Anticipatory postural adjustments are associated with single vertical jump and their timing is predictive of jump...

This study was carried out to determine the existence and the specificity of anticipatory postural adjustments (APA) associated with vertical jump. Single vertical jump is an upward-oriented movement with identical initial and final postures. A consistent backward shift of the center of pressure (CP) clearly demonstrated the existence of APA. This shift mainly resulted from a soleus (SO) deactivation and tibialis anterior (TA) activation. As in gait initiation, the main role of APA associated with vertical jump is to create the necessary disequilibrium to initiate the movement. A comparison of CP and CG (center of gravity) forward shifts between single jump and tip-toe rising allowed us to propose that forward disequilibrium ends when a critical 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 of the APA is to create the initial disequilibrium, their duration is greater than when their role is to compensate for the forthcoming postural disturbance 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 programmed. 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 were dependent on the peak amplitude of the jump. Nevertheless, there was no relationship between the backward CP shift amplitude and the vertical impulse. Thus, it appears that the amplitude of the jump is not fully programmed but must be adjusted after the onset of the movement, i.e., there is a sequential 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 of 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.