Back extensor muscle fatigability and its relationship to body mass index (BMI) was measured in 12 chronic nonspecific low back pain (CNLBP) patients (7 women and 5 men) and 12 healthy age-and gender-matched controls. Subjects performed SÃÿrensen back isometric endurance test until exhaustion while EMG spectral mean power frequency (MPF) over the lumbar erector spinae muscle and endurance time were recorded. The CNLBP patients had significantly shorter endurance time than healthy controls. Spectral MPF significantly declined as time of isometric contraction progressed. Relative decrease of the MPF per minute (MPF slope) for left and right side, and pooled MPF slope was significantly higher in CNLBP patients compared with controls. In CNLBP patients the isometric endurance time correlated significantly negatively with BMI (). In controls BMI correlated significantly positively with MPF slopes of left () and right () side, and pooled MPF slope ().