Work all of your ab muscles with this one classice move.
Credit: David Martinez

Lie back on a mat with legs extended and hands lightly touching the sides of your head. Draw left knee toward chest while simultaneously twisting upper body to the left―try to touch right elbow to left knee, but do not force. Next, reverse the motion, drawing right knee up as you twist to the right―try to touch left elbow to right knee. Continue alternating―as if pedaling a bicycle. Do as many as you can until you reach fatigue.