The mechanics of torso flexion: situps and standing dynamic flexion manoeuvres.

The mechanics of torso flexion has not been thoroughly investigated due to the lack of knowledge of tissue force-time histories during the performance of torso flexion exercises. An anatomically detailed, three-dimensional model that was sensitive to lumbar curvature and muscle activation patterns was used to assess tissue load distribution during the performance of both isometric and dynamic situps and standing flexion manoeuvres in 12 young men. Situps were performed starting with a flexed torso which was lowered to horizontal and then raised again (instrumentation restrictions did not allow subjects to rest their torsos when in the lowered position). Specific variables measured were air flow during inhalation-exhalation, intra-abdominal pressure, myoelectric activity of the torso muscles, intercostals, and rectus femoris, three-dimensional dynamic curvature of the lumbar spine and body segment displacements. Lumbar compressive loads greater than 3000 N were predicted for both straight leg and bent knee situps. No biologically significant differences were found between bent knee and straight leg situp techniques. RELEVANCE: The safety of situps as both a therapeutic modality and training exercise has been questioned for years but evaluation has suffered from a lack of knowledge of tissue loading. The load predictions in this study suggest that performing situps (both dynamic and quasi-static) imposes high levels of compressive loading (> 3000 N) on the low back. The issue of using straight legs or bent knees is probably not as important as the issue of whether or not to prescribe situps at all.