Non-local muscle fatigue effects on muscle performance and corticospinal excitability

Šambaher, Nemanja (2015) Non-local muscle fatigue effects on muscle performance and corticospinal excitability. Masters thesis, Memorial University of Newfoundland.

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Abstract

Fatigue is defined as any decrease in the ability to produce force whether or not the task can be sustained. Inability of the central nervous system to adequately drive muscles has been termed central fatigue, while any physiological changes that occur at the neuromuscular junction or distal to it, have been classified as peripheral fatigue. Contractions of one muscle group can have direct local effects on the exercised muscle as well as more global effects, influencing non-exercised muscle groups as well. Exercise induced fatigue can produce performance decrements in non-exercised muscles, and this “non-local” effect of fatigue has been termed non-local muscle fatigue. It is thought that decrements in performance of the non-local muscles are due to central fatigue mechanisms. Decrements in performance along with deficits in voluntary activation and no changes in the peripheral system, suggest that processes within spinal and/or supraspinal structures mediate these decrements. Measuring excitatory-inhibitory balance of these structures can provide insights into how central nervous system behaves in response to non-local muscle fatigue. However, due to methodological limitations of the studies that investigated this phenomenon so far, it is not possible to provide any suggestions on that matter. Exploring this phenomenon can provide us with further understanding of the complex central nervous system functioning, as well it can provide us with further insights into the origins of fatigue. Based on this data, important rehabilitation and athletic strategies can be employed. Non-local muscle fatigue research literature is not expansive and there are contradictory results between studies. Different fatiguing protocols, outcome variables, and population samples, are just a few factors that can explain inconsistent findings. Literature seems to suggest that non-local muscle fatigue might be muscle and contraction type specific. Studies have shown that dominant arms are more susceptible to non-local muscle fatigue, and that fatiguing protocols with higher intensity and higher volume are able to produce larger magnitude of non-local muscle fatigue. There are only a few studies that have investigated changes in corticospnal excitability in non-exercised muscles after non-local muscle fatigue. These measurements were usually taken during rest, which is not a good representation of what occurs during a contraction. Furthermore, all non-local muscle fatigue studies so far measured excitability of the corticospinal tract as a whole, without taking into account modulation of spinal and supraspinal excitability separately. To further the knowledge on this topic, we investigated the effect of knee extensor fatigue on dominant elbow flexor force output, voluntary activation, corticospinal excitability and muscle contractile properties. To answer these questions, subjects were required to come for two testing sessions consisting of the same pre- and post-test measurements. At pre-test, subjects performed three maximal voluntary contractions (MVCs), where baseline values for previously described variables were obtained. A fatiguing protocol (intervention condition) that consisted of 12 MVCs with 10 sec rest period between repetitions ensued (or rest for control condition), which was followed by the post-test protocol (measurements). The fatiguing protocol consisted of 5 sets of dynamic bilateral knee extension contractions until task failure, which has been shown previously to be successful in inducing cross-over fatigue. Based on the previous literature, it has been hypothesized that knee extensor fatigue will cause dominant elbow flexor decrements in force production, voluntary activation and corticospinal excitability, without affecting muscle contractile properties. The results demonstrated that knee extensor fatigue was successful in eliciting non-local muscle fatigue in the dominant elbow flexors. Knee extensor fatigue lowered dominant elbow flexor force production, voluntary activation and interestingly, it caused elbow flexor peripheral fatigue. Furthermore, knee extensor fatigue modulated corticospinal excitability differently compared with control, suggesting that complex changes in corticospinal excitability might have contributed to voluntary activation decrements.

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