Investigation of the task- or muscle-specific organisation of motor cortex representations of shoulder muscles in pain-free adults

Neuroscience. 2026 Jun 18:S0306-4522(26)00410-0. doi: 10.1016/j.neuroscience.2026.06.023. Online ahead of print.

ABSTRACT

Coordination between the deltoid and rotator cuff (including subscapularis) muscles is critical to maintain dynamic stability of the glenohumeral joint. Organisation of the primary motor cortex (M1) might provide insight into how this coordination is controlled. Task specific organisation of the motor cortex would imply that individual muscles are controlled by multiple cortical regions subserving different functions, and that multiple muscles involved in a task are controlled by a common region. Alternatively, the cortex might be organised in a muscle-specific manner. This study compared these alternatives. Transcranial Magnetic Stimulation (TMS) was used to investigate the motor cortex representations of the middle deltoid and subscapularis muscles during two tasks (isometric abduction and internal rotation) in nineteen pain-free adults. Muscle-specific hotspots and active motor thresholds were identified. TMS maps were generated for each muscle during the tasks. Novel measures were used to quantify the extent of overlap between TMS map pairs. Overlap was observed between deltoid and subscapularis maps during the same task - consistent with TASK-specific organization. Overlap was also evident within the same muscle between tasks - aligning with a MUSCLE-specific organization. Subscapularis had a greater bias to MUSCLE-specific organisation. Results suggest coexisting principles of MUSCLE- and TASK-specific representations in M1 for the control of shoulder muscles. Differences between organisation of the rotator cuff and deltoid muscles might have functional relevance. This study provides insight into the M1 cortical organization principles underlying shoulder motor control.

PMID:42314795 | DOI:10.1016/j.neuroscience.2026.06.023