Can local vibration alter the contribution of persistent inward currents to human motoneuron firing?

The response of spinal motoneurons to synaptic input greatly depends on the activation of persistent inward currents (PICs), which in turn are enhanced by the neuromodulators serotonin and noradrenaline. Local vibration (LV) induces excitatory Ia input onto motoneurons and may alter neuromodulatory inputs. Therefore, we investigated whether LV influences the contribution of PICs to motoneuron firing. This was assessed in voluntary contractions with concurrent, ongoing LV, as well as after a bout of prolonged LV. High-density surface electromyograms (HD-EMG) of the tibialis anterior were recorded with a 64-electrode matrix. Twenty males performed isometric, triangular, dorsiflexion contractions to 20% and 50% of maximal torque at baseline, during LV of the tibialis anterior muscle, and after 30-min of LV. HD-EMG signals were decomposed, and motor units tracked across time points to estimate PICs through a paired motor unit analysis, which quantifies motor unit recruitment-derecruitment hysteresis (ΔF). During ongoing LV, ΔF was lower for both 20% and 50% ramps. Although significant changes in ΔF were not observed after prolonged LV, a differential effect across the motoneuron pool was observed. This study demonstrates that PICs can be non-pharmacologically modulated by LV. Given that LV leads to reflexive motor unit activation, it is postulated that lower PIC contribution to motoneuron firing during ongoing LV results from decreased neuromodulatory inputs associated with lower descending corticospinal drive. A differential effect in motoneurons of different recruitment thresholds after prolonged LV is provocative, challenging the interpretation of previous observations and motivating future investigations. KEY POINTS: Neuromodulatory inputs from the brainstem influence motoneuron intrinsic excitability through activation of persistent inward currents (PICs). PICs make motoneurons more responsive to excitatory input. We demonstrate that vibration applied on the muscle modulates the contribution of PICs to motoneuron firing, as observed through analysis of the firing of single motor units. The effects of PICs on motoneuron firing were lower when vibration was concurrently applied during voluntary ramp contractions, likely due to lower levels of neuromodulation. Additionally, prolonged exposure to vibration led to differential effects of lower- vs. higher-threshold motor units on PICs, with lower-threshold motor units tending to present an increased and higher-threshold motor units a decreased contribution of PICs to motoneuron firing. These results demonstrate that muscle vibration has the potential to influence the effects of neuromodulation on motoneuron firing. The potential of using vibration as a non-pharmacological neuromodulatory intervention should be further investigated.

Neuromuscular fatigability during repeated sprints assessed with an innovative cycle ergometer.

PURPOSE: Repeated sprint ability is an integral component of team sports. This study aimed to evaluate fatigability development and its aetiology during and immediately after a cycle repeated sprint exercise performed until a given fatigability threshold. METHODS: On an innovative cycle ergometer, 16 healthy males completed an RSE (10-s sprint/28-s recovery) until task failure (TF): a 30% decrease in sprint mean power (Pmean). Isometric maximum voluntary contraction of the quadriceps (IMVC), central alterations [voluntary activation (VA)], and peripheral alterations [twitch (Pt)] were evaluated before (pre), immediately after each sprint (post), at TF and 3 min after. Sprints were expressed as a percentage of the total number of sprints to TF (TSTF). Individual data were extrapolated at 20, 40, 60, and 80% TSTF. RESULTS: Participants completed 9.7 ± 4.2 sprints before reaching a 30% decrease in Pmean. Post-sprint IMVCs were decreased from pre to 60% TSTF and then plateaued (pre: 345 ± 56 N, 60% 247 ± 55 N, TF: 233 ± 57 N, p < 0.001). Pt decreased from 20% and plateaued after 40% TSTF (p < 0.001, pre-TF = - 45 ± 13%). VA was not significantly affected by repeated sprints until 60% TSTF (pre-TF = - 6.5 ± 8.2%, p = 0.036). Unlike peripheral parameters, VA recovered within 3 min (p = 0.042). CONCLUSION: During an RSE, Pmean and IMVC decreases were first concomitant to peripheral alterations up to 40% TSTF and central alterations was only observed in the second part of the test, while peripheral alterations plateaued. The distinct recovery kinetics in central versus peripheral components of fatigability further confirm the necessity to reduce traditional delays in neuromuscular fatigue assessment post-exercise.

