A recently developed abdominal muscle test (AMT), using four levels of difficulty to assess abdominal muscle strength, was validated by examination of the functional interrelationship between the individual muscles during the AMT, and by determining the relationship between the AMT level of difficulty and electromyographical (EMG) activity. EMG data were collected from the upper and lower rectus abdominis, and the external and internal oblique in 22 subjects as they participated in the AMT. Typically, throughout the AMT, lower rectus abdominis was most active, followed by upper rectus abdominis, external oblique, and internal oblique. EMG activity increased significantly (p < .05) for all muscles from Levels 1 to 3. At Level 4, although upper and lower rectus abdominis remained more active than the obliques, the relative contribution from the obliques was increased. The results of this study show that the AMT is a valid indicator of ability to develop voluntary abdominal muscle tension in Levels 1 to 3, although, Level 4 is more indicative of the oblique muscles' functional abilities and therefore may be considered a distinctly different muscle test.