When you contract your quads, the hamstrings will automatically relax. That's because the quads are what's known as "antagonists" to the hamstrings. If a muscle has a job to do, the nervous system will tell its antagonist to relax. For example, when you're lifting that five-pound weight, with your biceps bending your elbow, your nervous system will tell the triceps, which straightens the elbow, to let go. Contracting the triceps would interfere with the work of the biceps. Applying that rule to Uttanasana, when you contract your quads, your nervous system tells the hamstrings to relax and let go.
Ease on Back
To understand the final muscular action in Uttanasana, let's look at the erector spinae, the group of smaller muscles that form a thick bundle running parallel up each side of the spine. When you roll down into Uttanasana, your erector spinae contract eccentrically in order to control the descent of your torso. When you roll back up, the erector spinae contract concentrically to bring you upright. Going into or coming out of Uttanasana, your pelvis rotates in the appropriate direction due to the action of the erector spinae.
If you lower into the pose with a straight spine, the erector spinae contract isometrically to hold the normal spinal curves as long as possible while the hamstrings contract eccentrically to rotate the pelvis forward. Coming back up with a straight back, the erector spinae again contract isometrically to maintain the normal spinal curves as the hamstrings pull down on the sitting bones, rotating the pelvis to bring you back to standing.
You can feel the hamstrings and erector spinae working hard in the transitions into and out of Uttanasana. Both can relax into the final pose if you support the weight of your torso by placing your hands on the floor or a prop. Over time the lengthening of the hamstrings will allow the pelvis to tilt a little more, the spine will curve into soft flexion as the erector spinae release their load, and your head will hang heavy. Your belly will be soft, and it will gradually lengthen down the front of your thighs as you release deeper and deeper into the pose. Remember though, in the midst of all that relaxation, your quads should still be working.
A physical therapist and Iyengar Yoga teacher, Julie Gudmestad runs a physical therapy practice and yoga studio in Portland, Oregon. She cannot respond to requests for personal health advice.