The roll up is one that will truly tell you if you have been getting a good contraction of the TA (transverses amdominis). In fact all 4 of the abdominal muscles are required. Your obliques and rectus (your six pack abdominals) are all needed. You are working to smoothly lift the spine from the floor and hinge forward at the hips. Then you return to the floor with control as each vertebrae lowers to your mat.