Trunk muscle co-activation of several muscles is considered necessary in achieving adequate spinal stability to prevent and treat low back injury [1]. Common exercise recommendations from health professionals include trunk exercises to prevent and treat low back injuries. Knowing the trunk muscle activation levels during exercises is important in the prescription and design of exercise programs that aim to increase the training intensity over time (progressive resistance model). Previous research has documented trunk muscle EMG during various exercises designed to train the trunk musculature and during functional activities [2-7]. Ng et al [7] found that abdominal and trunk muscles not only produce torque but also maintain spinal posture and stability during axial rotation exertions. Vera-Garcia et al [8] showed that performing curl-ups on a labile (moveable) surface changes the muscle activity amplitude required to perform the movement. Increases were greatest in the external oblique muscles. Mori [9] documented the trunk muscle activity during a variety of trunk muscle exercises on a Swiss ball. However, comparisons in muscle activity were not made with ground based exercises (no Swiss ball present), therefore, the influence of a Swiss ball on trunk muscle activity compared with ground based bridging is not known.