Active Plank

There are several variations you can do with a plank (for abdominal muscles), and you can find some by searching for past plank posts. One simple and effective variation is to get in the plank position, with your head, neck, back and hips in a level position, and slowly bring your hips and buttocks up while keeping your legs straight. Return slowly to the starting position. Repeat for four sets of 10 repetitions, or until you fatigue. 

When you finish, lie on your back and hug your knees for 10 seconds. Repeat as needed.