Influence of body position on fibularis longus and soleus Hoffmann reflexes.

It is widely accepted that Hoffmann (H) reflex amplitudes of the soleus decrease as complexity of body positions increases, but it is not known if this same mechanism of postural control is seen in other ankle muscles such as fibularis longus (FL). Our purpose was to assess if FL H-reflex changed in different body positions and if the adaptations were correlated to soleus H-reflex modulation. Fifteen healthy subjects had their FL and soleus H-reflexes measured in three positions (prone, bipedal, unipedal). Maximal H-reflexes (H-max) and motor responses (M-max) were collected bilaterally. The average H-max and M-max were used to calculate H(max)/M(max) ratios. To control influences of background muscle activity on H-reflex measures, the ratios were normalized to their corresponding mean EMG amplitudes over a 50-ms time epoch. H-reflex amplitudes of both muscles were significantly lower in unipedal stance than other positions. Additionally, there were strong correlations (R(2)>0.7) in H-reflex modulation between the two muscles when transitioning from prone to either bipedal or unipedal stance. Down-modulation of H-reflex when transitioning to unipedal stance was present in both FL and soleus suggesting that H-reflex modulation of both muscles may play a similar role in control of upright posture.