Abstract
BackgroundFollowing active muscle lengthening, there is an increase in steady-state isometric force as compared with a purely isometric contraction at the same muscle length and level of activation. This fundamental property of skeletal muscle is known as residual force enhancement (RFE). While the basic mechanisms contributing to this increase in steady-state isometric force have been well documented, changes in central nervous system (CNS) excitability for submaximal contractions during RFE are unclear. The purpose of this study was to investigate spinal and supraspinal excitability in the RFE isometric steady-state following active lengthening of the ankle dorsiflexor muscles.MethodsA total of 11 male participants (20–28 years) performed dorsiflexions at a constant level of electromyographic activity (40% of maximum). Half of the contractions were purely isometric (8 s at an ankle angle of 130°), and the other half were during the RFE isometric steady-state following active lengthening (2 s isometric at 90°, a 1 s lengthening phase at 40°/s, and 5 s at 130°). Motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and compound muscle action potentials (M-waves) were recorded from the tibialis anterior during the purely isometric contraction and RFE isometric steady-state.ResultsCompared to the purely isometric condition, following active lengthening, there was 10% RFE (p < 0.05), with a 17% decrease in normalized CMEP amplitude (CMEP/Mmax) (p < 0.05) and no change in normalized MEP amplitude (MEP/CMEP) (p > 0.05).DiscussionThese results indicate that spinal excitability is reduced during submaximal voluntary contractions in the RFE state with no change in supraspinal excitability. These findings may have further implications to everyday life offering insight into how the CNS optimizes control of skeletal muscle following submaximal active muscle lengthening.
Highlights
Residual force enhancement (RFE) is the increase in isometric steady-state force following active muscle lengthening when compared to a purely isometric contraction at the same muscle length and level of activation (Abbott & Aubert, 1952; Power, Rice & Vandervoort, 2012; Seiberl et al, 2015)
During submaximal contractions, only the cervicomedullary motor evoked potentials (CMEPs) amplitude decreased. These findings indicate a reduction in spinal excitability (i.e., CMEP amplitude), and increased supraspinal excitability (i.e., Motor evoked potentials (MEPs) to CMEP ratio) during maximal active muscle lengthening, and a reduction in spinal excitability during submaximal eccentric contractions (Gruber et al, 2009)
Dorsiflexion torque and muscle activity in the residual force enhancement (RFE) state following active lengthening Following active lengthening, steady-state isometric torque was significantly greater (paired t: df (10), t-value (-5.89), p = 0.00015, dz = 2.02) than the torque recorded during purely isometric contractions at the same muscle length (Fig. 3A), resulting in an average RFE across all participants of 10.0% ± 6.7%
Summary
Residual force enhancement (RFE) is the increase in isometric steady-state force following active muscle lengthening when compared to a purely isometric contraction at the same muscle length and level of activation (Abbott & Aubert, 1952; Power, Rice & Vandervoort, 2012; Seiberl et al, 2015). While there have been many investigations into the basic underlying mechanisms of RFE (Lee, Joumaa & Herzog, 2007; Joumaa et al, 2008; Koppes, Herzog & Corr, 2013; Power et al, 2013; Herzog et al, 2016), the implications of this history-dependent phenomenon on voluntary control of force production remain unknown It is unclear how spinal and supraspinal excitability are modulated in the isometric force enhanced steady-state during submaximal voluntary contractions. These findings may have further implications to everyday life offering insight into how the CNS optimizes control of skeletal muscle following submaximal active muscle lengthening
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