Evidence-based physiotherapy and rehabilitation resources
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Evidence-based physiotherapy and rehabilitation resources
Editorial & review policyHuman Anatomy · The trunk
A ring of bone carrying the whole weight of the trunk into the legs, closed underneath by a sheet of muscle most people never think about until it fails. The pelvis comes first and in detail. Then the glands — because the bone you have just learnt is one of the things they spend a lifetime regulating.
Every other region in this subject has a joint at its centre. The pelvis does not. It is a closed ring, and a ring behaves differently from a lever: you cannot loosen it in one place without changing every other place.
The ring is two hip bones, meeting each other in front at the pubic symphysis and meeting the sacrum behind at the two sacroiliac joints. Each hip bone is itself three bones — ilium, ischium and pubis — fused at the acetabulum by adult life.
A line from the sacral promontory round the arcuate line of the ilium to the top of the symphysis divides the ring into two storeys. Above is the greater pelvis, which is really part of the abdomen. Below is the lesser pelvis, the true bowl holding bladder, rectum and the internal reproductive organs.
| Region or opening | Bounded by | Why it matters |
|---|---|---|
| Greater pelvis the false pelvis |
The flared wings of the ilium, open in front | Abdominal territory, not pelvic territory. |
| Lesser pelvis the true pelvis |
Sacrum and coccyx behind, ischium and pubis at the sides and front | The bowl that holds the pelvic organs, closed below by the pelvic floor. |
| Pelvic inlet | Sacral promontory, arcuate line, pectineal line and pubic crest, upper border of the symphysis | The doorway between the storeys. In childbirth the head must pass it first. |
| Pelvic outlet | Lower border of the symphysis, ischiopubic rami, ischial tuberosities, sacrotuberous ligaments, tip of the coccyx | A diamond, not a circle. Its soft tissue closure is the perineum. |
Find two landmarks on a skeleton now, because the chapter keeps returning to them. The ischial spine projects into the cavity from each side. The ischial tuberosity is the lump you sit on.
Figure 1 · The bony pelvis, male and female
Illustration to be added
Four panels. Panel one: the female pelvis from above and slightly in front, showing the complete ring - both hip bones, sacrum and coccyx - with the pelvic inlet outlined as one continuous coloured line running from the sacral promontory along the arcuate line, the pectineal line and the pubic crest to the top of the symphysis. Shade the greater pelvis above that line in pale sand and the lesser pelvis below it in pale mist, so the two storeys read at once. Label ilium, ischium, pubis, acetabulum, sacral promontory, ischial spine and ischial tuberosity. Panel two: the male pelvis from the same viewpoint at the same scale, its heart-shaped inlet obvious against the rounder female one. Panel three: both pelvises from below and in front, side by side, with the subpubic angle drawn in each as two arms with an arc between them and no numeral - a narrow V in the male, a broad U in the female - and the diamond of the outlet outlined, its boundaries labelled. Panel four: a smaller posterior view of the female pelvis showing the posterior sacroiliac, sacrotuberous and sacrospinous ligaments, with the greater and lesser sciatic foramina formed between them shaded. Bone warm ivory with navy outlines, cartilage pale blue, ligaments navy, white ground. No text in the image beyond leader-line labels.
This is the one part of the skeleton where the sexes differ enough to be told apart reliably. Every difference points the same way. The female pelvis is shaped around the passage of a head; the male pelvis around carrying a heavier, more muscular frame.
| Feature | Male | Female | The reason |
|---|---|---|---|
| General build | Heavier, strong muscle marks | Lighter and smoother | Greater muscle bulk pulling on it. |
| Inlet | Heart shaped, promontory pushing in | Rounder, wider from side to side | A rounded doorway admits the head more easily. |
| Cavity | Deep and funnel shaped | Shallow, closer to a cylinder | A short straight passage beats a narrowing cone. |
| Subpubic angle | Narrow, a sharp V | Wide U, usually over eighty degrees | A wide arch enlarges the front of the outlet. |
| Ischial spines | Prominent, turned inwards | Less prominent | The gap between them is the narrowest part of the birth canal. |
| Greater sciatic notch | Narrow | Wide | The same widening applied to the back of the pelvis, which enlarges the posterior part of the cavity. |
| Sacrum | Longer, narrower, more curved | Shorter, wider, flatter | A flatter sacrum takes up less room in the cavity. |
One consequence you will use every week
A wider pelvis sets the hip joints further apart while the knees stay under the body's centre. Two things follow from that. The angle between the pull of quadriceps and the line of the patellar tendon widens. Each hip joint also sits further out from the midline, so body weight acts on a longer arm about that hip and the abductors must pull harder to keep the pelvis level. Anterior knee pain is commoner in women, and the wider angle is the traditional explanation offered for it. Prospective studies have not confirmed that the measured angle predicts the pain, so treat it as a partial explanation rather than the cause. The longer arm against the abductors is on firmer ground.
