Rate modulation of human anconeus motor units during high-intensity dynamic elbow extensions

Investigations of high-intensity isometric fatiguing protocols report decreases in motor unit firing rates (MUFRs), but little is known regarding changes in MUFRs following fatigue induced by high-intensity dynamic contractions. The purpose was to evaluate MUFRs of the anconeus (an accessory elbow extensor), and elbow extension power production as a function of time to task failure (TTF) during high velocity fatiguing concentric contractions against a moderately heavy resistance. Fine-wire intramuscular electrode pairs were inserted into the anconeus to record MUs in 12 male participants (25±3y), over repeated sessions on separate days. MUs were tracked throughout a three-stage, varying-load, dynamic elbow extension protocol designed to extend the task duration for >1 minute thereby inducing substantial fatigue. Mean MUFRs and peak power were calculated for three relative time ranges: 0-15% TTF (beginning), 45-60% TTF (middle) and 85-100% TTF (end). Mean duration of the overall fatigue protocol was ~80 seconds. Following the protocol, isometric MVC, highest velocity at 35% MVC load, and peak power decreased 37%, 60%, and 64% compared to baseline, respectively. Data from 20 anconeus MUs tracked successfully throughout the protocol indicated a reduction in MUFRs in relation to power loss from 36 Hz/160W (0-15% TTF) to 28 Hz/97W (45-60% TTF) to 23 Hz/43W (85-100% TTF). During these high-intensity maximal effort concentric contractions, anconeus MUFRs decreased substantially (> 35%). Although the absolute MUFRs were higher in the present study than those reported previously for other muscles during sustained high-intensity isometric tasks, the relative decrease in MUFRs was similar between the two tasks.

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