Dynamic transitions in stance support accompanying leg flexion movements in man.

The control processes underlying dynamic transitions in stance support during single leg flexion movements were investigated in human subjects as a function of the intended speed of movement, by examining the vertical and lateral horizontal components of the ground reaction forces, the frontal plane trajectory of the body center of mass (CM) recorded via motion analysis, and the electromyographic (EMG) recordings of selected lower limb muscles. For the slowest movements, the measured vertical force beneath the flexing and single stance limbs closely matched the vertical force-time history predicted by a quasi-static mechanical model, whereas, the more rapid natural and fast speeds showed progressively larger discrepancies between measured and predicted forces. The initial resultant horizontal force component was exerted in the flexing to stance limb direction but was proportionately greater (4:1) beneath the flexing versus the stance limb during fast and natural speeds, and became equivalent for slow movements. Speed related EMG differences included an early phasic recruitment of the lateral hip muscle of the flexing limb which always preceded the ground reaction force changes for fast and natural but not slow movements, and a considerably earlier onset of the stance leg knee extensor relative to the flexing limb knee flexor for slow versus fast and natural speeds. Overall, the findings suggested two different speed related strategies for linking the postural and intentional movement components, where the choice of the strategy selected appeared to reflect the mechanical requirements needed to overcome the inertial force of the body mass during transitions from bipedal to single limb stance support.