A ring with three joints in it would be free to change shape, and this one is not allowed to. So both kinds of joint here are strong, stiff, and built to pass force rather than to produce movement.
The sacroiliac joint is the odd one. Its front and lower part, between the ear-shaped, or auricular, surfaces of sacrum and ilium, is a true synovial joint with a cavity and a capsule. Its back and upper part has no cavity at all. There the bones are bound directly to each other by the interosseous ligament, a mass of short, immensely strong fibres. The synovial part allows the few degrees of movement. The fibrous part is where most of the load actually crosses.
| Ligament | Runs | What it does |
|---|---|---|
| Interosseous sacroiliac | Between the rough surfaces behind the joint | The strongest. It slings the sacrum from the two ilia, so weight hangs rather than presses. |
| Anterior sacroiliac | Across the front of the joint | A thin reinforcement of the capsule. |
| Posterior sacroiliac | Behind, in long and short bands | The long dorsal band is tender in many people with pelvic girdle pain, and easy to palpate below the posterior superior iliac spine. |
| Sacrotuberous | Sacrum and coccyx to the ischial tuberosity | Resists the sacrum tipping forward, and receives fibres from biceps femoris. |
| Sacrospinous | Sacrum and coccyx to the ischial spine | The same job. With the sacrotuberous it turns the sciatic notches into the greater and lesser sciatic foramina. |
| Iliolumbar | Fifth lumbar transverse process to the iliac crest | Anchors the lumbar spine to the pelvis. |
The pubic symphysis is a secondary cartilaginous joint. Each pubic surface is capped with hyaline cartilage, and a disc of fibrocartilage sits between them. Superior and inferior pubic ligaments hold it. Normal movement is a millimetre or two. In late pregnancy it softens and widens slightly, which is normal, useful and sometimes painful.
This is the idea to take away from the chapter. A sacroiliac joint is held shut in two different ways, and they behave quite differently in a patient.
Figure 2 · Form closure and force closure
Form closure is what the shape gives you for nothing. The sacrum is a wedge, broader above than below and broader in front than behind, so it jams between the ilia as soon as weight comes down through it. The surfaces are not smooth either: matching ridges and grooves grip one another. The harder you load it, the tighter it holds.
Force closure is the compression added by tension in muscle, ligament and fascia. Gluteus maximus on one side pulling against latissimus dorsi on the other, through the thoracolumbar fascia, presses the two halves of the ring together. So do the deep abdominal wall, the pelvic floor, and biceps femoris through the sacrotuberous ligament.
The clinical value is simple. Form closure is fixed. Force closure can be too little, too much or badly timed — and it is the part you can examine and change.
The sacrum is not quite still. It tips within the ring, through a few degrees, and the two directions have names.
Figure 3 · Nutation and counternutation
Nutation means nodding. The base of the sacrum tips forwards and downwards, while the tail end swings back. It happens when you stand, load the pelvis or bear down. It tightens the sacrotuberous and sacrospinous ligaments and wedges the sacrum deeper. It is the close-packed, most stable position.
Counternutation is the reverse. The base moves back and up, the tail comes forward, and the long dorsal sacroiliac ligament takes up the slack. It is the less stable of the two, and appears as load comes off the ring.
The same two movements in labour
Counternutation opens the inlet, which helps the head enter. Nutation opens the outlet, which helps it leave. That is the anatomical reason behind changing position in labour, and a neat demonstration that a joint with a few degrees of movement can still matter enormously.
The outlet of the true pelvis is a hole. Something must close it, hold up the organs above it, and still let three tubes through. That is the pelvic floor.
