The modified prone (face down) bridge or "plank" is a great exercise if you are trying to develop the core muscles. This exercise is unique in that it is done from a static position. There are no repetitive movements. The modified prone bridge strengthens the lower back, as well as several muscles in the upper body. A workout mat is recommended.
The starting position for the prone bridge is similar to the push-up, except the elbows are bent and the forearms are flat on the floor. The palms can either rest on the floor or they can be together with the fingers interlaced. Next, use your elbows and knees to hold the upper body up off the floor. It is important to keep your back flat, abs tight, and body completely straight. Hold this position for several seconds (between 30 and 60) and repeat. You can make this exercise more challenging by doing it from the toes instead of the knees.
From knees, get into plank position. Keep abs tight. Hold for 30 - 60 seconds.