Please Your Knees
You're balancing confidently in Ardha Chandrasana (Half Moon Pose), and the pose feels firm and steady. There's only one problem: You're hyperextending the knee of your standing leg. When you extend, or straighten, your knee beyond a straight line, it's called hyperextension, which can injure the knee and other parts of your body. It's quite common among yoga students at all levels, and certain asanas can exacerbate the condition if you repeatedly do them incorrectly. Fortunately, you can learn to practice in a way that both aligns and protects your knees and makes them stronger and healthier.
What's My Line?
When a knee that isn't prone to hyper-extension extends, its ligaments—the cords of connective tissue that join the thighbone to the shinbone—pull taut and stop the two bones at the point where they lie directly in line with each other. If your knee hyperextends, that means its ligaments are too long, and so they don't stop the bones until your leg has moved beyond a straight line. If you're uncertain about whether your knees hyperextend, stand sideways in front of a full-length mirror, gently press your knees backward until you are unable to move them back any further, and envision an imaginary line running down the side of your leg from your hip joint to your ankle. If the center of your knee ends up behind that line, it is hyperextended.
Standing with your knees locked back in hyperextension can cause a host of problems in your knees and also in your legs, hips, and spine. In addition to overstretching the ligaments, hyperextension stresses the front of the knee joint surfaces and weakens the quadriceps muscles. Over time, this misalignment may create deeper hyperextension, ligament strains or tears, cartilage degeneration (including meniscus damage), and arthritis of the knee joint or kneecap. What's more, if you push the knee back with enough force, you can tear a ligament, most likely the anterior cruciate. Standing in hyperextension puts excess pressure on your heels and the front of your shins, which can lead to inflammation. It may also tilt the top of your pelvis forward, which can stress your hip joints, overarch your lower back, and disturb your posture all the way up to your neck and head.
Some people develop hyperextended knees at an early age, so the condition may be partly genetic, but it's also likely that posture and movement habits (especially in activities like dance, gymnastics, or yoga) can exacerbate the condition. Even everyday habits can contribute: The soleus, a calf muscle, can pull the shinbone back. Tightness in this muscle—for example, from wearing high heels—might help create or worsen hyperextension.
Some yoga poses, such as Trikonasana (Triangle Pose) and Ardha Chandrasana, tend to strongly push the knees back toward hyperextension if you don't practice them with care. In Trikonasana, the angle of your front leg to the floor invites gravity to push your knee into extension, and as you sidebend over the leg, the weight of your torso magnifies the effect. In Ardha Chandrasana, you put all of your weight on one leg and then straighten it fully, so if your knee is even slightly hyperextended, your body weight will often push it back more. To keep your knees healthy, it's important to learn how to do these and similar poses safely.
Find Your Limits
The knee joint is the junction of the thighbone (femur) with the shinbone (tibia). It is formed by two convex bulges at the lower end of the femur (the femoral condyles) and two corresponding shallow depressions at the upper end of the tibia (the tibial condyles). The depressions are rimmed with tough, contoured rings of cartilage, called the medial and lateral menisci. These distribute the weight of the femoral condyles evenly over the tibial condyles, so no one spot gets too much wear. The knee is naturally unstable because the joint surfaces do not interlock deeply. In addition, the leg bones are long, giving them leverage to bend the knee back in unhealthy ways.
Four main ligaments bind the femur to the tibia, limiting some movements and allowing others. The medial and lateral collateral ligaments stabilize the inner (medial) and outer (lateral) knee. The anterior and posterior cruciate ligaments lie between the femoral and tibial condyles and work together to keep the condyles in close contact throughout the knee's entire range of motion.
Additional ligaments tie together other parts of the knee; two that are especially relevant are the popliteal ligaments that connect the back of the tibial condyles to the back of the femoral condyles. If you have a normal knee and extend it to the point where your femur and tibia form a straight line, all four of your main knee ligaments, plus your two popliteal ligaments, will become taut and will stop your knee from extending further. If you force the extension beyond this, you will overstretch these ligaments, and possibly tear some of them.
The knee ligaments are tremendously strong, but not strong enough to resist the immense force that the femur and tibia can wield. Fortunately, several powerful muscles send tendons across the knee to reinforce the ligaments. If you can learn how to access and use these muscles properly, you can prevent hyperextension while doing your poses.
The four quadriceps muscles help hold the front of the knee together by connecting the front of the thigh and pelvis to the front of the tibia, by way of the kneecap. The three hamstring muscles help hold the knee joint together from the rear by connecting the sitting bone and the back of the femur to the back of the tibia and fibula. The quadriceps straighten the knee, and if their force is unopposed, they can push the knee back into hyperextension. The hamstrings, with the help of several other muscles, bend the knee, so they can protect it from overzealous action by the quadriceps. To help keep the knee stable, it's crucial to balance the knee-extending strength of the quadriceps with the knee-flexing strength of the hamstrings and other knee-flexing muscles.
Get It Straight
Trikonasana is an ideal pose to practice to learn how to avoid hyperextending: You do this by using your muscles rather than your ligaments to hold the bones in correct alignment. Begin with your legs in a wide stance, turn your left toes in slightly, and your right foot out 90 degrees. Bend your right knee a little, and then contract your quadriceps and your hamstrings by imagining that your muscles are simultaneously hugging toward the bone and drawing up toward your pelvis. This will hold the leg firmly in place, so it can neither bend deeper nor straighten more. Continue contracting both sets of muscles, but allow your quadriceps to work a little harder than your hamstrings so that your knee begins to slowly straighten against the resistance of the hamstrings. Keep straightening until your thighbone and shinbone are in a straight line with each other, using feedback from a mirror or a friend.
Most students stop short of a straight line, so take extra care to ensure that you put your shinbone and thighbone in a 180-degree line. If you have hyperextended knees, your ligaments will not be taut when you reach your line. Otherwise, you will feel your ligaments just beginning to pull taut as you arrive at your line; be very careful to stop extending your knee once you feel mild tension on the ligaments.
Once your shinbone and thighbone are in line with each other (or are as close to it as possible), squeeze the muscles of your back calf and your front, back, inner, and outer thighs into the bones to hold them unshakably in place. Finally, still hugging your leg in this ideal alignment, bend sideways to the right into Trikonasana. It's OK to place your right hand on your right ankle or shin and bear weight on it, but if you do, subtly increase the action of your hamstrings, so that your hand does not push your knee into hyperextension.
To The Moon and Beyond
To move into Ardha Chandrasana from Trikonasana, bend your right knee, shift your weight over your front leg, and lift your left leg up off the floor. Balancing on one leg with the knee slightly bent, apply the same muscle action to your leg that you did in Trikonasana, straightening the leg slowly until it is in a 180-degree line. Carefully adjust the weight on your right foot by moving your hips forward or back, until the heel and the ball bear the same amount. Too much weight on the heel promotes hyperextension, while equal weight promotes a straight leg.
With Trikonasana and Ardha Chandrasana under your yoga belt, you now have the tools you need to protect your knees against hyperextension in other poses, such as Virabhadrasana III (Warrior Pose III), Parivrtta Ardha Chandrasana (Revolved Half Moon Pose), Vrksasana (Tree Pose), and more. The more you practice this way, the steadier and better aligned your knees will become. Remember: The road to strong, healthy knees runs in a straight line.
Roger Cole, PhD, is a certified Iyengar Yoga teacher and a sleep-research scientist in Del Mar, California. For more information, visit http://www.rogercoleyoga.com.