This study sought to examine any differences in upper rectus abdominis (URA) and lower rectus abdominis (LRA) muscle activity during four abdominal exercises, the curl-up, Swiss ball curl-up, Swiss ball jackknife and Swiss ball rollout. Fourteen healthy adults (7 males, 7 females, mean age+/-S.D.=21.8+/-3.8 years) performed abdominal exercises in a randomised order following maximal voluntary isometric contraction. Muscle activity of the URA and LRA was assessed using surface electromyography. Results indicated that activity of the URA was significantly greater than muscle activity of the LRA for the curl-up, Swiss ball curl-up and Swiss ball rollout. LRA muscle activity was greater than URA during the jacknife exercise. Muscle activity during the curl-up was significantly lower than muscle activity during the other exercises. To conclude, muscle activity was greater when exercises were performed on a Swiss ball in comparison to a stable surface and LRA muscle activity was maximized during the Swiss ball jacknife.