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Cross-sectional study of 63 patients with chronic low back pain (CLBP).To determine the relationship between the SF-36 physical functioning (PF) scale and physical performance on a modified symptom limited treadmill test, and to determine the effect of disability status and SF-36 mental health (MH) scores on self-reported physical functioning and on physical performance.Patients with CLBP report low levels of physical functioning. Because self-report measures are influenced by psychosocial factors, such as disability and low MH, a common recommendation is to supplement self-report with objective or physical performance testing. Little information exists regarding whether performance testing is also influenced by disability and low MH.Patients completed the SF-36 and performed a maximal, symptom-limited, modified treadmill test. Total walking time and heart rate on the treadmill were registered. The effects of disability and low MH on self-report and performance testing were calculated by unpaired Student’s t-testing and effect size estimation.A strong relationship between the PF scale of the SF-36 and treadmill walking time was found, but there was little evidence of overlap between these physical functioning domains. Both disability status and low MH had a much larger effect on self-report measures than on physical performance.Self-report measurements and performance-based assessments provide information about distinct, although related, domains of physical functioning. Disability and low MH are associated with lower SF-36 PF scores. Our results confirm that self-report measures require supplementation with objective performance testing to provide optimal assessment for patients with CLBP.