Axial rotation of the lumbar spine and the effect of flexion. An in vitro and in vivo biomechanical study. A series of experiments were performed on eight whole, cadaveric lumbar spines and on eight male volunteers to determine whether axial rotation changed with subjects bending forward compared with being in a neutral posture and whether rotation was affected by articular tropism. Kirschner wires were inserted into the spinous processes of the eight cadaveric lumbar spines, and the axial rotation of the wires was measured while the spine was rotated in a torsion apparatus. Similarly, Steinmann pins were inserted into the spinous processes of L3, L4, and L5 of the eight volunteers, and the axial rotation of the pins was measured while the subjects rotated in a torsion apparatus. Axial rotation was found to be less when combined with forward flexion, and articular tropism did not influence the amplitude of rotation. 