FIG. 5.

Regions of muscle activity and muscle action direction estimates in endpoint force plane. Top panels: normalized electromyographic data across all subjects, target directions, and target magnitudes, with a single cosine tuning fit to the grouped data. Bottom panels: polar histogram of action direction estimates across all subjects, target directions, and target magnitudes. A: the first dorsal interosseous (FDI) muscle is active in the left half of the endpoint force plane between extension and flexion and has an action direction of mostly abduction with some flexion (196°). B: the extensor indicis propius (EIP) muscle is active in the upper half of the endpoint force plane between adduction and abduction, and has an action direction of mostly extension with some adduction (70°). C: the extensor digitorum communis (EDC) muscle was active largely in the second quadrant of the endpoint force plane between extension, and has an action direction of mostly extension (90°).