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.
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.
Crunch Alternative #10 -- Cable Torso Twists
In a standing position with your feet firmly planted and knees slightly bent, hold a cable or elastic band at arm's length (keep your arms straight) and twist your body. To do this exercise properly, imagine your bellybutton pointing straight forward, then rotate your bellybutton approximately 30 degrees.
Crunch Alternative #9 -- Woodchopper
In a standing position, hold a dumbbell, medicine ball, elastic band or cable above your body and to the side of your shoulder, and diagonally rotate with extended arms from above your shoulder, down and across your body to the outside of your hips -- just as if you were holding an axe and chopping a block of wood. Also, try "reverse woodchoppers," which is the same motion in the opposite direction.
Crunch Alternative #8 - Front Planks
Get into a belly-down position with your elbows bent and your body supported by just your forearms and your toes. This is a popular position that you'll see in yoga and Pilates. From here, you can simply hold this plank position, or you can tap your feet or alternate between reaching with each arm.
Crunch Alternative #7 -- Side Planks
Get into a similar position as a front plank but support yourself on one arm, so that your bellybutton is turned to the side. You can simply hold this position, or you can also do more advanced exercises like raising a dumbbell with your non-supporting arm or rotations.
Crunch Alternative #6 -- Stability Ball Plank
If your forearms don't like the front plank position, the stability ball plank is a great option. Simply put your shins or toes up on a stability ball and support the rest of your body in a pushup position with your hands.
Crunch Alternative #5 -- Stability Ball Knee to Chest
After you've mastered the stability ball plank position, try pulling the ball closer to you by bending your knees to your chest, then extending your legs back into the starting position.
Crunch Alternative #4 -- Stability Ball Pike
For an even harder stability ball move, try pikes, in which you start in a stability ball plank position and then point your butt up towards the sky in a capital letter A position while keeping your legs as straight as possible.
Crunch Alternative #3 -- Bridges
As I wrote in "How to Get a Flat Stomach," when you exercise your abs, you also need to maintain your low back strength. You can do a low back strengthening bridge by lying on your back, then pushing your hips up towards the sky while keeping your feet flat on the ground. For more advanced variations, try a single leg bridge or a stability ball bridge.
Crunch Alternative #2 -- Medicine Ball Side Throws
These last two exercises are a bit more ballistic and athletic, but great for building abdominal strength. Grab a medicine ball, which is one of those small but very heavy balls you can find at the gym, and then find an area of the gym where you have a solid wall. Twist your body to one side and then twist back and throw the ball against the wall. Catch, then repeat. Once you've completed a series of throws for one side, face the other direction and complete another set of throws for the opposite side. These are very similar to cable torso twists, except you're throwing an object.
Crunch Alternative #1 -- Medicine Ball Slams
I've saved the best exercise for last. Hold the medicine ball overhead, then slam it into the ground as hard as you can, while keeping your abs tight and breathing out. This exercise is also a fantastic stress-reliever.
Now that you know 10 ways to avoid crunches, you'll never have to strain your credit card again -- and that's good advice for the one in your wallet, too.
Ben Greenfield is a fitness and triathlon expert and host of the Get-Fit Guy podcast on the Quick and Dirty Tips network. His book, "Get-Fit Guy's Guide to Achieving Your Ideal Body -- A Workout Plan for Your Unique Shape," will be published by St. Martin's Press in May 2012.
For more by Ben Greenfield, click here.
For more on fitness and exercise, click here.
Follow Ben Greenfield on Twitter: www.twitter.com/GetFitGuy