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. Our 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 ± 3 yr), 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 min 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 s. Following the protocol, isometric maximum voluntary contraction (MVC), highest velocity at 35% MVC load, and peak power decreased 37, 60, and 64% compared with 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/160 W (0-15% TTF) to 28 Hz/97 W (45-60% TTF) to 23 Hz/43 W (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.
School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and.
Brianna L
BL
Cowling
School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and.
Brad
B
Harwood
School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and.
David B
DB
Copithorne
School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and Department of Anatomy and Cell Biology, The University of Western Ontario, London, Ontario, Canada crice@uwo.ca.
Charles L
CL
Rice
D004576:Electromyography; D000072696:High-Intensity Interval Training / Q000379:methods; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009068:Movement / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D009469:Neuromuscular Junction / Q000503:physiopathology*; D010807:Physical Endurance / Q000502:physiology*; D005082:Physical Exertion / Q000502:physiology; D016059:Range of Motion, Articular / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology; D009435:Synaptic Transmission / Q000502:physiology; D055815:Young Adult
concentric; discharge rate; fatigue; motor neuron; velocity-dependent contraction