This paper evaluates task complexity as a task-related factor causing the development of psychologically mediated ('psychogenic') shoulder muscle tension. Eighteen subjects performed an experimental work session, responding to simple and complex reaction time tasks which were presented on a VDU screen. Most subjects generated low-level static muscle tension during the tests. On a group level the two tasks did not have a differential effect on muscle tension. However, a subgroup of eight subjects which consistently generated higher muscle tension in the complex tests, was identified. It is argued that for these subjects the difference in muscle tension is due to an increased mental effort invested, because of the higher computational demands in the complex task.