were instructed to remove their hands from the keyboard and relax back in their chair during the breaks. During the 3-minute breaks operators performed other non-VDU work. In the breaks and exercise condition the workers were instructed to perform one stretching exercise for 15 seconds and at least two during the three minute breaks. The operators were asked to complete a mood survey and a musculoskeletal discomfort survey 3 times per day. Results showed that there was no significant overall treatment effect for any measures at the larger site. There were, however, problems with compliance due to an incentive pay scheme and lack of break integration into the tasks. At the smaller site the results showed statistically significant improvement in eyes, legs and foot comfort and improved productivity with the breaks and exercise condition.

EMG recordings and discomfort rating were correlated with spontaneous pauses and compared with introduced pauses during VDU work by Hagberg and Sundelin (1986). Several regimes were studied: (i) Five hours continuous work with coffee breaks and lunch (ii) 3 hours continuous work with coffee breaks and (iii) 3 hours of work with introduced pauses every 6 minutes for 15 seconds. Discomfort ratings increased in all 3 regimes but were less when short pauses were introduced. EMG recordings showed no significant difference. Spontaneous short pauses were fewer when operators were reminded by music to pause. There was a negative correlation between the number of spontaneous short pauses taken by the operators and the static median loads on the upper trapezius. This means the more short pauses taken the less static load on the upper trapezius.

Discomfort ratings tended to be higher with passive pauses and operators tended to find pauses with activity more relaxing than passive pauses in a study comparing an active pause, a passive pause and a diverting pause in 3 30-minute work periods of VDU work (Sundelin and Hagberg, 1989). Each pause lasted 15-20 seconds and operators were reminded of it every 6 minutes with soft music. Results showed the static muscular load was low and did not change with different kinds of pauses. Swanson and Sauter (1992) studied the effect of frequent rest breaks with physical exercises in comfort and productivity in a repetitive VDU task. Breaks were taken for: (i) 30 seconds after 10 minutes work (ii) 3 minutes after 50 minutes work and (iii) 10 minutes after 100 minutes work, with a 45-minute lunchbreak. One group sat passively and the other did simple exercises. There was no difference between the 2 break groups in discomfort or mood state. However, the decrease in keystroke rate across the workday was less pronounced in the exercise group than in the passive group. The authors suggested that this might indicate that exercise breaks stabilise performance over time. In order to do this they need to be effectively integrated into the work schedule.

Janaro and Bechtold (1985) found that work output and rest breaks were both seen to increase with a set rest break policy compared to when subjects controlled their own rest-work schedule. This was mainly due to the fact that the set policy had 'front loaded' rest breaks so there were shorter work periods initially and this helped to decrease the accumulation of fatigue. This meant that output was greater and the workers enjoyed more rest breaks. Similarly, Henning et al (1989) found that subjects showed decreased output and increased errors when subjects regulated the length of their own pauses after twenty minutes work. They found that the pause length was predictive of correction rate and heart rate, that is, a long break resulted in a lower correction rate (better productivity) and a lower heart rate and vice versa. It was suggested that a single pause may not provide sufficient time for adequate recovery and that the discretionary pause may not be effective in controlling fatigue as the worker terminates it before recovering. Following on from this Henning et al (1996) found that VDU workers were able to manage pauses, at least 30 seconds every 10 minutes, more effectively with on-screen feedback and correction rates were lower than with a forced computer administered pause. A second experiment with onscreen feedback given to half the subjects to ensure pauses for at least 30 seconds over 8 minutes in a more demanding mental task resulted in longer and more frequent pauses than subjects without the feedback. Less back discomfort was also reported.

Conclusion

It seems that there is evidence to support frequent pauses but a continuing problem with introducing these pauses is the disruption from tasks resulting in loss of physiological and psychological adaptation to work (Rohmert, 1973b; Henning et al, 1989; Sundelin and Hagberg, 1989; Henning et al, 1997). However, VDU operators without on-screen pause feedback reported more task interruption than operators with on-screen pause feedback in the Henning et al studies (1996). This may indicate that one way to improve task integration is to provide on-screen feedback.

WMSD continue to be an problem for individuals, organisations and researchers in terms of disability, productivity losses, terminology, causation factors and prevention. Despite the ongoing debate the current literature concerned with causation, muscular endurance, fatigue, recovery, rest breaks and pauses generally suggest that VDU work productivity and comfort is enhanced by frequent short rest breaks if they can be integrated well into the task.

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The Author, Nicola Green