Comparison of the on-line effects of different motor simulation conditions on corticospinal excitability in healthy participants.

In healthy participants, corticospinal excitability is known to increase during motor simulations such as motor imagery (MI), action observation (AO) and mirror therapy (MT), suggesting their interest to promote plasticity in neurorehabilitation. Further comparing these methods and investigating their combination may potentially provide clues to optimize their use in patients. To this end, we compared in 18 healthy participants abductor pollicis brevis (APB) corticospinal excitability during MI, AO or MT, as well as MI combined with either AO or MT. In each condition, 15 motor-evoked potentials (MEPs) and three maximal M-wave were elicited in the right APB. Compared to the control condition, mean normalized MEP amplitude (i.e. MEP/M) increased during MI (P = .003), MT (P < .001) and MT + MI (P < .001), without any difference between the three conditions. No MEP modulation was evidenced during AO or AO + MI. Because MI provided no additional influence when combined with AO or MT, our results may suggest that, in healthy subjects, visual feedback and unilateral movement with a mirror may provide the greatest effects among all the tested motor simulations.

Reliability outcomes and inter-limb differences in ankle joint stiffness in children with unilateral cerebral palsy depend on the method of analysis.

Children with cerebral palsy (CP) present increased passive ankle joint stiffness, measured as the slope of the torque-angle curve relationship. However, large discrepancies in results exist among studies, likely because of various methodologies used. The purpose of this study was to determine the influence of different calculation methods on the outcomes and their inter-session reliability in children with unilateral CP (UCP). Thirteen children (mean age: 9.8 years) with spastic UCP underwent passive ankle mobilization at 2°/s on both legs using a dynamometer, on two occasions separated by one week. Passive ankle joint stiffness was calculated as the slope of the torque-angle curve using linear regression on three different relative ranges of torque (i.e. 30%-100%, 20-80% and 50-90% of maximal torque for method 1, 2 and 3, respectively) for both the paretic and non-paretic legs. Inter-session reliability was significantly lower on paretic leg (mean CV = 13.8%, ICC = 0.62) when compared to non-paretic leg (mean CV = 6%, ICC = 0.85), and method 3 presented lower reliability outcomes (mean CV = 11.7%, ICC = 0.75) than methods 1 (mean CV = 7.5%, ICC = 0.78) and 2 (mean CV = 6.6%, ICC = 0.79). Paretic values (0.24 Nm/°) were not different from the non-paretic leg (0.25 Nm/°), although significantly higher when considering the same angular sector (0.18 Nm/°). Passive ankle joint stiffness measurement can be reliably performed in children with UCP using method 1 and 2 while method 3 should be avoided. The non-paretic leg may be used for comparison with the paretic leg when taking into account differences in maximal dorsiflexion angle between legs. ClinicalTrials.gov Identifier: NCT02960932.

Two-dimensional and shear wave elastography ultrasound: A reliable method to analyse spastic muscles?

INTRODUCTION: Few data exist on the feasibility and reliability of measuring muscular atrophy in 2 dimensions (2D) by ultrasonography (US) and elasticity with shear wave elastography (SWE) in spastic muscles. METHODS: Fourteen patients with chronic stroke took part in 2 intersession reliability experiments performed with 1-week intervals between sessions. Pennation angle (PA), muscle thickness (MT), and shear elastic modulus (µ) were measured in spastic gastrocnemius medialis (GM) muscles at rest and at maximal passive stretching in paretic and nonparetic legs. RESULTS: On the paretic side, the coefficient of variation (CV) in GM was 6.30% for MT and 6.40% for PA at rest and was 7.53% and 8.26% for MT and PA, respectively, at maximal passive stretching. The reliability of the µ measurement was good only for GM at rest on the paretic side (CV = 9.86%). DISCUSSION: 2D US associated with SWE shows promise for assessing structural changes in muscles. With some methodological adaptations, this approach could help guide spasticity treatment. Muscle Nerve 57: 222-228, 2018.