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Re: Re: Is a recommendation for fewer hours sensible?

Mr. Darby raises some interesting points regarding the prevention of discomfort and pain among computer users. The main topic is whether the suggestion to limit computer time (IJmker et al., 2007) has validity in preventing hand-arm-wrist and neck-shoulder symptoms.

We agree with Mr Darby that reducing the duration of computer use without breaks may not be the solution for all workers. At present, there is uncertainty regarding the positive effects of breaks in preventing hand -arm-wrist and neck-shoulder symptoms, since high quality intervention studies are lacking and the results from the available evidence are inconsistent (Brewer et al., 2006; Bongers et al., 2006). In addition, a study by Veiersted (1994) among assembly line workers might fit into the idea that rest breaks might not be productive in preventing symptoms for some workers. He found that during forced machine stops future patients showed higher muscle tension compared to employees who remained healthy. In addition, Blangsted and co-workers (2003) showed that increasing the duration of rest breaks does not per se cause muscle relaxation. High quality studies are needed in this field.

Mr Darby also notes that reducing time alone may be “superficial�, given the supposed underlying mechanism (i.e. sustained muscle activation). Mr. Darby notes a number of factors that might relate to muscle tension during mouse use. We agree on these factors. However, in our opinion there are other factors to be considered as well, if we assume that sustained muscle activation plays a leading role in the development of symptoms. Factors to be added to the list might be cognitive demands (Waersted, Bjorklund and Westgaard, 1991) and psychosocial demands (i.e. time pressure, see Visser et al., 2004). Despite the indications from experimental lab studies, longitudinal field studies studying the additive and or multiplicative effect of these factors, in combination with the duration of mouse use per se, are lacking. Currently several longitudinal studies are in progress that use software to register computer use and associated factors. We hope that these studies will provide more insight into these issues.

Contrary to what Mr. Darby states, modern break reminder software already registers mouse use. This information is integrated with information on keyboard use into algorithms to provide feedback to the user to take a break from computer work. However this kind of software does not register all the factors listed by Mr Darby. For example, the gripforce while pointing the mouse is not registered.

Finally, we would like to note that the review included a limited number of studies, which where all based on self-reported duration of computer use. Future studies, especially those with objective recordings of computer use, might change our conclusions. Also, the pathophysiological link we made between mouse use, sustained muscle activation and symptoms, has been criticized (Knardahl, 2002).

References

IJmker S, Huysmans MA, Blatter BM, van der Beek AJ, van Mechelen W, Bongers PM. Should office workers spend fewer hours at their computer? A systematic review of the literature. Occup Environ Med. 2007;64(4):211-22.

Veiersted KB. Sustained muscle tension as a risk factor for trapezius myalgia. Int J Ind Ergon. 1994;14:333-339.

Blangsted AK, Sogaard K, Christensen H, Sjogaard G. The effect of physical and psychosocial loads on the trapezius muscle activity during computer keying tasks and rest periods. Eur J Appl Physiol. 2004;91(2- 3):253-8.

Brewer S, Van Eerd D, Amick BC 3rd, Irvin E, Daum KM, Gerr F, Moore JS, Cullen K, Rempel D. Workplace interventions to prevent musculoskeletal and visual symptoms and disorders among computer users: a systematic review. J Occup Rehabil. 2006;16(3):325-58.

Bongers PM, Ijmker S, van den Heuvel S, Blatter BM. Epidemiology of work related neck and upper limb problems: psychosocial and personal risk factors (part I) and effective interventions from a bio behavioural perspective (part II). J Occup Rehabil. 2006;16(3):279-302.

Waersted M, Bjorklund RA, Westgaard RH. Shoulder muscle tension induced by two VDU-based tasks of different complexity. Ergonomics. 1991;34(2):137-50.

Visser B, De Looze M, De Graaff M, Van Dieen J. Effects of precision demands and mental pressure on muscle activation and hand forces in computer mouse tasks. Ergonomics. 2004;47(2):202-17.

Knardahl S. Psychophysiological mechanisms of pain in computer work: the blood vessel-nociceptor interaction hypothesis. Work & Stress 2002;16(2):179-189.

Send letter to journal:
Re: Is a recommendation for fewer hours sensible?

The authors suggest that reducing the work time is an answer to the discomfort and pain that computer users often get.

If the discomfort and pain is due, however, to a combination of muscle tension and time, as comments in the section on biological plausibility seem to suggest, is this the only strategy possible?

Limiting the painful effects of computer use (clearly worthwhile) by time alone may be superficial and counterproductive. Many anecdotal reports from computer users indicate that having to limit the time at the keyboard, or being suddenly frozen out, can be frustrating - and even very frustrating.

Is there an alternative? A better solution could be to build a device into the mouse that integrates:

* the length of time for which the mouse is moved * the precision in the movements made * the distance over which it is moved * the acceleration of the mouse * the force with which it is gripped * the rate of clicking

Research would be needed to find out if tension in the arm muscles is reflected somehow in mouse movements and grip, and if a suitable weighting for each factor can be found. The display of feedback about performance would be an interesting challenge for a student of information design.

Some software is available presently that interrupts users by measuring the keystroke rate and the time spent keying and combining these in some way. The authors findings, indicating that mouse use is more associated with discomfort than keyboard use, suggests the device described could be more beneficial.

In the absence of such a device, does the suggestion to limit time have validity? As noted above, some approaches to achieving this can have disadvantages.

Frank Darby Technical Specialist Department of Labour Wellington New Zealand