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

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.

The influence of residual force enhancement on spinal and supraspinal excitability

“…By matching activation, despite the same motor neuron output (40% EMG) for the purely isometric contraction and the isometric steady-state following active lengthening, we were able to show 10% RFE and a 17% reduction in spinal excitability. These findings provide support that the history-dependence of torque alters corticospinal excitability through a neuromechanical coupling ( Hahn et al, 2012 ; Grant et al, 2017 ; Sypkes et al, 2017 ). However, there was no change in supraspinal excitability.…”

“…Lengthening contractions were also associated with increased supraspinal excitability (i.e., MEP/CMEP ratios). In contrast, following active muscle shortening in the torque depressed state, when the ability of the muscle to generate force is impaired, we recently demonstrated increased spinal excitability (i.e., normalized CMEP amplitude) and no change in supraspinal excitability (i.e., normalized MEP amplitude) ( Sypkes et al, 2017 ). The present study, together with the previously mentioned investigations, provides strong evidence for a neuromechanical coupling that may alter corticospinal excitability in a manner that may be related to the capacity of the muscle to produce force.…”

“…The following methods are used frequently in our lab and there is overlap in the language used throughout this methods section to that of Sypkes et al (2017) . A HUMAC NORM dynamometer (CSMi Medical Solutions, Stoughton, MA, USA) was used for all torque, angular velocity, and position recordings.…”

“…To determine the submaximal iEMG target for the activation matching contractions, as described previously ( Sypkes et al, 2017 ), participants were instructed to perform an 8 s maximal dorsiflexion contraction at an ankle angle of 90°. The average iEMG collected between 5.5 and 6.5 s was then used to determine the 40% submaximal iEMG target.…”

“…This current (150–325 mA) was used for the remainder of the experiment. A CMEP of 40% M max was selected to match our recent work ( Grant et al, 2017 ; Sypkes et al, 2017 ) and probe the excitability of low and high threshold motoneurons.…”