Thighs MatterWhether you are a veteran yoga practitioner or a beginner, you know that your quadriceps—the muscles on the front of your thighs—work hard in many postures. They are often tired and sore after bent-leg standing poses like Virabhadrasana I and II (Warrior Pose I and II), or repetitions of Setu Bandha Sarvangasana (Bridge Pose), or a long hold of Navasana (Boat Pose). If you regularly practice such poses, your quads will become stronger. But if you work these muscles without also stretching them, they will become shorter and tighter as well. So it's important to balance poses that strengthen your quads with those that stretch them.
The word quadriceps means "four heads," referring to the four distinct muscles that all join at a single tendon. Three of the four originate on the femur, or thighbone: the vastus medialis, on the inner front portion of the femur; the vastus lateralis, on the outer front; and the vastus intermedius, between the other two. The fourth, the rectus femoris, sits on top of the vastus intermedius and goes down the center of the thigh. It originates on the front of the pelvis just below the anterior superior iliac spine (often called the frontal hipbone or hip point in yoga classes). All four muscles join to insert, via the quadriceps tendon, on the patella, or kneecap. The strong patellar ligament then attaches the patella to the top of the tibia, or shinbone.
The four quadriceps muscles strongly extend (straighten) the knee. In poses in which the quadriceps straighten the knee entirely, like straight-legged standing poses and standing and seated forward bends, this knee-extending action is obvious. But the quadriceps also work hard in poses in which the leg remains bent, like Virabhadrasana I and II. In asanas like these, the pull of gravity on the torso tends to bend the knee ever more deeply, and the quads must engage strongly so that you don't simply sink down to the ground.
In addition to straightening the knee, the rectus femoris acts as a hip flexor, pulling the torso and the thigh toward each other. In Navasana, the rectus femoris performs both of these actions at the same time. It must work with other hip flexors, like the psoas, to create the V shape of the pose by holding up the weight of the legs and torso against the pull of gravity. Simultaneously, it works with the other three quad muscles to hold the knee straight.
Long, Strong Quads
Keeping the quads strong is important for several reasons. First, strong quads bring stability to the knee joints, which are inherently unstable and dependent on ligaments and muscles to protect them from injury. (For more on knee safety, see "Knee Deep in Yoga,".) Second, research has shown that quad weakness is a predictor of arthritis in the knees. Third, weak quads can decrease your ability to live independently in your later years—as the decades go by, the quads gradually become weaker if they aren't worked regularly, until eventually it becomes difficult to go up and down stairs and get up out of a chair.