OBJECTIVE: This study was designed to evaluate the contribution of anatomical factors to the stimulation intensity needed for functional electrical stimulation (FES) of shoulder girdle muscles, especially the supraspinatus. METHOD: Anatomical dimensions, including the length of the arm and scapular spine, were measured in twenty three normal subjects. Depth and thickness of the supraspinatus and trapezius muscle were measured ultrasonographically. FES was applied for supraspinatus muscles, and the minimal intensity required to induce contraction was recorded. Correlations of intensity with the anatomical dimensions were investigated statistically. RESULTS: The thickness of the supraspinatus muscle and the length of the scapular spine showed statistically significant correlations with the minimal intensity for FES of supraspinatus muscles. No other anatomical measurements showed significant correlation. CONCLUSION: The intensity required for FES was affected by the thickness and length of muscles, rather than other anatomical variables. The results of this study suggest that one of the major factors contributing to the determination of the intensity of FES is the size of muscles. If the intensity could be estimated before stimulation, based on the size of muscle, unnecessary discomfort of the patients would be avoided.