BL-40 (Wei Zhong) Bend Middle

What does BL-40 Wei Zhong do?

Wei Zhong BL-40 is the He-Sea point of the Bladder and the Command Point of the lower back, and its recorded action is correspondingly focused: it benefits the lumbar region and the knees, and it opens the channel to relieve pain. Almost everything else attributed to it follows from one of those two things — its command of the whole length of the Bladder channel down the back, or its position in the crease behind the knee.

• Benefits the lumbar region and the knees — the primary recorded action, and the reason the point exists in clinical practice. The Bladder channel runs the entire length of the back in two parallel lines, and BL-40 is the distal point classically held to command that whole territory.
• Activates the channel and alleviates pain — the second recorded action. It is used for pain, numbness, weakness and restricted movement anywhere along the back of the leg and the lower back.
• Clears summer-heat and cools the blood — the recorded basis for its classical use in acute febrile disorders with vomiting and diarrhoea, and in skin conditions with heat such as erysipelas. This action is bound up with the point's traditional treatment by bleeding rather than by ordinary needling.
• Relaxes the sinews — recorded for contracture and tightness of the tendons in the hollow of the knee, a local action determined by the anatomy directly beneath it.

Why does being the He-Sea point matter?

The He-Sea point is the last of the five transporting points, where the channel's qi is described as entering deeply, like a river reaching the sea. The Nan Jing assigns He-Sea points to counterflow qi and diarrhoea, and the Ling Shu states that He-Sea points govern the fu organs — which is why a point behind the knee carries indications for vomiting and diarrhoea that have nothing to do with the leg. BL-40 is also the Earth point of a Water channel, making it the child point of the Bladder and so the classical choice for draining an excess of that channel.

Source: Sacred Lotus sources & references (existing action record and point categories); Deadman & Al-Khafaji, A Manual of Acupuncture (p. 299); Chinese Acupuncture & Moxibustion (p. 184); Ling Shu and Nan Jing, on the five-shu points and the He-Sea points; cross-referenced against multiple online sources.

What is BL-40 Wei Zhong used for?

Wei Zhong BL-40 is used above all for low back pain — it is the classical command point for the lumbar region and the back. The indications carried in Sacred Lotus sources & references are listed below and cross-checked against the standard reference literature. They describe traditional use, not proven treatment, and this point carries the significant safety cautions set out under needling.

• Low back pain and lumbar stiffness
• Sciatic pain down the back of the leg
• Restricted movement of the hip joint
• Contracture and tightness of the tendons in the hollow of the knee
• Knee pain and swelling
• Pain, numbness and weakness of the lower limb
• Muscular atrophy of the leg
• Hemiplegia and one-sided weakness
• Abdominal pain
• Vomiting and diarrhoea, particularly in acute summer-heat disorders
• Erysipelas and other hot, red skin eruptions
• Nosebleed
• Heatstroke and acute febrile disease

Source: Sacred Lotus sources & references (existing indication record); Deadman & Al-Khafaji, A Manual of Acupuncture (p. 299); Chinese Acupuncture & Moxibustion (p. 184).

Where is BL-40 Wei Zhong, and how is it used in practice?

Wei Zhong sits exactly in the middle of the crease behind the knee — the popliteal crease — in the hollow midway between the two tendons that stand out on either side when the knee is bent against resistance. Those tendons are biceps femoris on the outer side and semitendinosus on the inner. The hollow between them is the popliteal fossa, and this is the point's whole identity: it is named for the bend of the knee and it sits at the centre of it. That same hollow is where the great vessels and the main nerve of the lower leg pass through, which is why this is one of the most safety-sensitive points on the limbs.

Which points is BL-40 combined with?

• With BL-23 Shen Shu and BL-25 Da Chang Shu — the standard low back combination, joining the distal command point to the Back-Shu points beside the lumbar spine. Both are held in this library.
• With BL-57 Cheng Shan — its nearest functional partner on the same channel, in the belly of the calf, for lumbar and leg pain and for the calf cramp both are classically indicated for. BL-57 is likewise a Heavenly Star point.
• With BL-60 Kun Lun — the Jing-River point at the ankle, the other great distal Bladder-channel point for the spine. The two divide the channel between them: BL-60 is weighted toward the neck and head, BL-40 toward the lumbar region.
• With GB-30 Huan Tiao and BL-54 Zhi Bian — for sciatic pain, joining the buttock points to the point behind the knee along the path of the pain.
• With DU-04 Ming Men and the extra point Yao Yan — for chronic lumbar ache, joining the governing-vessel point between the kidneys and the paired lumbar depressions to the distal command point. Both are held here.
• With GB-34 Yang Ling Quan — the Hui-Meeting Point of the Sinews, on the opposite side of the same knee, for contracture and restricted movement.
• With its Command Point peers — ST-36 Zu San Li, LI-04 He Gu and LU-07 Lie Que. The four divide the body between them and are the oldest formalised distal-point scheme in acupuncture.

