Intervertebral disc

An intervertebral disc is one of the cushions between the bones of your spine. An adult has 23 of them: six in the neck, 12 in the thoracic spine and five in the lower back. There is no disc between the top two neck bones, and the lowest sits where the lumbar spine meets the sacrum. Together the discs make up roughly a quarter to a third of the spine’s length (Waxenbaum et al., 2026).

What do intervertebral discs do?

Each disc joins the vertebra above to the one below. The discs let the spine bend while keeping it stable, spread compressive loads, and stop the bones of the spine from touching (Waxenbaum et al., 2026).

How is a disc built?

A disc has three parts (Waxenbaum et al., 2026):

• The nucleus pulposus, a gel-like core that is 66% to 86% water. A molecule called aggrecan holds much of that water in place. The core resists compression and lets each segment of the spine flex. More on the nucleus pulposus.
• The annulus fibrosus, a ring of 15 to 25 stacked sheets of collagen around the core. The fibres in each sheet run at an angle of about 60 degrees to those in the next, the same criss-cross idea used in radial car tyres, which makes the ring very strong. More on the annulus fibrosus.
• Cartilage endplates, which anchor the disc to the vertebrae above and below.

Why are you shorter in the evening?

Discs lose and regain fluid every day. In a small study of eight young adults, height varied by an average of 19.3 mm, about 1.1% of their height, across the day. More than half of the daily loss happened in the first hour after getting up, and about 70% was regained in the first half of the night (Tyrrell et al., 1985). A pressure sensor in one volunteer’s disc showed the same story from the inside: pressure rose from 0.1 to 0.24 MPa over seven hours of sleep as the disc took up fluid again (Wilke et al., 1999).

What does sitting do to your discs?

The best-known measurements come from that same volunteer, a 45-year-old man with a sensor in a healthy lumbar disc (Wilke et al., 1999):

• relaxed standing: 0.5 MPa
• sitting without a backrest: 0.46 MPa
• sitting relaxed, which the authors called “nonchalant”: 0.3 MPa
• sitting fully rounded forward: 0.83 MPa
• standing bent forward: 1.1 MPa

So sitting is not automatically harder on the discs than standing. Rounding the back, which is spinal flexion, did raise the pressure. The authors concluded that changing position often is important, because it helps fluid, and with it nutrition, move in and out of the disc. They were careful to note that this was one person.

A small MRI study scanned 12 people before and after four hours of sitting on two days. After continuous sitting, the disc at L4 to L5 lost a little height. On the day they changed position and stretched every 15 minutes, the change was not significant (Billy et al., 2014). Whether that matters for pain wasn’t tested. Practical ways to break up the day are in microbreaks, and the sitting time calculator shows how many hours you spend in a chair.

Can exercise strengthen your discs?

Discs were long thought to change too slowly to respond to exercise. A 2017 study found that regular runners had better-hydrated discs, richer in the water-holding molecules, and larger discs than people who didn’t run. In activity tracking, movement at the speed of brisk walking or slow running went with healthier discs, while high-impact activity, slow walking and static positions did not (Belavý et al., 2017). It compared groups at one point in time, so it can’t prove cause, but the authors called it the first evidence in humans that exercise can benefit the discs. See also Is walking good for back pain?

Discs and age

Disc changes on scans are very common in people without pain. In a review of 3,110 people with no back pain, disc degeneration showed up in 37% of 20-year-olds and 96% of 80-year-olds (Brinjikji et al., 2015). That is worth knowing before reading too much into a scan report mentioning degenerative disc disease or a bulging disc. When disc material pushes out and presses on a nerve, see herniated disc.