Slip characteristics and contact area effects on postural dynamics during isometric pushes performed by seated subjects

The aim of this study is to determine, during horizontal isometric pushes carried out with upper limbs, (1) the effects of slip characteristics of support surfaces on the task performance and dynamics of the postural, i.e. body segments not directly involved in a task, here the trunk and lower limbs, and (2) whether these effects vary according to the seat contact area.Four experimental conditions were analyzed: two support surfaces, which were chosen according to their frictional properties and two seat contact areas. Reaction forces exerted at seat (RxS and RzS), footrest (RxF and RzF) and on a dynamometric bar (Fx) were measured. Adherence ratio and risk of slipping were calculated at the seat and at the footrest.External forces and postural dynamics were influenced by the slip characteristics of the support surfaces. Since focal chain, i.e. body segments involved directly in the task, here the upper limb, is not in direct contact with the support surfaces, it appears that the environmental information is transferred along the postural chain, which in turn modulates the external force. When the seat contact area is modified, the slip characteristics of the support surface do not have the same effects on the task performance and postural dynamics vary. Adherence ratio and reaction forces, measured on the seat platform, are bounded by the risk of perturbing body balance.These results confirm the view that a motor act results from the interaction between the focal segments, the postural chain and characteristics of support surface. The dynamics of the subject depend on postural stability even in sitting posture.Relevance to industryThis study demonstrates that the interface between the body and the support surface appears to be an important constraint for the performance of a task. In this way, seat surface characteristics should be taken into consideration for design of wheelchair, and in ergonomy, for seats and supports used by the workers.