Comparison of soleus H-reflex gain from prone to standing in dancers and controls.

To examine the differences in soleus H-reflex gain between trained dancers and control subjects, the soleus H-reflex amplitude and background muscle activity of 9 trained dancers (means of 20.3 +/- 2.1 years of age, and 14.3 +/- 3.8 years of training) and 9 control subjects (mean 23.3 +/- 3.2 years of age) were compared at rest and at 10, 20, and 30% of a maximal voluntary soleus contraction during two conditions: prone and standing. The ratio of the maximal H-reflex (H-max) to the maximal motor response (M-max) was also measured during both conditions. Correlation was performed between background EMG and the resultant H-reflex to determine reflex gain. The results demonstrated that the control subjects and the dancers displayed a similar reflex gain during the prone condition (slope = 3.30 vs. 3.64, respectively). However, during the standing condition, dancers demonstrated a significantly lower reflex gain (slope = 1.78) than did control subjects (slop = 3.68). Furthermore, although both groups significantly decreased the H-max/M-max ratio from prone to standing, no differences were found between groups at either condition. This suggests that the differences in standing reflex gain between the dancers and control subjects were a product of differential control of reflex modulation involved in postural control. An initial hypothesis explaining the differences between the standing reflex gain of the groups relates to plasticity of central inhibitory control mechanisms, primarily presynaptic and/or reciprocal inhibition.