Both recruitment and rate coding are well-known mechanisms by which the human nervous system grades muscle force. This thesis has utilised the mechanism of rate coding to generate optimised stimulation patterns that enhance contractile responses in both a fatigued and non-fatigued state. Human motoneurones are known to fire with significant discharge variability (irregularity) during voluntary contractions. Yet many of the mechanisms as to why they fire in this manner, are still unclear. This thesis has tested the hypotheses that integrating physiological variability into trains of stimuli could offer some advantages to the human neuromuscular system that have not yet been explored. We have stimulated single motor axons and multiple motor units with long, short and continuous trains of stimuli that integrate discharge variability. The results reported in this thesis highlight the benefits that discharge irregularity offers to improving contractile responses and reducing fatigue in human leg muscles. While the entirety of this research has been conducted in healthy human subjects, there is a potential for this to translate clinically. Functional electrical stimulation (FES) or neuromuscular stimulation is a well-known therapy utilised after stroke or spinal cord injury to assist in restoring motor function. It can help to reduce muscle atrophy, improve contractility and increase muscle strength by electrically exciting the muscles via surface electrodes directly over the muscle belly. Current FES stimulation patterns often incorporate high frequency, constant-interval stimuli that do not resemble the physiological patterns that are exhibited during normal voluntary contractions. This thesis has used novel stimulation patterns that emulate the firing of volitionally active motoneurones in an attempt to increase contractile responses and reduce the magnitude of muscular fatigue
