Appearance
6.4 — The Trunk and Core
Before you lift your arm, the deep muscles of your abdomen and back contract — about 30 to 110 milliseconds before the shoulder muscle that actually moves the limb. Your body braces the trunk before it moves anything attached to it, without any conscious involvement, and in people with chronic low back pain that anticipatory timing is measurably delayed.
That is what "core stability" actually means, and it is more specific and more useful than the version sold in gyms.
The abdominal wall
Four muscles on each side, arranged in layers with their fibres running in different directions — the same principle as plywood, and for the same reason.
Rectus abdominis — the vertical strap running from the pubic bone to the ribs. Flexes the trunk. It is crossed by three or four fibrous bands, and those are what create the "six-pack" appearance — the segments are not separate muscles, they are one muscle with tendinous interruptions, which is a leftover of the segmental origin from Chapter 4.4.
Whether someone has visible abdominal definition depends almost entirely on the fat layer covering it, not on the muscle. This is why abdominal exercises alone never produce it.
External oblique — fibres running downward and forward, as if putting your hands in your front pockets.
Internal oblique — fibres running the opposite way, upward and forward.
Together the obliques rotate and side-bend the trunk. To turn to the left you use the right external oblique and the left internal oblique.
Transversus abdominis — the deepest layer, fibres running horizontally around the body like a belt. It does not move the spine at all. Its only action is to compress the abdomen, and it is the muscle most implicated in spinal stability.
All four converge at the front into a broad tendon sheet that wraps rectus abdominis and meets its opposite number in the midline at the linea alba, a fibrous line running from sternum to pubis.
Two clinically important consequences of that midline:
Diastasis recti — separation of the two rectus muscles as the linea alba stretches. Common in pregnancy and in central obesity. Most resolve within a year after delivery, and specific exercise helps; surgical repair is reserved for large persistent separations.
The linea alba is where surgeons make a midline laparotomy incision, because it is almost bloodless and cuts no muscle fibres. The cost is that it heals with scar tissue and incisional hernias are relatively common there.
What the abdominal wall is for
Containing the viscera — holding the intestines in place.
Generating intra-abdominal pressure, which is used for coughing, sneezing, vomiting, defecation, urination and childbirth. All of these depend on the abdominal muscles contracting against a closed glottis and a contracted diaphragm, and all of them are impaired when the abdominal wall is weak or painful.
This is why abdominal surgery is followed by chest complications. The person cannot cough effectively because it hurts, so secretions accumulate, so pneumonia follows. Adequate pain relief after abdominal surgery is a respiratory intervention, exactly as it is with rib fractures (Chapter 5.4).
Protecting the spine. Raising intra-abdominal pressure creates a rigid cylinder in front of the lumbar spine that shares the load, reducing compressive force on the discs by an estimated 20 to 40 percent. This is the physiological basis of bracing before a heavy lift, and it is why weightlifting belts work — they give the abdominal wall something to push against.
Movement — flexing and rotating the trunk, which is actually the least of its jobs.
Hernias
A hernia is the protrusion of an organ through a weakness in the wall containing it. The abdominal wall has several inherent weak points, all of them created by something having to pass through.
Inguinal hernia — through the inguinal canal, the passage in the lower abdominal wall through which the testis descended in male development, or the round ligament passes in females (Chapter 3.6). Around 27 percent lifetime risk in men against 3 percent in women, and the entire difference is that descent.
Femoral hernia — through the gap beside the femoral vessels in the groin. Commoner in women, and important because the opening is narrow and rigid, so femoral hernias strangulate far more often than inguinal ones and are repaired promptly rather than watched.
Umbilical hernia — at the navel, where the cord passed. Very common in infants and usually closes by itself by age three or four.
Incisional hernia — through a previous surgical scar. Occurs in around 10 to 20 percent of midline laparotomies.
Hiatus hernia — the stomach pushing up through the diaphragm's oesophageal opening. Chapter 9.2.
The complication that matters is strangulation. The sequence is: a loop of bowel enters the defect, becomes trapped (incarcerated), swells so it cannot return, and then its blood supply is compressed (strangulated). The bowel then dies within hours.
The warning signs are a hernia that has become painful, tense and irreducible, with vomiting and no bowel movements. This is a surgical emergency, and the practical rule is that a hernia which has always been soft and gone away when lying down, and has suddenly become hard and painful, needs assessment the same day rather than the next week.
The diaphragm
The principal muscle of breathing, and a dome rather than a flat sheet. Muscle fibres radiate from a central tendon out to the lower ribs, the sternum and the lumbar spine.
When it contracts, the dome flattens, increasing the vertical height of the chest cavity. It accounts for about 70 to 80 percent of the air moved in quiet breathing.
Three openings, at three different levels, each carrying named structures:
- The inferior vena cava, at the level of T8.
- The oesophagus with the vagus nerves, at T10.
- The aorta with the thoracic duct, at T12.
Its nerve supply is the phrenic nerve, from C3, C4 and C5, for the developmental reason given in Chapter 4.4 — the diaphragm forms in the neck and descends, dragging its nerve with it.
