w/ Foot Flexed
w/ Toes Pointing down
*You are never sitting straight up, you stay in the 45% angle and use your core to keep yourself stable.
Bonus: Keep your feet of the floor as you do this to work your lower abdominal muscles even more.
*Keep your core tight throughout the exercise (and on all plank and leg exercises)!
*If this is difficult for you, keep your knees on the floor rather than your feet throughout the exercise.