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Structure
Intermediate
9 min read

The diaphragm

A muscular dome separating the chest from the abdomen. It is the engine of quiet breathing and, at the same time, the ceiling of the cavity we pressurise to stiffen the trunk — two jobs competing for one muscle.
3D scene
The diaphragm inside the trunk
The scene places it in the deep plane, with the other muscles no outside view can show. Without the 3D, the picture fits in two sentences: a dome stretched across the trunk, its top reaching mid-height of the ribcage. Its rim follows the lower border of the ribs at the front and the sides, then descends markedly lower at the back, where its two crura reach the lumbar vertebrae.
Open in the explorer

A muscle shaped like a dome

The diaphragm has the shape of no other muscle in the body. It is neither a spindle nor a flat sheet: it is a dome, bulging upwards, closing the chest from below and separating the lungs and heart from the organs of the abdomen.
Its position often comes as a surprise. The top of the dome rises to mid-height of the ribcage, far above the rib margin you can feel: part of what we spontaneously place in the belly is in fact below the diaphragm, and therefore inside the chest. Its crura, by contrast, descend below the ribs to reach the lumbar vertebrae. The exact height of the dome changes constantly, since it is what breathing moves.
Diaphragm
A dome-shaped muscle stretched between the lower border of the chest and a central tendon. It separates the thoracic cavity from the abdominal cavity and is the main muscle of breathing.
diaphragmatic dome

Origin

Its fibres arise all around the lower opening of the chest, in three groups. The sternal part comes from the back of the xiphoid process. The costal part comes from the inner surface of the lower six ribs and their cartilages. The lumbar part reaches lowest: its two crura attach to the bodies of the upper lumbar vertebrae, right against the lumbar spine.

Insertion

All these fibres converge upwards and inwards, meeting on a fibrous sheet: the central tendon. This is a rare arrangement — the muscle ends on no bone, but on its own tendon, which does not change place with each contraction. The tendon acts as a mobile anchor for the fibres surrounding it.

Nerve supply

The nerve supply comes from the phrenic nerve, from the cervical roots C3 to C5. The route is long and unexpected: the nerve arises in the neck and descends through the whole chest to reach a muscle sitting far below. That distance is a remnant of embryonic development, where the future diaphragm forms high before descending.

What the diaphragm does

Breathing

When its fibres contract, the dome flattens and descends. The volume of the chest increases, the pressure inside it falls, and air enters. That is the whole mechanism of quiet inhalation: the diaphragm supplies most of it, with the intercostal muscles and the neck muscles contributing markedly only as demand rises.
Quiet exhalation, by contrast, requires no muscular work. The diaphragm relaxes, and the elasticity of the lung and the chest wall returns the volume to its starting point.
Certainty level · Established
The diaphragm is the main muscle of inhalation: its contraction lowers the dome, increases the volume of the chest and draws air in.
Descriptive anatomy and respiratory physiology describe the same mechanism, measured by the displacement of the dome and by pressure changes. The exact share of the other inspiratory muscles varies with posture, with the intensity of the effort and with the state of the airways.
Standring S (2020) · Neumann DA (2016)

Holding the trunk

The second role is less well known and bears directly on the rest of this corpus. The dome is the ceiling of a cavity closed below by the pelvic floor and at the sides by the abdominal wall. Contracting those three walls together puts the contents under pressure, and a pressurised cavity resists deformation: the trunk becomes stiffer.
That is exactly what the squat and horizontal push pages describe when they mention a brief breath hold before the effort. The breath is held not to save air, but to lock one of the walls of the cavity.
Certainty level · Established
The diaphragm takes part in postural control at the same time as in breathing: its activity increases with the acceleration of a moving limb, independently of the breathing cycle.
The observation rests on electromyographic recordings through intramuscular electrodes, in a small number of healthy subjects, during a repetitive arm movement. It establishes that the postural function exists and that it is modulated by mechanical demand; it does not quantify its contribution in a loaded exercise.
Hodges PW, Gandevia SC (2000)

Two jobs, one muscle

Here the diaphragm becomes a special case. No other muscle in the body has to carry out an obligatory rhythmic task and a holding task at the same time.

The two jobs of the diaphragm

Criterion
Breathing role
Postural role
Rhythm
Cyclic, permanent
According to mechanical demand
Effect sought
Vary the volume of the chest
Maintain abdominal pressure
Position of the dome
Descends then returns
Stays low and fixed
What interrupts it
Nothing, except voluntary apnoea
Rising respiratory demand
The two are not compatible indefinitely. Maintaining pressure calls for a stable dome; breathing calls for it to move. When respiratory demand rises, the postural share falls — which explains why a breath hold does not last, and why a long effort is conducted differently from a brief one.

