Diagnostic Ultrasound Shows Preferential Activation of Rectus Abdominis Segments with Exercises Targeting Upper Versus Lower Segments.

Preferential activation of rectus abdominis sections during crunch or leg lifts has long been disputed. The objectives of this study were to both explore the activation of the rectus abdominis during these exercises. This study used a randomised independent measures design utilising both EMG and diagnostic ultrasound to record changes in rectus abdominis activity. Fifteen participants each performed multiple repetitions of a 45° crunch, a 90° sit-up, and finally a 90° leg raise, sufficient for an 8-second ultrasound recording utilising M-mode of all four unilateral abdominal segments during each of the exercises, resulting in 12 images per participant. There was a significant interaction between segment number and type of exercise when testing for percent difference (F = 4.718, p < .01, η = .065). The mean thickness change of abdominis during a crunch manoeuvre was greater than the leg raise by 36.39 ± 3.21 % (p < .01). The mean thickness during the sit-up was also greater than the leg raise (32.49 ± 3.04 %). Rectus abdominis shortening and thickening was observed in all the exercises tested but was most pronounced in the abdominal crunch. Muscle recruitment seems to be biased closer to the load.