Motor Unit Firing Patterns during Fatigue

Abstract

The central nervous system may manipulate motor unit firing rates to maintain sub-maximal force output and delay the onset of fatigue. These strategies may be enhanced with endurance training. PURPOSE: To compare motor unit firing rates of the adductor pollicis (AdP) muscle in young, able-bodied adults during a sustained submaximal isometric fatiguing task before and after 4 weeks of endurance training. METHODS: Participants performed maximal voluntary contractions (MVCs) and a fatigue task prior to and following 4 weeks of muscular endurance training. The fatigue task consisted of a sustained voluntary isometric thumb adduction contraction at 20% MVC. The fatigue task was performed until endurance time, denned as the time when force output deviated by ±5% of the target force for 5s. Single motor unit potentials were recorded from the AdP muscle during the fatigue task with intramuscular fine-wire electrodes. Participants endurance trained their thumb every other day for 4 weeks. Each training session consisted of 3 sets of 7 1-minute isometric thumb adduction contractions at 20% MVC. Mean interspike intervals over 5s time bins were calculated every 5% of endurance time. Pre-post training mean motor unit firing rates were compared during the initial 5s, the 5s time bin with the lowest firing rate and the final 5s time bin of the fatigue task. RESULTS: Following endurance training, initial mean motor unit firing rates increased (2.1 ± 4.7 Hz) compared to pre-training. The lowest mean firing rates increased (2.1 ±2.8 Hz) and the final mean motor unit firing rates also increased (1.9 ± 8.2 Hz) following endurance training. Pre-training to post-training increases were also found for endurance time (78.1 ± 33.1%) and MVC (45.9 ± 55.2%). CONCLUSIONS: Endurance training leads to increased endurance time and motor unit firing rates. The results of this study suggest that neuromuscular adaptations, specifically increases in motor unit firing rates, may contribute to the enhancement of endurance time.