A motor unit, which is comprised of a motoneurone and the muscle fibres it innervates, forms the basic building block of motor control. Studying the behaviour of motor units affords insight into the properties of human motoneurones. Serotonin (5-HT) has strong modulatory effects on motoneurone excitability. 5-HT neurons form monosynaptic connections with motoneurones and converging lines of evidence indicate that 5-HT release onto motoneurones varies proportionally with the intensity of motor activity. Cellular preparations have indicated that the most significant modulatory effects that 5-HT has on motoneurones result from activation of somato-dendritic 5-HT2 receptors, which facilitate the generation of a type of electrical current that is resistant to inactivation known as a persistent inward current (PIC). However, a direct link between motor unit discharge, 5-HT2 receptor activity, and PIC activity, is yet to be demonstrated during voluntary muscle contractions in humans. Therefore, the purpose of this thesis is to investigate the competitive antagonism of 5-HT2 receptors in human participants and quantify the impact this has on motor unit activity as measured by high-density surface electromyography (HDsEMG). [...]