A hyperextend knee can cause injury, but yoga can help stabilize loose tendons and ligaments by strengthening the muscles around the joint.
Many yoga students are quite surprised to learn that joints can become too flexible. But in many joints, ligaments and tendons play a major role in preventing excessive motion; if those tissues become too loose, the joint can move in ways that cause damage or set the stage for injury. A joint with such laxity is said to be hypermobile, and the knee is particularly vulnerable to this problem. In fact, this joint is more or less just one long bone (the femur, or thighbone) stacked on top of another long bone (the tibia, or shinbone). Ligaments and tendons are all that holds these bones together.
Most people are all too aware of how easily and dramatically the knee's connective tissues can be injured by twisting. But fewer people are aware that knee hyperextension—too much opening at the back of the knee—also creates misalignment and bad movement habits that can pave the way for arthritis and more serious knee injuries.
Do you hyperextend?
Wearing minimal or tight-fitting clothes so you can clearly discern the alignment of your legs, stand sideways in front of a full-length mirror, far enough away that you can see your whole body. In normal standing alignment, the leg forms a straight line from ankle to hip, with knee over ankle and hip over knee. If your knee is hyperextended, however, the leg will appear to curve back, with the knee behind an imaginary straight line drawn from ankle to hip.
Since hyperextended knees are basically a problem of too-loose ligaments and tendons around the knee, you can cause or exacerbate such looseness through poor alignment in yoga poses. The soft tissues at risk of being overstretched include the cruciate ligaments deep inside the knee, the medial and lateral collateral ligaments on the inner and outer surfaces of the knee, and the popliteal ligaments, which cross the back of the knee. There are also several large tendons that cross the back of the knee and normally help prevent hyperextension: the hamstring tendons coming down from the back of the thigh and the gastrocnemius tendons coming up from the calf.
If you look at the hyperextended knee pictured at left, you can see that these tendons are overstretched. The overstretched knee ligaments and tendons are also usually accompanied by changes in adjacent muscle groups, including the soleus muscle, which is deep in the calf. The soleus originates on the upper tibia and fibula, then runs down the calf to attach to the heel. If it is short and tight, it will pull the upper ends of the tibia and fibula backward, contributing to hyperextension. So if you have hyperextended knees, it's important to regularly practice bent-knee calf stretches, like Malasana (Garland Pose) and simple squats.
Though yoga probably will not shorten overstretched knee ligaments, it can help stabilize your knees by strengthening the surrounding muscles.
Protecting Your Knees
Of course, you want to practice your poses in a way that won't increase hyperextension and knee instability. If you tend to hyperextend, typical calf and hamstring stretches done with a straight knee can aggravate the problem unless you take care to engage your quadriceps (front thigh muscles). People with hyperextended knees usually have weak quadriceps, or do not tend to engage them fully in straight-legged poses like Tadasana (Mountain Pose) and Trikonasana (Triangle Pose). Instead of engaging the quads to stabilize and protect the knees, people who hyperextend usually just push their knee joints back. To overcome this habit, they need to strengthen the quads in bent-knee postures such as Virabhadrasana I and Virabhadrasana II (Warrior Poses I and II) and Parsvakonasana (Side Angle Pose) and train the quads to be active and strong in straight-knee poses.
One of the best ways to start training the quads to work in the straight-knee position is by sitting on the floor with your legs stretched out straight in front of you. Contract your quads by gently pressing your thighbones down into the floor. If you have hyperextended knees, your heels will lift off the floor; if you watch carefully, you will observe that the part of your shinbone nearest each knee shifts back toward the floor. If that happens, your challenge is to learn to contract your quads without letting your knees hyperextend. To do this, gradually press your thighs down while keeping your heels on the floor; you will notice that now the upper shinbones do not drop toward the floor. Since you need to challenge a muscle regularly for it to build and keep strength, it's a good idea to practice this action a few times a week. Not only will you start building strength in your quads, but you'll also learn how to straighten the legs without hyperextending them.
Correct your alignment
Now, let us apply this awareness to some yoga poses. Hyperextended knees are often a problem in Trikonasana, for several reasons. If you are doing the pose to the right, your right leg is at an angle to the floor that makes it easy for gravity to pull the leg into hyperextension. If your right hand is pressing down on your right shin firmly, you're pushing the tibia back. (Especially if your hamstrings are quite flexible and their tendons overstretched, they won't offer much resistance to the movement of the tibia.) And finally, if your quads are untrained and/or weak (unfortunately true for many students new to yoga), these muscles will not contract enough to prevent hyperextension and help protect the knee.
If you do hyperextend your knees in Trikonasana, however, all is not lost. With careful practice, you can learn to do the pose with a strong, straight front knee. As you are learning this new alignment, it's helpful to watch yourself in a mirror or get feedback from a teacher with a good eye for structure; you want to make sure to correct your alignment enough to remove the hyperextension but not so much that you bend your knee.
If you usually place your hand on your shin in Trikonasana, the first step in correcting hyperextension should be placing your hand on a block instead. Then move the part of the shinbone nearest the knee away from the floor. You can get a good feel for this movement if you try to press the upper shinbone into a finger (either your own or someone else's).
As an alternate strategy, you can place a block or some other firm object 6 to 7 inches high under your calf, and make sure you do not let your calf press into the block as you move into the pose. Whichever approach you take, move the shinbone just enough to straighten the knee so that it no longer curves back, but not so much that the knee bends forward.
As you move the shinbone, you may notice that a little more weight shifts onto the ball of the foot and that there is less weight on your heel. Because many people prone to hyperextension lean too heavily on the heel, this is a good correction; remember that a goal in standing poses is to have your weight evenly balanced on the four corners of each foot (the inner and outer edges of the heel, and the inner and outer edges of the ball of the foot). If you press those four corners into the floor, your quads will contract, helping to stabilize your knee in its new, straight alignment.
To accompany your Trikonasana work, also practice your new knee alignment in Tadasana or at any other time you find yourself standing for a moment—in line at the store, waiting for the teakettle to boil, taking a shower. Whether in Trikonasana or any of these standing moments, move the upper tibia forward slightly. For most people, about a half inch is enough.
As you begin to correct your knee alignment, you may become aware that your hyperextended knees are part of a bigger posture problem. As the knees curve back, there's a tendency for the pelvis to push forward, the chest to collapse back, and the head to jut forward. These forward-and-back shifts form a system of compensation that can contribute not only to knee problems but also to lower back and neck pain. So as you work on moving your upper tibia forward, you may also want to move your pelvis slightly back and your chest up and forward. Your efforts to protect your knees will then coincide with important physical goals of your yoga practice: to create strong, healthy joints and a spacious, vertical posture.