LOCOMOTOR PATTERNS OF THE LEADING AND THE TRAILING LIMBS AS SOLID ANDFRAGILE OBSTACLES ARE STEPPED OVER - SOME INSIGHTS INTO THE ROLE OF VISION DURING LOCOMOTION

The issues explored in this article are the role of exproprioceptive i nput and the nature of exteroceptive input provided by the visual syst em in the control of limb elevation as obstacles are stepped over duri ng locomotion. In the first experiment, the differences in limb trajec tory of movements over solid and fragile obstacles of similar dimensio ns were examined. Subjects increased their toe clearance, vertical pos ition of the hip, and the hip vertical velocity when going over a frag ile obstacle with the leading limb. This suggests that in addition to visually observable properties of obstacles such as height or width, o ther properties, such as rigidity or fragility, which may be classifie d as visually inferred, also influence the limb trajectory. Part of th e first and the second experiment was focused on understanding differe nces in leading limb and trailing limb trajectory over obstacles. The toe clearance of he trailing limb was lower for smaller obstacles. The re was no consistent correlation between the toe clearance values of t he leading and trailing limbs. The variability in toe clearance was hi gh-rr for the trailing limb, which is attributable to lack of visual e xproprioceptive input about trailing limb movements and to the shorter time available following toe-off to fine-tune the trailing limb traje ctory. Because the body center of mass is moving toward the supporting foot when the trailing limb goes over obstacles and the trailing limb foot is moving up, the chances of a trip are minimized and recovery f rom an unexpected trip are more likely. These results highlight the ro le of exproprioceptive input provided by the visual system and possibl e cognitive influences on the limb trajectory as one travels over unev en terrains.