Janu Sirsasana (Head to Knee Pose)

Janu Sirsasana, Head to Knee Pose, is a seated forward bend over one straight leg while the other knee is bent with the heel drawn in to the groin. Janu means knee and sirsa means head, so the Sanskrit name and the English one both promise something the pose does not require: for a great many people the head should never go anywhere near the knee.

What it actually is, is a one legged hamstring and side stretch with a turn in it. The chest turns to face the straight leg, which is the instruction most often left out, and without that turn the whole thing becomes a lean towards the side rather than a fold over the leg.

It is not Paschimottanasana, and it is usually the better place to start. Folding over both straight legs asks both hamstrings and the whole lower back to cooperate at once, and most adults simply round the spine instead. With one knee bent the pelvis can tip forward more freely, and the load is halved.

Where it belongs is in the seated group, late in a practice, after the standing poses and the backbends. It is calming rather than stimulating, so it fits well in the evening and in the bedtime sequence. It is worth noting that seated forward bends are on the list of poses to leave alone on the yoga for back pain page, and that includes this one while a back is actually hurting: the reason is set out below, and it is the most important thing on this page.

Three adjustments turn this pose from a back strain into a hamstring stretch, and none of them involves trying harder.

Sit higher. Sit on the floor with the legs straight and see what the lower back does. In most adults, and in nearly everybody who sits at a desk, the pelvis tips backwards and the lumbar spine rounds before the fold has even begun. From there, folding forward simply loads a rounded spine. One or two folded blankets under the seat tips the pelvis forward again, and the pose becomes possible.

Turn the chest. Before folding, turn the trunk so the breastbone points along the straight thigh. The fold then travels down the straight leg. Without the turn the ribs go towards the space between the legs and the hamstring is hardly reached.

Let the knee bend. A slightly bent front knee lets the pelvis tip. A locked knee with a rounded back means the stretch has moved off the hamstring and onto the ligaments of the lower back. Nobody has ever been harmed by a soft knee.

• Belt around the sole of the straight foot, elbows bent, no pulling.
• Cushion under the bent thigh if that knee is uncomfortable.
• Stop where the fold stops coming from the hips, however early that is.
• Both sides the same number of breaths.

The habit of building the pose around the body rather than the body around the pose, with whatever props that takes, is the tradition described on the Iyengar yoga page.

A short hamstring pulls on the back of the pelvis, and a pelvis that cannot tip forward passes the job of bending to the lumbar spine. That is the whole mechanical story of the seated forward bend, and it explains both why the pose is useful and why it is on the caution list.

Useful, because lengthening the hamstrings gives the pelvis its movement back, and there is good reason to think held stretching changes reach: a randomised controlled trial in twenty three older adults improved the chair sit and reach test by 83 percent after eight weekly sessions, against 3 percent in the control group. Small trial, clear direction.

Needing care, because a rounded lumbar spine held under a forward stretch is one of the positions an irritated disc likes least. The Cochrane review of twenty one trials and 2,223 participants in chronic low back pain found small improvements in back related function from yoga compared with no exercise, and found that the most commonly reported harm was increased back pain, with no serious side effects reported in any trial. Seated forward bends are the obvious candidate for that reported harm, which is why this site's back pain page keeps them off the list until the pain has gone.

The practical resolution is not to avoid hamstring work. It is to do it with the lower back long: sit higher, bend the knee, fold shallowly, or use the lying version with a belt where the floor supports the spine for you. That last one is what I give to anybody who has had a disc problem and wants their hamstrings back, and it works perfectly well.

A stretch responds to time rather than to force, so the holds are long and the effort is low.

Five or six days a week is fine, because there is no load on anything and hamstrings respond to frequency. A minute a side most days will change your reach in a couple of months, which is the same principle as the guidance in how often should you do yoga.

Late, and warm. Hamstrings are noticeably longer after standing poses and after a few rounds of Surya Namaskar, so this pose goes in the seated group towards the end rather than at the beginning of a practice.

The counter pose is a gentle backbend. Bhujangasana for twenty seconds, or simply lying on the front and lifting the chest a little, returns the lower back to neutral after a long fold. Then Savasana to finish.

It pairs naturally with Baddha Konasana, which needs the same seated height and works the inner thigh instead of the back of the leg, and with a seated twist such as Ardha Matsyendrasana. Those three, done on blankets with a belt to hand, make a ten minute seated practice that suits an evening and needs no space at all.

Because it is calming, it is a good pose to put immediately before sitting for breathing. Two minutes a side, then sit up and practise Anulom Vilom, and both the hips and the mind are readier than they were. It is also a sensible pose to keep in a short practice when the reason for practising is stiffness rather than strength, which is most people's reason.

The changes matter more here than in most poses, because the pose is genuinely capable of irritating a back.

• An active back problem, a disc or sciatica. Leave the seated fold out. Use the lying hamstring stretch with a belt instead, which gives the same length with the spine supported, and see a doctor before returning to the seated version.
• Knees. Cushion under the bent thigh, and less bend in that knee. A rolled towel under the straight knee helps too.
• Hamstring strain. Leave it out until it has healed.
• The head below the heart. Glaucoma, a recent eye operation or a middle ear problem all argue for keeping the trunk higher: fold over a bolster instead of down to the leg.
• Pregnancy. Fold to the side of the abdomen rather than over it, keep it shallow, and stop when it stops being comfortable. The wide seated poses are usually more useful at that stage.
• Over forty and coming back to movement. Blankets, a belt and a bent knee, for two months, and read yoga for beginners over 40 before you decide how deep to go.

If you want somebody to look at whether your fold is coming from your hips, that is genuinely hard to judge from the inside. Come to a class and be watched, or start with a first class.