Muscle fibre orientation of abdominal muscles and suggested surface EMG electrode positions

The ability of surface electrodes to accurately detect the activity of a particular muscle relies not only on their being placed over the muscle but also on their position in relation to muscle fibre orientation. For optimal pick-up of electromyographic (EMG) signals, surface electrodes are best aligned in parallel with the fibre orientation of the underlying muscle. This study aimed to measure muscle fibre orientation and other parameters of muscle morphology of the abdominal muscles in relation to palpable bony landmarks. Thirty-seven embalmed cadavers (19 males and 18 females) were examined. Results showed that the fibres of obliquus externus abdominis were about 4 degrees more vertical than the lower edge of the eighth rib. Below the rib cage, the muscle fibres of obliquus externus abdominis were approximately 5 degrees closer to vertical than a reference line between the most inferior point of the costal margin and the contralateral pubic tubercle. In the anterolateral abdominal wall area below the anterior superior iliac spine (ASIS), the obliquus internus abdominis was superficial being covered only by the aponeurosis of obliquus externus abdominis. At the level of ASIS, the muscle fibres of obliquus internus abdominis were almost horizontally orientated but at 2 cm below ASIS were aligned about 6 degrees inferomedially to the horizontal. The muscle fibres of upper rectus abdominis were 2 degrees inferolateral to the midline while the lower rectus abdominis muscle fibres deviated inferomedially from the midline by about 8 degrees. The appropriate surface electrode placements which follows the muscle fibre orientation of the obliquus externus abdominis, obliquus internus abdominis and rectus abdominis have been suggested.