Ahhh, Uttanasana. A welcome rest between poses during an invigorating vinyasa sequence, or a relaxing break after vigorous standing poses. Hang down, relax, let it all go, we're instructed. But to get a deeply relaxing stretch in your legs, back, and neck in Uttanasana (Standing Forward Bend), you have to release your hamstrings. Sadly, those students with tight hamstrings and most in need of the stretch that Uttanasana offers are the ones most likely to tense up and contract the hamstrings rather than let them go.
Ideally, in Uttanasana you're stretching the hamstrings on the back of your thighs and contracting the quadriceps on the front of your thighs. The three muscles that make up the hamstrings—the biceps femoris, the semitendinosus, and the semimembranosus—originate on your ischial tuberosities, or sitting bones, on the bottom of your pelvis and insert below the knee on your lower leg. Like any other muscles, the hamstrings try to pull their points of origin and insertion closer together when they contract. This action results in the knee flexing (bending), or the hip extending (straightening, or bringing the torso and thighs into one line), or both. When you stretch the hamstrings, you are doing the opposite action—the knee is straight and the hip flexed. To get the best possible stretch in Uttanasana, you should be positioned so that the hamstrings can relax and lengthen into the stretch, not hold on tight or contract to do the work in the pose.
Three Kinds of Contractions
To understand how your hamstrings work in Uttanasana, you'll first need to understand the three types of muscle contractions—isometric, concentric, and eccentric. The biceps brachii muscle, on the front of the upper arm, is a good muscle for illustrating the types of contraction. Imagine that you're holding a five-pound weight in your right hand, with your elbow bent at about 90 degrees. If you just hold the weight there, the muscle is doing an isometric contraction. It's definitely working, but not changing length. If you bend the elbow more and lift the weight a little higher, the muscle gets shorter, which is a concentric contraction. Finally, if you set the weight down, the elbow moves from bent to straight, and the biceps lengthens but still contracts to control the descent of the weight. This is an eccentric contraction.
Now, let's look at the action of the hamstrings in Uttanasana. Start in Uttanasana, with your knees straight and your pelvis tilted forward, so your spine and head flow down toward the floor. As you start to rise up out of the pose, the hamstrings contract and pull down on the sitting bones, the pelvis comes into an upright position, and the torso comes into a vertical line with the legs. With this action, you've moved from hip flexion in the pose to hip extension in a standing position, and the hamstrings performed a concentric, or shortening, contraction. On the other hand, if you are standing upright and tilt the pelvis forward to move into the pose, the hamstrings do an eccentric contraction—they are lengthening but working hard to control the descent of the torso as the sitting bones gradually rotate upward.