Nucleus pulposus

The nucleus pulposus is the soft, gel-like core of each intervertebral disc, the cushions between the bones of the spine. It sits slightly behind the centre of the disc, resists compression and lets each segment of the spine bend (Waxenbaum et al., 2026).

What is the nucleus pulposus made of?

Mostly water: between 66% and 86%. The rest is largely type II collagen and proteoglycans, large molecules that hold on to water; one of them, aggrecan, is mainly responsible. There are very few cells (Waxenbaum et al., 2026).

Around it lies the annulus fibrosus, 15 to 25 layers of collagen fibres. The fibres in neighbouring layers run at different angles, a crossed design that makes the ring strong, much like the plies of a car tyre. Thin plates of cartilage anchor the disc to the vertebrae above and below. The adult spine has 23 discs; there is none between the top two vertebrae of the neck (Waxenbaum et al., 2026).

What sitting does to the nucleus pulposus

The classic measurements of pressure inside discs were made in the 1960s. In 1999, researchers repeated them by placing a pressure sensor in the nucleus pulposus of a healthy disc low in the lumbar spine of one 45-year-old man and recording it for about a day. Their results matched the older data for many activities, but not for the comparison of sitting with standing (Wilke et al., 1999):

• lying on the front: 0.1 MPa
• relaxed standing: 0.5 MPa
• sitting without a backrest: 0.46 MPa
• slouched, relaxed sitting: 0.3 MPa
• sitting and bending fully forward: 0.83 MPa
• lifting 20 kg with a rounded back: 2.3 MPa

Overnight, pressure rose from 0.1 to 0.24 MPa as the disc took up fluid again. The authors cautiously concluded that sitting may load the disc less than standing, and that changing position often helps fluid, and so nutrition, flow into the disc. With one person measured, these numbers are a guide, not a rule.

The daily cycle shows up on scans too. In MRI scans of 84 adults aged 18 to 83, a day’s loading compressed every region of the lumbar discs, and younger people lost more disc height over the day than older people (Newman et al., 2026). Morning back stiffness is covered in Why does my back hurt in the morning?

Herniated nucleus pulposus

In a herniated disc, disc tissue bulges or pushes outwards. It hurts if it presses on a nerve, and many people with one never know (NHS). A slipped disc pressing on a nerve root in the lower back is the most common cause of sciatica (NHS).

The body often clears escaped material. Pooling studies of people treated without surgery, a review found that the herniation shrank on its own in 96% of sequestrations (where a fragment has broken free), 70% of extrusions, 41% of protrusions and 13% of bulging discs (Chiu et al., 2015). Scan findings still need care: in adults aged 50 or under, bulges, protrusions and extrusions were more common in people with back pain than without, but they were found in pain-free people too (Brinjikji et al., 2015).

Does exercise help the discs?

Possibly some kinds. A review of 39 studies with 4,152 people found running linked with better disc health on MRI, with very low certainty; no other type of physical loading showed a clear link either way, and the combined results were mixed. Most of the studies were observational (Samanna et al., 2026). The NHS advice for preventing a slipped disc is to keep active, lift with a safe technique and not smoke, since nicotine weakens disc tissue (NHS). Chair setup is covered in office chair setup.

When to see a doctor

See a GP if painkillers aren’t helping, the pain hasn’t improved after a few weeks, it is worse at night, or it comes with a high temperature, unexplained weight loss or a swelling in your back. Call 999 or go to A&E if you have numbness around your bottom or genitals, can’t pee, lose feeling in one or both legs, or can’t control when you pee or poo (NHS).