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Loss of knee extensor torque complexity during fatiguing isometric muscle contractions occurs exclusively above the critical torque

Pethick, Jamie, Winter, Samantha L., Burnley, Mark (2016) Loss of knee extensor torque complexity during fatiguing isometric muscle contractions occurs exclusively above the critical torque. American Journal of Physiology: Regulatory Integrative and Comparative Physiology, 310 (11). R1144-R1153. ISSN 0363-6119. E-ISSN 1522-1490. (doi:10.1152/ajpregu.00019.2016) (KAR id:55011)


The complexity of knee extensor torque time series decreases during fatiguing isometric muscle contractions. We hypothesised that, due to peripheral fatigue, this loss of torque complexity would occur exclusively during contractions above the critical torque (CT). Nine healthy participants performed isometric knee extension exercise (6 s contraction, 4 s rest) on 6 occasions for 30 min or to task failure, whichever occurred sooner. Four trials were performed above CT (trials S1-S4, S1 being the lowest intensity), and two were performed below CT (at 50% and 90% of CT). Global, central and peripheral fatigue were quantified using maximal voluntary contractions (MVCs) with femoral nerve stimulation. The complexity of torque output was determined using approximate entropy (ApEn) and the Detrended Fluctuation Analysis ? scaling exponent (DFA ?). The MVC torque was reduced in trials below CT (by [Mean ± SEM] 19 ± 4% in 90%CT), but complexity did not decrease (ApEn for 90%CT: from 0.82 ± 0.03 to 0.75 ± 0.06, 95% paired-samples confidence intervals, 95% CI = –0.23, 0.10; DFA ? from 1.36 ± 0.01 to 1.32 ± 0.03, 95% CI –0.12, 0.04). Above CT, substantial reductions in MVC torque occurred (of 49 ± 8% in S1), and torque complexity was reduced (ApEn for S1: from 0.67 ± 0.06 to 0.14 ± 0.01, 95% CI = –0.72, –0.33; DFA ? from 1.38 ± 0.03 to 1.58 ± 0.01, 95% CI 0.12, 0.29). Thus, in these experiments, the fatigue-induced loss of torque complexity occurred exclusively during contractions performed above the CT.

Item Type: Article
DOI/Identification number: 10.1152/ajpregu.00019.2016
Uncontrolled keywords: non-linear dynamics; fractal scaling; exercise; central and peripheral fatigue
Subjects: Q Science > QP Physiology (Living systems)
Divisions: Divisions > Division of Natural Sciences > Sport and Exercise Sciences
Depositing User: Mark Burnley
Date Deposited: 18 Apr 2016 13:33 UTC
Last Modified: 04 Mar 2024 17:07 UTC
Resource URI: (The current URI for this page, for reference purposes)

University of Kent Author Information

Pethick, Jamie.

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Winter, Samantha L..

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Burnley, Mark.

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