Fuxi

What is BL38?

BL38 (Fuxi) is a Bladder-channel point on the back of the thigh, just above the crease behind the knee, towards the outer side.

Its Chinese name, Fuxi, is usually translated as "floating cleft", a reference to the gap beside the big tendon you can feel there. In the standard modern numbering used by the World Health Organization, BL38 sits one thumb-width above BL39 (Weiyang), on the inner edge of that tendon.

One warning before anything else. Older English textbooks used a different numbering for the Bladder channel, and in those books BL38 is Gaohuangshu, a point between the shoulder blades. If a video sent you to your upper back, it was using the old system. This page describes the modern BL38 on the leg.

In tradition, BL38 is a minor local point. Classical and modern texts list it for stiffness and pain around the back of the knee, cramping in the calf and hamstring, numbness along the outer leg, and sometimes for bowel or bladder complaints by way of the channel it sits on. It is taught in most curricula, but it is rarely a headline point.

The honest verdict on evidence: there is essentially no human research on BL38 specifically. Acupressure research as a whole is dominated by small, unblinded, short-term studies, and the few Bladder-channel points that have been trialled are others, not this one. So anything you read about BL38 is tradition plus the general, modest effect that steady touch has on a tight muscle.

Two things worth saying once, clearly. Long traditional use is not evidence that a point works. It is also not evidence that a point is safe, which matters here because BL38 sits close to the hollow behind the knee, where a major artery, vein and nerve run. Read the "Who should avoid it" points below before you press.

Where is BL38 and how do you find it?

BL38 lies about one thumb-width above the crease behind your knee, on the inner side of the thick tendon at the outer corner of that crease.

• Sit down and bend the knee. Sit on a chair with your foot flat on the floor and the knee bent to roughly a right angle. This softens the area and makes the landmarks pop out.
• Find the crease. Reach behind the knee and feel the horizontal fold that forms when the knee bends. That fold is your baseline.
• Find the big outer tendon. At the outer corner of the crease, on the same side as your little toe, you can feel a firm cord standing up. That is the biceps femoris tendon. It tightens if you press your heel back against the chair leg.
• Move up one thumb-width. Slide your finger up the tendon about 2 to 2.5 cm from the crease, which is the traditional one cun.
• Step just inside the tendon. At that level, drop off the hard cord towards the middle of the leg. BL38 is in the soft groove on the inner edge of the tendon.

You are looking for a slightly tender dip, not a lump. The tendon itself is hard and ropey, so if your finger is on something that feels like a guitar string, you have gone too far outwards.

Two checks before you press. First, you should not feel a pulse under your fingertip. The main artery of the leg runs through the middle of the hollow behind the knee, further in than BL38, and you must stay out of it. Second, you should not feel a buzzing or electric line running down into your calf or foot. If either happens, lift off and move your finger.

Find the point on both legs. Tradition uses BL38 on the side with symptoms, but comparing left and right helps you confirm you have the right spot.

How do you apply pressure to BL38?

Use your fingers only, with the knee bent and supported, and build pressure slowly because of what runs nearby.

1. Get into position. Sit with the foot flat and the knee bent to about a right angle, or lie on your front with a cushion under the ankle. Never press the area with the leg locked straight.
2. Place the pad of the thumb. Reach around the outside of the knee and settle the flat pad of your thumb, or two fingertips, in the groove on the inner edge of the tendon.
3. Press in gradually. Push towards the middle of the leg over three or four seconds. Aim for a firm, dull ache you would rate about 3 or 4 out of 10. Sharp pain means too hard or wrong spot.
4. Hold or circle. Either hold still for 30 to 60 seconds, or make slow small circles for about a minute while breathing out steadily.
5. Release slowly. Ease off over a few seconds rather than snapping your hand away, then rest the leg for a moment.
6. Repeat on the other leg. Doing both sides keeps things even and tells you what normal feels like for you.
7. Keep the dose sensible. Two or three short sessions a day for a few days is plenty. If nothing has changed after a week, stop and rethink.

Stop immediately and do not go back to the point if you feel a pulse beating against your fingertip, pins and needles or electric buzzing down the calf, numbness in the foot, or pain that lingers after you let go.

Do not use hard tools, massage guns or body weight here. A knuckle, elbow or a pressure gadget cannot feel what it is pressing on, and the back of the knee is not a place to find out the hard way. Fingers give you feedback and that feedback is the safety system.

What is BL38 traditionally used for?

• Stiffness behind the knee. The commonest traditional use, and the most plausible one, since BL38 sits right on the hamstring tendons. No trial has tested BL38 for this. Steady pressure on a tight muscle often eases it briefly, but that is general soft-tissue effect, not something special about the point.
• Calf and hamstring cramp. Classical texts list Fuxi for cramping and spasm in the back of the leg. The human evidence is absent. Gentle pressure and stretching are reasonable self-care for an ordinary cramp, whatever label you put on the spot.
• Numbness along the outer leg. Tradition links BL38 to numbness and weakness down the lateral thigh and lower leg. There is no research support, and this symptom often has a nerve or spine cause that deserves a proper assessment rather than finger pressure.
• Bladder and bowel complaints. Some texts list BL38 for difficult urination or constipation through its channel. This is channel theory, not a measured effect. Nothing in the human literature supports using BL38 for either.
• Local pain after long sitting. Modern practitioners often use BL38 as part of a leg routine for people who sit all day. Any benefit here is most likely the familiar short-term relief of attention, warmth and touch on a stiff area.
• General relaxation. Slow pressure with slow breathing tends to lower felt tension for a short while. Trials of acupressure for relaxation are small and unblinded, and none isolate BL38, so treat this as a comfort measure.

