Characteristics of the force applied to a pedal during human pushing efforts: emergent linearity

The force seated humans exert on a translationally fixed pedal (foot force) may be directed at any angle because the fixed distance between the seat and the pedal axis kinematically constrains the lower limb. The authors' objective in the present work was to characterize such force. Participants (N = 7) generated force with their lower limb by pushing against the pedal in the most comfortable manner. Pushing efforts were repeated randomly 3 times at each of 97 sagittal-plane pedal axis positions and 10 additional times in 9 of those positions (2,895 total pushes). In 87% of the pushes, the measured sagittal-plane force exerted on the pedal by the foot changed magnitude and direction through time, such that the path of the head of the force vector traced a straight line. The linearity of the foot force paths reflected directional invariance in the changes of the foot force vector as the magnitude of the vector increased. The orientation of those linear force paths varied with limb posture in a similar manner across participants. The authors conclude that the emergent linearity of the force path is consistent with minimization of path length in foot force space. Alternatively, the linearity of the force paths suggests a motor control strategy that simplifies the control to a monoparametric form.