Imagine your spine is a credit card. In the same way that repeatedly flexing and extending a credit card will eventually lead to wearing out of the plastic, repeatedly doing crunches can put damaging strain on your back.

Here's why:

Each of your spinal discs is only able to support a limited number of bending motions over the course of your lifetime before you get low back pain, a disc bulge or a disc herniation.

And since crunches involve lying on your back and repeatedly bending and extending your spinal credit card, they place excessive strain on the part of your low back that has the most nerves and is most prone to wear and tear.

Perhaps you've heard that if you pick a heavy object off the ground, and you don't want to hurt your spine, you should "Bend at the knees, and not at the back." But think about crunches this way: Anytime you do a crunch, you're bending at the back -- over and over and over again!

And that's not all: A crunch simply doesn't burn as many calories as necessary for you to get rid of any fat that is obscuring your stomach muscles -- so it's an inefficient way to get your gut looking good.

Ultimately, there are better ways to get a flat stomach. Here are my top 10 crunch alternatives that will avoid back pain and burn many more calories than crunches. For each exercise, choose a weight that allows you to do 10-15 repetitions and click on the exercise name for a demonstration.