Effects of linear movements on upright standing position

This study investigated the effects of antero-posterior movement on the upright standing posture in the humans. Subjects were asked to stand on a platform in a linear accelerator (sled) in the dark. At a fixed distance of sled movements in an antero-posterior direction, peak speed was changed by sinusoidal modes run at 0.02 G, 0.04 G, 0.06 G and 0.08 G. At lower speeds (0.02 G and 0.04 G) joints moved in parallel, but at higher speeds (0.06 G and 0.08 G) the hip flexed or extended in the direction of the sway. At lower speeds (0.02 G and 0.04 G) EMG in lower leg exhibited co-activated patterns in both forward and backward accelerations at lower speeds (0.02 G and 0.04 G), but at higher speeds (0.06 G and 0.08 G) TA was activated with reciprocal inhibition of GC to maintain the body in equilibrium in a reflexive manner. This reciprocal inhibition may be due to main reflex control elicited by motor functions as a result of the linear acceleration. The threshold of speed was assumed to be from 0.04 G to 0.06 G for all but one subject.