Control of the force exerted by the foot on the ground is critical to human locomotion. During running on a treadmill and pushing against a fixed pedal, humans increased foot force in a linear manner in sagittal plane force space. However, for pushes against a moving pedal, force output was linear for some participants but slightly curved for others. A primary difference between the static and dynamic pedaling studies was that the dynamic study required participants to push with varying peak effort levels, whereas a constant peak effort level was used for the fixed pedal pushes. The present study evaluated the possibility that force direction varied with level of effort. Seated humans pushed against a fixed pedal to a series of force magnitude targets. The force direction varied systematically with effort level consistent with the force path curvature observed for dynamic pedaling.
The authors are with the Department of Kinesiology at the University of Wisconsin-Madison, Madison, WI 53706-1189. Additionally, K.G. Gruben and L.M. Rogers are with the Department of Biomedical Engineering, and Gruben is with the Department of Mechanical Engineering—both at the University of Wisconsin-Madison.