Introduction

As the number of employees involved in computer work increases, neck and upper extremity complaints grow more common. Tight deadlines lead to high time pressure, which, in combination with precision demands, may increase the risk of musculoskeletal disorders. The physiological manifestation of such demands, however, is not entirely clear. In the present study, we examined local tissue oxygen saturation in the upper extremity as well as subjective stress responses during computer mouse work with and without time pressure and precision demands, and investigated whether these working situations have different effects on wrist position sense.

Material and methods

Twenty-four healthy, right-handed subjects (12 females, 12 males; age 19-28 years) participated in the study. Except for one subject who reported using a computer for 480 minutes per day, their average daily computer use was 84 minutes (SD 65 minutes). Subjects performed a 45-min mouse operated computer task on two occasions, separated by 3-5 days. The task consisted of painting squares that were presented on the screen. On one occasion, time pressure and precision demands were imposed on the task by limiting the time available for painting a square and introducing a scoring system based on precision of painting. On the other occasion, no such restraints were added. The order of the two task versions was randomized. During the task, tissue oxygen saturation in trapezius and extensor carpi radialis on the right arm was measured non-invasively by near-infrared spectroscopy, NIRS (Inspectra, Hutchinson Technology). In addition, subjective ratings of tenseness and strain, painting performance measures, and skin temperature were recorded. The position matching ability of the wrist was measured before and after the computer task. In the position matching tests, subjects attempted to actively reproduce target positions of horizontal movements about the right wrist joint. From a starting position of 30° of extension, target positions were randomized between 0° and 30° of flexion. The absolute value of the difference between presented target and reproduced position (AE) was used as outcome measure. Skin fold thickness at the locations of the NIRS electrodes was measured with a caliper at the beginning of the experiment.

Repeated measures analyses of variance were used to test for differences in oxygen saturation throughout the task, and for testing differences in position sense, as estimated by position matching ability, before and after the task. Subjective ratings during the task were analyzed by Wilcoxon’s non-parametric tests. Remaining parameters were tested for differences by t-tests or Wilcoxon’s non-parametric tests, depending on distribution of data. In all tests, p<0.05 was considered significant.

Results

A significant decline in oxygen saturation during mouse work under time pressure and precision demands was seen for extensor carpi radialis (F=4.68, p=0.036). This pattern was not present when mouse work was performed under more relaxed conditions. For trapezius, no difference in oxygen saturation between task versions was seen (F=0.01, p=0.932), although a general increase in saturation during work was found (F=10.35, p=0.002). Gender differences were apparent for extensor carpi radialis as well as for trapezius. Females showed an overall lower oxygen saturation in extensor carpi radialis than men (F=4.81, p=0.034). Furthermore, they showed a significantly different trend in trapezius oxygen saturation during work than men (F=6.27, p=0.016). Somewhat surprisingly, these gender differences could not be explained by differences in skin fold thickness at electrode positions (extensor carpi radialis: t=0.77, p=0.449; trapezius: t= 1.34, p=0.193). Subjects’ mean skin temperature changes during work was +0.41°C (SD 0.83°C).

AE measured before the computer task did not significantly differ between occasions (paired t-test: t=0.08, p=0.940), indicating that the time period between occasions was long enough for effects of work to wash out. A significant increase in AE was seen following the task (F=15.59, p<0.001), irrespective of task version (F<0.01, p=0.968). No gender differences in AE were found (F=0.08, p=0.774). Subjects’ ratings of tenseness and strain were significantly higher during work under time pressure and precision demands (Wilcoxon’s signed-ranks test: Z>3.41, p<0.001), compared to work without such demands. This is in agreement with the fact that subjects increased their work pace (squares painted in work with demands: 119, without demands: 84, paired t-test: t=8.38, p<0.001), and made an effort to paint as accurate and precise as possible (no. of times outside the square in work with demands: 2, without demands: 4, paired t-test: t=3.68, p=0.001) during the more demanding task version. No gender differences in subjective ratings (Mann-Whitney U-test: Z<2.14, ns) or performance variables (t-test: t<1.71, ns) were found.

Discussion

Subjects’ showed diminished oxygen saturation in extensor carpi radialis when working under time pressure and precision demands. This could be attributed to an increased mental load and/or a higher work intensity associated with this working situation. In support of this, subjective ratings of tenseness and strain scored higher during the more demanding task. It was also shown that subjects painted squares at a higher rate during the task. The fact that no differences in trapezius oxygen saturation between task versions was found might suggest that the physical strain associated with the work, being more prominent in extensor carpi radialis, could be a major contributor to oxygen saturation changes. Subjects’ tenseness ratings, however, would argue against it. Position sense, although poorer following work, was not affected by work type. One may speculate that physiological mechanisms involved in our measurement of position sense are not affected by local tissue oxygen saturation in extensor carpi radialis. The present data show considerable gender differences in oxygen saturation during rest as well as computer mouse work, that does not seem to be caused by skin fold thickness or painting performance.