Diverging intramuscular activity patterns in back and abdominal muscles during trunk rotation.

• STUDY DESIGN: An intramuscular electromyographic study was performed on trunk rotations during sitting and standing. OBJECTIVE: The aim was to provide new information on activation levels for deep trunk muscles in various unresisted and resisted trunk rotations. SUMMARY AND BACKGROUND DATA: Frequent daily trunk twisting and decreased maximal strength during trunk rotation have been associated with low back pain or sciatic pain. However, the involvement of deep trunk muscles during different trunk rotations is relatively unknown. METHODS: Ten healthy subjects participated. Fine-wire electrodes were inserted, under ultrasound guidance, into psoas, quadratus lumborum, the superficial medial lumbar erector spinae (ES-s, multifidus) and its deep lateral portion (ES-d, iliocostalis), iliacus, rectus abdominis, obliquus externus, and obliquus internus. RESULTS: The highest involvement for all muscles was observed on the ipsilateral side, in maximal trunk twists with shoulder resistance, except obliquus externus, which showed a dominant contralateral side, and rectus abdominis, which was little activated in all rotations. In contrast, maximal trunk twist without shoulder resistance, i.e., freely performed, resulted generally in lower levels for all muscles involved and in a shift of side dominance for the lumbar muscles quadratus lumborum, psoas, and ES-s. CONCLUSIONS: During trunk rotations the activity patterns for various trunk muscles could drastically change, and even be the opposite, between the two body sides, within the same type of task, depending on several factors such as initial position, effort level, sitting or standing, and external shoulder resistance.

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