The purpose of this study was to determine the relative electromyographic (EMG) activity of the upper and lower rectus abdominis and the external oblique muscles during 5 commonly performed abdominal strengthening exercises. Twenty-five healthy subjects participated in the study. EMG data were collected under isometric and dynamic conditions. The reverse curl resulted in the greatest amount of lower rectus activity, the v-sit and reverse curl exercises resulted in the greatest amount of external oblique activity, and the trunk curl, reverse curl, trunk curl with a twist, and v-sit all resulted in similar amounts of upper rectus EMG activity. The vacuum exercise resulted in moderate levels of external oblique EMG activity but very low levels of activity in the rectus abdominis. Our findings support the concept that abdominal strengthening exercises can differentially activate various abdominal muscle groups, but contradict some traditionally held assumptions regarding the effects of specific exercises.