title: Reduced firing rates of high threshold motor units in response to eccentric overload.
abstract: Acute responses of motor units were investigated during submaximal voluntary isometric tasks following eccentric overload (EO) and constant load (CL) knee extension resistance exercise. Ten healthy resistance-trained participants performed four experimental test sessions separated by 5 days over a 20 day period. Two sessions involved constant load and the other two used eccentric overload. EO and CL used both sessions for different target knee eccentric extension phases; one at 2 sec and the other at 4 sec. Maximal voluntary contractions (MVC) and isometric trapezoid efforts for 10 sec at 70% MVC were completed before and after each intervention and decomposed electromyography was used to measure motor unit firing rate. The firing rate of later recruited, high-threshold motor units declined following the 2-sec EO but was maintained following 2sec CL (P < 0.05), whereas MUFR for all motor units were maintained for both loading types following 4-sec extension phases. MVC and rate of force development where maintained following both EO and CL and 2 and 4 sec phases. This study demonstrates a slower firing rate of high-threshold motor units following fast eccentric overload while MVC was maintained. This suggests that there was a neuromuscular stimulus without cost to the force-generating capacity of the knee extensors.
authors.affiliation: Physiology, Exercise and Nutrition Research Group, University of Stirling, Stirling, Scotland, United Kingdom.
authors.forename: Tom G
authors.initials: TG
authors.lastname: Balshaw
authors.affiliation: Computing Science and Mathematics, University of Stirling, Stirling, Scotland, United Kingdom.
authors.forename: Madhu
authors.initials: M
authors.lastname: Pahar
authors.affiliation: Physiology, Exercise and Nutrition Research Group, University of Stirling, Stirling, Scotland, United Kingdom.
authors.forename: Ross
authors.initials: R
authors.lastname: Chesham
authors.affiliation: Physiology, Exercise and Nutrition Research Group, University of Stirling, Stirling, Scotland, United Kingdom.
authors.forename: Lewis J
authors.initials: LJ
authors.lastname: Macgregor
authors.affiliation: Physiology, Exercise and Nutrition Research Group, University of Stirling, Stirling, Scotland, United Kingdom a.m.hunter1@stir.ac.uk.
authors.forename: Angus M
authors.initials: AM
authors.lastname: Hunter
mesh_terms: D000200:Action Potentials; D000328:Adult; D001696:Biomechanical Phenomena; D004576:Electromyography; D006801:Humans; D007537:Isometric Contraction; D007717:Knee / Q000502:physiology; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D052097:Quadriceps Muscle / Q000294:innervation / Q000502:physiology*; D011999:Recruitment, Neurophysiological; D055070:Resistance Training; D055815:Young Adult
keywords: Decomposed electromyography; MVC; lengthening contractions