Cortical voluntary activation can be reliably measured in human wrist extensors using transcranial magnetic stimulation

Health and Exercise Science, School of Medical Sciences, Faculty of Medicine, University of New South Wales, Sydney, Australia.
Clinical Neurophysiology (Impact Factor: 3.1). 05/2008; 119(5):1130-8. DOI: 10.1016/j.clinph.2007.12.018
Source: PubMed


A twitch interpolation technique using transcranial magnetic stimulation (TMS) was recently developed to measure motor cortical drive to human elbow flexors. Here, we described studies designed to test the applicability and reliability of the technique for the human wrist extensors and to provide new evidence regarding the sensitivity of the technique to inadvertent antagonist activation.
Study 1: we measured amplitudes of superimposed twitches (SITs) produced by TMS during wrist extension at intensities from rest to maximal voluntary contraction on two occasions (n=9). Study 2: we assessed the impact of inadvertent antagonist activation by TMS on measurement of voluntary activation using a muscle potentiation technique to increase mechanical efficiency of the wrist flexors (n=6).
The SITs decreased linearly between 25% and 100% MVC and voluntary activation could be reliably estimated across days (ICC(2,1)=0.963, p<0.001). Prior potentiation of the wrist flexors had little impact on extension SITs and voluntary activation.
TMS allows valid and reliable measurement of voluntary activation of the wrist extensors.
TMS can be used to assess effects of supraspinal fatigue, pathology and rehabilitation interventions on cortical activation in upper limb muscle groups.

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    • "Cortical voluntary activation and supraspinal fatigue were first measured in the elbow flexors (Todd et al., 2003, 2004). Recently, the TMS voluntary activation technique has been validated for the wrist extensors (Lee et al., 2008) and back extensors (Lagan, Lang, & Strutton, 2008). However, none of these muscle groups lend themselves well to the study of dynamic locomotor exercise. "
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    ABSTRACT: Abstract The aim of this commentary is to provide a brief overview of transcranial magnetic stimulation (TMS) and highlight how this technique can be used to investigate the acute and chronic responses of the central nervous system to exercise. We characterise the neuromuscular responses to TMS and discuss how these measures can be used to investigate the mechanisms of fatigue in response to locomotor exercise. We also discuss how TMS might be used to study the corticospinal adaptations to resistance exercise training, with particular emphasis on the responses to shortening/lengthening contractions and contralateral training. The limited data to date suggest that TMS is a valuable technique for exploring the mechanisms of central fatigue and neural adaptation.
    01/2014; 14(sup1):S332-S340. DOI:10.1080/17461391.2012.704079
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    • "Voluntary activation of human elbow flexors has been tested with using transcranial magnetic stimulation (TMS) of motor cortex but not peripheral nerve magnetic stimulation where electrical stimulation is typically more prevalent in use (Todd et al., 2003; 2004). However, the evoked twitches produced by a cortical stimulus superimposed during maximal isometric contractions test degree of volitional drive to muscle for supraspinal contributions, while the evoked twitch force to peripheral stimulation assesses spinal neural drive (Lee et al., 2008). No detailed comparison between these two methods of peripheral nerve stimulation has been undertaken for elbow flexors with respect to the use of twitch interpolation for the assessment of voluntary activation-force relationship. "
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    ABSTRACT: The study compared peripheral magnetic with electrical stimulation of the biceps brachii m. (BB) in the single pulse Interpolation Twitch Technique (ITT). 14 healthy participants (31±7 years) participated in a within-subjects repeated-measures design study. Single, constant-current electrical and magnetic stimuli were delivered over the motor point of BB with supramaximal intensity (20% above maximum) at rest and at various levels of voluntary contraction. Force measurements from right elbow isometric flexion and muscle electromyograms (EMG) from the BB, the triceps brachii m. (TB) and the abductor pollicis brevis m. (APB) were obtained. The twitch forces at rest and maximal contractions, the twitch force-voluntary force relationship, the M-waves and the voluntary activation (VA) of BB between magnetic and electrical stimulation were compared. The mean amplitude of the twitches evoked at MVC was not significantly different between electrical (0.62 ± 0.49 N) and magnetic (0.81 ± 0.49 N) stimulation (p > 0.05), and the maximum VA of BB was comparable between electrical (95%) and magnetic (93%) stimulation (p > 0. 05). No differences (p >0.05) were revealed in the BB M-waves between electrical (13.47 ± 0.49 and magnetic (12.61 ± 0.58 stimulation. The TB M-waves were also similar (p > 0.05) but electrically evoked APB M-waves were significantly larger than those evoked by magnetic stimulation (p < 0.05). The twitch-voluntary force relationship over the range of MVCs was best described by non-linear functions for both electrical and magnetic stimulation. The electrically evoked resting twitches were consistently larger in amplitude than the magnetically evoked ones (mean difference 3.1 ± 3.34 N, p < 0.05). Reduction of the inter-electrodes distance reduced the twitch amplitude by 6.5 ± 6.2 N (p < 0.05). The fundamental similarities in voluntary activation assessment of BB with peripheral electrical and magnetic stimulation point towards a promising new application of peripheral magnetic stimulation as an alternative to the conventional ITT for the assessment of BB voluntary activation.
    Journal of sports science & medicine 12/2012; 11(4):709-18. · 1.03 Impact Factor
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    • "dall et al . , 2009 ) . Indeed , TMS is less effective at activating motoneurons at lower contraction levels because of the reduction in corticospinal excitability ( Todd et al . , 2003 ) . This is characterized by a curvilinear relationship between SIT and voluntary torque when using contraction strengths below 50% MVC ( del Olmo et al . , 2006 ; Lee et al . , 2008 ) . It may also be impossible to obtain a SIT at high - contraction intensities ( >75% MVC ) ( del Olmo et al . , 2006 ) . Therefore , if a SIT can be evoked at high - contraction intensities and if the relationship between SIT and force ( 50 – 100% MVC ) appears to be linear ( r P 0 . 9 ) , then it is appropriate to estimate resting tw"
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    ABSTRACT: This review aims to characterize fatigue-related changes in corticospinal excitability and inhibition in healthy subjects. Transcranial magnetic stimulation (TMS) has been extensively used in recent years to investigate modifications within the brain during and after fatiguing exercise. Single-pulse TMS reveals reduction in motor-evoked potentials (MEP) when measured in relaxed muscle following sustained fatiguing contractions. This modulation of corticospinal excitability observed in relaxed muscle is probably not specific to the fatigue induced by the motor task. During maximal and submaximal fatiguing contractions, voluntary activation measured by TMS decreases, suggesting the presence of supraspinal fatigue. The demonstration of supraspinal fatigue does not eliminate the possibility of spinal contribution to central fatigue. Concomitant measurement of TMS-induced MEP and cervicomedullary MEP in the contracting muscle, appropriately normalized to maximal muscle compound action potential, is necessary to determine the relative contribution of cortical and spinal mechanisms in the development of central fatigue. Recent studies comparing electromyographic responses to paired-pulse stimuli at the cortical and subcortical levels suggest that impaired motoneuron responsiveness rather than intracortical inhibition may contribute to the development of central fatigue. This review examines the mechanical and EMG responses elicited by TMS (single- and paired-pulse) and cervicomedullary stimulation both during and after a fatiguing exercise. Particular attention is given to the muscle state and the type of fatiguing exercise when assessing and interpreting fatigue-induced changes in these parameters. Methodological concerns and future research interests are also considered.
    Neuroscience 01/2012; DOI:10.1016/j.neuroscience.2012.10.058 · 3.36 Impact Factor
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