"I can't squat - I have bad knees..."
Needless to say, we've heard this one a few times before. We're not saying that this statement doesn't hold some validity, rather people often simply reiterate what they've heard or what an uninformed doctor has told them. The fact is, squatting is not only excellent for strength training, it is also a fantastic exercise for both rehabilitation and for injury prevention. The squat builds muscle, increases leg strength, increases hip felxibility and strength, and increases knee stability through strength.
On the flipside, if you only perform partial squats where the crease of the hip doesn't sink below the depth of the knee, the majority of the force is placed on the tibia as it sinks down and forward. As the tibia is pulled forward, the hamstrings fail to reach full stretch. This puts the ligaments of the knee (where the quadricepts connect to the front of the tibia) in shear and often result in patellar tendonitis.
The often injured anterior cruciate ligament (ACL) works alongside the hamstrings to prevent the tibia from moving forward of the femur. Because of this, some ACL injuries can be attributed to underdeveloped hamstrings. By maintaining healthly hip mobility and practicing full depth squats, you can squat without any stress being placed on the ACL. Instead, you are strengthening the posterior chain and stabilizing the knees, rehabilitating old injuries and preventing future ones. The key is to squat correctly with proper depth.