Seeing the diaphragm

The diaphragm inside the trunk

The scene places it in the deep plane, with the other muscles no outside view can show. Without the 3D, the picture fits in two sentences: a dome stretched across the trunk, its top reaching mid-height of the ribcage. Its rim follows the lower border of the ribs at the front and the sides, then descends markedly lower at the back, where its two crura reach the lumbar vertebrae.
Current step
Deep and medial muscles
Beneath the visible groups work muscles no silhouette shows. The rotator cuff holds the head of the humerus against the shoulder blade, serratus anterior keeps the shoulder blade flat on the ribcage, the iliopsoas links the lumbar spine to the femur, and the adductors fill the inner thigh. Deeper still, the diaphragm closes the trunk as a dome, inside the ribs. They orient and stabilise more than they move.
Scene description
A three-dimensional diagram of a standing human body, arms by the sides, built from simplified paper-coloured volumes. Twelve priority muscle groups are laid over this outline in terracotta: at the front, pectoralis major, the deltoid, biceps brachii, the abdominal wall and the quadriceps; at the back, the trapezius, latissimus dorsi, the erector spinae, triceps brachii, the glutes, the hamstrings and the calves. A simplified skeleton — skull, spine, ribcage, pelvis, humerus, femur and tibia — can be shown beneath the outline to reveal the bony levers these muscles pull on. Each group is selectable and opens its own reference page. This is a teaching diagram with stylised volumes, not an exact anatomical reconstruction.
Visible structures
  • Pectoralis major
    A thick sheet running from the clavicle, the sternum and the upper costal cartilages to the lateral lip of the bicipital groove of the humerus. It draws the arm towards the midline and forwards, and drives horizontal pushing.
  • Deltoids
    Three heads — anterior, lateral and posterior — running from the clavicle, the acromion and the spine of the scapula to the deltoid tuberosity of the humerus. Together they raise the arm; separately they carry it forwards, out to the side or backwards.
  • Biceps brachii
    Two heads arising from the scapula — the supraglenoid tubercle and the coracoid process — joining on the radial tuberosity. It flexes the elbow and supinates the forearm, and contributes to every pulling movement.
  • Abdominal wall
    Rectus abdominis, the external and internal obliques and transversus, running from the ribs and sternum to the pubis and iliac crest. They flex and rotate the trunk and, above all, stabilise the spine by regulating intra-abdominal pressure.
  • Quadriceps
    Four heads — rectus femoris, vastus lateralis, vastus medialis and vastus intermedius — joining through the quadriceps tendon onto the patella and then the tibial tuberosity. They extend the knee, and rectus femoris also flexes the hip.
  • Trapezius
    A diamond-shaped muscle spanning from the occiput and the cervical and thoracic spinous processes to the clavicle, the acromion and the spine of the scapula. Its upper, middle and lower portions elevate, retract and depress the scapula.
  • Latissimus dorsi
    A broad fan arising from the lower thoracic vertebrae, the thoracolumbar fascia, the iliac crest and the lowest ribs, converging on the bicipital groove of the humerus. It pulls the arm down and back, and drives vertical pulling.
  • Erector spinae
    Muscular columns — iliocostalis, longissimus and spinalis — running from the sacrum and iliac crest to the ribs, the vertebrae and the skull, on either side of the spinous processes. They extend the spine and control the trunk as it lowers.
  • Triceps brachii
    Three heads — long, lateral and medial — arising from the scapula and the back of the humerus and joining on the olecranon of the ulna. It extends the elbow, and its long head also assists shoulder extension.
  • Glutes
    Gluteus maximus, medius and minimus, arising from the iliac wing and the sacrum and inserting on the femur and the iliotibial tract. They extend, abduct and rotate the hip, and drive the pelvis upright in the hinge and the squat.
  • Hamstrings
    Biceps femoris, semitendinosus and semimembranosus, running from the ischial tuberosity to the tibia and fibula. They extend the hip and flex the knee, and control the trunk as it lowers in the hip hinge.
  • Calves
    The gastrocnemius and soleus, joining through the Achilles tendon onto the calcaneus. They plantarflex the ankle, and the gastrocnemius, which crosses the knee, also assists knee flexion.
  • Rotator cuff
    Four short muscles running from the shoulder blade to the top of the humerus. Their tendons merge into a sheet that holds the head of the bone against its socket while the deltoid moves the arm.
  • Serratus anterior
    A sheet hooked onto the upper ribs by triangular slips, passing under the shoulder blade and attaching to its inner border. It holds the blade against the ribcage and rotates it.
  • Iliopsoas
    The only muscle linking the lumbar spine directly to the femur. It descends from the vertebrae and the iliac fossa, curves around the rim of the pelvis and attaches to the lesser trochanter.
  • Hip adductors
    Five muscles running from the pubis to the back of the femur in overlapping layers. They draw the thigh towards the midline and control the pelvis in single-leg stance.
  • Forearm muscles
    Masses gathered near the elbow, continued by long tendons to the wrist and fingers. They drive the hand from a distance and carry grip strength.
  • Tibialis anterior
    A muscle pressed against the outer surface of the tibia, its tendon swinging towards the inside of the foot. It lifts the toes on every step and brakes the forefoot after heel strike.
  • Diaphragm
    A muscular dome running from the lower border of the ribs, the sternum and the lumbar vertebrae to a central tendon. As it contracts it descends, the chest gains volume and air enters: it is the engine of quiet breathing, and the ceiling of the abdominal cavity.
Guided steps
  • 1/5
    The front of the body. Pectoralis major covers the upper chest, the deltoid caps the shoulder, the biceps fills the front of the arm, the abdominal wall links the ribs to the pelvis, and the quadriceps fills the front of the thigh. Further out on the limbs, the forearm muscles drive the wrist and hand, and tibialis anterior lifts the foot.
  • 2/5
    The back of the body. The trapezius links the neck to the shoulder blades, the latissimus dorsi fans down to the pelvis, the erector spinae flank the spine, and the triceps fills the back of the arm. Glutes, hamstrings and calves form the posterior chain from hip to ankle.
  • 3/5
    Beneath the visible groups work muscles no silhouette shows. The rotator cuff holds the head of the humerus against the shoulder blade, serratus anterior keeps the shoulder blade flat on the ribcage, the iliopsoas links the lumbar spine to the femur, and the adductors fill the inner thigh. Deeper still, the diaphragm closes the trunk as a dome, inside the ribs. They orient and stabilise more than they move.
  • 4/5
    In a push, force starts at the ground, travels through the quadriceps and the abdominal wall, and then pectoralis major, the anterior deltoid and the triceps drive the load away from the trunk. The skeleton acts as the lever: no group works alone, and the quality of the movement comes from their sequencing.
  • 5/5
    In a pull, the latissimus dorsi and trapezius bring the arm and shoulder blade towards the trunk while the biceps flexes the elbow. Glutes, hamstrings and erector spinae hold the pelvis and spine so that force transfers without the trunk collapsing.
Model licence · Z-Anatomy et BodyParts3D — CC-BY-SA 4.0

