In vivo axial rotations and neutral zones of the thoracolumbar spine.

Fourteen normal male subjects with mean age 23 (range 20-32) years and mean body weight of 69.6 kg underwent right and left axial rotation in a special machine--axial rotation tester (AROT). The AROT was designed and fabricated such as to allow uninhibited coupled axial rotation and lateral flexion while preventing flexion and extension. The range of rotation (ROR) and neutral zone (NZ) were recorded during active rotation (.A), active rotation with blindfold (.B), and passive rotation with blindfold (.P). Finally, bilateral axial rotation was tested with 6, 12, 18, 24, 30, and 36 Nm rotary torque (.T). There was no significant difference between ROR.A and ROR.B, both being approximately 140 degrees. However, there was a significant difference between NZ.A and NZ.B (p < 0.01). ROR.P was approximately 30 degrees greater than ROR.A. The trunk structures showed a nonlinear viscoelastic behavior with progressive rotary torque application. The neutral zone in axial rotation did not show significant difference between different loads. It is reported that the rotary neutral zone varies between subjects, and it is suggested to be contributory to spinal laxity.