Effects of sustained cycling exercise on corticospinal responsiveness

Simranjit Kaur Sidhu (2012). Effects of sustained cycling exercise on corticospinal responsiveness PhD Thesis, School of Human Movement Studies, The University of Queensland.

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Author Simranjit Kaur Sidhu
Thesis Title Effects of sustained cycling exercise on corticospinal responsiveness
School, Centre or Institute School of Human Movement Studies
Institution The University of Queensland
Publication date 2012-03
Thesis type PhD Thesis
Supervisor Associate Professor Timothy Carroll
Professor Andrew Cresswell
Total pages 99
Total black and white pages 99
Language eng
Subjects 110603 Motor Control
110602 Exercise Physiology
110903 Central Nervous System
110601 Biomechanics
Abstract/Summary The purpose of this thesis was to investigate the responsiveness of the human corticospinal pathway to sustained cycling exercise. The application of non-invasive stimulation techniques during sustained single joint exercise suggests that the responsiveness of the corticospinal pathway has the capacity to change during exercise. A better understanding of changes in corticospinal responsiveness during sustained locomotor exercise, that involves rhythmic activation of muscles at multiple joints, should contribute to a broader knowledge of the sites and mechanisms of central changes during sustained exercises of this type. The technique of transcranial magnetic stimulation (TMS) has been widely used to probe the central nervous system and provide direct evidence of changes in responsiveness of the human corticospinal tract during sustained single joint exercise. Because of the inability to dissociate cortical versus spinal contributions to any changes in TMS-evoked responses, stimulation of the descending tracts at the cervicomedullary junction (CMS) can be used to account for changes in excitability of spinal motoneurons. In addition, TMS at very low intensities can be used to quantify the contribution of the human motor cortex to muscle activation. This method works by activating intracortical inhibitory circuits, which can produce a consequent reduction in output from the corticospinal cells and a reduction in ongoing EMG activity. The first experimental study (Chapter 3) investigated corticospinal contributions to locomotor drive during normal cycling. In the study, TMS at very low intensities was applied during cycling to activate intracortical inhibitory interneurons in the motor cortex. Because the subthreshold stimulation suppressed the ongoing cycling EMG, the results provided evidence that the motor cortex is involved in generating locomotor muscle activity. In the same study, TMS and CMS were applied during cycling, and the results showed that the phase-dependent modulation of corticospinal responses was driven mainly by spinal mechanisms. However, since the MEP responses were slightly more facilitated than CMEP responses prior to the EMG burst, there may also be small increases in cortical excitability prior to muscle activation. The results of the study also confirmed the applicability of these techniques to locomotion to allow the exploration of changes in responsiveness of the motor cortex to interventions such as sustained cycling exercises. To investigate the effects of sustained cycling exercise on the responsiveness of the human corticospinal tract, TMS and CMS were applied during the sustained cycling exercise (Chapter 4). It was shown that, at task failure of a 30 minute sustained cycling exercise, responses elicited by TMS were comparable to control responses whereas responses elicited by CMS were larger than control responses. The results suggest that the responsiveness of motor cortical neurons was reduced and the responsiveness of spinal motoneurons was increased. This pattern of corticospinal modulation contrasts markedly to that observed previously during single joint contractions, where an increased cortical and decreased spinal responsiveness is observed. In a separate experiment (Chapter 5), it was shown that sustained locomotor exercise also alters the responsiveness of the intracortical inhibitory interneurons. Specifically, the amount of suppression in the ongoing cycling EMG elicited via TMS at subthreshold intensities was larger during the last 5 minutes of a 30 minute exercise when compared to the first 5 minutes. Because a similar increase was shown in a recent study involving sustained single joint contraction, it was proposed that the increase in intracortical inhibition during sustained contractions is independent of the mode of exercise. Overall, the data from the thesis provide new evidence that sustained cycling exercise has the capacity to modulate the responsiveness of the neurons within the human motor cortex. While the differences in responsiveness of the corticospinal cells to locomotor versus single joint exercise are likely to be due to differences in systemic responses such as increases in brain temperature and muscle metabolic changes to the two modes of exercises, an acute exercise-induced response that is common to fatiguing exercise of any nature, such as an accumulation of fatigue metabolites, is likely to explain the similar responsiveness of the intracortical inhibitory interneurons during locomotor versus single joint exercise.
Keyword Central fatigue
endurance cycling exercise
transcranial magnetic stimulation (TMS)
Motor Cortex
intracortical inhibition

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Created: Mon, 18 Jun 2012, 09:58:13 EST by Ms Simranjit Kaur Sidhu on behalf of Library - Information Access Service