Spinal reflex adaptation in dancers changes with body orientation and role of pre-synaptic inhibition

Dancers undergo specific activity-dependent neuromuscular adaptations following long-term training that allow them to develop the refined motor skills required for success in dance. The spinal stretch reflex circuit has demonstrated specific adaptations following prolonged dance training. Adaptations in the spinal stretch reflex can be studied using H-reflex methodology, first described by Paul Hoffmann in 1910. This article discusses H-reflex methodology and presents data that examine the neural mechanisms that contribute to adaptations in the spinal stretch reflex with dance training. Two groups of subjects, modern dancers (N = 5) and untrained controls (N = 5), were tested. On one-half of the trials common peronal nerve (CPN) conditioning of the soleus H-reflex was used to assess one spinal mechanism, pre-synaptic inhibition; the other half tested the soleus H-reflex only (unconditioned). The dependent variables were the H(max)/M(max) ratios, unconditioned and with CPN conditioning, expressed as percent values. The results revealed three main findings: 1. Modern dancers had smaller H(max)/M(max) ratios than control subjects; 2. The H(max)/M(max) ratio was smaller in standing posture than in prone among both dancers and controls; and 3. Pre-synaptic inhibition was not different between dancers and controls in standing. In conclusion, modern dancers have smaller H-reflexes than untrained controls, but pre-synaptic inhibition does not appear to explain this difference.