Passive and active lower-limb movements delay upper-limb balance reactions

This study investigated the influence of rhythmic lower-limb activity on the timing of upper-limb balance reactions. Compensatory grasping reactions were evoked in healthy subjects by rapid sagittal tilts of a chair under three conditions: (1) active leg pedaling, (2) passive (motor-driven) leg pedaling, and (3) no lower-limb movement (control task). Compared with control trials, both active and passive pedaling resulted in similar delays in the initiation (43-47 ms) and execution (12-17 ms) of grasping reactions. The similarity between effects due to active and passive movement suggests that the conditioning arose predominantly from sensory discharge associated with lower-limb movement. These results may have important implications for understanding the influence of locomotion or other ongoing movement on the control of stability.