A medium firm mattress is actually the best for back pain despite the fact that the standard advice for a long time has been to sleep on a firm mattress.
A 2003 study by the Lancet Medical Journal provided the findings and over the last few years the mattress industry has begun to adjust.
The theory behind the firm mattress advice was that you need support and too soft a mattress, while comfortable, does not give that much needed support to your back and joints.
It is still the case that too soft is detrimental. However it was found that by moving a small degree towards medium was much better. The reason is that the body needs a little bit of give, too rigid a mattress is bad as well.
This was an old wives tale but it was also advice that was being given out by orthopedic surgeons. However this randomized double blind trial demonstrated otherwise.
It is easy to be skeptical of medical studies as it always seem that scientists are changing their mind or that new evidence is found to refute the claim or there were flaws in the study. Further studies may change the hypothesis again but at this point I think the medium-firm recommendation is spot on. The reason is that is just makes good sense.
You need firmness in order to support your spine and joints. However the reason we sleep on a mattress but not concrete floor is that we need a bit of give. The pelvis, shoulders and head are heavier than other parts of the body and need to be allowed to depress the mattress a little. If the mattress is too rigid the spine actually doesn't sit properly aligned.
For those who don't have back pain, for the most part it does not. If you have a healthy back and are strong and flexible you probably won't notice the difference too much with different mattresses. It won't be ideal to sleep long term on something too firm or too soft but if you don't have back pain you would be able to manage.
While a mattress alone is not likely to cause back pain, if you already have it you become much more sensitive to your mattress. While getting the right mattress won't cure the back pain it can stop it getting worse. A medium firm mattress might be an option to consider.
The trouble with the medium firm mattress theory is that mattress firmness is a very subjective thing. What is firm or medium to you may be different to somebody else, particularly if you have a significant weight difference. Weight is something that is very rarely mentioned but it is a major factor you should be considering, particularly with firmness.
Not everybody in a study is going to produce the same result. Even if the vast majority sleep better on medium-firm that does not mean all people do. Some people in the study slept better on something soft or something firm. What if you are one of those people?
So bear in mind what studies have shown to be the best result for most people, but do not take that as gospel. You need to do some research on yourself, your sleeping habits and your needs before you make a decision about which firmness to choose.