Where does BL-40 sit among its peers?

BL-40 is one of the four Gao Wu Command Points — and Sacred Lotus sources & references confirm that exactly four points in this library carry that designation: ST-36 Zu San Li, LI-04 He Gu, LU-07 Lie Que and BL-40 Wei Zhong. The scheme comes from a rhymed classical couplet assigning one distal point to each region of the body: the abdomen to ST-36, the lower back to BL-40, the head and nape to LU-07, the face and mouth to LI-04. It is also one of Ma Dan-yang's Heavenly Star Points. Among the Bladder channel's sixty-seven points it is, with BL-60 and BL-57, one of the three distal points that carry real clinical weight for the back — but it is the only one of the three that commands the lumbar region by name.

Source: Sacred Lotus sources & references (location, categories and cross-referenced point records — the four Gao Wu Command Points were confirmed by query, not from memory); Deadman & Al-Khafaji, A Manual of Acupuncture (p. 299); Chinese Acupuncture & Moxibustion (p. 184); Gao Wu, Zhen Jiu Ju Ying (1529), on the Four Command Points; cross-referenced against multiple online sources.

Where is BL-40 located?

• A Manual of Acupuncture (p. 299): At the back of the knee, on the popliteal crease, in a depression midway between the tendons of biceps femoris and semitendinosus.
• Chinese Acupuncture and Moxibustion (p. 184): Midpoint of the transverse crease of the popliteal fossa, between the tendons of m. biceps femoris and m. semitendinosus.

How is BL-40 Wei Zhong needled, and what is the risk?

Stated plainly: BL-40 sits directly over the popliteal artery and vein, with the tibial nerve running through the same hollow. Deep needling here has caused documented vascular injury, including popliteal artery pseudoaneurysm requiring surgical repair. An MRI study in 124 people put the mean safe depth at 18.5 mm — under three-quarters of an inch. Everything below is recorded as neutral reference data describing what the textbooks and published imaging and case-report literature state. It is not instruction, and this page does not teach technique. Needling is carried out only by a qualified, licensed practitioner.

The angle, depth and technique the texts record

• Sacred Lotus sources & references record perpendicular insertion of 0.5 to 1.0 inch, or pricking the popliteal vein with a three-edged needle to cause bleeding.
• Deadman & Al-Khafaji (p. 299) and Chinese Acupuncture & Moxibustion (p. 184) record perpendicular insertion in a comparable range and both document the bleeding technique as a distinct traditional method at this point.
• The reference literature is consistent that the popliteal artery is the structure the recorded depth is set by, and that the pulse in the fossa is a landmark to be identified rather than a target.

What the imaging evidence actually shows

The safe depth at this point has been measured directly, which is unusual and makes the figures worth stating precisely.

• MRI study of safe needling depth and angle at BL40. Investigators measured the point on knee MRI scans in 124 participants, locating BL-40 by World Health Organization standards and treating 70 per cent of the distance from the skin to the popliteal artery as the safe depth. Mean safe depth overall was 18.51 ± 3.56 mm (95% CI 17.88 to 19.14). By body mass index it was 17.24 ± 3.14 mm in the low and normal group, 18.76 ± 2.90 mm in the overweight group and 22.01 ± 3.71 mm in the obese group. The artery was not directly under the point in most people: 111 of 124 (89.5 per cent) showed outward deviation of the angle to the artery, mean 9.75° ± 3.46°, while 13 (10.5 per cent) showed inward deviation, mean 8.78° ± 2.92°. The authors' stated conclusion was that with straight insertion a 25 mm needle can be advanced roughly 12.5 mm, or about 17.5 mm in people with a BMI above 28, and that depth should be reduced further in people of low body weight or height (Hou HK, Liu CZ, Lin LL, Acupunct Med 2021;39(4):343–50, PMID 33032445).
• The practical reading. Those figures place the safe depth below the upper end of the range the classical texts record, and they show that the margin narrows in slighter people — the opposite of an intuitive assumption that a smaller leg means a shallower needle is automatically safe. They also show that individual variation, not a single textbook number, is what governs the hazard.