The muscular sheet, the pelvic diaphragm, is levator ani and coccygeus with their fasciae. It is funnel shaped rather than flat, sloping down and inwards from the side walls to meet in the midline.
| Part | Runs | What it does |
|---|---|---|
| Puborectalis | Back of the pubis, round behind the anorectal junction as a sling | Holds a forward bend at the junction. That bend does much of the work of solid and gas continence. |
| Pubococcygeus | Pubis backwards to the coccyx and to a tough midline seam of fibrous tissue, the raphe, running from the anus to the coccyx | The main bulk of the sheet. Its medial fibres pass beside the urethra, and in the female the vagina. |
| Iliococcygeus | From a thickened cord in the fascia over obturator internus, the muscle lining the side wall of the pelvis, back to the coccyx and the raphe. The sheet hangs from that cord like a curtain from a rail | The thin lateral part, lying almost flat. It takes the downward pressure from above rather than gripping the openings, so it is the part that sags first in prolapse. |
| Coccygeus | Ischial spine to the side of the coccyx and lowest sacrum | Completes the sheet behind and pulls the coccyx forward again after emptying the bowel or giving birth. It lies straight on the sacrospinous ligament, the ligament the pudendal nerve wraps around, and backs it with muscle. |
| Perineal membrane | A tough triangular sheet between the ischiopubic rami, below the levator | Not muscle. It anchors the external genitalia and gives the urethra a firm collar. |
| Perineal body | A fibromuscular knot in the midline in front of the anal canal | The central anchor. Several muscles converge on it, so damage here undermines the whole floor. |
Levator ani is supplied by a branch from the fourth sacral nerve, with a contribution from the pudendal nerve. Coccygeus takes branches from the fourth and fifth. The floor is a sacral muscle throughout, so a sacral root problem weakens it.
Figure 4 · The pelvic floor and the perineum
Illustration to be added
Three panels. Panel one: the female pelvic floor from above, looking down into the true pelvis, with levator ani drawn as a funnel-shaped sheet and its three parts in different tints - puborectalis as a sling passing behind the anorectal junction, pubococcygeus as the main bulk from pubis to coccyx and midline raphe, iliococcygeus as the thin lateral part arising from the tendinous arch on the obturator internus fascia. Show coccygeus completing the sheet behind, lying on the sacrospinous ligament, and mark the three openings for urethra, vagina and anal canal. Panel two: the same floor in sagittal section from the side, showing the funnel shape, the forward bend held at the anorectal junction by the puborectalis sling, and the perineal membrane lying below and in front of the levator sheet. Panel three: the perineum from below, the diamond of the outlet divided by a line between the ischial tuberosities into the urogenital triangle in front and the anal triangle behind, with the perineal membrane filling the front triangle, the perineal body as a midline knot between them, and the fat-filled ischio-anal fossa on each side of the anal canal. Trace the pudendal nerve in gold from the greater sciatic foramen below piriformis, round the back of the ischial spine, forward through the lesser sciatic foramen into the perineum, and on in the pudendal canal, giving off its inferior rectal, perineal and dorsal branches. Muscle brick, fascia and ligament navy, bone warm ivory, nerves gold, white ground. No text in the image beyond leader-line labels.
Below the floor lies the perineum, the diamond filling the pelvic outlet. Draw a line between the ischial tuberosities and it splits into two triangles.
| Triangle | Contains | Note |
|---|---|---|
| Urogenital, in front | The perineal membrane, the external urethral sphincter above it, the roots of the external genitalia below it | The urethra pierces it in both sexes; the vagina as well in the female. |
| Anal, behind | The anal canal with its external sphincter, and a wedge of fat on each side in the ischio-anal fossa | The fat is there so the canal can distend. |
The main nerve of the perineum is the pudendal nerve, from the second, third and fourth sacral nerves. It is not quite the only one. The anterior perineum, and the front of the scrotum or labia, takes ilioinguinal and genitofemoral fibres, and the skin at the back and side takes perineal branches of the posterior femoral cutaneous nerve. That is why a pudendal block does not numb the whole perineum for delivery.
The pudendal course is one of the strangest in the body. It leaves the pelvis through the greater sciatic foramen below piriformis, hooks round the back of the ischial spine and the sacrospinous ligament, and passes through the lesser sciatic foramen into the perineum. It then runs forwards in the pudendal canal, a fascial tunnel on the side wall of the ischio-anal fossa, which lies below the pelvic floor and outside the pelvic cavity.
