Author(s): VITRY, F., MARTIN, A., ENJALBERT, G., SORDET, J., PAPAIORDANIDOU, M., Institution: INSERM U1093 - UNIVERSITÉ DE BOURGOGNE FRANCHE-COMTÉ, Country: FRANCE, Abstract-ID: 1005

INTRODUCTION:
Electrical stimulation can be delivered over a nerve trunk or muscle belly, and can generate contractions by activating motor (direct recruitment) and sensory (indirect recruitment) axons. The aim of the present study was to investigate the impact of the stimulation mode (nerve trunk vs muscle belly) on the extent and origin of neuromuscular fatigue of the triceps surae.
METHODS:
Fifteen participants visited the laboratory for two experimental sessions. Each session consisted of delivering 25 stimulation trains (100 Hz, 20s ON – 20s OFF) at an intensity evoking 10% of maximal voluntary contraction (MVC), either applied over the triceps surae muscles or over the posterior tibial nerve. Before (PRE) and after (POST) the 25 stimulation trains, MVC, muscle contractile properties, as assessed by the twitch torque (Pt) and neural changes, as assessed by the voluntary activation level (VAL), were evaluated. For each stimulation train, the torque-time integral (TTI) was determined. Moreover, three twitches at the intensity of each stimulation train (PtIstim) were delivered before each 25-train session to identify the initial motor unit (MU) recruitment (direct vs indirect, [1]). Finally, the sum of the TTI of the 25 stimulation trains was calculated to determine the total TTI developed by each subject during each of the two sessions (∑TTI).
RESULTS:
Results showed that MVC, VAL and Pt were significantly decreased (P < 0.05) after the 25 stimulation trains for both muscle and nerve stimulation (-9.5 and -7.6% for muscle and nerve stimulation, respectively). The ∑TTI was not different between the two modalities of stimulation (P = 0.72). When the EMG associated to the PtIstim was considered to discriminate the trials according to the initial MU recruitment, independently of the stimulation modalities, 17 trials out of the 30 were considered as inducing a direct MU recruitment and the rest 13 trials were considered as inducing an indirect MU recruitment. Statistical analysis revealed a significant decrease of MVC after the 25-stimulation trains only after the trials inducing a direct MU recruitment (P < 0.001). Finally, ∑TTI developed was not significantly different between trials inducing direct and indirect MU recruitment (P = 0.21), although mean TTI was higher in trials inducing indirect (6270 ± 4282 Nm) comparatively to those inducing direct MU recruitment (4539 ± 3124 Nm).
CONCLUSION:
These results indicate that muscle and nerve stimulation induce the same amount of neuromuscular fatigue that can be attributed both to neural and muscle alterations. It appears that the site of stimulation is not a discriminating factor for fatigue development. On the other hand, the loss of MVC reported only for the direct MU recruitment suggests that when an indirect MU recruitment is induced, fatigue development is less pronounced. It appears thus that the level of induced fatigue seems more linked to the initial type of MU recruitment than the site of stimulation.

1 - Vitry et al, 2019