Rate Modulation of Human Anconeus Motor

19 Investigations of high-intensity isometric fatiguing protocols report decreases in motor 20 unit firing rates (MUFRs), but little is known regarding changes in MUFRs following 21 fatigue induced by high-intensity dynamic contractions. The purpose was to evaluate 22 MUFRs of the anconeus (an accessory elbow extensor), and elbow extension power 23 production as a function of time to task failure (TTF) during high velocity fatiguing 24 concentric contractions against a moderately heavy resistance. Fine-wire intramuscular 25 electrode pairs were inserted into the anconeus to record MUs in 12 male participants (25 26 ± 3y), over repeated sessions on separate days. MUs were tracked throughout a three27 stage, varying-load, dynamic elbow extension protocol designed to extend the task 28 duration for >1 minute thereby inducing substantial fatigue. Mean MUFRs and peak 29 power were calculated for three relative time ranges: 0-15% TTF (beginning), 45-60% 30 TTF (middle) and 85-100% TTF (end). Mean duration of the overall fatigue protocol was 31 ~80 seconds. Following the protocol, isometric MVC, highest velocity at 35% MVC 32 load, and peak power decreased 37%, 60%, and 64% compared to baseline, respectively. 33 Data from 20 anconeus MUs tracked successfully throughout the protocol indicated a 34 reduction in MUFRs in relation to power loss from 36 Hz/160W (0-15% TTF) to 28 35 Hz/97W (45-60% TTF) to 23 Hz/43W (85-100% TTF). During these high-intensity 36 maximal effort concentric contractions, anconeus MUFRs decreased substantially (> 37 35%). Although the absolute MUFRs were higher in the present study than those reported 38 previously for other muscles during sustained high-intensity isometric tasks, the relative 39 decrease in MUFRs was similar between the two tasks. 40 NEW AND NOTEWORTHY 41 by 10.0.33.3 on S etem er 2, 2017 http://jaysiology.org/ D ow nladed fom We recorded single MUFRs during a high-intensity fatiguing dynamic elbow extension 42 task. Due to unique features, the anconeus muscle (an accessory elbow extensor) 43 facilitates tracking of units during rapid maximal effort concentric contractions at a 44 moderately-high load. Despite baseline dynamic rates being higher than those recorded 45 during isometric maximum contractions, there was a decrease in MUFRs over the ~80s 46 fatiguing protocol indicating rate reduction is a common feature to both isometric and 47 dynamic high-intensity fatiguing tasks. 48