Knee pain while squatting is often a symptom of injury or dysfunction in another joint. There are several possible causes for the pain. These causes often involve lack of mobility at the ankle or hip joints. This immobility can cause you to shift more of the squatting stresses to your knees, leading to overuse injuries. Consult your physician for a proper diagnosis of your pain.
Full vs. Partial Range-of-Motion Squats
You may have heard the advice that you should not squat any lower than where your thighs reach parallel. The fear was that the more acute knee angle you reach below parallel increases unnecessary pressure on your patella. However, halting the squat at parallel or above does not engage your hip flexor muscles and concentrates the sheer forces across your knee. For this reason, it is important to develop a full range-of-motion squat, even before using weight.
If you find your heels lifting off of the floor as you reach the bottom of a squat, your ankles lack the necessary flexibility for the movement. Ankle immobility can cause the angular rotation that should occur across the ankle joint to shift upward to the knee joint, increasing the workload of your knees for each squat repetition. Inflexible ankles can also forcibly reduce your range of motion. Reduced range of motion again increases the shear forces across your knees.
Your gluteal muscles are a complex of muscles that work to extend legs at the hip joint, and to externally rotate your femur. Inactivity of the gluteal muscles is very common if you spend the majority of your day sitting down. If these muscles are inactive, the role of hip extension is assumed by muscles that rotate your femur internally. In these cases, your knees will have a tendency to collapse inward when squatting, which can cause unnatural stresses to your knee joint, ultimately resulting in pain.
Hip Flexors and Patellar Tendonitis
Like the inactivity of your glutes, tightness in your hip flexors is a common problem that arises from sitting all day. Your rectus femoris muscle is one of the heads of the quadriceps, responsible for extending your knee. The rectus femoris also assists in flexing your hip; as such, it crosses both your knee and your hip. Tightness in this muscle can cause uneven tension on your knee cap, resulting in pain or even patellar deviation.