Postural constraints modify the organization of grasping movements

Reaching and grasping movements have been studied mostly in seated contexts. There are several tasks, however, that also impose specific postural constraints that may affect the organization of these movements. For example, in rock climbing, a successful grasp is conditional of an accurate static and dynamic postural control. The purpose of our study was to analyze if postural constraints, imposed by rock climbing, would mask speed-accuracy tradeoff normally observed in pointing and grasping movements. More specifically, it was predicted that climbers, in order to reduce the duration of the presumably less stable tripodal phase, would shorten the duration of their reachings whatever the size of the hold. This time reduction would be incongruous with the longer end-phase control observed in seated conditions when precision requirements increase. Seven expert rock climbers performed grasping movements in two different postural conditions (easy and complex) towards holds of various depths (0.8, 2 and 5 cm). As expected, the hold depth (precision requirements) did not influence the organization of movements until contact with the hold. More specifically, movement duration and velocity-time profiles of the reachings were not affected by the accuracy requirements but were shortened by an increased postural difficulty. The duration of force adjustments once a contact with the hold was established increased with a decreased target size and were not affected by the postural complexity. The present findings suggest a hierarchical processing of postural constraints and precision constraints. (C) 1999 Elsevier Science B.V. All rights reserved.