Eccentric contractions are commonly executed in daily activities and frequently utilised in both sporting and rehabilitation settings due to several advantages, including their lower metabolic cost compared to concentric and isometric actions, while eliciting favourable adaptations. However, when individuals engage in a bout of unfamiliar eccentric exercise (EE), symptoms of exercise-induced muscle damage (EIMD) are often experienced in the days following exercise. Interestingly, if the same EE is repeated in the weeks following the initial exercise, EIMD symptoms are attenuated, a phenomenon known as the repeated bout effect (RBE). One proposed mechanism for this phenomenon is neural in nature, whereby a shift in activation strategy is suggested to protect the muscle from further damage in a repeated bout, but the precise neural changes are relatively unknown. The aim of this Thesis was to investigate single motor unit (MU) alterations using high-density electromyography during and in the days after repeated bouts of EE performed with the tibialis anterior (TA). Additionally, this work aimed to assess the reliability and validity of the StartReact method to indirectly assess reticulospinal tract function and its potential role in the recovery of force after a bout of EE. Study 1 (Chapter 3) investigated whether MU properties are altered during a repeated bout of EE. Numerous MU alterations occurred with EE; specifically, we demonstrated a reduction in the coefficient of inter-spike intervals (CoVISI), signifying a reduction in discharge rate variability. We also found a greater reliance on lower threshold MUs during the repeated bout of EE. These findings suggest an adaptation during a second bout of EE, possibly playing a role in the attenuation of EIMD symptoms following a repeated bout. Study 2 (Chapter 4) confirmed significant disruptions in markers of EIMD following the initial bout of EE and confirmed the presence of an RBE, evidenced by a quicker recovery of markers of EIMD in the days following Bout 2 compared to Bout 1. Following both bouts of EE, MU discharge rate increased, accompanied by the reduction in MVIC, likely in compensation for the associated exercise-induced force loss. Moreover, the increase in DR during Bout 2 was less pronounced compared to Bout 1. Additionally, force steadiness and CoVISI was less variable in Bout 2. Furthermore, derecruitment thresholds increased over time in Bout 1 but no change was present in Bout 2. In Study 3 (Chapter 5), we found that a small StartReact effect can be assessed in the TA, but the methods were not reliable between-days. Additionally, reticulospinal drive was not affected by EIMD, suggesting that this tract may not play a role in the recovery of force from EIMD, at least in the TA. Collectively, the findings from this Thesis extend the knowledge of neurophysiological alterations associated with damaging EE and contribute to understanding the “neural” component of the RBE.
- Motor Unit
- Exercise induced muscle damage
- High-density electromyography
- Reticulospinal Tract
Neurophysiological responses during/following single and repeated bouts of eccentric exercise
Hayman, O. (Author). 19 Dec 2024
Student thesis: Doctoral Thesis