Learn Supta Padangusthasana properly: how to stretch your hamstrings safely with a strap, how long to hold and why stretching is partly tolerance.
How far do you have to straighten your leg for reclining hand-to-big-toe pose to work? The honest answer: not all the way. An even pull in the back of the leg with a flat hip matters more than a straight knee. Supta Padangusthasana ("reclining hand-to-big-toe pose") is one of the milder stretches (practice assessment): you lie down, a strap provides the reach and gravity helps. The pose comes in two parts: variation I pulls the leg up, variation II opens it to the side.
What actually happens when you stretch? In a meta-analysis of 77 studies, stretching increased flexibility in the long term, most through static stretching or PNF (alternating tensing and releasing) compared with ballistic stretching. Neither volume nor intensity nor frequency played a demonstrable role [1]. And a study of 20 healthy people showed that four weeks of daily stretching (20 minutes per weekday) did not change muscle length, but raised tolerance for the pull by 8 degrees [4]. So you partly train the head, not only the muscle.
Lie on your back, both legs long. Bend one leg, loop the strap around the ball of the foot and stretch the leg toward the ceiling as far as is comfortable.
Pull the leg toward you with the strap. The other hip stays on the floor, the other foot actively flexed and the ribs soft. If the back of the leg is very tight, bend the knee slightly until the pull is moderate.
Milestone: You hold for 20 seconds with a flat hip and without a rounded back.
Stay 30 to 60 seconds per side. The times are practice values. In 62 people around age 85, 60 seconds of hold time brought more progress per week than 30 or 15 seconds [3], which is not a statement for young adults. In the meta-analysis, the length of a single session played no demonstrable role [1][2]. All that is proven: stretching works, and the length or hardness of a session made no demonstrable difference.
Switch legs. If you like, open the raised leg to the side in variation II, with one hand on the strap and the other hip on the floor, and then return to the center. Do not overdo it.
Milestone to move on: You hold 1 minute per side with a flat hip and calm breath. Then you are ready for the standing version, which challenges the back of the leg with body weight.
The back of the leg is a known problem area in yoga: a review of yoga injuries names the hamstrings alongside neck, shoulders, lower back and knees [7]. Strains usually happen when you stretch too fast or too deep. A stretch stimulus is a moderate pull, never a stab. If you "breathe through" the pain, you only raise tolerance, not muscle length [4], and you risk a strain exactly where the signal is missing. How much of it is tolerance is shown by a study of 90 healthy people: in men, pain tolerance mainly explained how high the straight leg came [6]. This guide explains and motivates; it does not replace supervised training.
Briefly tense the extended leg. Gently press the leg against the strap for 5 to 10 seconds, release and then pull a little further. This is a simple trick from PNF technique, which together with static stretching led the meta-analysis for long-term flexibility [1]. Do not do it with full force, but with about a third.
A practice rule is 30 to 60 seconds per side. For flexibility, meta-analyses found no demonstrable difference from the length and hardness of a single session [1][2]. In very old people (average 85 years), 60 seconds brought more than 30 or 15 [3].
No. A slightly bent knee is perfectly fine. Then you feel the stretch in the back of the leg and not in the knee. The studies suggest that part of the progress is stretch tolerance [4][6].
In 31 young adults, a 7-week Hatha program improved the hamstrings by about 4 degrees on average [5]. That is little and shows it takes patience. Your result depends on starting value and regularity.
Use a belt, a scarf or a towel. Even a hand around the calf will do, as long as the hip stays on the floor. The strap is a tool for reach, not a requirement.
Yes, with a strap and a bent knee. You lie down and gravity works for you. What matters is that you never go into pain but look for a moderate pull.
Cow Face Pose (Gomukhasana): Opening Hips and Shoulders With a Strap