Intervertebral disc

Function

A pad between each pair of vertebrae

An intervertebral disc is a pad of fibrocartilage that fills the gap between the bodies of two neighbouring vertebrae¹. It is anchored to both¹. The adult spine has 23 discs: 6 in the neck, 12 in the mid-back and 5 in the lower back². The lowest sits between the last lumbar (lower back) vertebra and the sacrum, the triangle-shaped bone at the base of the spine².

Discs are thin in the neck¹. They are thickest in the lower back, the region that bears the most body weight¹. A lumbar disc is about 7 to 10 mm thick and about 4 cm from front to back³.

A gel core inside a wall of rings

Each disc has three main parts²:

• The nucleus pulposus is the inner part, a gel-like core that sits slightly behind the centre of the disc².
• The annulus fibrosus is the outer part, built from 15 to 25 rings, one inside the next^2,3.
• The cartilage endplates anchor the disc to the vertebrae above and below². Each is a thin layer of hyaline cartilage (a smooth, glassy kind of cartilage), usually under 1 mm thick³.

Within each ring of the annulus the collagen fibres lie parallel, at about 60 degrees to the vertical³. Collagen is a strong, rope-like protein. The fibres lean the opposite way in the next ring³. This collagen network gives the disc its tensile strength, its strength against being pulled apart, and anchors it to the bone³.

The nucleus is about 80% water by wet weight, and the annulus about 70%³. The nucleus also holds more proteoglycan, a large protein-and-sugar molecule that holds water³. Aggrecan, the main proteoglycan of the disc, keeps the tissue hydrated through the osmotic pressure its sugar chains create³. Osmotic pressure is the pull that draws water toward dissolved substances.

What the disc does

The discs are the spinal column’s main joints³. They constantly pass on loads from body weight and muscle activity, and they let the spine bend and twist³. They spread compressive (squeezing) loads along the spine and keep neighbouring vertebral bodies from touching bone to bone².

The gel-like nucleus lets the disc change shape as one vertebra rocks on the next¹. Bending forward compresses the front of the disc and expands the back¹.

How much pressure a disc carries

One set of figures comes from a single 45-year-old male volunteer with a pressure sensor in a healthy disc⁴. The sensor sat in his L4/L5 disc, between the fourth and fifth bones of the lower back⁴. MPa stands for megapascals.

Position or task, one volunteer	Pressure in the L4/L5 disc
Lying face down	0.1 MPa4
Sitting unsupported	0.46 MPa4
Relaxed standing	0.5 MPa4
Sitting fully flexed (bent)	0.83 MPa4
Standing bent forward	1.1 MPa4
Lifting 20 kg held close to the body	1.1 MPa4
Lifting 20 kg with flexed knees	1.7 MPa4
Lifting 20 kg with a rounded, flexed back	2.3 MPa4

How water and nutrients move

The changing loads on the spine over a day alter the water content and height of the discs⁹. In eight young adults, whole-body height varied by an average of 19.3 mm over the day¹⁰.

The disc has no blood supply of its own¹¹. Its cells rely on diffusion, the slow spread of a substance through a tissue, from blood vessels at the edges of the disc¹¹. Nutrients reach the nucleus from capillaries (tiny, thin-walled blood vessels) by diffusing through the cartilage endplate³. Nucleus cells can lie up to 8 mm from those capillaries³.

The disc has no blood supply of its own.

Breakdown

Discs change early, and in most people scanned in one study

The disc shows degenerative and ageing changes earlier than any other connective tissue in the body³. The Wakayama Spine Study scanned the whole spine of 975 people aged 21 to 97 with MRI¹². It found degeneration somewhere in the spine in 71% of men and 77% of women under 50¹². Over 50, the figure was more than 90% in both¹².

The largest chemical change in degeneration is loss of proteoglycan, which lowers the osmotic pressure of the matrix and so its water content³. The matrix is the material around the cells. Degenerated discs hold less water than normal discs of the same age³. Under load they lose height and fluid faster, and they tend to bulge³.

What studies link to degeneration

Four sources looked at what goes with degeneration^5,6,12,13:

• Inheritance. An MRI study scored disc degeneration in 172 identical and 154 non-identical twins not selected for back pain⁵. Heritability of the overall score was 74% in the lumbar spine and 73% in the neck, after adjusting for age, weight, smoking and other factors⁵. A narrative review (a summary of research written without a set, checkable search method) covered the Twin Spine Study¹³. It concluded that genetic influences appeared to determine degeneration in great part¹³.
• Physical loading. In the same review, twin siblings whose physical loading at work and leisure differed greatly through adulthood showed surprisingly little difference in disc degeneration¹³.
• Several causes together. A review of the literature named four underlying causes: genetic inheritance, age, poor transport of nutrients and wastes, and loading history⁶. It held that each can weaken a disc enough for it to fail structurally during everyday activity⁶.
• Age and obesity. The Wakayama Spine Study found age and obesity associated with degeneration in every region of the spine¹².

