Doing exercises the right way is essential. Even the most basic of exercises can be harmful if done wrong.
* Sit-ups: Make sure your legs are bent, that you don't raise more than 30 degrees from the floor and fold your hands over your chest instead of putting them behind your head to achieve the maximal workout for your abs. Keeping your legs straight can be harmful to your lower back.
* Toe-touches: You can easily hurt your back if not done properly. Make sure your legs are bent slightly, slowly bend toward the ground and hang for about 15 seconds. Slowly come back up and repeat. Make sure not to bounce.
* Leg-lifts: Should be done while on your side and supported by your arms. Slowly raise and lower your leg in a straight up and down motion. You should not do leg-lifts while lying on your back. You can cause injury to your pelvis.
* Knee-bends: Make sure your thighs stay parallel to the floor. Knee-bends can cause knee injury and/or pain.
Take special care when dealing with exercises that involve the knees and lower back. If you find that doing an exercise is painful - don't do it.