Hi LAH7,
This is generally a “symptom” of a comfort layer that is too soft or thick where the midback sinks in more than the lighter shoulders which are being “held up” more. As you mentioned this would be different from the floor where the area between the shoulder blades wouldn’t sink in at all (the floor would “stop” any sinking in) so you would sink in more evenly. From the sounds of it (and keeping in mind that I don’t know your specific body type) it sounds like you may need a little bit firmer layer on top of your mattress … not the extra softness/thickness that would come with a topper. If you add a topper then the risk is higher that it would affect the alignment of your pelvis/lower back. You can read a little more about “hunched” shoulders and other symptoms and fine tuning a mattress in post #2 here and in post #6 here and in post #6 here.
Mattress issues can be very complex and are often counterintuitive and difficult to “diagnose”. In general there are three types of “issues” with a mattress. One is connected to pressure relief, one is connected to alignment, and finally there are issues which are more about “feel” which is much more subjective. The first two generally have specific symptoms where people describe their experience as “I have” pain or discomfort in a specific area while the third (feel) is generally described as “if feels like” where they describe more subjective experiences such as “it feels like I’m sinking in too far or not enough” in a particular area or other more subjective descriptions. In general it’s easier to deal with specific symptoms that are more defined while the “it feels like” descriptions are often more a matter of personal preferences and the overall subjective experience on the mattress. Are you actually experiencing pain or discomfort in your upper back or is it more of a “feeling” that you are uncomfortable with.
You could easily check if your mattress is the correct one and has the correct foam layer on top by looking at the law tag, measuring the height of your mattress, or by checking the internal layers on the side of the mattress for thickness by feel. I should also mention though that I have talked with at least one retailer that I respect that had a customer where the mattress they received was clearly firmer than the mattress in the showroom and they suspect that the extra firmness is in the rebonded base layer … not in the comfort layer of latex. They actually went to their customers home to check their mattress in person. The firmer support layer would probably be more noticeable with heavier people that “went through” the softer comfort layers more. This would usually lead to more of a “feeling” of firmness more than actual pressure issues for back sleepers and a thin topper may compensate somewhat for this although increasing the thickness of the comfort layers by adding a topper could also lead to alignment issues because the real issue in this case would be in the firmness of the support layers. In this case a topper as thin as possible (say an inch or so) and slightly firmer would have less risk of alignment issues than a thicker topper and would still add to the softness of the sleeping surface and isolate you more from the firmness of the support layer.
Walking on latex will have a smaller effect and could help slightly but not as much as other foams such as memory foam that have more of a tendency to break in.
This makes more sense to me now. From your first description I thought you were describing that somehow your shoulders were sinking in more than your midback area which was confusing to me. This uneven sinking in where the heavier pelvis sinks in too far relative to the upper body is the risk of adding a topper to a mattress that already has several inches of softer foam on top. There is always a balance between “allowing” the upper body to sink in enough while at the same time “stopping” the heavier pelvis from sinking down too far.
A wool topper can “pad” the pelvic area which adds some surface area and can reduce how much it sinks down slightly while at the same time it can add some surface softness which may “feel” better for you although how wool interacts with foam underneath it can be fairly complex and depends on the specifics of body type, body shape, and the softness of the foam underneath it. It can be a little less risky topper than foam in terms of alignment issues. I think it may well be worth a try because it may even out how you sink in somewhat however I would also consider trying a wool topper that you are able to return if it doesn’t work because there are so many variables and unknowns involved in more complex “theory” and layering that only your own experience can really know if it’s successful. I would personally choose wool over cotton because cotton will compress more and be much firmer over time because wool is much more resilient than cotton … even when it’s compressed.
A relatively thin but firmer latex or polyfoam topper or a thin higher density memory foam topper may also be worth a try although trying to “fix” a mattress that is already to soft or thick in the comfort layers (vs the support layers) is not nearly as easy as adding more softness on top of your mattress. I know that it “feels” like your mattress is too firm but I think the issue may be in the support layer and not in the thickness of the comfort layer (assuming that you verify that the comfort layer is the right thickness) so you would really be trying more of a “workaround” in the hopes it will help. Sometimes a softer comfort layer over a support layer that is very firm can “feel” like the comfort layer itself is too soft but using a firmer comfort layer can actually “feel” softer because it can isolate you more from the firmness of the layers below it.
Perhaps the most effective suggestion of all would be to talk with your retailer to see if they have any suggestions as well and they may even be willing to come to your home and see if they can also feel the difference in your mattress compared to the floor model. It may also be well worth talking to Pure Latex Bliss itself because in my experience they are quite responsive to consumer issues wioth their mattresses.
Some of the other possibilities that would also be worth checking to rule out other possibilities besides the mattress itself are in post #2 here.
All of this can require some detective work so that you are confident that you have identified the actual underlying problem correctly which would give you much better odds of correcting it.
Fixing the “wrong” problem in the “wrong” layer can sometimes lead to a different set of symptoms or cause more issues than it solves.
Phoenix