BACKGROUND: Objective measurement of trunk muscles strength in ankylosing spondylitis (AS) may provide clinical information that can be used to evaluate functional disability. OBJECTIVE: To compare the isokinetic strength of trunk muscles in AS patients with healthy subjects. METHODS: Thirty-four patients with AS and thirty-one healthy subjects were enrolled. Patients with spinal ankylosis and bridged syndesmophytes in the lumbar and dorsal vertebrae were excluded. Trunk flexor and extensor strength was measured at: 60 and 90°/sec. RESULTS: When compared with the control group, AS patients demonstrated 38% and 24% decrease in the extensor and flexor strength, respectively (p< 0.05). In addition, the flexor/extensor strength ratio was 1.0 and 0.8 in the patients and control subjects, respectively. CONCLUSIONS: In view of the relatively selective extensor weakness we recommend assessment of trunk muscle strength in these patients for guiding clinicians regarding possible reconditioning exercises in the early period of AS. [ABSTRACT FROM AUTHOR]