Anticipatory postural adjustments depend on final equilibrium and task complexity in vertical high jump movements.

Anticipatory postural adjustments (APA) associated with high vertical jumping were studied in five normal men. The dependence of APA on final equilibrium and task complexity was studied by analyzing the characteristics of single versus repetitive jumps. Subjects performed series of 25 single jumps and 25 repetitive jumps on a force platform, in response to a Go signal. Surface EMG from the Soleus (SO), Tibialis anterior (TA), Vastus lateralis (VL) and Biceps femoris (BF) muscles were recorded simultaneously with the three components of the ground reaction forces. The existence of APA associated with both single and repetitive jumps was evidenced by a backward shift of the center of pressure, an SO deactivation, and a TA activation which preceded the onset of movement by hundreds of milliseconds. In repetitive jumps, SO deactivation occurred later than in single jump, whereas the center of pressure shift started earlier, lasted longer and had a greater amplitude. Moreover, repetitive jumps were initiated later with respect to the Go signal. These data show that the central nervous system anticipates not only the equilibrium disturbance caused by an initial movement but that caused by a whole sequence of movements. They also show that the corresponding APA are based heavily on a single functional ankle synergy.