A functional subdivision of hip, abdominal, and back muscles during asymmetric lifting.

STUDY
DESIGN: An experimental study of muscle recruitment patterns during asymmetric lifting in healthy individuals.
OBJECTIVE: To investigate muscle recruitment patterns during asymmetric lifting, representing a common daily living activity, to determine whether systematic differences exist between functioning of the local and global muscle systems. SUMMARY OF BACKGROUND DATA: The normal function of the local muscle system is to provide sufficient segmental stability to the spine. The global muscle system provides general trunk stabilization and enables the static and dynamic work necessary for daily living and sports activities. Current knowledge about these two muscle groups appears to be specifically derived from anatomic findings and experiments conducted under artificial circumstances. To the authors' knowledge, the recruitment patterns of both muscle groups have not been investigated in daily living activities.
METHODS: Twenty-nine healthy individuals performed different variants of asymmetric lifting activities. Electromyographic data were collected from seven hip, abdominal, and back muscle pairs. In addition, trunk kinematics were measured by means of an ultrasonic movement analysis system.
RESULTS: The left and right obliquus internus, rectus femoris, and multifidus showed symmetric co-contraction in all variants of activities. In contrast, significant left/right differences were observed in the external oblique, gluteus maximus, iliocostalis lumborum pars thoracis, and latissimus dorsi.
CONCLUSIONS: The results of this study show a symmetric activation of the local muscles during the performance of low-load, asymmetric lifting tasks, which suggests that these muscles play a stabilizing role during these manoeuvres. The global muscles, however, hand show asymmetric patterns of activation during the same tasks, supporting their role as global stabilizers and prime movers.