In Hands To Feet Pose (Padahastasana) we bend the upper body down to lay flat against the thighs and legs, stretching the back of the legs, especially the hamstrings. Ideally, this is not a back stretch. The pelvis should tilt forward as much as possible, keeping the back nearly straight and moving the stretch into the upper hamstrings.
(In the picture to the left, you'll notice that due to the tilt in my pelvis, there is gentle rounding in the back. Over time, this rounding should decrease as my hamstrings lengthen and the pelvis is free to tilt forward/down more, so the sit-bones will go higher and the back will be straighter.)
In Bikram's class, we are instructed to place our fingers underneath our heels and shift our body weight forward. When we shift our weight forward, the backside of our legs engage - the calves, hamstrings and glutes - to keep us from falling forward. These are the very muscles that we are trying to stretch, so we should relax them as much as possible.
Try this: don't put your fingers underneath your heels. Place your hands around the back of your ankles or heels, but leave your feet flat on the ground. Then, instead of shifting your weight forward, shift it slowly backward. See if your hamstrings and calves don't relax, allowing you to stretch them.
This journal honors my ongoing experience with the practice, study and teaching of yoga.
1) Sridaiva Yoga: Good Intention But Imbalanced
2) Understanding Chair Posture
2) Why I Don't Use Sanskrit or Say Namaste
3) The Meaningless Drudgery of Physical Yoga
5) Beyond Bikram: Why This Is a Great Time For Ghosh Yoga