Bent Knee Sit-Ups

Sit-ups are one of the traditional forms of abdominal training. When performed correctly, a sit-up tones and strengthens your abdominal and oblique muscles. Bent-knee sit-ups, also called crunches, can be challenging for those with weak abdominal muscles or people who are not accustomed to regular exercise. Start slowly to avoid injury.

Begin a bent-knee sit-up by lying on the floor or a mat on your back. Bend your legs so your heels are planted firmly on the floor. Your toes should be slightly elevated off the floor. Place your feet between 12 and 18 inches away from your buttocks. Stretch your arms out in front of you or cross them on your chest for an easier bent-knee sit-up. Keeping your back straight and chin tucked slightly toward your chest, lift your upper back and head off of the floor as you tighten your core muscles. Perform the lift in a progressive curling motion rather than one swift, jerky movement. Hold for a second and ease yourself back down to the ground.

Alternatives to the traditional bent-knee sit-up may make the exercise more doable for beginners. Have a friend or family member hold your feet down to keep you from moving during the sit-up, as movement can negate the force with which you are strengthening your abs and obliques. If your stomach muscles are not ready for a tough workout, start with straight-leg sit-ups. As you become accustomed to the exercise, incorporate the bent-knee pose into your routine. Another variation of the bent-knee sit-up is to rest your lower legs on a low stool or chair. Still keeping your knees bent, perform the sit-up with your thighs at a 45-degree angle.

Those in good shape who are ready for a more challenging bent-knee sit-up can adjust the placement of their arms during the exercise. Bend your elbows and place your hands behind your head. This will increase the resistance used to work your obliques and abdominal muscles.

According to Samuel Homola, author of "The Chiropractor's Self-Help Back and Body Book," the bent-knee sit-up puts your hip flexor muscles in a shortened position. To maintain optimum flexibility and range of motion in your hips, perform a hip flexor stretch after doing bent-knee sit-ups. Lie on your back with both legs straight. Bend one knee. Grasping the back of your thigh with your hands, bring your bent knee up as high as you can toward your chest. Hold for 30 seconds before relaxing. Repeat the stretch two to five times with each leg.