Weizhong (BL40): The Back-of-the-Knee Point for Lower Back Pain

Where Weizhong Sits

This point sits at the back of the knee, in the middle of the skin crease, halfway between the two firm cords that frame the hollow.

The line this point belongs to is the longest on the traditional map. It starts at the inner corner of the eye, climbs over the top of the head and runs down the back in two parallel lines on each side of the spine. Those lines cross the buttock and run down the back of the thigh. They meet in the crease behind the knee, and the single line then runs down the calf, behind the outer ankle and along the outer edge of the foot to the little toe.

Vietnamese and Chinese texts call it the Bladder channel. This is its fortieth stop, at the place where its two branches join.

The old texts place it at the midpoint of the crease at the back of the knee. The hollow behind the knee is framed by tendons: on the outer side the tendon of the outer hamstring (the biceps femoris), on the inner side the tendons of the inner hamstrings (the semitendinosus and its neighbours). Bend the knee a little and both stand out as firm cords. The point is on the crease, halfway between them.

What lies underneath is the reason for care. The hollow behind the knee is a passage for the main artery and vein of the leg (the popliteal artery and vein) and for the large nerve that serves the calf and sole (the tibial nerve). Fat and connective tissue cushion them, but they are not far below the skin. That is why this page asks for light pressure only, and why you must move off any spot where you feel a pulse.

The skin over the crease is served by the posterior cutaneous nerve of the thigh, from the first to third sacral roots at the base of the spine. Those roots also serve the skin of the buttock and the back of the thigh. The tibial nerve below it carries fibres from the lower lumbar and upper sacral roots, the same roots that feed the sciatic nerve. That shared wiring is one reason pressing here can be felt as a pull up the back of the thigh or down into the calf.

Why This Spot

From the anatomical side, the hollow behind the knee is a crossroads. The big nerve to the back of the leg passes through it. The calf muscles begin right below it and the hamstrings end at its edges. When the lower back is tight and the hamstrings are pulled short, this crease is often tender. People who sit for long hours, or walk with the knees slightly bent to protect a sore back, load this area all day.

The landmarks are also clear. The two cords at the sides are easy to feel when the knee is bent, and the crease marks the level. You can reach it from a chair by wrapping your hands around the knee, without bending forward. That matters when bending forward is the very thing your back will not allow.

In this tradition the point is the sea point of its channel, where the flow runs deepest. It is also the lower sea point of the bladder. Most important for readers with back pain, it is one of a small group of points used as commands for whole regions of the body. For the back and the lower back, the command point is this crease. Its name, the middle of the bend, describes exactly where to look.

What the Old Texts Say About Weizhong

The place of this point is tied to one of the oldest measurements of the leg. The World Health Organization's 2008 standard sets the distance from the fold under the buttock to the knee crease at fourteen equal units. It credits that figure to the Illustrated Manual of the Bronze Man, the Tongren Shuxue Zhenjiu Tujing, written in 1026 by the court physician Wang Weiyi. Several points on the back of the thigh are still measured from the crease where this point sits.

The Vietnamese handbook of points and channels by Ngo Trung Trieu, published by Hong Duc, lists it for knee joint pain, cramping of the calf muscles, pain of the back and the lower back, and pain running down the sciatic nerve. Almost every entry describes the same thing: pain or tightness along the back of the body, from the waist to the calf.

The book's finding method is plain. Take the midpoint of the line between the outer and inner corners of the diamond-shaped hollow behind the knee, where the artery runs. It names the artery as a landmark. For pressing at home, we read it as a warning: the point is close to the pulse, never on it.

There is also a short teaching rhyme, memorised by generations of students, that pairs four regions of the body with four points. For the back and the lower back, it sends the student to this crease. The rhyme did more than any book to make the point famous. It proves nothing about how anything works. It does show that, for centuries, people with aching backs were sent to the back of the knee, not only to the back itself.

Part of the reason is practical. When the lower back is in spasm, the back itself is often too sore to touch. The crease behind the knee, on the same channel, can be reached gently while the back is left alone.

Points That Work With Weizhong

The first partner is lower on the same line, in the middle of the back of the calf. Stand on tiptoe and look for the upside-down V where the two bulges of the calf muscle meet the tendon below. The point sits at the tip of that V. It is commonly paired with this one for tight, cramping calves and pain running down the back of the leg.

The second sits at the hip. Lie on your side and find the large bony bump at the top of the thigh. The point lies in the hollow of the buttock behind and above it, about a third of the way toward the tailbone. It is used with this one when the pain runs from the buttock down the back or side of the leg.

The third is at the ankle, in the hollow between the tip of the outer ankle bone and the Achilles tendon behind it. It is used when the pain runs right down to the heel, and it carries a caution in pregnancy, so leave it alone if that applies to you.

How to Find and Press Weizhong

Getting Into Position

Sit on the front edge of a firm chair, feet flat on the floor, knee bent to about a right angle. A slightly bent knee lets the tissues behind it go soft. Do not stretch the leg straight out; that pulls everything tight over the point. Wrap both hands around the knee from the sides, so your fingertips meet in the hollow behind it.

How to Find It

With your fingertips behind the knee, feel for the two firm cords at the edges of the hollow: one on the outer side, one on the inner side. They are the tendons of the hamstrings. Now find the skin crease that runs across between them. The area is on that crease, halfway between the two cords.

