Done correctly, lunges effectively work your abs, butt, and thighs. However, done incorrectly they can aggravate and even injure your knees. Proper form can protect your knees and the rest of your body from aches and pains and even from serious injury. Prior to doing any lunges, check with your doctor to make sure they are safe for you.
The first step to safe lunges is a proper starting position. Stand with your feet about hip-width apart and both toes pointing forward. You should have a slight bend to both of your knees. Pull your abdominal muscles in toward your spine to protect your back muscles and to avoid putting too much weight on your knees. Hold your abs in the entire time you are performing lunges. Relax your shoulders by imagining your shoulder blades gliding down your back toward your hips. Dangle your hands at your sides.
To initiate your lunge, take a big step forward. Your front heel should hit the ground first, followed by your toe so your entire front foot is flat on the ground. When your front foot lands, your legs should be wide enough apart that you can keep both legs at 90-degree angles as you lunge down. Because your step was so big, you back heel will need to rise up off the ground as you lunge.
Proper alignment of your entire body, but especially of your legs, is key to protecting your knees. As you lunge, your shoulders should be directly over your hips, and your hips should be directly over your back knee. Your torso should remain tall and straight, forming a continuous line with your back thigh. Do not lean forward as you lunge. After your first forward step, movement in a lunge is down and up. Minimize any forward motion during the downward and upward phases of your lunge.
The final way to protect your knees in lunges is by monitoring the height of your back knee. Your ultimate goal should be to get your back knee to just a few inches off the ground. Do not lower so far that your back knee touches the ground. As you lower your body to the ground, stop your descent if you feel any pain in either of your knees. While getting your knee almost all the way to the ground is your goal, you should not suffer knee pain to achieve that goal.