Corticospinal Responses to Sustained Locomotor Exercises: Moving Beyond Single-Joint Studies of Central Fatigue
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REVIEW ARTICLE
Corticospinal Responses to Sustained Locomotor Exercises: Moving Beyond Single-Joint Studies of Central Fatigue Simranjit K. Sidhu • Andrew G. Cresswell Timothy J. Carroll
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Published online: 1 March 2013 Ó Springer International Publishing Switzerland 2013
Abstract There is substantial evidence that fatiguing exercise is accompanied by changes within the central nervous system that reduce the force that can be produced by working muscles. Here we review studies that used noninvasive neurophysiological techniques to show that sustained single-joint contractions have the capacity to increase corticospinal responsiveness and reduce motoneuronal responsiveness. We contrast these findings with new evidence from our laboratory regarding corticospinal responsiveness during sustained cycling exercise. There seems to be a similar increase in responsiveness of the intracortical inhibitory interneurons during sustained locomotor and single-joint exercise which might be due to acute exercise responses that are common to fatiguing exercise of any nature, such as local accumulation of fatigue metabolites. In contrast, the pattern of changes in corticospinal responsiveness is fundamentally different between the two modes of exercise which might be due to greater systemic fatigue responses to locomotor exercises. 1 Introduction Muscle fatigue is defined as an exercise-induced reduction in the ability of a muscle or muscle group to produce S. K. Sidhu A. G. Cresswell T. J. Carroll Centre for Sensorimotor Neuroscience, School of Human Movement Studies, The University of Queensland, Brisbane, QLD 4072, Australia S. K. Sidhu (&) Department of Internal Medicine, George E. Whalen VA Medical Center, School of Medicine, University of Utah, GRECC 182, Building 2, Rm 1D27A, 500 Foothill Dr, Salt Lake City UT 84148, USA e-mail: [email protected]
maximum force [1]. Historically, it was assumed that muscle fatigue largely or exclusively stemmed from changes occurring distal to the neuromuscular junction, also known as peripheral fatigue [2]. However, there is now good evidence to demonstrate that some fatigue processes occur at or proximal to the neuromuscular junction and reside within the central nervous system [1]. Essentially, central fatigue occurs when maximum muscle force is reduced via insufficient motor unit recruitment and/or suboptimal motor unit firing rate. This may be due to modulations in input to the motoneuron pool from spinal reflex circuits and/or descending motor pathways [1]. It is apparent that the relative importance of central factors of fatigue is largely influenced by the type of exercise performed (for a review, see [3]). While central mechanisms of fatigue are well understood in the context of single-joint exercise [1, 4, 5], less work has been done in the context of locomotor exercises involving multiple limb muscles. The investigation of central mechanisms of fatigue during locomotor exercise is important because intramuscular and systemic fatigue properties are likely to substantially d
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