How a disc herniates

A disc herniation is when the soft centre of a disc pushes out through its tough outer ring. The disc bulges or ruptures backward, or backward and to one side³. It then presses on nerve roots inside the spinal canal, the channel in the spine that holds the spinal cord³. A nerve root is the part of a nerve where it leaves the spinal cord. Herniation is most common at the L4/L5 and L5/S1 discs, the two lowest lumbar levels^1,14.

In laboratory tests, a healthy disc herniates only under forces larger than those normally met³. In most such tests the vertebral body fails before the disc, and some degenerative change seems to be needed first³. Three studies looked at how discs fail:

In laboratory tests, a healthy disc herniates only under forces larger than those normally met.

• Cadaver (donated body) spines. Fifty-two lumbar segments were fatigue-loaded (loaded over and over) in compression and bending, and six discs herniated gradually¹⁵.
• Pig neck segments. In the laboratory, herniation at the back of the annulus occurred with modest compression but highly repetitive bending forward and back¹⁶.
• People having surgery. In 181 consecutive patients having surgery for a lumbar herniation at one level, 117 had failed where the disc joins the endplate¹⁷. That was more common than rupture of the annulus¹⁷.

Herniation of the L4/L5 or L5/S1 disc can cause sciatica, a widespread pain that spreads from the lower back into the leg^1,18. The disc herniation page covers who is affected and what trials found, and the sciatica page covers the leg pain.

Recovery

A tissue that renews slowly

One review of the literature states that structural failure of a disc is irreversible, because adult discs have limited potential to heal⁶. In 21 adult sheep given a 5 mm cut in the outer annulus of lumbar discs, the outermost annulus showed the ability to heal¹⁹. The inner annulus failed progressively in all the sheep, in most discs within 4 to 12 months of the operation¹⁹.

A 2011 evidence review states that on repeated MRI scans the herniated part of a lumbar disc tends to shrink over time²⁰. A 2024 meta-analysis (a study that pools earlier studies) covered 2,233 people who had non-surgical care for a lumbar disc herniation²¹. Its pooled overall incidence of disc resorption, the herniated material being absorbed, was 70.39%²¹.

Disc cells continually break the molecules of the matrix down and make new ones³. Disc collagen, though, turns over very slowly, and more slowly with age²². A laboratory study of collagen from normal discs estimated an average half-life of 95 years between ages 20 and 40²². A half-life is the time it takes for half of something to be replaced or broken down.

In seven healthy men, 21 days of head-down bed rest changed disc volume and height²³. Those measures had not returned to their starting values 153 days later, although back muscle size had recovered²³.

What studies of loading and running found

Studies in runners, twins and animals bear on how discs respond to loading^24,25:

• Runners. A cross-sectional study, one that looks at a group once, compared adults aged 25 to 35²⁴. Long-distance runners and joggers had lumbar disc T2 times, an MRI measure, 11.4% and 9.2% higher than people who did no sport²⁴. The authors read this as better hydration, and the two running groups did not differ significantly²⁴.
• Twins. The narrative review of the Twin Spine Study reported some indications that routine loading may have some benefits for the disc¹³.
• Animals. A brief narrative review reported that animal studies suggest a dose-response relationship between loading and the disc²⁵. High loading at high volume and frequency might speed degeneration or injure the disc²⁵. High loading at low volume and low frequency appears to switch on potentially regenerative mechanisms²⁵.

Connections

Each disc joins the bodies of the vertebrae above and below it, and the discs are the main joints of the spine^1,3. Five discs sit in the lower back, the region of the lumbar vertebrae². Neighbouring vertebrae also meet at the facet joints, the small joints that guide and limit movement². The disc joints between the vertebral bodies carry most of the weight the spine bears².

A spinal nerve leaves through the intervertebral foramen, the opening formed between neighbouring vertebrae¹. A nucleus bulging backward can press on the nerve there^1,3. Herniation of the L4/L5 or L5/S1 disc can cause sciatica, and the disc herniation page covers that condition^1,18. In the Wakayama Spine Study, low back pain was associated with degeneration in the lumbar region¹².

Related exercise pages: good morning, child’s pose, knee-to-chest and running. Four male nationally ranked weightlifters did good mornings with a mean of 67.5 kg on the bar²⁶. A model estimated the mean peak compressive force at L5/S1 at 6712.7 newtons²⁶.

One published description of child’s pose is a forward bend with the spine rounded²⁷. In one trial in people with chronic low back pain, the flexion-based (forward-bending) lumbar exercise programme included knee-to-chest²⁸. Bending forward compresses the front of a disc and expands the back¹.

Sources

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Every source the library cites

Changes

• October 9, 2026 First published.