The influence of residual force enhancement on spinal and supraspinal excitability

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.

Corticospinal excitability remains unchanged in the presence of residual force enhancement and does not contribute to increased torque production

“…Contrary to these findings under submaximal contraction intensities, Hahn et al (2012) found larger MEPs, but unchanged CMEPs, following stretch of the plantar flexors during maximal voluntary contractions. Both the results of Hahn et al (2012) and Sypkes et al (2018) could be interpreted as an increase in cortical excitability in the presence of rFE. Further, enlarged V-waves during rFE in the Hahn and colleagues (2012) study suggest a greater motoneuronal output and/or an increase in stretch reflex excitability after active stretch (Hahn et al 2012).…”

“…Two further studies have used motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) to investigate modulations to the excitability of cortical and spinal sites in the presence of rFE (Hahn et al, 2012; Sypkes et al, 2018). During submaximal dorsiflexion contractions, Sypkes et al (2018) found unchanged TA MEPs, but reduced TA CMEPs, following active muscle stretch compared with fixed-end reference contractions. This is in line with the idea of reduced motor unit recruitment (Altenburg et al 2008) and might be caused by tension-related afferent feedback as assessed by tendon-evoked inhibitory reflexes (Contento et al 2019).…”

“…Based on previous research (Hahn et al, 2012; Sypkes et al, 2018), we predicted that inhibiting motor cortical neurons by subthreshold TMS (Davey et al, 1994; Petersen et al, 2001; Zuur et al, 2010) would lead to a larger suppression in muscle activity (EMG) and plantar flexion torque in the presence of rFE compared with fixed-end reference contractions with a matched background muscle activity. Conversely, facilitation of cortical interneurons and pyramidal neurons by suprathreshold TMS (Rossini et al, 2015; Rothwell, 1997) was predicted to result in greater superimposed twitches in the presence of rFE compared with the fixed-end reference contractions with a matched background muscle activity.…”

“…However, whether neural inhibition occurs during and following active stretch under voluntary activation remains controversial (Hahn, 2018). The potentially unique control of stretch-hold contractions and the observation of AR following active stretch has motivated a number of studies to investigate the neural control and/or neural modulations that occur during the isometric steady state following active muscle stretch (Altenburg et al, 2008; Contento et al, 2019; Hahn et al, 2012; Sypkes et al, 2018). For example, Altenburg et al (2008) examined single motor unit behaviour of the vastus lateralis muscle during AR after active muscle stretch.…”

“…These authors found similar discharge rates of VL motor units between STR and REF conditions, which led them to conclude that a derecruitment of motor units might have occurred during AR (Altenburg et al 2008). Two further studies have used motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) to investigate modulations to the excitability of cortical and spinal sites in the presence of rFE (Hahn et al, 2012; Sypkes et al, 2018). During submaximal dorsiflexion contractions, Sypkes et al (2018) found unchanged TA MEPs, but reduced TA CMEPs, following active muscle stretch compared with fixed-end reference contractions.…”