Muscle activities at 15 sites were compared within a group of healthy young adults to evaluate their relative intensities during six abdominal exercises: partial and full sit-ups on a firm surface (floor) and on an exercise ball that was either stabilized or unstabilized. The most strenuous abdominal exercise overall (i.e., whole body workout) was the full sit-up on a firm surface which included significant muscle activities in the lower extremities. Exercise intensity was also high in the full and partial sit-ups when performed on a ball. The partial sit-up on the floor was the least strenuous of the six exercises. The greatest effect on the abdominal muscles was observed in the partial sit-up on a ball (stabilized and unstabilized). Results suggest that, although abdominal exercises on a ball may be gentler on the hip and lower back, overall exercise intensity is not necessarily lower than that on the floor. Moreover, partial sit-ups, both on the floor and on a ball, also required greater neck muscle activities than full sit-ups. In deciding what type of sit-up to do, exercise surface and different muscular activities between the partial and full sit-ups should be considered.