OBJECTIVE: To study the ability of a widely used isometric back endurance test to measure lumbar back erector muscle fatigue and to assess the influence of age, sex, and body mass index (BMI) on back and hip extensor muscle fatigability (EMG spectral indices). DESIGN: Cross-sectional study of men and women without back problems. SETTING: Occupational health center and rehabilitation clinic in Finland. SUBJECTS: Experiment 1 consisted of 233 consecutive occupational health center customers (133 women, 100 men) without back problems. Experiment 2 consisted of 20 healthy women. INTERVENTION: Subjects performed the isometric Sørensen back endurance test up to 240sec in experiment 1 and to the limit of endurance in experiment 2. OUTCOME MEASURES: Raw surface EMG was recorded bilaterally over the belly of lumbar erector spinae muscles at L1-L2 and L4-L5 levels in experiment 1, and bilaterally over the medial paraspinal muscles at L1-L2, L3-L4, and L5-S1 levels and over the major hip extensor muscles (gluteus maximus and biceps femoris) in experiment 2. In both experiments, time to endurance was recorded (in experiment 1 up to 240sec). The EMG spectral median frequency (MF) decrease over time was used for the assessment of back and hip extensor fatigability. RESULTS: In experiment 1, the rate of change in paraspinal MF was greater in men than in women, indicating greater paraspinal fatigability in men. Multiple regression analysis indicated that the rate of MF decrease (fatigue) during the test was dependent on age and BMI in both sexes and that the effects of age and BMI were more pronounced in women than in men. Correlation analysis revealed that the rate of paraspinal muscle MF decrease was associated with endurance time and BMI in women and with endurance time and age in men. In experiment 2, the paraspinal muscles, as well as the hip extensor muscles, biceps femoris, and gluteus maximus, showed clear decreases in MF during the isometric endurance test in women. MF decrease was highly related to endurance time and BMI in women. CONCLUSIONS: Lumbar paraspinal muscle fatigability during the Sørensen test is influenced by subject characteristics. Further, the hip extensor muscles also significantly fatigue, indicating load sharing between back and hip extensor muscles during the test. According to these results, the validity of this widely used back endurance test in specifically measuring lumbar paraspinal muscle endurance is questionable, as is the direct comparison of test results between women and men.