Hi pcunite and VAsleeper,
The short answer is that this is a myth.
You can see a
study here
that is frequently referred to and suggests that a medium firm mattress is the 'best" choice but even this really has little meaning because they don't differentiate between the firmness of the comfort layers and the firmness of the support layers which each have different functions in a mattress or specify the combinations of comfort and support layers that may be most suitable for different body types and sleeping styles. The study was also only based on two mattresses and because there isn't an industry standard for mattress firmness it really has little meaning except to say that "firmer isn't always better".
The real goal of a mattress is to keep the spine and joints in good alignment and re-distribute pressure away from the bony prominences and pressure points. Since each person has a different body type and sleeping style, the balance between the firmness of the support layers which provides the primary support of the mattress and "stops" the pelvis from sinking down too far (which in turn controls pelvic tilt and the curve of the lumbar spine) and the thickness and softness/firmness of the upper comfort layers which "allows" the bony prominences to sink in enough and fills in the gaps in the sleeping profile (which provides the lighter or secondary support needed under the recessed parts of the body) is unique and can be different for each person.
A mattress that is too firm to allow the body to sink in evenly with good alignment and fill in the gaps in the body profile or a mattress that is too soft and allows the heavier pelvis to sink down too far can both lead to poor spinal alignment (see
this diagram
)
The other side of the coin though is that testing a mattress for comfort is very unlikely to produce good results. Comfort is very subjective and
this study
also indicates that the odds of success and choosing the most suitable mattress for those who choose a mattress based on "comfort" are less than random chance alone.
Of all the factors in a choice of mattress ... good alignment is the most important (because it requires the body prominences such as the shoulders to sink in enough which also tends to relieve pressure and the hips to be "stopped" before they sink in too far) but is also the most difficult to test for.
The "bridge" between the two (choosing the firmest possible mattress which tends to lead to choices which are too firm and testing for "comfort" which tends to lead to choices which are too soft) is testing objectively, carefully, and specifically for alignment and pressure relief rather than just "firmness" or "comfort" (which is mostly connected to pressure relief and not alignment).
The firmness and elasticity of the support layers along with the softness, thickness, and elasticity of the comfort layers and how they interact with each different body type and sleeping style are the key to success in a mattress choice and it would be nice if each mattresses had two ratings (one for the support layers and one for the comfort layers) rather than just one "overall" rating which would be a much more accurate way of assessing the suitability of a mattress.
In the highly managed environment of many mattress showrooms where subjective perceptions of "comfort" can be easily controlled in the short term and are relative to your current circumstances and mood and how you feel ... and what you tested immediately before and after each mattress (there are techniques that are used such as bracketing , testing order, or verbal cues that can change what you feel on a mattress and are used as a sales tool) ... only good objective testing with a specific goal in mind of what you are testing for has good odds of success.
Phoenix