The Etiology Of Muscle Fatigue Differs Between Two Electrical Stimulation Protocols

Abstract

PURPOSE: This study aimed at analyzing from combined electrophysiological and metabolic measurements, the mechanisms involved in force decrease throughout two electrical stimulation (ES) protocols for which the motor units (MUs) recruitment by efferent and/or afferent pathways differs. METHODS: The triceps surae of eleven healthy subjects was activated by ES applied over the tibial nerve. Two ES protocols (CONV: 20 Hz - 0.05 ms vs. WPHF: 80 Hz - 1 ms) were used consisting of 40 trains (6 s on- 6 s off) delivered at an intensity (IES) evoking 20% of maximal voluntary contraction. To assess the contractile properties of the solicited MUs, force-frequency relation was evoked at IES before and after each protocol. For both protocols, a single twitch was delivered at IES after each train, and the force and the electrophysiological responses (∑EMG) were recorded to analyze the behavior of the MUs. Phosphocreatine (PCr) depletion was assessed using 31P-MR spectroscopy. RESULTS: MUs were predominantly recruited through efferent pathways for CONV protocol, whereas afferent pathways were more involved in the MUs recruitment during WPHF. Despite this different MU recruitment, both the force reduction (≈-26%) and PCr depletion (≈8%) were similar between the two protocols. The CONV protocol induced a rightward shift of the force-frequency relationship, whereas a significant reduction of the ∑EMG evoked at IES was observed only for the WPHF. CONCLUSIONS: These results suggest that a decreased number of active MUs mainly contributed to WPHF-induced force decrease while intracellular processes were most likely involved in force decrease during CONV stimulation.