The purpose of this study was to determine the influence of test postures on trunk muscle strength, and to clarify the difference between concentric and eccentric strength of healthy subjects and patients with chronic low-back pain (CLBP). Volunteered and controlled trials took place at a university hospital, which is a referral center and is also open to the general community. The patients were selected from the outpatient care in the university hospital. Fifty healthy subjects (25 men and 25 women; average age 25.7 years, ranging from 23 to 34 years) and 48 patients with CLBP (26 men and 22 women; average age 33.5 years, ranging from 26 to 39 years) volunteered for this study. The main outcome measures were flexor and extensor peak torque/body weight, and flexor/extensor peak torque ratio during concentric and eccentric contractions; Eccentric/concentric peak torque ratio in flexor and extensor. Trunk-muscle strength was significantly greater in the sitting posture when measured with the subjects' feet against the floor (p < .05). Some patients complained of a slight low-back pain during testing, although the pain was not aggravated in any patients. In both concentric and eccentric contraction modes, the patients with CLBP had greater flexor/extensor ratios than the healthy subjects (p < .01). This implies that extensors are more affected in these patients. However, a great imbalance between concentric and eccentric strength of the patients was found, not only in extensors, but also in flexors (p < .01). The current study shows that one should consider the test posture carefully when measuring trunk-muscle strength.(ABSTRACT TRUNCATED AT 250 WORDS)