Comparing different core isolation exercises, Youdas et al. (2008) measured rectus abdominis muscle activity in the ab roll out, bent - leg curl up, supine double - leg lowering, and side bridge. (strengthandconditioningresearch.com)
Rectus abdominis muscle activity increased with increasing trunk flexion angles during the trunk flexion sit up but decreased with increasing hip flexion angles during the hip flexion sit up. (strengthandconditioningresearch.com)