Squatting, like every exercise, comes with inherent risks. Many of these risks come from poor technique, lifting a weight that you lack greater than your strength or continuing to attempt to lift despite fatigue. Before squatting, practice your technique with a broomstick to ensure good control through a full range of motion. Never lift without a spotter. Consult a health-care professional before beginning any strength-training program.
Injuries to the lower back can occur in training, but this can result from forward lean, the hips coming up too soon or rounding your back. All of these can be avoided by properly strengthening the muscles that support your torso — your abdominals and lower back. Additional work with light weight to build good technique should allow you to squat heavy weights without rounding or forward lean. Squatting properly develops the strength of your spine, according to a 2000 study in the "International Journal of Sports Medicine."
There is a great debate about the safety of squatting and the effect on your knee joints. Squatting with poor technique, allowing your knees to move in and out while you are moving up and down, is a recipe for disaster. Collapsing into the bottom of the squat is also an issue, but not the fault of the exercise itself. Powerlifters and weightlifters, both of whom squat extensively — and heavily — exhibit very few knee injuries. A 2002 study in the "American Journal of Sports Medicine" revealed that knee injuries, while rarely occurring, were slightly more prevalent in weightlifters. Weightlifters also perform the snatch, and the clean and jerk, movements that subject the knee to more stress than the squat.
Allowing the bar to roll down your back can hurt your shoulders. This can be avoided by not putting anything between yourself and the bar. Padding not only raises your center of gravity, forcing you to work harder to balance the bar, it leaves less of the bar in contact with your upper back. The more of the bar that is in contact with your body, the greater the friction to help hold it in place. To keep the bar firmly fixed, pull your elbows down and then pull your hands forward, as if you were trying to bend the bar over your back.
Getting stuck under the bar is a valid concern, because sooner or later you will miss a lift. This is why you should never squat outside of a power rack or squat cage. Set the pins or rack high enough that they are just below the level of the bar at the bottom of your squat. If you need to dump the bar off of your back, dump it backwards, to avoid leaning forward with the squat.