Twisting mobility of the human back in flexed postures.

The twisting mobility of the back was measured using an electromagnetic measuring device, the 3SPACE Isotrak, in ten healthy subjects in five postures: standing upright, in two forward-leaning positions, and two seated positions. In the forward-leaning positions the subjects flexed with mean values of 28 degrees and 53 degrees and had no difference in their active twisting compared with that experienced in the upright position. The two seated positions induced mean flexion values of 32 degrees and 44 degrees and were associated with statistically significant increases in active twisting compared with the upright position of 38% and 46%, respectively, at the level of P < 0.0001. These increases suggest that the wedge morphology of the zygapophysial joints permits greater twisting when the intervertebral joints flexed, but postural muscle action in forward leaning restricts active twisting. This study suggests a mechanism for increased vulnerability of the posterior anulus to injury when twisting is combined with flexion.