A straight leg is the foundation to several postures in the Bikram Method, the concept is simple and still I field the most questions in  regards to it. It’s all about your buns!
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You should lock your knee because it is anatomically the safest and most therapeutic position for all of your leg joints. In particular, it  creates eustress (beneficial stress) for the knee joint.
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The misunderstanding about this locked  leg is that the position somehow hurts the knee joint or creates hyperextension. Actually, it’s the movement of your leg bone The femur bone)  that will stop any hyperextension in the knee. If you are hyperextending your knee, your buttocks muscles are completely relaxed. Never bend your knee in this weight bearing position, you will create distress instead and cause injury over time.
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Here are 3 things to do in order to create the leg position you want for this incredibly effect posture:
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1. Straighten your leg. Push your knee back until the joint has no more movement. For those of you that hyperextend, you’ll see it occur now. Notice that your knee cap is turning in. This is originating  from the position of your femur bone (that long bone in your thigh) in your hip joint. Wait! We will get there!

2. Contract all leg muscles. Squeeze the muscles in your calf and thigh. Notice your knee cap lifts when you contract your quadriceps. Notice your knee cap is still turning toward the center of your body. This is because of the position of your femur bone. Wait! Almost there, stay with me!

3. Contract your buttocks muscles. Squeeze the standing leg buttocks muscles as much as possible. Notice the knee cap now turn forward and the hyperextension is gone! Your deep 6 rotator muscles externally rotate the femur bone in the hip joint. If you are hyperextending your knee, your buttocks muscles are completely relaxed.

And there you have it. When your buttocks muscles relax, your femur bone turns toward the center of your body and your knee moves beyond normal range of motion. When you contract your buttocks muscles the femur externally rotates and creates  straight position for the leg. Most of us have weak buttocks muscles from all the sitting we do. If you want to learn how to use them, try an Inferno Hot Pilates class. After they are sore from class, you’ll feel them contract when you are balancing on one leg!!