The knee is so vulnerable and sensitive to alignment because it is a shallow, basically unstable joint. Imagine two long columns stacked atop each other, and you've got the thigh bone (femur) and the shin bone (tibia). The flat surfaces of the bones make the knee dependent on ligaments (which join bone to bone) and tendons (which join muscle to bone) to hold it together. Any side-bending or twisting forces endanger these supporting tendons and ligaments. For example, standing poses done with improper alignment can put great strain on the knee. The best indicators of knee alignment in standing poses are the relative positions of the foot and kneecap. The foot acts like a pointer showing the rotation of the shin and lower leg, while the kneecap shows the rotation of the femur. In Trikonasana (Triangle Pose), for example, the front leg kneecap should point over the center of the foot. If the kneecap points toward or even inside the big toe, you know that the columns are twisting. In Trikonasana, all yogis need a strong outward rotation of the femur bone in the hip socket to align the femur with the shin and foot.
Bent-leg standing poses can also stress the knee. As the knee bends, it should function like a hinge, with no sideways movement. In Virabhadrasana II (Warrior II), a common misalignment is for the front knee to point inside the big toe. In this position, the columns of the leg are not only twisting, they are also bent to the side at their junction. This widens the gap between the bones at the inner knee, straining the ligaments there, and compresses the outer knee, which abrades the joint surface and contributes to arthritis. As in Triangle, a strong outward rotation of the front leg femur is needed.
Seated poses may also put strain on the knee. In Virasana (Hero Pose), if your toes angle out to the sides instead of pointing straight back, you're twisting and straining your knees. The foot and lower leg are rotating out while the femur is, relatively speaking, rotating in. While Virasana does require the femur to rotate in slightly and while the knee can safely allow some twist in a bent, nonweightbearing position, Virasana with the feet turning out is an excessive twist which will damage the knee ligaments.
Cross-legged sitting poses such as Padmasana (Lotus Pose) can also be problematic for the knees. To sit well in Padmasana, the femurs must be able to deeply externally rotate within the hip sockets. When this rotation is limited, the knees will stick up in the air. If you then attempt to place the foot on top of the opposite thigh, the outer knee ligaments will be stretched and the inner knee surfaces compressed, causing pain, and if the pose is forced, injury. Before students attempt Full Lotus, they should increase their hip flexibility so the knees come near the floor when they sit cross-legged. To help improve external rotation, try this variation of Baddha Konasana (Bound Angle Pose). Sitting tall with your back to a wall, place the soles of the feet together and draw the heels in toward the hips. Either allow gravity to pull the knees down or very gently press the hands on the thighs, lengthening the thigh bones out of the hip sockets and down toward the floor. Sit in this position for two or three minutes, so the muscles and connective tissue around the hip joint can soften and release.
Hip joint flexibility can also be helped by lying on your back and crossing the right ankle over the left knee. Hold your hands behind the left knee and gently pull the legs toward the chest. You should feel a stretch in the back of the right hip, not at the knee. Because the muscles and fascia (connective tissue) of the hip joints are so strong, it may take months of work to improve hip flexibility enough to do Padmasana without knee strain. If your hip flexibility is adequate and you still experience knee pain in sitting poses, it may be due to previous knee injuries or strains. If that is the case, it can help to create a long thin roll with a washcloth or small towel. Holding each end of the roll, pull it deep into the back of your partially bent knee; hold the roll in place as you continue to bend the knee fully. Then try Virasana, Padmasana, or some other bent-leg sitting pose. The roll helps to keep the bones in their natural alignment, without twisting or side bending, and keeps a little space open inside the joint, avoiding compression.
All these cautions may sound alarming, but you really only need to keep a few simple principles in mind: always check your alignment, and if you ever feel strain in your knees, back out of the pose and experiment until you feel the stretch in your hips or groins instead. Practiced with care, asanas can contribute to the long-term health of your knees by strengthening your quadriceps, opening your stiff hips, and teaching your body improved alignment and movement patterns that transfer into your everyday activities.