What the model simplifies

Limit
A dome without its surroundings
The diaphragm works between two sets of organs: the lungs and the heart above, the liver, the stomach and the spleen below. None appears in the scene — the anatomical sources Body Lab uses do not include the visceral system.
The dome is also frozen there. In a body it travels several centimetres with each breathing cycle, and more still under effort. The model shows a position, not a movement.
Finally, the pelvic floor, which closes the cavity from below, does not exist as a separate part either. The abdominal wall page already noted this: the cylinder described here is complete in no model.
  • The abdominal wall forms the side and front walls of the same cavity.
  • The erector spinae hold the spine while the pressure builds.
  • The lumbar spine receives the crura of the diaphragm on its upper vertebrae.
  • The aerobic system depends on the ventilation this muscle provides.

A subject that calls for care

Caution
What this page does not say
Holding the breath against a load raises arterial pressure substantially and transiently. Body Lab proposes here no breathing technique, no protocol and no recommendation on when to use one: the decision depends on an individual context a page does not have.
Chest pain, unusual breathlessness, faintness during effort or dizziness call for medical advice, and are addressed nowhere on this site.

Sources

Main sources

  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Neumann DA (2016). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 3rd edition. Elsevier.
  • Hodges PW, Gandevia SC (2000). Activation of the human diaphragm during a repetitive postural task. The Journal of Physiology.
Put it into practice in Shapier

Practise the squat while bracing the trunk

The squat technique, in which the diaphragm locks the top of the abdominal cavity before the effort, is followed step by step in Shapier; Body Lab only explains what this bracing is for.
Practise the squat while bracing the trunk
Body Lab explains; Shapier lets you act and track.

Check my understanding

Where does the top of the diaphragmatic dome reach?
What does quiet exhalation require?
What is the brief breath hold before an effort like the squat for?
Choose an answer

Read next

  • Abdominal wall
    The abdominal wall brings together four stacked muscles whose fibres cross. It flexes and rotates the trunk, but its most constant role is to resist movement and to manage internal pressure.
    With a 3D scene
  • The lumbar spine
    Five vertebrae, five discs and a set of ligaments. Each level moves only a few degrees: it is their sum that gives the trunk its range.
    With a 3D scene
  • The aerobic system
    The pathway that supplies most of the energy as soon as an effort lasts. It burns carbohydrate and fat with oxygen, produces a great deal but slowly, and never switches off.
    With a 3D scene

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Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • The scene shows the dome alone. Lungs, heart, liver and stomach rest against it on both sides and appear in none of our anatomical sources.
  • The model freezes the dome in a single position, although it travels several centimetres with every breathing cycle.
  • The pelvic floor, which closes the abdominal cavity from below, does not exist as a separate part in the sources used either.
  • This page describes an anatomy and a mechanism. It proposes no breathing technique and establishes no course of action.
Sources
  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Neumann DA (2016). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 3rd edition. Elsevier.
  • Hodges PW, Gandevia SC (2000). Activation of the human diaphragm during a repetitive postural task. The Journal of Physiology.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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