Spinal excitability is increased in the torque-depressed isometric steady state following active muscle shortening.

Caleb T Sypkes,Benjamin Kozlowski,Geoffrey A Power,Chris J Mcneil,Leah R Bent,Jordan Grant

doi:10.1098/rsos.171101

Caleb T Sypkes, Benjamin Kozlowski + Show 4 more

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https://doi.org/10.1098/rsos.171101

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Abstract

Torque depression (TD) is the reduction in steady-state isometric torque following active muscle shortening when compared with a purely isometric contraction at the same muscle length and level of activation. The purpose of the present study was to assess spinal and supraspinal excitability in the TD state during submaximal contractions of the dorsiflexors. Eleven young (24 ± 2 yrs) males performed 16 contractions at a constant level of electromyographic activity (40% of maximum). Half of the contractions were purely isometric (8 s at an ankle angle of 100°), whereas the other half induced TD (2 s isometric at 140°, a 1 s shortening phase at 40° s−1 and 5 s at 100°). Motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs) and compound muscle action potentials (M-waves) were recorded from tibialis anterior during the TD steady-state and purely isometric contractions. When compared with values in the purely isometric condition, following active shortening, there was a 13% decrease in torque (p < 0.05), with a 10% increase in normalized CMEP amplitude (CMEP/Mmax) (p < 0.05) and no change in normalized MEP amplitude (MEP/CMEP) in the TD state (p > 0.05). These findings indicate that during voluntary contractions in the TD state, the history-dependent properties of muscle can increase spinal excitability and influence voluntary control of submaximal torque production.

Highlights

It was shown that following active muscle shortening, there was a significant increase in cervicomedullary motor evoked potentials (CMEPs) amplitude when compared with the purely isometric contractions
The present study investigated alterations to the excitability of the central nervous system in the torquedepressed state as indicated by changes in Motor evoked potentials (MEPs) and CMEP amplitude
Using a paradigm of maintained motor neuron output, it was shown that following active shortening, steady-state isometric torque was significantly less than the torque produced during the purely isometric contractions

Summary

Introduction

When compared with values in the purely isometric condition, following active shortening, there was a 13% decrease in torque (p < 0.05), with a 10% increase in normalized CMEP amplitude (CMEP/Mmax) (p < 0.05) and no change in normalized MEP amplitude (MEP/CMEP) in the TD state (p > 0.05) These findings indicate that during voluntary contractions in the TD state, the history-dependent properties of muscle can increase spinal excitability and influence voluntary control of submaximal torque production. It is well established that the steady-state isometric torque following a shortening muscle action is reduced when compared with that produced during a purely isometric contraction at the same muscle length and level of activation [1] This shortening-induced reduction in torque has been termed torque depression (TD), and as a manifestation of the history-dependent nature of muscle contraction, TD has been investigated extensively for over half a century [2]. These differences seem to reflect a neuromechanical coupling response to changes in muscle force production capacity under different modes of contraction, and may apply to the shortening-induced torque reduction in TD

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