Before you probe, feel lightly with flat fingers. If you feel a pulse beating, or a soft bulging vein, that is not the place to press; move a little to the side, or skip the point. Then probe with the tip of your middle finger, nail cut short. A thumb is awkward to use behind the knee; a rounded-tip tool works if you can hold it there comfortably. Move in small steps along the crease with light pressure. One spot will sting: a small, sharp, pinpoint tenderness. That is the point. Behind the knee it takes much less force to find than elsewhere.

How to Press It

Keep your fingertip or tool on the stinging spot and rub in small, light circles, about the size of a fingernail, roughly one each second. Pull gently forward toward the front of the knee rather than pushing hard. Keep your hands wrapped around the knee so the work comes from the fingers, not the arms. If you feel a pulse under the tip at any time, stop and move off it.

How Much Pressure

Here more than anywhere, less is more. A small sting under the tip is right, and behind the knee it appears with light pressure. The test is what happens when you lift off: the sting should fade within moments. If the pain stays after you lift your finger, spreads, or the area turns swollen, hot or red, stop and leave it alone. Stop at once for any electric, tingling or numb feeling running down the calf or into the foot. If you are holding your breath, you are pressing too hard. Never press deep into the hollow.

How Long

Work each leg for one to three minutes. In the first sessions keep to about one minute with very light pressure. As the days go on, add a little time, up to three minutes, but do not add force. Do both legs, even when only one side of the back hurts. Once or twice a day is plenty. Pressing longer does not help more.

When to Do It

Morning suits a back that is stiff on getting up. Late afternoon suits tightness that builds after sitting or standing. Before bed is a good time too, especially if your calves cramp at night: a minute or two here, then a gentle calf stretch against the wall. Do not press in the middle of the night. If a cramp wakes you, leave this point alone until the morning, and tell your doctor if cramps wake you often.

What This Won't Do

It will not remove the reason your back hurts. Lower back pain in later life usually comes from wear in the discs and joints of the spine, weak muscles around the trunk, long hours sitting, or a fall years ago. Pain down the back of the leg often comes from a nerve pinched higher up. A crease behind your knee will not out-argue any of that. Pressing here is a comfort measure that works best alongside walking, gentle strengthening, changing position often, and the advice of the person who examined your back.

It will not replace a diagnosis either. Back pain with loss of control of the bladder or bowel, numbness around the groin or buttocks, or weakness in both legs is an emergency; get help immediately. So is back pain with fever, after a fall, or with weight loss you cannot explain. One calf that is suddenly swollen, warm and painful needs urgent assessment, because it can mean a clot in a deep vein. Do not press anywhere on that leg. A swelling behind the knee that you can feel as a lump should also be checked, not pressed.

Safety First

The hollow behind the knee carries a large artery and vein, so use light pressure only. Never press on a pulsing spot or a bulging vein. If you have varicose veins, have ever had a blood clot in the leg, or one calf is suddenly swollen, warm and painful, see a doctor first; the last one is urgent. Skip the point if the knee is hot and swollen, or you had knee surgery recently. Do not press over broken, bruised or rashy skin, or over a lump behind the knee. If you take blood thinners or bruise easily, ask your doctor before working here at all.

Stop at once if the pain stays after you lift off, spreads, or if the area turns swollen, hot or red. Stop too for any electric or numb sensation. Nothing here requires you to push through pain. For the full list of cautions and the situations where pressing on yourself is not appropriate, read the full safety guide before you begin.

Questions People Ask

Where is the Weizhong (BL40) pressure point? Weizhong (BL40) sits at the back of your knee, in the middle of the skin crease. Bend the knee a little and feel the firm cord on each side of the hollow behind it. The point is on the crease, halfway between those two cords. Probe there gently until one spot stings. In English it is called the Middle of the Bend; in Vietnamese, Ủy Trung.

Why does this page say light pressure only? A large artery, a large vein and a major nerve all pass through the hollow behind the knee. Pressing hard there is not safe. The point responds to light pressure, so there is no need to dig. If you feel a pulse under your finger, move off it.

How long should I press Weizhong? One to three minutes on each leg, with light pressure. Keep the first sessions to about a minute, then add a little as the days go on, up to three. Once or twice a day is enough.

My calves cramp at night. Should I press this point when it happens? No. Do not press in the middle of the night. Work the point during the day and again before bed, along with a gentle calf stretch. If cramps wake you often, tell your doctor, because some medicines and low minerals can cause them.

Why would a point behind the knee help my lower back? In this tradition the channel that runs down both sides of the spine passes through this crease on its way to the little toe. A well-known teaching rhyme sends the lower back here for that reason. On the anatomical side, the big nerve that serves the back of the leg also passes this hollow. We make no promise; many people find it eases a tight back and leg.

Can I press it if I have varicose veins? Not without asking your doctor first. Never press on a bulging vein or anywhere you have had a clot. If one calf is suddenly swollen, warm and painful, do not press at all; get urgent medical help.

Other Points in This Library

Each of these sits within reach of the same hand. They are often worked in the same sitting.

This page shares general information about a traditional practice. It is not medical advice and does not replace what your doctor has given you. If you are pregnant, managing a health condition, or unsure whether this applies to you, check with your doctor first.

The whole inner edge of the foot, laid out on one page with photographs, is in The Foot Map — the free book. For every point in this library, see the full pressure point index.

(Point shown: BL40 · Weizhong · The Middle of the Bend · Ủy Trung.)