Comparison of Surface to Indwelling Extrinsic Finger Muscle EMG during use of Computer Pointing Devices

Differences between indwelling and surface electromyography (EMG) were quantified by simultaneously recording extrinsic muscle activity of the index finger (flexor digitorum superficialis (FDS), flexor digitorum profundus (FDP), extensor indicis proprius (EIP), and extensor digitorum communis (EDC)) in a laboratory-based study. Eleven subjects performed computer-related pointing and dragging tasks using a mouse, pen and tablet, touchpad, and trackball. Surface electrodes poorly estimated FDP activity as demonstrated by low and variable cross-correlation values (0.47–0.64) and variable, high estimated error (RMS difference=11–46%MVC). Surface electrodes estimated FDS activity well (cross-correlation=0.79; RMS difference=11%MVC) when hand and forearm posture changes were minimized. Differences in forearm supination and pronation reduced the quality of FDS muscle activity estimation. Surface electrodes estimated EIP and EDC activity well with high cross-correlation values (0.77–0.84) and low RMS differences (8–12%MVC). Surface EMG can be used to assess EIP and EDC activity, and FDS activity when posture changes are minimized, during pointing device tasks. Surface EMG does not accurately assess FDP activity.