Modulation of anticipatory postural adjustments associated with unloading perturbation: effect of characteristics of a motor action.

The purpose of the study was to determine whether characteristics of a motor action affect anticipatory postural adjustments (APAs). Standing subjects held a load between their hands with arms extended in front of their body. Next, subjects performed bilateral shoulder abduction movements (motor action) of three amplitudes at three instructed speeds. This motor action led to the release of the same load, inducing unloading perturbation in the sagittal plane. Electromyographic activities were recorded for the leg and trunk muscles. A change in the background muscle activity in these muscles was observed prior to the unloading perturbation and was quantified as APAs. APAs were dependant on instructed speed of the motor action; larger APA activities were observed in the leg and trunk muscles with a faster speed instruction. Meanwhile, the modulation of APAs was not observed by altering the movement amplitude. Moreover, experiments showed that motor action itself without a load release did not generate APA activity. Therefore, we concluded that the central nervous system selects information within a motor action (i.e., speed instruction) to approximate the magnitude of the forthcoming perturbation and modulate APAs, even when the unloading perturbation was unchanged.