Cross-talk from adjacent muscle has a negligible effect on surface electromyographic activity of vastus intermedius muscle during isometric contraction

The purposes of this study were to attempt to record surface electromyography (EMG) from the superficial region of vastus intermedius (VI) and to investigate the influence of adjacent muscle activity on surface EMG of VI. First, serial axial magnetic resonance imaging of the thigh was performed for 45 healthy young men to determine morphological characteristics of the VI. Second, surface EMG activity of the VI and other quadriceps femoris (QF) muscle group components were recorded in maximum voluntary contraction during isometric knee extension from 11 healthy young men. To test cross-talk of EMG signals between VI and the nearest adjacent muscle, vastus lateralis (VL), we applied cooling for 20-min on VL to selectively alter activity. Cooling the skin above a muscle is known to decrease median frequency (MF) of EMG signal of the muscle. All subjects displayed a superficial region in VI sufficiently large (14 cm2) to record surface EMG. Surface EMG of VI could be detected in the same scale as other QF muscle group components. Cooling induced a significant MF decrease only in VL (from 92.5 to 44.2 Hz, p < 0.001), but no significant change was observed in VI (from 63.8 to 61.7 Hz). From this result, we concluded the muscle activity of VL is negligible on surface EMG detected from VI during isometric contraction.