A method was developed to quantify erector spinae fatigue resulting from repetitive dynamic lifting in the sagittal plane. This method was tested with the data from eight male subjects lifting inertial loads of 19 kg and 17 kg during sessions of 20 min and 2 h, respectively. Surface EMG electrodes were applied over sites representing the bilateral lumbar and thoracic erector spinae and external oblique muscles. Maximal and submaximal isometric trunk extensor contractions were performed at the start, intermittently throughout, and at the end of the dynamic lifting trials, within an apparatus designed to control spine posture in the upright standing position. These exertions were used to assess the decreases in strength, endurance and EMG mean power frequency (fw) as well as the increases in EMG amplitude that have been shown to accompany muscle fatigue. The average of the group for extensor strength decreased 17% and 21% (P < 0.05) and the endurance times decreased 60% and 62% (P < 0.01) for the 2-h and the 20-min session, respectively. The average endurance time decreased at least 10% for each subject in each session. Strength decreased at least 10% in all but 2 of 16 cases (both in the 20-min session). The average decreases in fw were 12% (lumbar) and 17% (thoracic; P < 0.05) in the 2-h sessions and 20% (lumbar; P < 0.05) and 14% (thoracic) in the 20-min sessions. There was also a significant increase in EMG amplitude (P < 0.05) for both muscle group in both sessions.(ABSTRACT TRUNCATED AT 250 WORDS)