Comparing abdominal hollowing and pelvic tilt, Drysdale et al. (2004) assessed the differences in these actions on external oblique muscle activity in the following positions: bent - leg with feet on floor, bent - leg with feet in the air (hips and knee at 90 degrees), and bent - leg with feet on a bench. (strengthandconditioningresearch.com)
There was a trend towards greater external oblique muscle activity in the long lever plank with posterior pelvic tilt compared to the long lever plank without posterior pelvic tilt (148 vs. 111 % of MVC). (strengthandconditioningresearch.com)
Comparing different core isolation exercises, Youdas et al. (2008) measured internal oblique muscle activity during the roll out, bent leg curl up, supine double leg lowering, and side bride. (strengthandconditioningresearch.com)