What the Studies Show
The most comprehensive study of mattress firmness and back pain (Kovacs et al., 2003) found that medium-firm mattresses resulted in less back pain than firm mattresses for participants with chronic non-specific lower back pain. This study is frequently cited to argue for medium-firm mattresses universally — but it compared medium-firm to firm, not to soft. A 2015 follow-up found that the optimal firmness varied meaningfully by body weight and sleep position.
The Limitation of Generic Recommendations
A 140-pound side sleeper and a 280-pound back sleeper should not be sleeping on the same firmness. The clinical literature on mattresses and back pain consistently finds that optimal firmness is not a universal prescription — it depends on body weight, sleep position, existing pathology, and individual preference. 'Medium-firm is best for back pain' is a marketing simplification of genuinely nuanced research.
Pressure Mapping Evidence
Pressure mapping studies (which measure pressure distribution across the body surface while sleeping) show that adequate pressure relief at the hips and shoulders reduces spinal compression in side sleepers. For back sleepers, lumbar support — the mattress maintaining contact with the lumbar region rather than allowing it to sink or bridge — is the key variable. This has led to the 'zoned support' designs used by Helix, Purple, and others.
Practical Implications
For people with chronic lower back pain: medium-firm (5–6.5) is a reasonable starting point, but the trial period exists for good reason — test your specific response. For people with hip or shoulder pain from side sleeping: softer surfaces (3.5–5) that allow those joints to sink tend to show better outcomes. For people with no existing pain: the mattress that results in the deepest, most refreshing sleep is probably the right one — there's no evidence that sleeping on a 'wrong' firmness causes injury in healthy adults.