Exercise 1: Plank on elbows
Assume a pushup position, but with your elbows bent and your weight resting on your forearms. Your body should form a straight line. Now brace your abs as if someone were about to punch you in the gut. Hold for 30 seconds. Rest 30 seconds, and repeat once.
Exercise 2: Mountain climber with hands on bench
In pushup position with your hands on a bench, brace your abs and slowly lift your left knee toward your chest. Pause 2 seconds, lower it slowly, and then raise your right knee. Alternate for 30 seconds, rest 30, and repeat once.
Exercise 3: Side plank
Lie on your left side and prop your upper body up on your left forearm. Raise your hips until your body forms a straight line from ankles to shoulders. Now brace your abs and hold for 30 seconds. Roll over onto your right side and repeat. Rest 30 seconds, and do 1 more set.
Exercise 1: Plank with feet elevated
Use the guidelines for the beginner version of the exercise, but with both of your feet on a bench.
Exercise 2: Mountain climber with hands on Swiss ball
Follow the beginner instructions, but place your hands on a Swiss ball instead of a bench.
Exercise 3: Side plank with feet elevated
Do this the same way as the beginner version, but with both of your feet on a bench.