UNLABELLED: There was much enthusiasm about the development of computerized dynamometry in providing large quantities of data to objectively assess muscle performance. However, a much more basic issue arose questioning what these machines actual measure, particularly in pain populations. The purpose of the present study was to determine whether patients' self-reported disability and pain expression, as evaluated with simple questionnaires, were related to isokinetic performance in low back pain patients. METHOD: Oswestry Disability Questionnaires and pain drawings were collected from 76 patients undergoing isokinetic testing upon entering a physical rehabilitation program. Isokinetic trunk testing was performed in the standing position and results recorded for flexion and extension at speeds of 50 degrees, 100 degrees, and 150 degrees per second. RESULTS: Patients indicating minimal disability on the Oswestry questionnaire performed better than those indicating greater levels of disability (P < 0.05; ANOVA with Tukey adjustment for multiple comparisons). Patients with greater pain drawing scores, indicating unusual pain patterns, performed more poorly during isokinetic testing than those with normal drawings (P < 0.05). Regression analysis revealed that the lifting question from the Oswestry questionnaire, pain drawings scores, and sex were all significantly related to isokinetic performance and could account for 37.6-48.1% of the variance in performance (varying with speed of test). CONCLUSIONS: The results of this study indicate that isokinetic test values are significantly influenced by a patient's self-reported disability and pain expression, which can be evaluated using simple tools such as pain drawings and the Oswestry questionnaire. This study supports the supposition that dynamometry testing is related to factors other than muscle performance.