Anticipatory postural adjustments associated with arm movements during balancing on unstable support surface.

This study aims to investigate the pattern of anticipatory postural adjustments associated with arm movements during balancing on unstable support surface as a mechanism of coordination between movement and posture in this specific motor task. The experiments were arranged in two series: (1) the subjects stood on a force platform, and (2) on unstable platform (see-saw). They performed self-paced arm elevations as fast as possible for each support condition. EMG activity of deltoideus anterior (DA), soleus (SOL), tibialis anterior (TA), biceps femoris (BF) and erector spinae (ES) muscles were recorded. The mean EMG of SOL and TA muscles during balancing and during the last 100 ms prior to DA EMG onset were measured. For both series the anticipatory postural adjustment of BF preceded the ES activation. During balancing on see-saw the following differences from the stable condition were found: (1) co-contraction of SOL and TA muscles; (2) significantly earlier recruitment of BF; (3) delayed related activity of ES; (4) decrease of SOL and increase of TA mean EMG activities during balancing and just prior to the onset of DA EMG activity. These data showed that posture control during balancing on unstable support surface consists mainly in adapting the motor program according to the postural requirements rather than in changes of the postural strategy.