Anticipatory control related to the upward propulsive force during the rising on tiptoe from an upright standing position

The purpose of the present investigation was to explore whether and how the amplitude and/or duration parameters of anticipatory postural adjustments (APAs) affect the upward propulsive force (DeltaFZamp) during rising on tiptoes from upright standing position. Seven subjects initiated the movement of rising on tiptoe from a strictly regulated initial body position at various velocities at the subjects' own spontaneous pace. In this experiment, electromyographic (EMG) activities of the muscles in the tibialis anterior (TA) and soleus, the goniogram in the ankle joint, and the anteroposterior and vertical ground reaction forces were simultaneously recorded. The average amplitude and the duration of the EMG activities and ground reaction force were measured, and the displacement of the center of body mass and the center of foot pressure (CP) were calculated and reported as the mean value plus its standard deviation. DeltaFZamp correlated positively and significantly with the maximum backward CP shift, the average amplitude of the backward loading and the average EMG amplitude in the TA in the anticipation phase, whereas DeltaFZamp did not correlate positively or significantly with the anticipatory duration of the backward CP shift of the backward loading and of the EMG activity in the TA. These results suggest that the amplitude parameters of the APAs associated with rising on tiptoes are dependent on the upward propulsive force, and that APAs may be directly involved not only with compensation of postural disturbance but also with increasing performance of the focal movement.