It gives an inferior rectal branch to the external anal sphincter, a perineal branch to the perineal muscles, the external urethral sphincter and the skin, and a dorsal nerve to the penis or clitoris. Because it wraps the ischial spine so predictably, that spine is the landmark used when the nerve is blocked for delivery.
S2, 3, 4 keeps the pelvis off the floor
An old mnemonic, and worth keeping. The same three roots supply the floor, the sphincters and perineal sensation. That is why saddle numbness, a change in bladder or bowel control and loss of anal tone are taken together as a serious sign in low back conditions.
Four jobs, and only the first is obvious.
Support. It holds the bladder, the rectum and, in the female, the uterus and vagina against gravity and against every rise in pressure from above.
Continence. Puborectalis holds a bend at the anorectal junction, and the muscle around the urethra keeps it closed. Both must let go at the right moment and hold at every other moment.
Sexual function. The superficial perineal muscles and the floor take part in erection, engorgement and orgasm. The muscles are pudendal; the vascular event of erection itself is parasympathetic, carried by the pelvic splanchnic nerves from the same second, third and fourth sacral segments.
Trunk stability. This is the job that belongs to physiotherapy, and it joins this chapter to the two before it.
Figure 5 · The floor as the base of a canister
Think of the trunk as a closed canister. The diaphragm is the domed roof, met in the thoracic chapter. The deep abdominal wall, transversus abdominis above all, is the front and side wall, met in the abdominal chapter. Multifidus and the thoracolumbar fascia make the back wall. The pelvic floor is the base.
Cough, lift or push, and pressure inside that canister rises. If all four walls hold, that pressure stiffens the trunk and helps transfer load between the legs and the arms. If the base gives way, the pressure escapes downwards. That is the mechanism behind leaking on a cough, and it is why floor and breathing pattern are examined together.
| Problem | Anatomy behind it | What you find |
|---|---|---|
| Stress urinary incontinence | A floor and fascia that no longer support the urethra when pressure rises above them | Leaking on coughing, sneezing, lifting or running. Dry when sitting still. |
| Pelvic organ prolapse | Failure of the levator sheet and of the fascia slung across it | Dragging or bulging, worse through the day and on standing. Bladder, rectum or uterus may descend. |
| Pelvic girdle pain | Load transfer through the ring failing, with tenderness at the sacroiliac joints, the long dorsal ligament or the symphysis | Pain between the back of the iliac crest and the gluteal fold, or over the symphysis. Worse on single leg loading, stairs, turning in bed. |
| Pregnancy and delivery | Softened ligaments, a greater and changing load, and stretch of the levator and the pudendal nerve during a vaginal birth | Symphysis and sacroiliac pain in pregnancy; floor weakness, altered sensation or perineal body damage afterwards. |
| Pudendal neuralgia | The nerve irritated where it wraps the ischial spine, or within the pudendal canal | Burning perineal pain in the nerve's territory, worse sitting and easier standing. |
| Coccydynia | The coccyx, its joint with the sacrum, and what attaches to it | Pain at the base of the spine, worst sitting and rising from sitting. |
The pelvis is anatomy at its most mechanical: wedges, ligaments, levers and load. The endocrine system is the opposite. Nothing to palpate, no levers. A gland releases a chemical message into the blood and it reaches every cell a minute later.
The two belong together for a real reason. The bone you have just read about is itself a target. Whether that pelvis is still strong in thirty years depends on hormones from four different glands. So does whether a wound heals, whether a tendon tolerates loading, and whether a patient can finish the session you planned.
