Spinal reflex plasticity during maximal dynamic contractions after eccentric training.

PURPOSE: The aim of the study was to use eccentric strength training of the plantar flexor muscles to investigate the plasticity of the spinal reflexes during maximal voluntary isometric, concentric, and eccentric contractions. METHODS: Eighteen healthy male subjects were divided into an eccentric strength training group (N = 10) and a control group (N = 8). The training program consisted of 18 sessions of eccentric exercise for a 7-wk period. All subjects were tested before, during, and after the training program. Soleus (SOL) and medial gastrocnemius (MG) spinal reflexes (H-reflex and V-wave) and M-waves were evoked at the same angular position during passive isometric, concentric, and eccentric actions (i.e., Hmax and Mmax, respectively) and during maximal voluntary isometric, concentric, and eccentric plantar flexion (MVC) (i.e., Hsup, V-wave, and Msup, respectively). RESULTS: : Both SOL and MG Hmax/Mmax ratios remained unchanged whatever the action type after training. The Hsup/Msup ratio was increased only during eccentric MVC for the SOL (P < 0.01) and regardless of the contraction type for the MG (P < 0.05). The eccentric SOL Hsup/Msup ratio was not different from the isometric and concentric Hsup/Msup ratios after 7 wk of training. The V/Msup ratios were increased during isometric and eccentric contractions for the SOL and regardless of the contraction type for the MG after training. CONCLUSION: : In conclusion, the present results suggest that the increase in voluntary torque induced by eccentric training could be ascribed, according to the contraction type, to an increased volitional drive from the supraspinal centers, which may induce neural adaptations at the spinal level. Changes in the regulation of the balance between excitation and inhibition affecting the motoneuron pool were suggested to explain the plasticity of the spinal reflexes.