Rebuttal from Taylor

Abstract

POINT-COUNTERPOINTRebuttal from TaylorPublished Online:01 Jul 2009https://doi.org/10.1152/japplphysiol.91220.2008bMoreSectionsPDF (45 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat Twitch interpolation measures the proportion of maximal possible muscle force produced during voluntary contraction. Drs. de Haan, Gerrits, and de Ruiter conclude this in their second paragraph, but two of three objections to the validity of twitch interpolation are based on the premise that voluntary activation represents something more central, perhaps motoneuron firing or even input to motoneurons.They first object that sensitivity at near maximal contraction intensities is low. The cited model (4) uses a sigmoidal input-output relationship for motoneurons, such that large changes in excitation to the motoneurons produce only small changes in force at high contraction strengths. Hence, twitch interpolation, like voluntary force, has low sensitivity to altered excitatory drive to the motoneuron pool at high forces. However, the model shows a linear relationship between voluntary force and the interpolated twitch and supports the validity of twitch interpolation. Experimental evidence given for low sensitivity is a relatively large EMG increase for the increase in force above 80% maximum in the knee extensors (5). The implication is that small increases in force require large increases in motor unit firing, but other factors like synchronization can also increase EMG (6). Furthermore, other muscles do not show similar EMG-force relationships (2).The second objection that voluntary activation cannot be generalized from one situation to another seems a positive rather than a negative. Under different loading conditions different firing rates will be required for production of full force and one would not expect the same maximal voluntary activation.On the third objection, I concede that nonlinearities in the relationship between the interpolated twitch and voluntary force are common in practice, and extrapolation to predict maximal force is not recommended. The paper cited (5) shows extreme nonlinearity for individuals' knee extensors. Similar curves were seen for biceps brachii where methodological contributors were postulated (1). Such possibilities, including stimulation of antagonists, muscle lengthening, and variable synergist or antagonist contribution to voluntary torque, also hold for knee extension. Triplet stimulation may also contribute to nonlinearity as multiple pulses produce more torque than single pulses during weak but not strong contractions (1). Similarly, short interpulse intervals increase force greatly in unrecruited muscle, but not during repetitive firing (3).Thus twitch interpolation using nerve stimulation cannot measure motoneuronal input or output but only how well the muscle is driven to produce force. Although never ideal in humans, it usefully discriminates altered drive to muscles.REFERENCES1 Allen GM, McKenzie DK, Gandevia SC. Twitch interpolation of the elbow flexor muscles at high forces. Muscle Nerve 21: 318–328, 1998.Crossref | PubMed | ISI | Google Scholar2 Gelli F, Del Santo F, Popa T, Mazzocchio R, Rossi A. Factors influencing the relation between corticospinal output and muscle force during voluntary contractions. Eur J Neurosci 25: 3469–3475, 2007.Crossref | PubMed | ISI | Google Scholar3 Griffin L, Godfrey S, Thomas CK. Stimulation pattern that maximizes force in paralyzed and control whole thenar muscles. J Neurophysiol 87: 2271–2278, 2002.Link | ISI | Google Scholar4 Herbert RD, Gandevia SC. Twitch interpolation in human muscles: mechanisms and implications for measurement of voluntary activation. J Neurophysiol 82: 2271–2283, 1999.Link | ISI | Google Scholar5 Kooistra RD, de Ruiter CJ, de Haan A. Conventionally assessed voluntary activation does not represent relative voluntary torque production. Eur J Appl Physiol 100: 309–320, 2007.Crossref | ISI | Google Scholar6 Yao W, Fuglevand RJ, Enoka RM. Motor-unit synchronization increases EMG amplitude and decreases force steadiness of simulated contractions. J Neurophysiol 83: 441–452, 2000.Link | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 107Issue 1July 2009Pages 357-358 Copyright & PermissionsCopyright © 2009 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.91220.2008bHistory Published online 1 July 2009 Published in print 1 July 2009 Metrics Downloaded 217 times