The rate of decline in the median frequency (MF) of the surface EMG power spectrum (MFgrad) has been evaluated for its use in monitoring the fatigability of the erector spinae muscles during the performance of submaximal isometric contractions. MFgrad consistently displayed a highly significant relationship with endurance time for the task (p < 0.05), giving a slightly better correlation for contractions performed in lordotic postures, with the back muscles in a shortened position, than in flexed postures. No significant difference existed between mean MFgrad values recorded from the right and left erector spinae muscles, at either thoracic or lumbar levels. The back extensor muscles of men were more fatigable than those of women, and this has been discussed in relation to corresponding differences in muscle fiber type relative size. Greater erector spinae muscle fatigability was associated with both the existence of, and the risk of developing, serious low back pain. Possible mechanisms for the association, from retrospective and prospective points of view, have been advanced.