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Searched for: back to some of the simpler issues of "real life" outside of "rough math"
03 Jul 2012 17:25
  • phoenix
  • phoenix's Avatar
Hi dudley9007,

Your question is very broad and can really only be answered in general terms without going into great length and including information that is very technical and could be a degree course in mattress manufacturing, physiology, materials science, and mattress ergonomics. I'll provide a more generic answer and then include a more technical answer for those who are more technically inclined.

The simple version ...

Each person has a range of tolerance for both pressure relief and alignment (the two main functions of a mattress) and there are many methods that are part of mattress theory and design to deal with both of these and how they interact so that both are within the range of what someone needs and prefers in their mattress. While pressure is never relieved down to "zero" ... as long as it is within the tolerance of the individual both in terms of their perception and comfort and in terms of any restriction of the capillaries and blood flow in the areas of the "bony prominences" then different types of mattress construction and materials can achieve both the pressure relief and alignment that someone needs yes. There's lots more information about this in the "mattresses" section of the site and in the more detailed information at the end of this post that describes how and why different materials and types of construction can be used to achieve both.

While this can involve a combination if technical knowledge and intuition in many cases ... the fact that almost everyone has found a good combination of both .. at least at some point in their lives (maybe not at the moment while they are looking for a new mattress) ... says more than anything else that both pressure relief and alignment as well as the many other personal preferences that are unique to each individual but also need to included as part of the mattress design are possible for the vast majority of people that don't have health or other issues that puts them outside of needs and preferences range of the majority of the population.

Of course the help of someone that is both knowledgeable and experienced can be a huge help for the minority of the population that are much more sensitive than average and have a much more narrow range of acceptable comfort and support needs and preferences. Finding someone like this who can identify what you need and prefer and either makes or sells mattresses that can be customized to finer levels can be much simpler than trying to learn all the details and technical information that it may take to do it on your own.

Beyond this you are into the range of medical issues or special needs that are much less common and can have other types of solutions than a "regular" mattress.

Phoenix

The much more complex and "rough math" version ...

This information is probably more technical than most people would want to know about mattress pressure relief in particular and how it interacts with alignment and may be of more interest for those who are more interested in technical details that can be overwhelming for most but it will give a sense of how sometimes some seemingly simple questions can have very complex answers.

There is little consensus information about pressure relief ... even in a hospital setting that deals with treatment of pressure ulcers and there is a great deal of information that needs further research. Even this information takes some liberties to make a very complex subject more understandable and some of the "assumptions" are not completely accurate ... but it gives a sense of the many factors involved from a more technical perspective.

Lets say an average person has an average body length (measured from a position a few inches above the knees to the top of the shoulders where almost all the weight is) of about 36" (I'm about 40" and 6'5 1/2" so probably more than most). This section of the body would include about 85% of the body weight. Lets also assume that an "average" person has body width in a side profile of about 12" (average from front to back) and that an average weight is about 180 lbs (male). I know these numbers are not really accurate and that there is a huge variance between people and body shapes but they're close enough for the purposes of this post. This means that this 36" section of the body would weigh about 153 lbs (excluding their head and legs from just above the knees to the feet).

This would also mean that the number of square inches in this side profile would be 12 x 36 = 432 (assuming this was all flat which of course it isn't). If every point of this area was being supported equally and the pressure was distributed equally along the body surface, then the average pressure along the body surface would be 153 lbs / 432 sq in = .35 psi.

To qualify as a "pressure reducing support surface", a mattress needs to reduce pressure to below 32 mmHg (also called a Torr) on "most bony prominences most of the time". Bony prominences are the bones and joints in each sleeping position that protrude and have less tissue between them and the surface. To qualify as a "pressure relieving support surface" pressure needs to be reduced to below 32 mmHg (Torr) on "all bony prominences all of the time". This is also not quite accurate as fixed numbers like this are not considered to be completely accurate or valid anymore as research continues but again for the sake of this post it will do. 32 Torr = .62 psi so if the weight was distributed equally throughout the entire side profile ... there would be no detectable pressure anywhere. This would be below the sensitivity threshhold of any individual and would not restrict blood flow because capillary pressure would be enough to maintain blood flow.

Very low levels of pressure for longer periods (levels below most very sensitive people's detection threshold) would be in the low 20's a good part of the time so lets say 23 Torr or .44 psi.

Lets assume that 60% of this body weight (92lbs) was concentrated in the hips/lower body and 40% (61 lbs) in the shoulders/upper body (and of course weight distribution varies between people).

