High postural constraints affect the organization of reaching and grasping movements.

In the vast majority of prehension experiments, subjects are requested to grasp a static object and to transport it with respect to the body. There are grasping tasks, however, that have different grasping requirements. For example, in rock climbing, the goal is to grasp a hold to transport the body about the grasped object. Unlike simple reaching and grasping movements, the successful hand grip is not the goal of the task per se, and it is the postural stability that primarily determines the success or failure of the task. In the present experiment, we wanted to examine the influence of such postural constraints on the organization of reaching movements. Four expert climbers participated in the study. A climbing simulator, equipped with strain gauges, measured the vertical forces applied by the hands. The subjects performed grasping movements (one-reach and two-reach conditions) towards holds of various depths (easy and difficult holds) in two different postures (standing and climbing conditions). The results showed that the duration of the right-hand transport phase was similar whatever the condition. Unexpectedly, in the climbing condition, the duration of the left-hand transport phase was shorter when the subjects grasped a difficult than when they grasped an easy hold. This result suggests that, in extreme cases such as rock climbing, the postural constraints may prevail over accuracy constraints, since increasing accuracy requirements did not yield an increased duration of the transport phase.