Abstract:  Body posture can be controlled by the central nervous system (CNS) in three ways: the feed-forward control (FFC), the feed-back control (FBC), and the voluntary control (VC). These neural control strategies not only enable rapid postural muscle responses but also stabilize the center of body gravity. While many studies indicated that visual anticipation (VA) and focus of attention (FOA) can affect the commands from the CNS, the effect of VA/FOA on posture control is not clear. The present study employed the classic ball hitting test (BHT) to simultaneously examine the effect of the VA/FOA-caused pre-activations in the lumbar postural muscle and upper motor muscle. 24 college students (10 males, 14 females) participated in the present study. The BHTs were conducted using a 2 by 2 within-subject design. That is, the participants either anticipated (with eyes open) or not anticipated (with eyes closed) the hitting ball, and they either focused on stabilizing the center of body mass (COM) or focused on the holding pan. A ball of 1.5 kg was used as a load to trigger a perturbation. To study the effects of external perturbation on APAs and CPAs, homemade ball-hitting platform and surface EMG equipment were used to study the response latency and response intensity of the target muscles. Electric activities from the lumbar multifidus muscle (LM) and from the biceps brachiimuscle (BB) were collected. The response latency, response intensity of the APAs and CPAs of the target muscles were used to evaluate the effects on the postural and focal muscles. The results showed (1) VA significantly affected the APAs’ latency of the LM, and also affected the APAs’ latency, APAs’ AEMG, and CPAs’ AEMG of the BB; (2) FOA significantly affected the CPAs’ latency of the LM, and the APAs’ latency of the BB; (3) Interactions between VA/FOA and APAs’ latency and APAs’ AEMG of BB were significant. These results showed that CNS may use different postural strategies to control postural and motor muscles. That is, VA and FOA may only affect temporal characteristics of the postural muscle, but they can affect both temporal and intensity parameters of the motor muscle. The CNS postural-control system is controlled by both the postural and motor muscles under external perturbation. These findings shed light on the mechanism of the CNS posture-control system.

Key words:  visual anticipation, focus of attention, anticipatory postural control, compensatory postural control, external postural perturbation