Reticulospinal tract adaptations to resistance training in humans

Atkinson, Elliott (2024) Reticulospinal tract adaptations to resistance training in humans. Doctoral thesis, Northumbria University.

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Abstract

Resistance training is a potent means for increasing muscular strength, and even short periods of training elicit discernible functional improvements. Central nervous system adaptations which underpin the initial increases in strength have been well researched, yet the locus of these changes is not well-understood. Research has focused on the corticospinal tract (CST), but with equivocal results. The reticulospinal tract (RST) has been recently implicated as a possible neural substrate, yet much of this work is inferred from secondary observations, animal models or cross-sectional data. The aim of this thesis was to investigate the role of the RST in adaptation to resistance training, by studying the RST function of those who are already chronically resistance trained (CRT), and the time-course of RST adaptations during a period of resistance training. Study 1 (Chapter 4) investigated the test-retest reliability of indirect testing methods for assessing RST function, enabling the design of a suitable testing battery. Study 2 (Chapter 5) examined differences in RST function of the dominant elbow flexors and extensors between CRT and untrained (UT) individuals. Lastly, Study 3 (Chapter 6) investigated the time-course adaptations of RST function to 6-weeks of isometric resistance training in the dominant elbow flexors. In Chapter 4, the most reliable indirect measures of human RST function were pairing cervical motor evoked potentials (CMEP) from cervico-medullary stimulations with startling auditory stimuli (CMEPCON) in the elbow flexors and extensors (ICC ≥ 0.66, CV ≤ 16%), alongside all StartReact reaction times in the same muscles (ICC ≥ 0.78, CV ≤ 10%). All other measures were unreliable. In Chapter 5, CRT RST excitability was greater as measured by CMEPCON (22% greater than UT p = 0.037) and StartReact times in both elbow flexors (13% faster than control, p = 0.026) and extensors (17% faster than control, p = 0.016). In Chapter 6, isometric strength increased (Δ11%, p = 0.006) following a 6-week period of isometric resistance training, concurrent with reductions in StartReact reaction time (31%, p = 0.004), and an increased RST Gain at week 1 (22%, p = 0.043), compared to controls. However, there were no cortical or CST changes. Collectively the findings of this work demonstrate that RST adaptations contribute to the increased muscular strength observed after resistance training.

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