Two consequences follow, and both are clinically decisive.
A cervical spinal cord injury above C3 paralyses the diaphragm and requires permanent ventilation; below C5 spares it. The teaching phrase is "C3, 4, 5 keeps the diaphragm alive."
And irritation of the diaphragm is felt in the shoulder tip. Blood, pus or inflammation under the diaphragm sends signals along the phrenic nerve, and the brain interprets them as arising from the C4 dermatome — the skin over the shoulder. Shoulder tip pain in someone with abdominal trauma means blood under the diaphragm until proven otherwise, and the same referral explains the shoulder pain after laparoscopic surgery, where gas is left under the diaphragm.
Hiccups are involuntary diaphragmatic spasms with abrupt closure of the vocal cords, which is what makes the sound. Usually trivial. Persistent hiccups lasting more than 48 hours are not, and can indicate irritation of the phrenic or vagus nerve anywhere along its course — from the ear canal to the diaphragm.
The back muscles
Arranged in layers, from superficial ones that move the arm to deep ones that move and stabilise the spine.
Superficial — trapezius, latissimus dorsi, rhomboids, levator scapulae. Despite lying on the back, these are functionally arm muscles, moving the shoulder girdle and the humerus. Chapter 6.5.
Intermediate — the serratus posterior muscles, which assist respiration.
Deep (intrinsic) — the true back muscles.
Erector spinae — three vertical columns running the length of the spine. They extend the spine and, more importantly, control its flexion eccentrically. When you bend forward, gravity does the bending; the erector spinae pay it out under control. When you lift something, they are working against a very unfavourable lever.
The arithmetic is worth doing, because it is the whole argument for lifting technique. Bending forward to lift a 20 kg object, the object sits perhaps 50 cm from the lumbar spine's pivot point. The erector spinae attach only about 5 cm from that pivot. The lever arm ratio is therefore about 10 to 1, so the muscles must generate roughly ten times the load's force — around 200 kg of force, and that entire force is compressed through the L5/S1 disc, on top of the weight of the upper body.
Bring the object closer and the number falls proportionally. Holding it at 20 cm instead of 50 cm cuts the muscle force by more than half. This is why "lift close to the body" matters far more than "bend your knees" — the knee position changes the load modestly, the distance changes it enormously.
Multifidus — small deep muscles spanning two to four vertebrae. These are the spine's fine stabilisers, and they are the muscle that most consistently shows changes in chronic low back pain: they atrophy and become fatty on imaging, and their activation is delayed.
Core stability, stated accurately
The "core" is a cylinder: the diaphragm above, the pelvic floor below, transversus abdominis and the obliques around the front and sides, and multifidus and the lumbar fascia behind.
Its function is not to move — it is to hold the spine steady while the limbs move. A limb muscle can only pull the limb effectively if its other end is anchored. If the trunk is unstable, force applied at the hand is partly wasted moving the trunk instead, and the spine is loaded in uncontrolled directions.
Two findings anchor this. The anticipatory activation described at the start of this page is real and measurable, and it is delayed in people with chronic low back pain — though whether that is cause or consequence is not settled. And multifidus atrophy after an episode of back pain does not recover spontaneously when the pain resolves; it recovers with specific exercise.
What the evidence actually supports, stated without the fitness-industry embellishment:
Exercise helps chronic low back pain. This is well established. Which exercise matters much less than commonly claimed — several trials comparing specific core stabilisation programmes with general exercise have found little difference. What consistently helps is doing something regularly.
Bracing is useful for heavy lifting and probably not something to maintain all day.
And crunches are a poor choice for most purposes: they load the lumbar spine in repeated flexion, which is the direction that stresses the disc most (Chapter 5.3), and they train a movement rather than the stabilising function. Isometric holds and anti-rotation work load the core in the way it actually works.
The pelvic floor, and how the cylinder works together
The pelvic floor closes the bottom of the core cylinder (Chapter 5.6). The diaphragm, abdominal wall and pelvic floor work as a coordinated unit, and they must, because they are enclosing a fluid-filled space.
When intra-abdominal pressure rises — a cough, a lift — the pelvic floor must contract at the same instant, or the pressure pushes the pelvic organs downward.
This is exactly the mechanism of stress incontinence: pressure rises, the pelvic floor does not respond quickly or strongly enough, and urine is forced out. And it explains why heavy lifting with poor pelvic floor function worsens prolapse, and why pelvic floor training is prescribed alongside any return to lifting after childbirth.
Supervised pelvic floor muscle training cures or substantially improves stress incontinence in the majority of women, and it remains under-offered relative to pads and surgery.
What the next page fixes
The trunk is the anchor. Chapter 6.5 covers what it anchors: the muscles of the upper limb, from the four rotator cuff muscles that hold the shoulder together to the tendons in the forearm that let you type — including why tennis elbow is not usually caused by tennis and why the carpal tunnel contains nine tendons and one nerve in a space that cannot expand.