Influence of event anticipation on postural actions accompanying voluntary movement.

The central organization of anticipatory postural adjustments was investigated by examining the influence of preparatory set on the temporal relationship between postural and arm (focal) muscle activation. Surface EMG was recorded from the right tibialis anterior, lateral gastrocnemius, anterior deltoid and posterior deltoid muscles when pushing or pulling on a stiff handle. Preparatory set was manipulated by informing the subject of the upcoming direction of responding with a 80, 50 or 20% certainty. This created high, neutral and low levels of preparatory set, respectively. All six subjects showed activation of postural muscles in advance of focal muscles for both push and pull responses. However, only three subjects showed the expected effect of preparatory set on reaction time performance, i.e., an increase of reaction time with decreasing response probability. For these three subjects, the time between the activation of postural and focal muscles was the same for the high and neutral levels of preparatory set, but increased with a low level of preparatory set. The increased postural-focal latency for the low preparatory set condition was due to a longer delay for the activation of the focal muscles but not the postural muscles. This finding suggests that anticipatory postural adjustments and the activation of focal muscles are triggered by separate motor commands.