BACKGROUND CONTEXT: Many studies report an association between low back pain (LBP) and reduced back muscle endurance and consider this to indicate muscular dysfunction. However, few have investigated the potentially confounding influence of psychological factors on performance during such endurance tests. PURPOSE: This study examined whether psychological factors were associated with "underperformance" on the Biering-Sørensen (BS) test (ie, not performing as well as one is physiologically capable of). STUDY DESIGN/SETTING: Cross-sectional study of the baseline data of patients with chronic (>3 months) nonspecific LBP (cLBP) before participation in a clinical trial of exercise therapy. PATIENT SAMPLE: One hundred forty-eight patients with cLBP (43% men; age, 45±10 years). OUTCOME MEASURES: The time for which the modified BS isometric endurance test could be performed to exhaustion minus the time that would have been predicted based on the rate of decline in median frequency of the surface electromyographic (EMG) signal recorded bilaterally from the erector spinae at L3 and L5. METHODS: Back pain and disability, psychological disturbance, catastrophizing, fear-avoidance beliefs, back beliefs, and exercise self-efficacy were measured using validated questionnaires. Patients performed the BS test to exhaustion while physiological muscle fatigability was measured from continuous surface EMG recordings. RESULTS: Multivariable regression analysis controlling for gender revealed that greater psychological disturbance (p=.003) and more negative back beliefs (p=.015) were unique predictors of the extent of "underperformance," accounting for 22.3% variance in expected endurance time minus actual time. CONCLUSIONS: It is important that the underlying nature (psychological or physiological) of performance deficits be identified during such tests because this may influence the interpretation of prospective studies reporting risk factors for LBP and dictate the particular treatment or interventional approach required to remedy the situation in individuals with LBP.