Mapping Motor Point Response Areas on the Skin for Optimisation of Neuromuscular Electrical Stimulation

Background Calf neuromuscular electrical stimulation (C-NMES) activates muscle and enhances venous return to prevent thromboembolism, support rehabilitation, and improve muscle strength. Its clinical efficacy, however, is limited by anecdotal and suboptimal electrode sizing and placement, causing discomfort and reduced effectiveness. The anatomical motor point (MP) denotes where a motor nerve enters the muscle belly but is not directly accessible from the skin. In practice, NMES activates broader skin regions; yet their spatial extent remains poorly defined, limiting guidance for electrode design. This study aimed to map the responsive skin area surrounding the MP during calf-NMES at the minimal intensity required for plantar flexion (PF) and assess its reproducibility, as well as potential associations with participant characteristics. Methods Thirty healthy adults underwent three weekly C-NMES sessions on the same leg. In each, the same MP was stimulated at the lowest intensity eliciting plantar flexion (PF). 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. Results 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 median was 35.6 cm². All zones demonstrated consistent size across sessions. Higher physical activity and older age showed inverse associations with Zone 2 size. Conclusion MP response areas can be reliably mapped and are reproducible. Rather than targeting a single focal point, electrodes should cover the surrounding response area around the MP. Electrodes sized between ~ 8–43 cm² are recommended for optimised, comfortable C-NMES delivery.