The squat, when performed correctly, is the safest leg exercise for the knees. It also produces a more stable knee than any other leg exercise. "Correctly" is "deep." Deep is the hips dropping below level with the top of the patella. Correctly is full range of motion. A squat that is not deep is a "partial" squat. Partial squats are awful because they stress the knee and the quads without stressing the glutes (butt), the adductors (inner thighs) and the hamstrings (back of thighs). The hamstrings, groin muscles and glutes perform their function in the squat when the hips are stretched to the point of full flexion, where they get tight - the deep squat position. The hamstring muscles and the adductors reach a full stretch at the very bottom of the squat, where the pelvis tilits forward with the torso, stretching the ends of the muscles apart. At the stretched position they provide a slight rebound out of the bottom, which will look like a "bounce." The tension of the stretch pulls the tibia (shin bone) backwards, balancing the forward-pulling force produced by the quads, which pull from the front. The hamstrings finish their work, with help from the adductors and glutes, by straightening out the hip. In a partial squat, which fails to provide a full stretch for the hamstrings, most of the force against the tibia is upward and forward. This produces a forward-directed sliding force on the knee, with the tibia being pulled forward from the patellar tendon and without a balancing pull from the opposing hamstrings. This "shearing force" is the biggest problem with partial squats. It's the leading cause of patellar tendinitis (pain and swelling in the knee) in non-basketball players, but the majority of patients, thinking their poor form is actually correct, blame "squats" for their problems. Partial squats also cause problems because they help you move very heavy loads, due to the short range of motion and the greater mechanical efficiency of the quarter and half squat positions. This sets you up for back injuries as a result of the extreme back loading occurring when you put a weight on your back that could be up to three times the weight you can safely handle in a full squat. A lot of people are fond of partial squats as it allows them to claim 600+ lbs "squats." Your interest should be in getting strong, not playing silly number games. If you can't squat the weight below parallel, it's bad for your back.
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