Head to knee posture is a particular adaptation of the forward bending posture. In head to knee, you round your spine so that the forehead touches your straightened knees, using the hands as an aid.
The Mechanics of Head to Knee
Head to knee posture can be done in either a seated or standing position, depending on your level of balance. As a warm up, cool down, or if you have been injured, practice head to knee sitting down. To challenge yourself or for a more intense stretch across the shoulders, practice from a standing position.
When sitting, start by sitting up very straight with your core muscles engaged, legs together with the muscles fully engaged so that the kneecaps are lifted and the thighs are firm. Raise your arms over your head to align your spine, then bring your hands forward to your legs. Reach as far as you can, placing your fingers under the knees, behind the calves, or holding the ankles.
The position of the elbows is important in this posture. Elbows reach out to the sides in an exaggerated "chicken wing" pose. This helps the back to curve and maximizes the stretch across the shoulders and the lower back.
When standing, start with a firm foundation. Open your toes and make sure the balls of the heels touch and the toes face forward. All the muscles in the legs and gluteus should be firm and kneecaps lifted. With the legs fully engaged, lift the arms over the head to align the spine and then bend forward as the arms arc out to the sides and then down, so that the hands hold behind the elbows, knees, or heels as is comfortable.
Cautions
For flexible folks, when doing the head to knee variation of the forward bend, don't reach out to grasp under your feet. Doing so straightens the spine and encourages you to look forward, which lessen the curved-back position desired in this adaptation.
For less flexible folks, go easy and be gentle with yourself. This is a satisfying position for beginning yogis and yoginis. Begin with your hands behind your knees. Practice the position twice, holding for 20 second each, then see if you can reach your hands down behind your calves. If so, do it two more times, coming back to the beginning position between each repetition. If you find that position comfortable after two repetitions, try reaching for your ankles. Do this entire sequence every time you practice yoga, and you'll find yourself becoming more flexible over time.