There is much debate over Bikram's "locking the knee" in balancing postures, especially in Standing Head to Knee. There are a couple good reasons to straighten the standing leg, but oddly none of them have to do with the strength of the quadricep or the health of the knee. I will write about those another time.
There are three significant and concrete reasons not to lock the knee, especially for beginners.
1) Locking the knee engages the quadriceps on the front of the thigh which sympathetically relaxes the back side of the thigh, our hamstrings. But our hamstrings actually carry a lot of the weight of this posture. As we bend forward our center of gravity shifts forward. In order to hold the posture, the back side of our body becomes tight to transfer the weight through our torso into the standing leg and then the ground. People talk a lot about hyperextension of the knee, which is a symptom of this hamstring dis-engagement. The hamstrings should actually be engaged and strong in any standing posture.
2) A slightly bent knee shifts the center of gravity backward, centering it over the standing foot and allowing for even engagement of all the muscles of the body. If we try to shift the weight forward, putting the hips directly over the standing foot, we put tremendous strain on the body as it tries to overcome the laws of physics to keep the body from falling forward. If we bend the knee slightly, the hips shift back and the posture becomes much more balanced and effortless.
3) A slightly bent knee allows for easier balance. When the knee is bent and mobile, it becomes another point that can move and adjust to aid in the balance of the posture. Quite simply, if we immobilize the knee, any balance adjustment has to come from somewhere else. We become perched upon a 3 foot rod (our locked leg) that makes balancing very difficult. For this reason, "locking the knee" is one of the most advanced things we can do in Standing Head to Knee. It demands tremendous control and balance.
This journal honors my ongoing experience with the practice, study and teaching of yoga.
1) Sridaiva Yoga: Good Intention But Imbalanced
2) Understanding Chair Posture
2) Why I Don't Use Sanskrit or Say Namaste
3) The Meaningless Drudgery of Physical Yoga
5) Beyond Bikram: Why This Is a Great Time For Ghosh Yoga