INVESTIGATION OF NEURAL CONTROL IN DISUSE AND DISEASE AND DEVELOPMENT OF THE NECESSARY TOOLS

The motor unit (MU) is the basic functional component of the neuromuscular system and consists of an alpha motoneuron, its axon, and all the muscle fibres it innervates. The MU acts as a neuromechanical transducer and plays a pivotal role in generating muscle contractions by converting descending neural inputs into muscle forces. Thanks to recent technological advances, the High-Density surface Electromyography (HD-EMG) technique can be used for the non-invasive estimation of MUs behaviour and properties, and for the study of the neuromuscular control of muscle force production. This thesis starts with an introduction on the fundamentals of neuromuscular physiology, followed by a detailed description of the HD-EMG technique. This comprehensive understanding will lay the groundwork for the following three experimental chapters investigating the physiological adaptation of neural control to disuse and disease, and proposing advancements to the analysis of HD-EMG recordings. The first experimental chapter explores the effects of unilateral lower-limb suspension (ULLS) on the neural control of force production in the vastus lateralis muscle. Ten participants volunteered for 10 days of ULLS followed by 21 days of active recovery. The findings reveal changes in MU properties during unloading, particularly in the discharge rate of lower-threshold MUs. Remarkably, these alterations are reversed by the subsequent active recovery period, highlighting the plasticity of the neuromuscular system. The second experimental chapter investigates the early consequences of type 1 diabetes on the neural control of muscle force production. MUs activity was recorded from the vastus lateralis muscle of eight participants with type 1 diabetes and eight controls. The results unveil distinct neural control strategies in the diabetic group, detectable even in absence of any functional manifestations. These novel findings suggest that type 1 diabetes has early consequences on the neuromuscular system and highlights the necessity of a better characterization of neural control in this population. The third experimental chapter presents a comprehensive tutorial on how to investigate central and peripheral MUs properties using HD-EMG. Additionally, it also introduces the openhdemg framework, a free and open-source user-friendly project that simplifies the analysis of MUs properties. openhdemg enables researchers, regardless of coding proficiency, to study neural control. The conclusion section of the thesis will discuss the similarities and differences observed between the studied conditions and will present some research questions that emerged from our findings.