Mapping motor point response areas in the calf during transcutaneous neuromuscular electrical stimulation

Neuromuscular electrical stimulation (NMES) delivered transcutaneously activates skeletal muscle; its effectiveness is determined by stimulation parameters and by how skin-applied currents recruit underlying neuromuscular structures. Motor points (MPs) are commonly used to guide electrode placement; however, they are typically treated as discrete skin locations, and the spatial extent and reproducibility of surrounding responsive regions remain poorly characterised. Moreover, the minimum intensity required to elicit visible motor responses, and its variability over time, are seldom reported. The calf represents an appropriate model to examine these issues, as posterior calf stimulation produces visible ankle plantar flexion (PF). This study aimed to map stimulation response patterns in the skin associated with an identified MP in the posterior calf, assess their reproducibility, and determine the stimulation intensity required to elicit PF.

Thirty healthy adults underwent three weekly NMES sessions on the same calf. In each session, the same MP was stimulated at the lowest PF-eliciting intensity. A circular grid (5 cm radius) classified the surrounding skin as eliciting PF with muscle contraction (Zone 1), isolated muscle contraction (Zone 2), or no response (Zone 3). Areas were compared across sessions. Temporal stability was assessed using non-parametric tests and bootstrapping. Logistic regression examined associations between participant characteristics and response area size.

A concentric pattern of diminishing responses from the MP was observed. Median Zone 1 (MP response area) was 8.1 cm² (Interquartile range (IQR): 4.7-12.6), Zone 2 was 30.4 cm² (IQR: 15.7-44.8), and combined Zone 1 + 2 (functional MP response area) measured 42.9 cm² (IQR: 21.5-57.6). Zone 3 was 35.6 cm². All zones showed consistent size across sessions. Higher physical activity and older age were inversely associated with Zone 2 size.

MP activation during transcutaneous NMES is better described as a finite and reproducible cutaneous response area rather than a single skin location. PF is confined to a small, stable core region, while surrounding areas elicit weaker or absent motor responses. These findings have implications for how MPs are conceptualised and assessed in NMES protocols.