Mettler, J. A., & Griffin, L. (2007). Motor unit firing patterns during fatigue: Adaptations following endurance training. ACSM Annual Meeting New Orleans, Presentation Number, 1925.

This study compared motor unit firing rates of the adductor pollicis muscle in young, able-bodied adults during a sustained submaximal isometric fatiguing task before and after four weeks of muscular endurance training. Ss performed maximum voluntary contractions and a fatigue task prior to and after training. The fatigue task consisted of a sustained voluntary isometric thumb adduction contraction at 20% maximum voluntary contractions. The fatigue task was performed until muscular endurance time (defined as the time when force output deviated by ±5% of the target force for five seconds). Single motor unit potentials were recorded from the adductor pollicis muscle during the fatigue task. Ss' endurance-trained their thumb every other day for four weeks. Each training session consisted of three sets of seven 1-minute isometric thumb adduction contractions at 20% maximum voluntary contraction.

Compared to pre-training, initial mean motor unit firing rates increased following muscular endurance training. The lowest mean firing rates increased and the final mean motor unit firing rates also increased. Pre-training to post-training increases also were found for muscular endurance time and maximum voluntary contractions.

Implication. Muscular endurance training leads to increased endurance time and motor unit firing rates. Neuromuscular adaptations, specifically increase in motor unit firing rates and may contribute to the enhancement of muscular endurance time.

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