Investigating the Neural Control of Muscle Torque Using High-Density Surface Electromyography

The use of high-density surface electromyography (HDsEMG) decomposition presents an opportunity to study the behaviour of individual motor units, the smallest functional components of the neuromuscular system involved in movement. The present thesis aimed to investigate the neural control of muscle torque during isometric exercise of the ankle dorsiflexors in healthy adults using decomposition of the tibialis anterior HDsEMG signal. The first experimental study (Chapter 4) investigated the methodological aspects of HDsEMG signal acquisition including the skin-electrode interface, ground electrode configuration, and decomposition processing criteria. No differences in motor unit yield were observed between skin treatments, and at least four ground electrodes resulted in significant reductions in baseline signal noise. The use of adjusted acceptance criteria during the decomposition process resulted in a greater motor unit yield when compared to the original conditions. Furthermore, the low motor unit yields observed with increasing contraction intensities meant that contractions in subsequent studies were performed below 50% maximal voluntary contraction (MVC). Study 2 (Chapter 5) then investigated the effects of fatiguing intermittent exercise performed with arterial occlusion at 30% MVC on the complexity of muscle torque output and the motor unit cumulative spike train. Reductions in the complexity of the tibialis anterior motor unit cumulative spike train were concomitant with a loss of dorsiflexor muscle torque complexity, indicating an increase in the transmission of common synaptic input relative to independent synaptic input to the muscle as task failure approaches. In Study 3 (Chapter 6), the critical torque concept was extended to the ankle dorsiflexors, and the end-test torque from the 5 min all-out test was compared to the critical torque estimated from the conventional method involving multiple submaximal exercise bouts performed until task failure. The 5 min all-out test overestimated the critical torque compared to the conventional method, and the critical torque of the dorsiflexors was shown to be higher than previously reported for the knee extensors. In the final study (Chapter 7), the performance of 6 min prior heavy-intensity exercise had no discernible effects on tibialis anterior motor unit behaviour, suggesting the 'priming' effects previously observed for the knee extensors are absent in the dorsiflexor muscles. Collectively, the findings of this thesis confirm the dorsiflexors as a highly fatigue-resistant muscle group, and suggest that the findings from HDsEMG studies involving the tibialis anterior are not necessarily generalisable to other muscles (such as the knee extensors).