Functional modulation of the human flexion and crossed extension reflexes by body position.

The effects of body position on the electrically evoked flexion (FR) and crossed extension reflexes (CER) were investigated in humans. The FR area in the ipsilateral tibialis anterior muscle was significantly smaller during sitting than supported stance by 36% (P < 0.01). In contrast, the excitability of extensor muscles on both sides was enhanced in standing. For instance, twice as many subjects manifested a response in the ipsilateral vastus lateralis (VL) and the contralateral VL and/or soleus muscles (i.e. the CER) in standing than sitting. The FR and CER modulation observed seems to be dictated by the difference in functional demand between sitting and supported stance.