All good questions Sarah Without being in front of the person/body it's hard to pin-point exactly what that person would require, but typically it is due to a tight low back that compromises weak core/abs even more by not allowing the necessary pelvic tilt required to roll/peel the sacrum/lumbar spine off the mat. Indeed tight hamstrings effect the back as well. Bent knees assists in getting the low back to curl under (posterior tilt) and to control eccentrically on the 2nd phase of the exercise. Thanks for watching & hope that helps :)