The documented vascular injuries

Popliteal artery injury after acupuncture is rare but is a recurring entry in the surgical literature, reported across several decades and several countries.

• A false aneurysm of the popliteal artery complicating acupuncture, reported in the Australasian surgical literature (Aust N Z J Surg 1996;66(9):645–7, PMID 8859171).
• A popliteal artery pseudoaneurysm following acupuncture for knee osteoarthritis, repaired surgically; the authors noted that only four such surgically treated cases had previously been reported worldwide (Kao CL, Chang JP, Tex Heart Inst J 2002;29(2):126–9, PMID 12075870, PMC116740).
• A pseudoaneurysm after acupuncture treated by deployment of a stent graft in the popliteal artery (J Cardiol 2007;50(3):213–8, PMID 17941198).
• A popliteal pseudoaneurysm with arteriovenous fistula after acupuncture — that is, damage to both artery and vein (J Mal Vasc 2015;40(1):58–62, PMID 25623503).
• A 2025 case of surgical repair of an acupuncture-related popliteal artery pseudoaneurysm with a Gore-Tex graft, in a patient who also had an abdominal aortic aneurysm (Asteguieta S et al., J Surg Case Rep 2025;2025(1):rjae837, PMID 39780957, PMC11710739).

Read together, these reports do not suggest that this is a common event — the total published series over thirty years is small. What they establish is that the injury is real, that it can present late as a pulsatile swelling behind the knee rather than immediately, and that it may require vascular surgery.

The bleeding technique

Our own record documents the classical alternative at this point: pricking the visible superficial veins in the popliteal fossa with a three-edged needle to release a small quantity of blood. This is the traditional method behind the point's summer-heat, skin and acute-fever indications. It is recorded here as a documented traditional technique, not described procedurally. Bleeding techniques carry their own hazards — the popliteal vein lies deep in the same fossa, and the vessels being pricked are the small superficial ones, not that vein.

Overall risk context

A meta-analysis of prospective clinical studies estimated serious adverse events from acupuncture at approximately 1 per 10,000 patients and around 8 per million treatments (Bäumler P et al., BMJ Open 2021;11:e045961, PMID 34489268). Both things are true at once: the overall rate is low, and the events that do occur cluster at a small number of anatomically exposed points, of which BL-40 is one of the very few on the limbs.

Source: Sacred Lotus sources & references (existing needling record); Deadman & Al-Khafaji, A Manual of Acupuncture (p. 299); Chinese Acupuncture & Moxibustion (p. 184); PubMed PMID 33032445, 8859171, 12075870, 17941198, 25623503, 39780957, 34489268 — each checked as active and not retracted.

What does the name Wei Zhong mean?

Wei Zhong means "Bend Middle" or "Middle of the Crook" — wei, to bend, bow or crook, and zhong, the middle or centre. The name is purely topographical: the middle of the bend behind the knee. It is one of the plainest names in the point nomenclature, and its plainness is itself a clue to the point's antiquity — the oldest layer of point names tends to describe where a point is rather than what it does. The neighbouring points share the same root: Wei Yang BL-39, "Bend Yang", sits at the outer end of the same crease, and Wei Zhong at its centre. The point is also recorded in the classics under the alternative names Xue Xi, "Blood Cleft", and Xi Zhong, "Middle of the Knee" — the first of which points directly at its bleeding technique.

What are the Four Command Points, and why is BL-40 one of them?

The Four Command Points — the Si Zong Xue — are set out in a rhymed couplet in Gao Wu's Zhen Jiu Ju Ying of 1529, assigning one distal point to each great region of the body: ST-36 Zu San Li for the abdomen, BL-40 Wei Zhong for the lumbar region and back, LU-07 Lie Que for the head and nape, and LI-04 He Gu for the face and mouth. Sacred Lotus sources & references record exactly these four and no others as Gao Wu Command Points. The scheme is the earliest formal statement of the distal-point principle — that a point far from the complaint can command the region where the complaint is — and it is still the first thing taught about low back pain in acupuncture education. BL-40's inclusion is the single fact that most explains its clinical prominence.

