Original Articles: PDF OnlyIn Vivo Axial Rotations and Neutral Zones of the Thoracolumbar SpineKumar, Shrawan; Panjabi, Manohar M.*Author Information *Department of Physical Therapy, University of Alberta, Edmonton, Alberta, Canada; and Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, Connecticut, U.S.A. Address correspondence and reprint requests to Dr. Shrawan Kumar, Department of Physical Therapy, University of Alberta, Rm 3-75, Corbett Hall, Edmonton, Alberta T6G 2G4, Canada. Results of this article were presented at the combined meeting of the Orthopaedic Research Societies of the United States, Japan, and Canada—October 21-23, 1991, Banff, Alberta, Canada. Journal of Spinal Disorders: August 1995 - Volume 8 - Issue 4 - p 253-263 Buy Abstract Summary: 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 ˜ 140°. However, there was a significant difference between NZ.A and NZ.B (p < 0.01). ROR.P was ˜30° 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. © Lippincott-Raven Publishers.