Effect of reflexive activation of motor units on torque development during electrically-evoked contractions of the triceps surae muscle
Résumé
The aim of the study was to identify stimulation conditions permitting the occurrence of extra-torque (ET) and to examine their impact on spinal and corticospinal excitabilities. Twelve subjects received stimulation trains over the tibial nerve (20 sec duration, 1ms pulse duration) that were delivered at three stimulation frequencies (20, 50 and 100 Hz) and at 5 intensities (110, 120, 130, 140 and 150% of the motor threshold). Torque-Time Integral (TTI) of each stimulation train was calculated. Spinal (Hmax/Mmax) and corticospinal (MEPmax/Mmax) excitabilities were assessed at rest before and after each stimulation train by tibial nerve stimulation and by transcranial magnetic stimulation, respectively. Moreover, a twitch at each stimulation intensity was delivered before and after each stimulation train. The EMG associated with this twitch was analyzed in order to identify the initial motor unit (MU) recruitment pathway before each stimulation train and discriminate trials to H-trials (indirect recruitment) and M-trials (direct recruitment). TTI was higher for H-trials compared to M-trials for all tested frequencies. There was a decrease in Hmax/Mmax for the 20 Hz-H trials and an increase for the 100 Hz-H trials, while MEPmax/Mmax remained unchanged at post measurements. Present results demonstrate that the initial MU recruitment pattern plays a main role in the ET occurrence, with the indirect recruitment via the afferent volley being substantial for its development. The modulations of Hmax/Mmax without changes in MEPmax/Mmax, suggest that the ET development affects spinal excitability and that these changes are frequency-dependent.