Medium-firm or firm mattress for a bad back? What the evidence says

If your back has started having opinions, someone will have told you to buy a firm mattress. Possibly a very firm one, possibly with the word "orthopaedic" on it. This advice has been passed down since the 1970s and it is mostly wrong. Here is what the evidence actually says, why a hard mattress often makes a sore back worse, and what to look for instead.

One thing before we start: a mattress supports your back, it does not treat it. If you have ongoing pain, see a professional. This guide is about not making it worse.

The one trial worth knowing about

Most mattress advice is opinion. There is one properly run study that is not. In 2003 a Spanish research group published a randomised trial in The Lancet: 313 adults with chronic low back pain were given either a firm mattress or a medium-firm one, without knowing which, and followed for 90 days. The medium-firm group reported less pain in bed, less pain on getting up, and less disability than the firm group. Not by a landslide, but consistently, and in the direction most people would not expect.

Since then, reviews of the smaller studies that exist have tended to land in the same place: medium-firm comes out ahead of firm for people with low back pain, and both come out ahead of soft.

Why hard mattresses go wrong

Your spine is not straight. Lying on your side, your shoulders and hips are wider than your waist. Lying on your back, there is a curve at your lower back. A good mattress lets the wide bits sink slightly so the spine stays level, and holds the narrow bits up.

A hard mattress does not let anything sink. On your side, your shoulder and hip take all the weight, your waist is unsupported, and your spine bends towards the mattress. On your back, your lower back arches over a gap. Either way you wake up stiff, and you toss more in the night trying to find a position that does not hurt, which is why people on hard mattresses often sleep worse without knowing why.

A soft mattress fails the other way: the hips, which are the heaviest part of you, sink deepest, and the spine sags into a hammock.

What "support" actually means

Support is not firmness. Support is whether your hips end up level with your shoulders. That is decided by the spring unit or foam core, not by how the surface feels.

This matters because it means you can have a mattress that feels comfortable on the surface and still holds you level underneath. The premium end of the trade has sold mattresses this way for a century: a plush or medium top over a firm, supportive base. Cheap mattresses cannot do it, because they only have one layer doing both jobs, so firm has to mean hard.

What to look for

Zoned springs. A spring unit where the centre third, under your hips and lower back, is firmer than the shoulder area. It stops your hips sinking lower than your shoulders when you lie on your side, which is the single most useful thing a mattress can do for a sore back. Look for "zoned", "3-zone" or "5-zone" on the spec, and ask how the zones are made (heavier wire or shorter springs, both fine).

A real comfort layer. Around 4 to 6 cm of pressure-relieving material on top, so your shoulder and hip can settle. Memory foam of 55 kg/m³ or more, or latex. Less than 3 cm on a firm spring unit is a hard mattress with a blanket on it.

A firm edge. You will sit on the edge to put socks on and get up. A steel-rod edge or a dense foam rail stops the side collapsing, which is where a lot of "my mattress has gone soft" complaints actually come from.

Medium-firm, not firm. On a ten-point scale where ten is a board, aim for around seven to eight. If you are heavier than about sixteen stone, or sleep on your front, go firmer in the support (a thicker wire, denser base) while keeping the comfort layer.

A base that is not the problem. Slats more than 7 cm apart, a broken slat, or an old divan that has gone soft will make any mattress feel wrong. Check it before you buy.

What about "orthopaedic" mattresses?

"Orthopaedic" is a marketing word. There is no standard behind it, no test, and no body that certifies it. It almost always means "firm", and often "hard", which is the opposite of what the evidence suggests. We do not use the word, and we would be careful of anyone who leans on it.

How we built ours

The Bad Back Years is medium-firm on purpose. A 2,000-spring unit in five zones with a firmer centre third, 5 cm of 60 kg/m³ memory foam on top for pressure relief, and a steel edge so the sides stay up. It comes in Medium and Firm tension, which changes the support underneath without changing the feel on top, and in king and super king you can have one tension on each side. If it is too soft after 30 nights, we swap it.

The short version

• The best evidence says medium-firm beats firm for chronic low back pain.
• Hard mattresses leave your waist and lower back unsupported.
• Support comes from the springs, not the surface. You can have both.
• Look for zoned springs, a real comfort layer, a firm edge and a sound base.