Core (anatomy)

The core, also called the trunk, is the axial part of the body, comprising the thorax, abdomen and pelvis and, in a broader academic sense, the neck and head.

In everyday speech core is often treated as a synonym for torso, but anatomists use it for the central column that transmits loads between the limbs and the axial skeleton.

Core muscles stabilise the spine, rib cage and pelvis, generate intra-abdominal pressure and provide the proximal anchor for limb movement.^[1]

Definition

• The core is the axial portion of the body, as opposed to the appendicular portion formed by the limbs.
• In clinical and sports anatomy the core is usually defined functionally, as the region that controls the orientation and stability of the trunk.
• The bony core includes the vertebral column, rib cage, sternum and pelvis, and these bones form the lever system through which core muscles act.
• The core is not a single muscle or a single compartment, and no consensus exists on exactly which muscles belong to it.
• Some definitions include the shoulder girdle and neck, while others restrict the core to the trunk between the diaphragm and the pelvic floor.

Muscles

• The principal core muscles are the pelvic floor, transversus abdominis, multifidus, internal and external obliques, rectus abdominis, erector spinae and diaphragm.
• The diaphragm forms the roof of the abdominal cavity, and its contraction lowers intrathoracic pressure during inspiration while contributing to intra-abdominal pressure.
• The transversus abdominis is the deepest abdominal muscle, and it increases intra-abdominal pressure and tensions the thoracolumbar fascia.
• The multifidus consists of short deep fibres that span one to three vertebral levels and provide segmental stiffness to the spine.
• The pelvic floor muscles form a funnel that supports the pelvic viscera and contributes to continence.
• The internal and external obliques produce trunk rotation and lateral flexion, and they also contribute to abdominal compression.
• The rectus abdominis flexes the trunk and, with the obliques, forms the anterior abdominal wall.
• The erector spinae, especially the longissimus thoracis, extends the spine and resists flexion moments during bending and lifting.
• Minor core muscles include the latissimus dorsi, gluteus maximus and trapezius, which connect the trunk to the limbs and modify core mechanics.
• The deep fibres of quadratus lumborum and the rotatores, rectus capitis anterior and lateralis and longus colli are sometimes included as core muscles.^[2]

Functions

• The core stabilises the thorax and pelvis during dynamic movement and provides a stable base for the limbs.
• Core muscles generate intra-abdominal pressure, which supports the viscera, reduces spinal compression and assists forced expiration.
• Intra-abdominal pressure contributes to continence by compressing the urethra and rectum during exertion.
• During labour the transversus abdominis and pelvic floor muscles help expel the fetus, although uterine contractions are the primary force.
• The Valsalva manoeuvre combines a closed glottis with abdominal compression, and it is used in lifting, pushing, defecation and childbirth.
• Core stability also supports posture by aligning the spine, ribs and pelvis against gravity.
• Core muscles act as a feed-forward system, and they often contract before a limb movement to prepare the trunk for the reaction force.
• Core function is not purely muscular, because fascia, ligaments and the thoracolumbar fascia also transmit and store elastic energy.

Static core function

• Static core function is the ability to align the skeleton to resist a force that does not change, such as holding a rifle in the prone position.
• In prone shooting the shooter must relax the muscles and let the aligned skeleton support the rifle, because muscular force would produce wobble.
• Static stability therefore depends more on bony alignment and ligament tension than on continuous muscle contraction.
• The core still contributes by aligning the spine, pelvis and rib cage so that the shoulders and arms can rest on a stable base.
• Static core tasks are used in rehabilitation to train awareness of alignment before dynamic loading is introduced.

Dynamic core function

• Dynamic core function resists forces that change direction, and it depends on timely muscle activation rather than pure skeletal rigidity.
• During walking on a slope, the legs provide propulsion but the core must continuously adjust the trunk to keep the centre of mass over the base of support.
• Dynamic stability requires feed-forward activation, rapid reaction time and the ability to modulate stiffness according to load.
• The thoracolumbar fascia and abdominal wall form a tensioned system that stores and releases energy during running and throwing.
• Poor dynamic core control is associated with compensatory limb strategies and increased energy cost of movement.

Clinical significance

• Chronic low back pain is associated with altered timing and endurance of multifidus and transversus abdominis, although causality is debated.
• Stress urinary incontinence is linked to pelvic floor weakness, and pelvic floor muscle training is first-line conservative treatment.
• Diastasis recti is separation of the rectus abdominis bellies, and it can impair trunk stability and abdominal compression.
• Core weakness may increase the risk of groin, hamstring and lumbar injury in athletes, but evidence for prevention is mixed.
• Abdominal binders and belts are used after surgery to support the abdominal wall, but their effect on core muscle activation is uncertain.
• In pregnancy the growing uterus changes core mechanics, and the abdominal muscles lengthen while the pelvic floor bears greater load.
• Core stability exercises are widely used in rehabilitation, but programmes differ in exercise selection, dosage and outcome measures.

Comparative anatomy

• Quadrupeds use the trunk mainly to transmit propulsive forces between the limbs, and their core muscles are organised around a horizontal spine.
• Bipedalism in humans shifts the core's role toward vertical load bearing and balance, which is reflected in the large gluteal and lumbar muscles.
• Whales and dolphins have a reduced pelvis and a core dominated by axial musculature for swimming, while primates have a more mobile lumbar spine.
• The diaphragm is a mammalian innovation, and its role in core pressure is not shared by reptiles or birds in the same form.

History

• The idea of core stability became prominent in the 1990s, when physiotherapists linked deep abdominal and multifidus dysfunction to low back pain.
• Earlier biomechanical work treated the trunk as a lever system, and later models added muscular co-contraction and intra-abdominal pressure.
• The term core was popularised by fitness and rehabilitation industries, which broadened it beyond its anatomical meaning.
• Terminologia Anatomica does not define a core region, because the term is functional rather than a named anatomical part.

Research and controversies

• Whether core stability training prevents low back pain or only improves performance in already symptomatic people is still unresolved.
• The relative contribution of transversus abdominis, multifidus and the diaphragm to spinal stability is difficult to measure and remains debated.
• Some studies find that general exercise is as effective as specific core training, which challenges the emphasis on isolated deep-muscle activation.
• Fascial tension and intra-abdominal pressure are increasingly studied, but their clinical measurement is not standardised.