Taken together, BL38 is a reasonable spot to rub if the back of your knee feels tight, and an unreasonable thing to rely on for anything systemic. Use it alongside movement, not instead of a diagnosis.

Are there tips for using BL38 effectively?

• Warm the leg first. A few minutes of walking, or a warm shower, makes the tendons and the groove beside them far easier to feel. Cold, tense hamstrings hide the landmarks.
• Let the knee bend. A bent, supported knee slackens the structures at the back of the joint. Pressing into a straight, locked leg is both harder to aim and less comfortable.
• Use the flat pad, not the tip. The broad pad of the thumb spreads pressure and gives you more information. A fingernail or a pointed fingertip concentrates force on a small area that may not be the point at all.
• Breathe out as you press. Pressing on a slow exhale and easing off on the inhale keeps the pressure steady and stops you bracing against your own hand.
• Compare sides. Find the spot on the comfortable leg first. Knowing what normal tissue feels like makes the tender side much easier to identify honestly.
• Pair it with movement. A gentle hamstring stretch or a short walk after a session tends to do more for a tight leg than the pressure alone. Tradition and physiotherapy agree on this one.
• Keep a simple note. Write down the date, how long you pressed and how the leg felt an hour later. A week of notes will tell you whether it is doing anything for you far better than memory will.
• Set a stop date. Decide in advance that you will stop after a week if nothing changes. Open-ended self-treatment is how people delay getting a problem properly looked at.
• Keep the pressure light the first time. Try half the pressure you think you need on the first session and see how the leg feels the next day. You can always go firmer later.

What do people get wrong with BL38?

• Using the wrong numbering system. In older textbooks, BL38 is Gaohuangshu between the shoulder blades. If your source points at the upper back, it is on the old system, and the point described here is a different place entirely.
• Pressing into the middle of the hollow. The centre of the space behind the knee carries the popliteal artery, vein and tibial nerve. BL38 is out to the side, on the inner edge of the tendon, and that distance is the whole point.
• Landing on the tendon itself. People find the hard cord, decide it must be important, and grind on it. The point is in the soft groove beside it, not on the ropey structure.
• Confusing it with BL39 or BL40. BL39 sits at the crease and BL40 in the middle of it. BL38 is a thumb-width above the crease. Being a few centimetres out puts you on a different point in a riskier spot.
• Going too hard. Digging in with a knuckle, an elbow or a massage gun can bruise the area and irritate the nerve. Firm thumb pressure is the ceiling here.
• Ignoring a swollen, hot calf. A calf that is swollen, warm, red or newly painful can mean a clot. Massaging it is dangerous. That is a same-day call to a doctor, not a pressure point.
• Expecting a cure. BL38 will not fix a torn hamstring, a trapped nerve, arthritis or a bladder condition. Expecting it to is the mistake that delays real treatment.
• Treating one-sided numbness as a tight muscle. Numbness or weakness on one side, especially with back pain, needs assessment. Pressing a point is not the answer to a nerve problem.

What does the research on BL38 actually say?

There is no useful human research on BL38 specifically. Searches of the acupressure and acupuncture literature turn up no controlled trial, and no meaningful observational study, that tests this point on its own for any condition.

That absence is not unusual. Most of the 361 classical points have never been studied alone. Trials tend to use whole prescriptions of several points at once, which means even a positive result cannot be traced back to any single location. When Bladder-channel points do appear in research, they are usually the back-shu points of the lower back or BL40 and BL60, not Fuxi.

The broader acupressure evidence base deserves a plain description too. Most of it is made up of small trials, often with fewer than a hundred people, usually unblinded, and almost always measuring outcomes over days rather than months. Unblinded studies of a hands-on treatment reliably overstate benefit, because both the participant and the person assessing know what was given.

There is one clear exception worth knowing, because it shows what decent evidence looks like. PC6 on the inner wrist has repeatedly reduced nausea and vomiting in controlled human trials, including after surgery and in pregnancy. That is a genuine, specific, replicated finding. Nothing of the kind exists for BL38, and saying otherwise would be dishonest.

Some published trials do needle points in this region of the leg for knee and hamstring complaints. Those are reports of what researchers did in a clinical setting with sterile technique. They are not instructions, and they do not transfer to finger pressure at home.

What this means for you in practice: treat BL38 as a traditional landmark and a comfortable place to rub a tight leg. Judge it by whether your own leg feels better, and give it a short trial rather than an open-ended one. If a symptom is new, worsening, one-sided or stopping you walking, the useful next step is a clinician, not more pressure.