However, if you decide to stop halfway down (or halfway up if you're coming out of the pose), with your fingertips several inches from the floor, the hamstrings will be contracting isometrically. If they didn't contract, your pelvis would tip all the way over and you'd fall to the floor. So they hold you in position, neither shortening nor lengthening, but working hard. And therein lies the problem for students with tight hamstrings: When they come into the pose, their fingers don't reach the floor, so the hamstrings contract to hold the pelvis in place. Then, as the teacher exhorts the class to relax and let go into the hamstring stretch, the tight students are led astray by thinking that what they feel—actually a contracting muscle—is relaxation. And this is a bad situation for the tight students who may already have trouble learning to relax.
Practicing Uttanasana with your hands dangling toward the ground isn't a good idea, not only because of inaccurate learning but also because of the huge strain it can put on the muscles and disks of your lower back; and while the hamstrings are contracting instead of stretching, their flexibility won't improve.
However, there is an easy fix for the problem. Simply put a yoga block or a folded blanket (or a chair, if you're really tight) under your hands, so the weight of your body is supported through the arms to the prop and then to the floor. Because the hamstrings no longer need to support your torso, they can relax, lengthen, and let go instead of contracting. You can visualize the hamstrings lengthening from the back of your knee up the length of your thigh to the sitting bones as you relax into the stretch.
While you're fine-tuning your Uttanasana alignment, make sure your hips are over, not behind, your ankles. When your legs lean back, you take the stretch out of the calf muscles (gastrocnemius and soleus). This happens because these muscles perform plantar flexion (that is, they raise your body up on tip-toe so your foot is in line with your leg) as they contract and shorten the calf muscles. When your legs are perpendicular to the floor, your ankles are bent about 90 degrees, which lengthens and stretches the calf muscles.
Work Your Quads
In Uttansana, like most other forward bends, the knees should be straight. If left to their own devices, the hamstrings will avoid a full stretch by keeping the knees bent. To counteract the tendency, you must contract the quadriceps muscles. The "quads" are made up of four muscles—the vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris. One of the actions of the quads is to extend, or straighten, the knee. So, in Uttanasana the quads need to contract with moderate firmness to counteract the tendency of the hamstrings to bend the knee.
There is an easy way to test whether your quads are contracting correctly. Sit on the floor with both legs straight out in front of you. Put your thumb on one side of a kneecap (patella) and your fingers on the other side. If your quads are relaxed, you can wiggle the patella up and down and side to side. When the quads contract, they hold the patella firmly and you won't be able to move it. To check the quads' action in Uttanasana, just feel your patella.