The control of foot force during pushing efforts against a moving pedal.

The muscle component of the force applied to a bicycle pedal (foot force) by seated humans provided insight into the organization of the motor system. Healthy adults ( n=11) pedaled a stationary cycle ergometer while attempting to match peak foot force magnitude to visually presented force targets (200, 250,., 650 N). Pedaling cadence was maintained at 60 rpm by a motor. Measurements of the foot force, pedal angle, and crank angle were recorded. The experimental design and data analysis allowed the isolation of the muscle component of the foot force from the contributions due to gravity and inertia. A graphical representation of the muscle component of the foot force (force path) was created for each of several crank angles throughout the extension phase of the pedaling cycle. The force paths showed several highly conserved characteristics across participants and crank angles. Each force path occupied a narrow range in force space despite the ability of the participants to produce force in a wide region of force space. Three control strategies were observed in the geometry of the force paths. Eighty five percent of the force paths were linear for six of the participants, and 79% of the force paths had second-order curvature for the other five participants. The curvature was concave to the posterior for four of the participants and concave to the anterior for one participant. The linear force paths were consistent with the previously reported linear nature of the force paths for pushes against a quasi-static pedal. The observation of simple force path geometry for two tasks with dissimilar dynamic characteristics suggests that this aspect of foot force control may be common to a range of lower limb tasks and may reflect a mechanism by which the nervous system organizes the control of foot force.