They found increased rectus abdominis muscle activity when performing trunk flexion exercises with added elastic resistance from a portable device (Perfect Abs). (strengthandconditioningresearch.com)
Assessing the effect of joint range of motion (ROM), Andersen et al. (1997) compared rectus abdominis muscle activity between sit ups performed with trunk flexion (at the lumbar spine) with sit ups performed with hip flexion. (strengthandconditioningresearch.com)
A variation on the sit up that only involves trunk flexion and does not involve hip flexion is known as the curl up or crunch. (strengthandconditioningresearch.com)