There is some evidence that the fatiguing characteristics during isometric back extension tasks may assist in identifying differences between individuals with and without low back pain (LBP). During these tasks, especially in standing, other abdominal trunk muscles are also active. The abdominal trunk muscles acting across multiple segments of the lumbar spine function in isolation or in synergy to create flexion torques. It is suggested that co-activation patterns of the trunk muscles are able to control the axis of rotation of the extension torque and also provide multi-segmental stability of the spine. The purpose of this study was to examine the fatigue responses in 4 asymptomatic individuals to a sustained isometric extension task of the trunk muscles evaluating the shifts in the median frequency of the electromyographic (EMG) signal. This study suggests that in asymptomatic subjects, the more superficial abdominal muscles (External Oblique and Rectus Abdominis) increased in activity as the test progressed. There was large inter-individual variation in both amplitude and median frequency changes. Rectus abdominis and the back extensors demonstrated characteristics of fatigue during the task. Studies to test for any characteristic trends in whether specific trunk muscles fatigue in standing is a feature in chronic LBP, invites a formal investigation. (C) 2001 Harcourt Publishers Ltd.