Lumbar axial torque actively induces trunk axial rotation during sidestep cutting manoeuvre: Insight to expand the trunk control concept

Core stability is widely recognised as ‘the body's ability to maintain or resume an equilibrium position of the trunk after perturbation’. As such, large excursions of the trunk during controlled activities are believed to be the result of poor trunk control. Here, we show that the axial torque actively induces the trunk axial rotation (the thoracic rotation relative to the pelvis) rather than minimise the axial rotation during sidestep cutting. We analysed the kinematic and kinetic data of 90° sidestep cutting with maximal effort by 10 physically active men. The thorax rotated toward the objective direction prior to the pelvis, resulting in the trunk axial rotation with the peak angle of 21.0 ± 6.0°. Lumbosacral axial torque was exerted toward the objective direction during the early stance phase, and it was then exerted inversely during the late stance and flight phases, which was consistent with the increase/decrease in the trunk axial rotation velocity. In the early stance phase, the absolute integrated component of the lumbosacral axial torque for pelvic rotation (0.074 ± 0.033 Nms/kg) was significantly larger than any other integrated component. In the late stance and flight phases, the lumbosacral axial torque mainly rotated the pelvis. The results indicate that the axial torque is exerted to actively induce the trunk axial rotation rather than minimise the trunk movement, suggesting that the trunk control concept probably should include not only stabilising but also actively moving the trunk.