Mapping Neural and Motor Unit Adaptations to Strength Training Using Advanced Neurophysiological Techniques

Neural adaptations are integral to the efficacy of strength training, particularly during its early phases when hypertrophic changes remain negligible. These adaptations encompass increased excitability within both the corticospinal and reticulospinal pathways, alongside modifications in motor unit recruitment, discharge rates, synchronisation, and firing variability. A precise understanding of the underlying neural mechanisms driving these adaptations is important for optimising strength training protocols and rehabilitation strategies, particularly for populations with neurological impairments or motor dysfunctions. Despite significant advancements in neurophysiology, the specific loci within the central nervous system (CNS) that undergo plasticity to enhance force production remain insufficiently characterised. The interplay between corticospinal and reticulospinal pathways, as well as their distinct contributions to motor unit behavior, necessitates further investigation. This project seeks to comprehensively map neural plasticity across multiple CNS sites using advanced neurophysiological techniques, including transcranial magnetic stimulation (TMS), conditioned TMS responses to startling auditory stimuli (SAS), and high-density surface electromyography (HD-sEMG). By systematically tracking temporal changes in corticospinal and reticulospinal excitability across various stages of strength training, this research will offer a nuanced understanding of the roles these pathways play in facilitating early- and late-phase motor adaptations. A key research gap within strength training literature pertains to the impact of pacing strategies, metronome-paced strength training (MPST) versus self-paced strength training (SPST)—on neural excitability and motor unit adaptations. MPST, characterised by externally regulated timing, may enhance motor unit synchronisation and corticospinal excitability by imposing consistent temporal demands on neural circuits. In contrast, SPST, which allows for self-selected pacing, may induce greater variability in motor unit recruitment patterns, thereby promoting adaptive plasticity across both corticospinal and reticulospinal pathways. This study aims to examine how these pacing strategies differentially modulate neurophysiological mechanisms underpinning strength adaptation, with implications for both motor control and functional performance. The research is structured around four primary objectives. First, a systematic review of motor unit adaptations will synthesise existing literature on how strength training influences motor unit behavior. This review will consolidate findings from prior studies to identify patterns in recruitment thresholds, discharge rates, synchronisation, and firing variability, thereby providing a foundation for subsequent experimental research. Second, the study will investigate the temporal dynamics of neural adaptations, focusing on corticospinal and reticulospinal changes across different time points within MPST and SPST protocols. This approach will elucidate the progression of neural plasticity throughout training and determine whether pacing strategies exert distinct influences on these adaptations. The third study will assess comparative neural adaptations in strength versus power training, examining how different training modalities impact motor unit recruitment and neuroplasticity. Strength training, typically involving high-load, low-velocity contractions, is hypothesised to primarily drive corticospinal adaptations, whereas power training, which emphasises low-load, high-velocity movements, may preferentially enhance reticulospinal contributions. Finally, the study will explore corticospinal-reticulospinal interactions using conditioned SAS-TMS responses to probe the interplay between these pathways and their respective roles in strength development. Understanding the synergistic contributions of these pathways will offer deeper insights into the neural control of force production, ultimately informing evidence-based training strategies designed to optimise neuromuscular performance. This project is poised to generate novel insights into strength-related neural plasticity, with profound implications for both athletic performance enhancement and clinical rehabilitation. By leveraging cutting-edge neurophysiological methodologies, this research will establish evidence-based recommendations for targeted training interventions that enhance neural adaptations. The findings will contribute to rehabilitation neuroscience, motor learning, and sports performance science, serving as a foundational framework for precision training strategies tailored to specific neural deficits.