Hi feeblewitz,
This would often indicate that the comfort layers may be too thin or too firm to allow the hips or shoulders to sink into the mattress far enough to fill in the lumbar curve or the waist. This is what I call secondary support (see post #4 here). The support layers themselves may still need to be firmer to “stop” the heavier pelvis from sinking down too much (primary support). Zoning can also help with more difficult situations or body types because they can be softer under the shoulders which are lighter and need to sink in more but still provide firmer support under the heavier pelvis (see post #11 here).
I would be a little cautious though about getting too involved in mattress theory and design which can sometimes lead to information overload and all that’s really important is careful and objective testing for PPP where your body will tell you which mattress is the best “match” for you much more effectively than specs or theory.
There are hundreds or even thousands of different designs of latex mattresses or latex hybrids that will each have a different feel and perform differently but it may be worthwhile testing some Dunlop latex which is less springy and different types of quilting layers can also reduce the surface resilience of a latex mattress.
There are certainly other alternatives to latex in comfort layers but if “chemical free” is important to you rather than just “safety” then natural fibers (cotton and wool) may be worth considering although they can be firmer than foam and innerspring/natural fiber mattresses can be quite costly. The issues surrounding “chemical free”, safety, natural, synthetic, organic, green, fir retardants, and all the other related issues can be very complex and confusing and the industry is filled with misinformation about all of these but a good starting point that can help each person answer the question of “how safe is safe enough for me?” on an individual level is post #2 here and the other posts and sources of information it links to. I would be forewarned though because this is a very complex and confusing issue where there are often no clear answers available and there are many opinions that are more about selling mattresses than they are about providing accurate information to consumers.
Phoenix