Figure 6 · The endocrine glands in place
Illustration to be added
Six small anatomical views on a white ground, not diagrams. Panel one: a midline sagittal section through the base of the skull, the pituitary sitting in the hypophysial fossa of the sella turcica and hanging from the hypothalamus by the infundibulum, the sphenoid sinus below and the optic chiasm immediately above and in front; tint the anterior and posterior lobes differently and show short portal veins running down the stalk to the anterior lobe only. Panel two: the front of the neck with the thyroid in place, two lobes and an isthmus crossing the second to fourth tracheal rings, larynx above. Panel three: the same region from behind with the lobes lifted forward, showing four parathyroid glands on their posterior surfaces and the recurrent laryngeal nerve on each side among the branches of the inferior thyroid artery - on the left running up within the groove between trachea and oesophagus, on the right ascending more obliquely and lying lateral to the groove. Panel four: the two adrenal glands on the posterior abdominal wall, the right pyramidal and capping the upper pole of the right kidney, the left crescentic and lying along the medial border of the upper half of the left kidney, with a cut section beside them showing the three cortical zones as concentric bands and the medulla as a clearly different core. Panel five: the pancreas in outline with an enlarged inset of one islet, a pale rounded cluster among darker exocrine tissue, a capillary through it and no duct. Panel six: an ovary beside the uterus and a testis in the scrotum at the same scale, each artery traced back to the aorta at kidney level, plus a small inset of the brain in sagittal view with the pineal marked on the roof of the third ventricle. Bone warm ivory, glandular tissue soft warm tan, nerves gold, arteries red, veins blue, navy outlines. No text in the image beyond leader-line labels.
| Gland | Where it sits | What to remember |
|---|---|---|
| Pituitary | In the hypophysial fossa of the sella turcica, a hollow in the sphenoid, hanging from the brain by the infundibulum | Two parts, two origins. The anterior lobe is glandular tissue that grew up from the roof of the primitive mouth. The hypothalamus controls it through a short set of veins running down the stalk to this lobe alone: a private line, so tiny amounts of hormone arrive undiluted. The posterior lobe is nervous tissue grown down from the brain, releasing hormones made in the hypothalamus and carried here along axons. |
| Thyroid | In front of the trachea, two lobes joined by an isthmus over the second to fourth tracheal rings | Pretracheal fascia ties it to the larynx, so a thyroid lump moves up when the patient swallows. That one sign separates it from most neck lumps. |
| Parathyroids | Usually four, on the back of the thyroid lobes | They control blood calcium and act on bone to get it. Small, pale and easily removed by accident. |
| Adrenal glands | The right caps the upper pole of the right kidney; the left is crescentic and lies along the medial border of the upper half of the left kidney. Both are behind the peritoneum | Two organs in one capsule. The cortex makes steroids in three layers: salt-holding hormone outside, cortisol in the middle, sex steroids inside. The medulla makes adrenaline and noradrenaline. |
| Pancreatic islets | Tiny clusters scattered through an otherwise digestive gland | A very small fraction of the pancreas, with no ducts at all. Their insulin, glucagon and somatostatin go straight into the blood. |
| Gonads | Ovaries in the pelvis beside the uterus; testes in the scrotum | Both make gametes and hormones at once. Both developed high on the back wall of the abdomen, which is why their arteries still leave the aorta up there. |
| Pineal | A small cone on the roof of the third ventricle | Releases melatonin on the light and dark cycle. It often calcifies with age, making it a useful midline marker on imaging. |
Two relations that decide surgical risk
The recurrent laryngeal nerve runs up in or close to the groove between trachea and oesophagus, directly behind the thyroid lobe and near the branches of the inferior thyroid artery. It lies reliably within the groove on the left. On the right it ascends more obliquely and often lies to the side of the groove, which is one reason the right nerve is the more often injured. It supplies almost every muscle that moves the vocal cords. Injury on one side leaves a hoarse, weak voice; on both sides it is an airway emergency. Above the pituitary lies the optic chiasm, so a pituitary tumour growing upwards takes the outer half of each visual field.
One link back. The adrenal medulla does not develop like a gland at all. It comes from the neural crest, the same migrating cells that produced the sensory ganglia in the development chapter. Its cells are sympathetic nerve cells that lost their axons and secrete into the blood instead. That is why a fright feels the way it does: it is the same system, working two ways.
