Weakening of the trunk muscles is thought to be one disadvantage of prolonged lumbar orthotic use. This study examines weakness of the trunk flexor and extensor muscles in patients who are wearing lumbar orthotics for extended periods. Strength of the trunk flexor and trunk extensor muscles was tested in 24 individuals, using the Kinetic computer. Both concentric and eccentric forces were recorded. Four groups of patients were studied. Group 1 (n= 6) consisted of patients with low back pain who had used a lumbar orthotic for a prolonged period of time. Group 2 (n= 6) consisted of hospital employees with no history of low back pain, who wore lumbar orthotics prophylactically, for back protection. Group 1C (n= 6) consisted of healthy controls, with no history of either back pain or lumbar orthotic use, who were individually age- and gender-matched to each patient in Group 1. Group 2C (n= 6) consisted of healthy controls matched in the same fashion to each patient in Group 2. After consultation with a statistician, statistical analysis was performed using the Wilcoxon's test. Nonparametric statistics were chosen because of the lack of evidence of a normal distribution of the parameters being studied. This analysis revealed significant weakness in concentric flexion (P= 0.0464), concentric extension (P= 0.0277), and eccentric extension (P= 0.0464) in Group 1 compared with matched controls in Group 1C. The only significant weakness compared with controls in Group 2 was in eccentric flexion (P= 0.0277). Trends were toward weakness in the orthotic users for the other motions studied, with aPvalue of less than 0.1 for eccentric extension. Prolonged use of lumbar orthotics may be associated with trunk muscle weakness in the population studied. Prescribers should continue to limit duration of use when possible and to consider strengthening exercises when prolonged use is anticipated.