Impact of fatiguing exercise on corticospinal excitability and motor performance in young and older adults

Abstract

The impact of fatigue on corticospinal excitability and GABAergic inhibitory activity has been relatively well studied in young adults. However, this is yet to be established in older adults. Furthermore, the role of fatigue on motor skill performance remains largely understudied with a majority of the studies ranging between the 1970s-90s in young adults. The purpose of this thesis was to identify the age-related differences in corticospinal mechanisms and motor performance with isometric single joint fatiguing exercise. This was achieved by applying single and paired-pulse transcranial magnetic stimulation (TMS) prior to, during, and after exercise, as well as performance of a speed-accuracy movement task in both age groups. The first experimental study (chapter two) evaluated the effect of fatigue induced by a fifteen-minute sustained submaximal isometric contraction (15% of maximum electromyography [EMG]) of the first dorsal interosseous (FDI) muscle on corticospinal excitability, short- (SICI) and long- (LICI) interval intracortical inhibition in young and older adults. While no change in SICI was identified in both age groups, an age-related reduction in amount of LICI was seen suggesting a compensatory decline in GABAB mediated inhibition in older adults. Nevertheless, a varying magnitude of fatigue was observed between young and old which required further investigation. The second experimental study (chapter three) implemented a larger muscle group (elbow flexor muscles) and a submaximal isometric contraction held at 30% of their maximum force to task failure to achieve a similar amount of fatigue in both age groups. Contrary to chapter two, an identical decline in GABAB mediated inhibition was observed in both age groups when a similar amount of fatigue was induced, indicating that fatigue related changes in GABA modulation may be task and muscle dependent. Using a similar exercise model, the third experimental study (chapter four) investigated age-related differences in performance of a speed-accuracy task with fatigue. While there was an attenuation in motor skill performance with increasing task difficulty, there were no differences across age groups. Given the noted impact of fatigue on corticospinal excitability and motor performance in chapters two, three and four, investigation of transcranial direct current stimulation (tDCS) as a possible intervention was assessed as the final step. TDCS can be used to manipulate fatigability either as a priming tool or in conjunction with a fatiguing task. In the final study (chapter five), the impact of cathodal primed anodal tDCS on corticospinal excitability, fatigability, and motor skill performance was assessed in both age groups with an unexpected suppression in corticospinal excitability and greater attenuation in GABAB observed following anodal tDCS. However, cathodal priming as standalone had no direct impact on corticospinal excitability, GABA modulation, fatigability, or motor skill performance. This thesis provides novel evidence of an age-related retention in the ability to modulate corticospinal excitability and motor performance when a similar amount of fatigue is induced in young and older adults.