Begin by placing your hands on a medicine ball or weighted exercise ball and getting into a pushup position. Keeping your chest tall and core tight, bring one knee up towards the ball, flexing your abs. Bring the knee back, then alternate with the other. As you're performing this movement, keep your hips high, back straight, and have tension in your stomach the entire time.
Perform 3 sets of 10 reps on each leg.Start this exercise by wrapping a resistance band and tying a knot over a sturdy pole or beam above you. Stand below the band, and grab it with both hands and pull it down to your chest. Keeping your core tight and legs straight, perform a standing crunch, curling all the way down, and flexing your abs hard at the bottom. Resist on the way back up before performing another rep. Complete 3 sets of 15 to 25 reps.