Morphology of the abdominal muscles in ballet dancers with and without low back pain: a magnetic resonance imaging study.

OBJECTIVES: To evaluate the morphology of transversus abdominis and obliquus internus abdominis muscles and the ability to "draw in" the abdominal wall, in professional ballet dancers without low back pain, with low back pain or both hip region and low back pain.

DESIGN: Observational study.

METHODS: Magnetic resonance images of 31 dancers were taken at rest and during voluntary abdominal muscle contraction. Measurements included the thickness of transversus abdominis and obliquus internus abdominis muscles, lateral slide of the anterior extent of the transversus abdominis muscles (transversus abdominis slide) and reduction in total cross sectional area of the trunk.

RESULTS: The transversus abdominis and obliquus internus abdominis muscles were thicker in male dancers and the right side was thicker than the left in both genders. There was no difference in muscle thickness as a proportion of the total thickness, between dancers with and without pain, although there was a trend for female dancers with low back pain only to have a smaller change in transversus abdominis muscle thickness with contraction than those without pain. Transversus abdominis slide was less in female dancers than in male dancers. When gender was ignored, the extent of transversus abdominis slide was less in dancers with low back pain only. Reduction in trunk cross sectional area with contraction was not different between genders or groups.

CONCLUSIONS: This study provides evidence that the abdominal muscles (transversus abdominis and obliquus internus abdominis) are asymmetrical in dancers and although the abdominal muscles are not different in structure (resting thickness) in dancers with LBP, there is preliminary evidence for the behavioural change of reduced slide of transversus abdominis during the 'draw in' of the abdominal wall.