Rapid Multimuscle Cortical Mapping of the Upper Limb: Activity Reveals Intralimb Gradients

Transcranial magnetic stimulation (TMS) mapping provides valuable insight into corticospinal organisation and plasticity, but conventional protocols are often too time-consuming to be practical in clinical settings. This study aimed to refine a rapid, multimuscle TMS mapping protocol capable of comprehensively characterising upper limb motor representations, including both distal and proximal muscles, and to examine how stimulation intensity and voluntary muscle activity shape cortical map metrics and motor evoked potential (MEP) latency. Nineteen healthy adults underwent neuronavigated TMS mapping of eight upper limb muscles using a rapid random walk protocol. Cortical maps were acquired at multiple stimulation intensities referenced to the motor threshold of the first dorsal interosseous muscle, both at rest and during a task requiring low levels of muscle activity. Map area, volume and centre of gravity were quantified using relative and absolute thresholds, alongside automated analysis of MEP onset latency. Cortical representations showed stable somatotopic organisation across conditions, with proximal-muscle centres of gravity located more medially than distal muscles. Increasing stimulation intensity preferentially expanded distal muscle map area and volume, whereas voluntary muscle activity selectively facilitated proximal-muscle representations, even after controlling for background muscle activity. These effects revealed a clear proximal-distal gradient in the physiological modulation of cortical maps. MEP latencies were shorter for proximal muscles and during active contraction. At rest, latency increased with distance from the map centre for several muscles, but this spatial gradient was abolished or inverted during voluntary activity. These findings demonstrate that rapid, multimuscle cortical mapping is feasible and yields physiologically meaningful metrics across the upper limb. The results highlight a functional dissociation between proximal and distal representations and underscore the importance of combining high-intensity resting and active mapping conditions to obtain a complete assessment of corticospinal integrity, with direct relevance for future clinical studies in neurological populations.