OBJECTIVE: The purpose of this study is to evaluate concentric and eccentric trunk muscle strength in patients with chronic low back pain and compare it with healthy controls to determine the role of trunk muscle in chronic low back pain METHOD: Subjects were 20 chronic low back pain patients with the duration at least 6 months and 20 normal adults. Concentric and eccentric isokinetic trunk muscle test was performed at specific speeds (30degrees, 60degrees, 120degrees/sec) with Cybex 770 machine in back pain patients and healthy control. RESULTS: The strength of trunk flexor and extensor muscles of chronic low back pain patients was significantly weaker than that of normal controls. Eccentric peak torque was significantly higher than concentric peak torque in both patients and healthy control. A concentric and eccentric ratio of patients was lower than that of normal controls. The correlation between concentric and eccentric variables was high. Our results show that concentric peak torque has the highest correlation with eccentric peak torque. CONCLUSION: Isokinetic evaluations of the trunk muscle offer objective and quantitative data of patients with chronic low back pain and will be clinically useful in muscle power measurement of lumbar muscles.