You will not treat an endocrine disorder. You will constantly treat people who have one, and it changes what their tissues can do.
| Situation | The mechanism | What it changes for you |
|---|---|---|
| Diabetes and exercise | Contracting muscle takes up glucose by a route that does not need insulin | Glucose can fall during a session and for hours after. Ask when they last ate, what medication they take, and what their warning symptoms are. |
| Diabetes and healing | Small vessel disease slows delivery, sugar binds to collagen and stiffens it, and the inflammatory response is blunted | Slower healing, stiffer connective tissue, and far more frozen shoulder and limited joint mobility. |
| Diabetic neuropathy | The longest nerves fail first, taking protective sensation from the feet. Autonomic fibres may go too | Look at the feet before prescribing walking. A blunted heart rate response makes pulse a poor guide to effort. |
| Underactive thyroid | Every metabolic process slows, including contraction and relaxation | Fatigue out of keeping with the load, aching muscles, proximal weakness, cold intolerance, slow-relaxing reflexes. |
| Overactive thyroid | Metabolism and bone turnover both run fast | Weight loss, tremor, heat intolerance, fast pulse, and a proximal myopathy that can look like deconditioning. |
| Long-term corticosteroid | Bone formation is suppressed while resorption continues; type two fibres atrophy; tendon cells make less collagen for a time after a local injection | Fracture risk rises early. Expect proximal weakness, and be careful with heavy tendon loading soon after an injection. |
| Osteoporosis | Oestrogen restrains resorption. After the menopause that restraint goes and resorption outruns formation. Excess parathyroid hormone, an overactive thyroid and excess cortisol push the same way | The bone chapter's rule holds: bone answers to load. The endocrine background tells you how much margin is left. |
Only its front and lower part is. Behind and above it is a fibrous union, and that is where most of the load crosses.
A few degrees, no more. Do not look for range there. Look at how well the ring is held together.
It is the base of a canister roofed by the diaphragm. Breath holding, a rigid trunk and a floor that will not let go usually travel together.
The pelvic diaphragm is muscle, above. The perineal membrane is a fibrous sheet, below, and it spans only the front triangle.
It leaves the pelvis, wraps the ischial spine and enters the perineum below the floor, never returning to the pelvic cavity. Every clinical fact about it follows from that hairpin.
An underactive thyroid, poorly controlled diabetes and long-term steroid all cause genuine weakness. If the response to training makes no sense, say so.
Ten questions on this chapter. Tap one to see the answer and the reasoning.
Answer: (B) The inlet runs from the promontory along the arcuate and pectineal lines to the top of the symphysis. The coccyx and sacrotuberous ligament belong to the outlet.
Answer: (C) A wide subpubic angle goes with a rounder inlet, a shallower cavity and a wide sciatic notch. The other three describe a male pelvis.
Answer: (C) The auricular surfaces form a true synovial joint. Behind and above, the interosseous ligament binds the bones directly, and most load crosses there.
Answer: (B) Form closure is what the shape gives you for nothing. Muscle and fascial tension is force closure, and that is the half you can change.
Answer: (B) Nutation means nodding. The base tips forward and down, the big posterior ligaments tighten, and the joint reaches its close-packed position.
Answer: (C) Puborectalis passes from the back of the pubis round behind the junction and holds a forward bend there, which does much of the work of continence.
Answer: (C) S2, 3 and 4 — the same roots that supply the floor, the sphincters and perineal sensation.
Answer: (B) It is a tough fibrous sheet between the ischiopubic rami, filling the front triangle only, and lying below the pelvic diaphragm rather than replacing it.
Answer: (B) On the left it runs up the tracheo-oesophageal groove itself, immediately behind the lobe and close to the branches of the inferior thyroid artery. On the right it ascends more obliquely and often lies to the side of the groove, which is one reason the right nerve is injured more often. Injury leaves a hoarse voice.
Answer: (B) Its cells come from the neural crest and behave like sympathetic neurons secreting into the blood. The cortex develops from mesoderm.
Everything on this page, in one screen
| Book | What it adds here |
|---|---|
| Anatomy and Human Movement Palastanga, Field and Soames |
The pelvic ring as a mechanical problem, with the sacroiliac joint given the space it deserves. |
| B D Human Anatomy, Volume 2: Lower Limb, Abdomen and Pelvis Chaurasia |
The floor, the perineum and the abdominal glands at examination level. Thyroid, parathyroid, pituitary and pineal are in Volume 3. |
| Clinical Anatomy by Regions Snell |
The clinical consequences of injury to the floor and the pudendal nerve. |
| Gray's Anatomy for Students Drake, Vogl and Mitchell |
The clearest account of the pelvic walls, the triangles and the fascia. |
| Last's Anatomy: Regional and Applied Sinnatamby |
Concise, precise notes on the pelvis and on the glands in their surgical relations. |
Reviewed by the Physiotherapist India Team. · Human Anatomy contents