BACKGROUND: Biomechanical changes of the trunk might be an important factor contributing to the pathogenesis and poor recovery of lumbar disc herniation.OBJECTIVE: To study the biomechanical changes of the trunk of patients with lumbar disc herniation by isokinetic test of the protruded lumbar disc.DESIGN: Non-randomized controlled retrospective study of concurrent patients.SETTING: Department of Rehabilitation Medicine, Changhai Hospital, Second Military Medical University of Chinese PLA.PARTICIPANTS: Thirty patients with lumbar disc herniation, admitted in the clinic of Department of Rehabilitation, Changhai Hospital affiliated to Second Military Medical University between February 2001 and January 2002, were enrolled in this study, with another 30 concurrent patients without lumbar disc herniation serving as the control group. Informed consent was obtained from all patients involved.METHODS: A Biodex Multi-joint Testing System was employed for measuring the peak torque(PT), peak torque to body weight(PT/BW), time to peak torque(TPT), torque at 0. 2 s(T@ 0. 2), total work(TW), average power(AP) and flexion to extension(F/E) ratio of the trunk muscles of the patients. The results were analyzed by t test using SPSS 9.0 software.MAIN OUTCOME MEASURES: Main outcome: isokinetic evaluation of the lumbar and dorsal flexors and extensors; secondary outcome: F/E ratio.RESULTS: The strength of the trunk flexors and extensors decreased significantly in patients with lumbar disc herniation at each testing speed of retraction, and the bursting strength and indices for work efficiency of the muscles also exhibited obvious changes. The flexors showed greater reduction in muscle strength than the extensors. The F/E ratios during isokinetic concentric contraction at 60°/s and 180°/s were 57.99 ±5.68 and 65.74 ± 8.12, respectively, in patients with lumbar disc herniation, in comparison with the ratios of 95.25 ±5. 18 and 83.03 ±7.61 in the control patients, showing significant difference between the two patient groups( P ＜ 0.01 ).CONCLUSION: Biomechanical changes of the trunk muscles of patients with lumbar disc herniation are definite, and proper rehabilitative treatment of these patients should consists of specific training protocols to restore the mechanical balance of the trunk and break the vicious cycle on the basis ofaccurate evaluation of such changes.