In Bikram yoga, to "lock the knee" means 3 things: Straight leg (full extension of the knee joint) + thigh muscles (including quads) contracted + body weight even on the foot. This position provides a solid support for the knee joint, giving it lateral stability as well as allowing the hamstring on the back of the leg to be long and stretchable. Since your quadriceps were not "designed" to hold a contraction for a long period of time, during the posture, it's important to train them by sending the message over and over to them to contract, thereby improving their strength and stamina. All this leg work is a big part of what gets your heart rate up in this pose! Check out this video for more about locking the knee:
Want to deepen your understanding and get more goody out of your Bikram yoga practice? Check out these posts below for more videos from Kay, or search the archives for "tips!"
Boss lady, yoga-doer and life-lover, Kay D.