MSDs within a multifactorial approach

MSDs are disorders of the musculoskeletal system in general, for which the professional activity can play a role in the arisen, the preservation, even the worsening. These disorders can result in pain, incapacity or long-lasting handicap. MSD’s approach requires looking at the professional risk factors, among which the biomechanical factors (efforts, repetitive movements, uncomfortable postures, prolonged static work), the risk factors concerning vibrations, exposure to cold and warm environment and stress. Some of these risk factors can also be present in the extraprofessional environment. The factor multiplicity makes the evaluation and treatment very complex. INRS intervened about the tram at the request of the CHSCT (committee for health and safety at work) of a transport company within the framework of an evaluation of the biomechanical risk factors. An ergonomics office analysed the other risk factors. We noticed that the drivers’ work was very static and intensified by the use of the “dead man” system, which requires pushing buttons frequently on both sides of the dashboard and thus a constant positioning of hands nearby immediate of the “dead man” buttons. The postures sometimes generate extreme angles for the wrist, in hyperextension with a support of the heel of the hand on the dashboard. The repetitiveness of the movements of flexion-extension of wrists, measured by means of electro-goniometers, is situated between 0 and 5 movements per minute. On the other hand, the fingers support on the “dead man” buttons is very frequent, much more than recommended in the specifications, with a support on average every 1,2 seconds. The frequency of support was variable during the ride: at the approach of crossroads or platforms, the tendency is to strongly increase the frequency of support, what was interpreted as the need to escape on the attentionnal level in this requirement of answer to “dead man” system, as a automatism to concentrate better its attention on the immediate environment and guarantee traffic safety. The visual attention is very strong throughout the route, and we know that attentional demand is a risk factor of MSDs, notably because it favors muscular contraction, in particular at the level of the neck and the shoulders. Our study showed that numerous aspects, others than the “dead man” system raised problem and have to be taken into account within the framework of the current reflection on the re-conception of the drivers cabs of the trams.