Anticipatory postural adjustments are associated with single vertical jumpand their timing is predictive of jump amplitude

This study was carried out to determine the existence and the specificity o f anticipatory postural adjustments (APA) associated with vertical jump. Si ngle vertical jump is an upward-oriented movement with identical initial an d final postures. A consistent backward shift of the center of pressure (CP ) clearly demonstrated the existence of APA. This shift mainly resulted fro m a soleus (SO) deactivation and tibialis anterior (TA) activation. As in g ait initiation, the main role of APA associated with vertical jump is to cr eate the necessary disequilibrium to initiate the movement. A comparison of CP and CG (center of gravity) forward shifts between single jump and tip-t oe rising allowed us to propose that forward disequilibrium ends when a cri tical CP position is reached. As in gait initiation, the duration of jump's APA was several hundreds of milliseconds. This showed that when the role o f the APA is to create the initial disequilibrium, their duration is greate r than when their role is to compensate for the forthcoming postural distur bance due to the movement. Since the goal of the voluntary movement was to jump at a given height, one expected that this parameter could be programme d. APA's timing depended on the amplitude of the vertical jump: the time of onset of SO deactivation and TA activation and that of backward CP shift w ere dependent on the peak amplitude of the jump. Nevertheless, there was no relationship between the backward CP shift amplitude and the vertical impu lse. Thus, it appears that the amplitude of the jump is not fully programme d but must be adjusted after the onset of the movement, i.e., there is a se quential programming of forward and upward movement. It is also of interest to stress that the SO-TA synergy, which is shown here to be at the basis o f the APA associated with single vertical jump, has been also found in many forward-oriented movements. Thus, the present data reinforce the idea that SO-TA synergy is part of a functional synergy used by the central nervous system to simplify the postural motor command.