Postural adjustments associated with a fast voluntary arm movement at various foot positions

The aim of the study was to further investigate postural adjustments associated with a fast voluntary arm elevation at various foot positions. The subjects stood upright on a force platform and participated in 3 sessions according to foot position: horizontal position, dorsal and plantar flexion of 6 degrees. EMG activity of anterior deltoid muscle (DA) and of ipsilateral and contralateral soleus (SD, SS), biceps femoris (BFd, BFs) and ipsilateral anterior tibial (TAd) muscles, initial acceleration (Acc) and stabilogram change (Sg) were recorded. The results showed that postural preadjustment was provided by a co-contraction of SOLd and TAd, preceding 31-37 ms the time onset of DA, observed in all foot positions. Time onset of EMG activity in BFd and BFs followed 105-134 ms after EMG onset of DA, showing some dependence on foot position. The co-contraction of SD and TAd presumably has postural function with the role to maintain stability of the body against the increased postural requirements associated with a very fast arm movement. The preceding activity of BFd and BFs and their participation in postural preadjustments is probably not necessary, since the co-contraction of SD and TAd has ensured fixation of the ankle joint.