We appreciate the careful reading of our editorial [1] by Drs. Mikkelsen and Andersen. We regret our omission of the one published NUDATA study available at the time our editorial was submitted [2]. That study reported significant associations between mouse usage time collected with memory resident software and both, acute neck pain and acute shoulder pain, among 2146 technical assistants. However, because i) median mouse usage time was 5.2 hours/week and median keyboard usage time was 0.9 hours/week, and ii) rates of moderate or greater musculoskeletal pain were very low among the participating computer users, we are concerned about the generalizability of the observed associations to workers with greater mouse and keyboard use.

Regarding differences in associations with musculoskeletal disorders (MSDs) observed across studies using self-reported estimates of computer use versus memory resident software documentation of computer use, we made no argument that one was correct and the other was incorrect. Rather, we raised the concern that these two exposure assessment methods capture different (but not totally unrelated) aspects of computer use relevant to MSD risk. The absence of perfect correlation between self-reported estimates of computer use and memory resident software documentation of computer use may be due to error in self report, differences in the kind of exposure information captured, or both. The claims of methodological objectivity and validity presented by Mikkelsen and Andersen do not address this fundamental question. We continue to believe, as noted in our editorial, that a better understanding of the attributes of work captured by self report and by computer registration software will clarify what appear to be inconsistent results reported by studies using them.

1. Gerr F, Fethke N. Ascertaining computer use in studies of musculoskeletal outcomes among computer workers: differences between self- report and computer registration software. Occup Environ Med 2011; 68: 465 -66.

2. Andersen JH, Harhoff M, Grimstrup S, et al. Computer mouse use predicts acute pain but not prolonged or chronic pain in the neck and shoulder. Occup Environ Med 2008;65 :126-31.

Conflict of Interest:

None declared

In a recent editorial Gerr et al.[1] discuss computer work and musculoskeletal outcomes based on self-reported exposure versus objective recordings using computer software. They state that only one small study (n=27) using objective recordings was published before a large study by Ijmker et al.[2], published in the same issue as the editorial. They failed to consider the results of two NUDATA papers based on more than 2000 study participants, one of them published in the OEM[3]. The results of the study of Ijmker et al. and the NUDATA studies consistently indicate that sustained or severe pain outcomes were not related to objective computer work recordings. The editorial argues that the results of the study of Ijmker et al does not invalidate the much larger literature in which self-reported computer use was associated with musculoskeletal symptoms. The main argument seems to be that objective recordings do not capture the relevant exposures, e.g. holding the hands over the keyboard without keying and that different cut points for such non-activity periods may invalidate the objective recordings. However, objective software-based computer work recordings are in very good accordance with other objective measures like video- recordings, and much better than self-reported exposure. Furthermore, within reasonable limits, different cut-off values for non-activity periods do not change these relations or computer times very much. This is consistent evidence from several studies and not from "preliminary investigations", as stated in the editorial. Finally, exposure times based on different cut off's are highly correlated, and their relation to musculoskeletal outcomes will not vary much with different cut-offs [4]. Contrary to the editorial, we find it very unlikely that retrospective self-reports about computer use during several months should capture biologically important aspects of computer work which are not captured by a validated objective method, which prospectively collects exact computer use data on a daily basis.

References

1. Gerr F, Fethke N. Ascertaining computer use in studies of musculoskeletal outcomes among computer workers: differences between self- report and computer registration software. Occup Environ Med 2011; 68: 465 -66

2. IJmker S, Huysmans MA, van der Beek AJ, et al. Software-recorded and self-reported duration of computer use in relation to the onset of severe arm-wrist-hand pain and neck-shoulder pain. Occup Environ Med 2011; 68: 502-9

3. Andersen JH, Harhoff M, Grimstrup S, et al. Computer mouse use predicts acute pain but not prolonged or chronic pain in the neck and shoulder. Occup Environ Med 2008;65 :126-31.

4. Mikkelsen S, Lassen CF, Vilstrup I, et al. Does computer use affect the incidence of distal arm pain? A one-year prospective study using objective measures of computer use. Int Arch Occup Environ Health 2011 May 24 [Epub ahead of print]

Conflict of Interest:

None declared