Why is BL-40 classically bled?

Few points carry a bleeding technique in the standard texts, and BL-40 is one of the most prominent that does. The reasoning is anatomical and classical at once: superficial veins are often visible in the popliteal fossa, and the tradition holds that releasing a little blood there clears summer-heat, cools the blood and resolves the acute vomiting-and-diarrhoea patterns of hot weather, as well as hot skin eruptions. Its alternative classical name Xue Xi, "Blood Cleft", records the same idea. This is the historical basis of an indication list that otherwise looks incongruous for a point behind the knee.

Is there research on BL-40 Wei Zhong?

There is a small but genuinely point-specific research literature on BL-40, and — unusually — the strongest single study on this point is an anatomical safety study rather than a clinical one. The clinical trial literature on low back pain that involves BL-40 almost always uses it inside a multi-point protocol, so it measures the protocol and not the point. That distinction is worth keeping.

• Safety anatomy — the strongest study. The MRI study of safe needling depth and angle at BL-40 in 124 participants, described under needling, is directly about this point and reports measured figures rather than general caution (PMID 33032445). For BL-40, as for several anatomically exposed points, the safety literature is the substantial research literature.
• Point specificity, human. A randomised controlled trial allocated 140 healthy adults to four arms — acupuncture at BL-40, acupuncture at LU-05 Chi Ze, moxibustion at BL-40, moxibustion at LU-05 — for a single 30-minute session, and measured lumbar skin temperature by infrared thermal imaging. Acupuncture at BL-40 produced a significantly greater rise in mean lumbar temperature than acupuncture at LU-05 (difference 0.29 °C, 95% CI 0.17 to 0.41) and than moxibustion at BL-40 (0.24 °C, 95% CI 0.08 to 0.41), with a significant interaction between point and method (Zheng SY et al., J Integr Med 2025;23(2):145–51, PMID 39863500; registered as NCT05665426). This is a well-designed test of the classical claim that a point behind the knee reaches the lumbar region — but the outcome is a physiological measurement in healthy volunteers, not clinical benefit in people with back pain, and it should not be read as the latter.
• Mechanism, animal. Electroacupuncture at BL-40 in rats with injured lumbar multifidus muscle was studied for effects on macrophage polarisation during repair (J Tradit Chin Med 2025;45(2):335–47, PMID 40151120). An earlier experiment examined pricking-bleeding at BL-40 in a rat model, measuring sciatic nerve conduction velocity and interleukin-1α content in the lumbar nucleus pulposus (Zhen Ci Yan Jiu 2007;32(5):319–22, PMID 18050623) — notable as one of the few studies to test the point's traditional bleeding technique rather than ordinary needling. Both are animal mechanism studies and neither is evidence of clinical effect in people.

What is not there: no adequately powered randomised trial isolates BL-40 as the tested variable in low back pain. Trials of acupuncture for low back pain that include BL-40 among their points are studies of a protocol, and this site does not present them as though they were about this point.

How does BL-40 differ from the points near it?

BL-40 sits among several Bladder-channel points held in this library. BL-39 Wei Yang lies at the outer end of the same popliteal crease and is a quite different point — the Lower He-Sea point of the Sanjiao, weighted toward urinary difficulty and abdominal fullness rather than the back. BL-38 Fu Xi sits just above it on the outer side. BL-55 He Yang lies below the crease in the upper calf, and BL-57 Cheng Shan further down in the belly of the calf is BL-40's closest functional partner for lumbar and leg pain and, like it, a Heavenly Star point. BL-54 Zhi Bian, on the buttock, and GB-30 Huan Tiao beside it take the upper end of the same sciatic territory. On the ankle, BL-60 Kun Lun is the other great distal Bladder point for the spine — weighted toward the neck and head where BL-40 is weighted toward the lumbar region, and unlike BL-40 it carries a classical pregnancy caution.

Source: Sacred Lotus sources & references (name, categories and cross-referenced point records); Deadman & Al-Khafaji, A Manual of Acupuncture (p. 299); Chinese Acupuncture & Moxibustion (p. 184); Gao Wu, Zhen Jiu Ju Ying (1529), on the Four Command Points; PubMed PMID 33032445, 39863500, 40151120, 18050623, each checked as active and not retracted; cross-referenced against multiple online sources.