Determining the contribution of the corticospinal and reticulospinal tract responses to resistive exercise? An exploratory study in non-trained and strength-trained participants.

It is well established that the human neuromuscular system can modify its function in response to physical activity or experience. This response has been termed ‘plasticity’ and involves reorganisation of neural circuits in the primary motor cortex (M1) that control movement. In several different ways, resistive exercise has been shown to influence plastic changes in the neuromuscular system. However, little is known regarding the modulation and the plasticity of the neural pathways interconnecting elements of the neuromuscular system and skeletal muscle in resistant-trained individuals. Further, additional important descending motor pathways, such as the cortico-reticulospinal or cortico-propriospinal pathway, could also serve as important neural pathways that contribute to neural drive and thus force production. There is little information regarding these potentially important pathways in humans, and no information regarding the efficacy of strength training to affect these pathways. These pathways can be stimulated via transcranial magnetic stimulation (TMS), which produce muscle twitches referred to as a motor-evoked potential (MEP). The amplitude of a MEP is a crude measure of the excitability of the neurons that contribute to these descending motor pathways. Typically, activation of the cortico-reticulospinal pathway produces MEPs ipsilateral to the motor cortex that is stimulated, suggesting that uncrossed descending motor pathways could be involved in the cortical control of ipsilateral muscle activity. Importantly, the cortical control of ipsilateral proximal upper limb muscles is rarely studied in healthy people, which is surprising, given that ipsilateral control of the upper limb is evident in healthy people and the recovery of motor function after adult stroke and cerebral palsy. Because strength training results in an increase in force output from the trained muscle(s), examining the ipsilateral MEPs between trained and non-strength trained individuals may provide important new knowledge on the neuromuscular mechanisms of strength development. Therefore, this project will examine the effects of resistive exercise on modulating the corticospinal and reticulospinal tract, which will provide new insights to the contribution of different descending motor pathways and their training-induced effects in increasing force output.