ON THE EFFECTIVENESS OF FORCE APPLICATION IN GUIDED LEG MOVEMENTS

In guided leg movements (e.g., in cycling or wheelchair propulsion), t he kinematics of a limb are determined by the object on which a force is applied. As a consequence, the force direction can vary and may dev iate from the movement direction, that is, the effective direction. In the present study, the relation of effective force application and ma ximal power output was examined. Subjects (n = 5) performed guided leg tasks on a special dynamometer. They were instructed to exert a maxim al force against a moving forceplate in the direction of the movement, as if they were pushing the plate away. Three different movement dire ctions were tested: perpendicular to the horizontal, rotated 30 degree s backward, and rotated 30 degrees forward. For each trial, force and position data were recorded. The results of the experiments showed tha t in the extreme movement directions (both 300 conditions), the force vector deviated significantly from the direction of the movement. Appa rently, maximal power output was achieved with a low force effectivene ss in these tasks. The background of this phenomenon was revealed by u sing the kinematics of one of these tasks in a simulation model. The s timulation level of 6 leg muscles was optimized toward a maximal effec tive force component (a) without a constraint on the direction of the total force or (b) with a constraint on the force component perpendicu lar to the effective force. The muscle stimulation pattern that result ed in the highest effective force coincided with a low force effective ness. Apparently, this is a prerequisite for maximal power transfer fr om the muscles to the plate in these guided movements.