Like the inner workings of an engine, knees are complicated, says Arturo Peal, who teaches yoga, anatomy, kinesiology, and therapeutic touch in New Orleans. “As the midway point between feet and hips, knees are responsible for translating your movement through the world, absorbing the pressures of gravity, and buffering the impact of walking and jumping. When they function well, they are shock absorbers.” And just like the shock absorbers on your car, they can wear out.
The key to the smooth operation of these vital joints: understanding how they work so you can treat them with care.
See also: Still Icing Your Creaky Knees? These Poses Are a Better Fix
The knee is a hinge joint; it extends forward and back. Four bones make up the joint: the femur (thigh bone), tibia (inner shin bone), fibula (outer shin bone), and patella (kneecap). The first three form the main knee joint, the tibiofemoral joint. At the front of the knee, the patella, which is shaped like a shallow saucer, rests in a groove on the front of the femur to form the patellofemoral joint.
Two C-shaped pieces of tough, rubbery, cartilage called menisci provide cushioning between your tibia and femur. Tough fibrous ligaments connect the bones, control the knee’s movement, and keep the joint strong and stable. Along with the knee tendons—strong tissues that connect muscle to bone—these ligaments stabilize the knee and limit sideways movement. (If your knee ever moves significantly left or right, it’s likely injured and should be looked at by a professional.) The synovial capsule surrounds the knee and is filled with synovial fluid, which nourishes and lubricates the joint so it can move freely.
All knees aren’t built the same
There’s a huge variety in the shape of the joint and the angle at which the knee is set. Multiple factors influence knee design, Peal says, including genetics, bone shape, and daily activities—for example, doing repetitive movements that favor one leg.
Your knees are also affected by the mobility of your hips and feet. In most people, the knee is aligned directly below the hip and above the ankle. But some people are knock-kneed—their knees angle inward toward the midline of the body—while others are bow-legged, with knees that angle outward. In both cases, the position of the legs causes stress on the ligaments at the side of the knee. More commonly, people hyperextend their knees. When standing, their knee joint presses backward, which can cause tissue damage and swelling.
“Even on the individual level, your knees are often not symmetrical,” Peal says. While we seek balance in yoga practice, both knees may not operate the same. “That’s why it’s important to listen to your own body and honor your own anatomy. If you’re attempting a pose and it causes pain in the joint, try a modification or use props.”
Although knee injuries are common, the joint is not as fragile as you might think, says Ariele Foster, a physical therapist and yoga teacher based in Washington, DC, and founder of the Yoga Anatomy Academy. In fact, your knees suffer more from too much sitting than from being well used. “We would do well as a yoga culture to build up fortitude, rather than creating a mindset of fragility around the knee,” she says. For example, worrying that you’ll twist your knee too much might cause you to avoid using its full range of motion.
See also: Know Your Knees to Help Prevent Pain & Injury
Case Study: Lotus Pose
A common cause of knee injury in yoga practitioners is forcing yourself into poses that acutely bend and twist the knee, says Ray Long, MD, a board-certified orthopedic surgeon, the founder of Bandha Yoga, and the author of several anatomy books. One pose in which this commonly happens is Padmasana (Lotus Pose). The movements used to enter Lotus—bending your knee, turning out your leg out from the hip, and lifting your shin to cross your foot over your opposite leg—create a rotational force that can put your knee in an unhealthy position, says Long. Lotus also affects the fit of the femur on the tibia. Rotating the tibia on the femur in this pose can tear the meniscus, damage the cartilage, or overly stretch the ligament on the outside of the knee, he says.
To do Lotus safely, Long suggests increasing the range of motion in your hip by doing poses that open the hips and strengthen the quads, hamstrings, and calf muscles before entering the pose. For example, practicing Supta (Padangusthasana (Reclining Hand-to-Big-Toe Pose) allows you to work the front and back of your thigh; flexing and pointing the foot in this pose engages the calf muscles.
“Muscles are the source of flexibility, not ligaments. You protect the knee joint by using the muscles around the knee,” Long says. The hamstrings and the abductor muscles that move the leg away from the body help to stabilize the knees. “You can use the muscles around the knee to form a protective brace to prevent it from going any which way,” Long notes.
If done correctly, Lotus can actually help to regenerate the knee because the pose stimulates the circulation of synovial fluid, which contains cells that nourish and repair the joint. But there may be only a millimeter of difference in the bend of the knee between helping or hurting it. Don’t force yourself into Lotus. Approach the pose with gentle attention to how it feels. If it feels like a strain or begins to be painful, back away from the pose.