Eccentric training at long muscle lengths induces greater corticospinal and spinal reflex plasticity than eccentric training at short muscle lengths.

It is well-established that resistance training generates neural adaptations. These may be greater when exercises mainly include eccentric contractions or when muscles are trained at long length. However, it remains to be clarified whether the length at which muscles are trained influences neural adaptation following eccentric training. We trained 28 healthy young individuals with eccentric exercises for 9 weeks (24 sessions) at either short (n = 13) or long (n = 15) plantar flexor lengths. Participants were assessed once before and once after this training. Estimates of corticospinal excitability and short-interval intracortical inhibition were obtained using transcranial magnetic stimulation and by analysing conditioned or non-conditioned motor evoked potentials. Effectiveness of Ia afferents to discharge α-motoneurons, and post-activation depression induced by primary afferent depolarization were estimated using peripheral tibial nerve stimulation conditioned or not by fibular nerve stimulation, and by analysing Hoffmann reflex amplitude. Maximal plantar flexor torque and voluntary activation were also assessed. The increase in corticospinal excitability and effectiveness of Ia afferents to discharge α-motoneurons were significantly greater after training at long muscle length than at short muscle length (+24.03% and +16.1%, respectively, P < 0.001), without between-group differences in adaptations for short-interval intracortical inhibition, post-activation depression by primary afferent depolarization, maximal torque or voluntary activation level. These results suggest that eccentric training performed at long muscle lengths induces greater adaptations in corticospinal and spinal reflex plasticity. It is crucial to consider muscle length during eccentric training to optimize neuronal plasticity and potentially enhance daily task performances.