Back muscle endurance is considered important in low back pain (LBP) rehabilitation. Specific training of multifidus may also be necessary to restore normal low back function. The reliability of surface electromyogram (EMG) to assess endurance of the multifidus muscle during intermittent isometric exercise was evaluated. Multifidus endurance was monitored in the four-point kneeling exercise position using the power spectral analysis method. Twenty healthy volunteers were tested on three separate occasions. Subjects performed repeat 10 s high-intensity voluntary contractions of multifidus for 3 min. The median frequency (MF) and the integrated-rectified (I-R) EMG signal displayed the fatigue pattern of multifidus. Intraclass correlation coefficients indicated fair-good reproducibility for MF (0.48-0.67) but poor reliability for IR-EMG. In conclusion, problems concerning functional testing protocols for the back muscles remain and careful development is necessary for more realistic rehabilitation monitoring. © 2001 Harcourt Publishers Ltd.