The knees give you the ability to do flexion and extension. Flexion takes place when you decrease the angle between your lower and upper leg. Extension takes place when you return your legs to a straight position. Both of these motions are involved with knee crunches, but the main focus of the exercise is in the midsection of the body.
Function of Knee Crunch
The abdomen contains a large muscle in the center known as the rectus abdominis. Knee crunches, also known as reverse crunches, target the lower region of this muscle. The abs are part of your body's core. By strengthening this area, you will be able to perform daily tasks with more efficiency. A strong core also contributes to a more stable spine.
Form
The knee crunch is performed from a face-up, or supine, position on the floor. When doing this exercise, it is important to pay attention to your form. This will not only bring you better results, but it will also prevent injury. After moving your arms out to your sides, lift your legs, bend your knees and level your shins to the floor. Steadily move your knees back toward your chest as you lift your hips. Squeeze your abs forcefully for a full second, move your legs back out and repeat. To make the exercise more challenging, fully extend your legs and let them hover above the ground during the lowering phase.
Secondary Muscles
Any time you decrease the angle between your thigh and stomach, you perform hip flexion. This causes you to work your quadriceps and hip flexors. If you extend your legs on the lowering portion, the quads will get even more work. The knee crunch also targets the obliques which sit on the sides of the stomach. These get activated as secondary muscles to stabilize the spine.
Variations
By doing a variation, you can change the emphasis of knee crunches or shift the focus on the muscles. Alternating knee crunches for example, shift the emphasis to your obliques. Perform these the same way as regular knee crunches, except pull one knee in while extending the other leg out. Do this in a steady, alternating motion with your head and shoulders slightly lifted off the floor.
Hanging knee crunches increase the challenge and they are performed while grabbing a hold of a pullup bar. Position your hands about shoulder-width apart on the bar, then pull your knees up toward your chest and slowly lower them down.
Training Tips
If at any point you feel tension in your lower spine, place your hands flat on the floor under your tailbone. This will take pressure away from your lower back. Although the weight of the legs is resistance enough to make progress, you might find it too easy at some point. The best way to increase your resistance is to wear ankle weights.
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