Lets also assume that someone is sleeping on a mattress where the pelvic/hip area sinks into a mattress' upper layers enough to distribute this weight over an area of 12" x 12" sleeping on the side.

This would mean that there was 92 lbs spread over 144 sq in or .64 psi ... right at the level of pressure relief for most people. If this same person sank in even more and some of the weight was taken up by the more recessed areas of the body outside this 12" by 12" area ... then the pressure on the hips/pelvis would be further reduced.

If the hips only sank into the upper layers enough to distribute pressure over an area of 6" x 6" (too firm or thin upper layers) then the same 92 lbs would be spread over 36 sq. in. or 2.56 psi ... which for many people would equal pressure points and discomfort. of course it's also important to remember that this area supporting weight doesn't have even pressure because the surface is rounded not flat so there may be areas inside the larger areas that have more or less pressure.

If someone sank into the comfort layers enough to spread the pressure out over an even larger area of 12" x 18" (say mid thigh to lower waist) it would be 92 lbs spread over 216 sq in or .43 psi ... which "on average" would be well below detectable for pretty much everyone. Of course there would be smaller areas of more or less pressure inside this area as well.

So in this hypothetical case, all you would have to do would be to have upper comfort layers that were just soft and thick enough to let someone sink in just enough that their weight was spread over an area just large enough to bring the pressure to a level below their individual discomfort threshhold. This threshhold would vary with the person depending on their sleeping style, their health, and their physiology. The greater the area needed for pressure distribution for each person ... the softer/thicker comfort layer they would need and this could be very different for different people. Even minor changes in sleeping position ccan change the surface area that is in contact with the mattress and would change the pressure distribution. For almost everyone though, a larger 12 x 18 area (upper thigh, hips and some waist/lumbar) would give complete pressure relief. This sinking in can be into a thicker comfort layer or it can use a slightly thinner comfort layer and the top part of the next layer. Either way the pressure relief could be very similar.

I know these numbers are not accurate and don't take all the variables into account because the body is rounded and doesn't respond exactly to a "formula" but it shows how "sinking in" to a "conforming" (point elastic) upper layer can spread weight over a larger area and relieve pressure to levels below detection.

The same general idea and "rough math" would apply to the shoulder area.

All of this is also for the most difficult challenge for side sleepers. On the back you would have probably closer to 24" (twice as wide) x 36" surface area which is 864 sq in and if even only half of this surface was supporting all the weight you would have 153 / 432 = .35 psi which would not present a pressure problem for anyone.

The type of materials used for pressure relief can also affect support and alignment. The same sinking in that forms the mattress cradle and relieves pressure should also offer more gentle support (fill in the gaps) in the waist (on the side) or recessed part of the lumbar (on the back) so this more gentle support is normally looked after as a "side effect" of dealing with pressure distribution issues. Only people who were completely insensitive to pressure and didn't want to sink in at all (they liked and could tolerate sleeping on a floor) would likely have support issues in the more recessed parts of the body except in cases where a much higher percentage of their weight was "in the middle" or they were too light and/or thin to sink in enough to "fill in the gaps" in which case other options could be used to create firmer support in these areas (reverse zoning for example). Some people are even quite sensitive to more pressure in these areas than they are used to and they would need to shift some weight back onto other areas of the body to be comfortable. This is often why lower density foams are used in quilting layers on top of more highly resilient foams.

So if you can create just enough "sinking in" to distribute pressure points over a wide enough surface area around the body prominences using comfort layers (and the upper surface of the middle layers if necessary) of various types and which will deal with the support needs of the recessed areas of the lumbar, then the only remaining issues are issues of alignment. This is controlled by the middle and deeper layers of the mattress and they need to prevent or "stop" any further sinking down of the heavier areas than are necessary (hips and pelvis primarily) to prevent them from sinking in further than necessary for pressure relief which could put the spine out of it's natural range of good alignment.

In other words ... the upper layers are about "allowing" enough sinking in to create the mattress cradle which is a larger area of surface contact with the mattress to relieve pressure and support the recessed areas of the spine. The deeper layers are about controlling any further compression than necessary for pressure relief to "hold up" the heavier parts of the body and keep the spine in its range of natural alignment. The amount of sinking allowed by lower layers has less effect on pressure relief in other words (in most types of mattress construction) but controls alignment. This is why when people change the firmness of upper layers to solve support issues or the lowr layers to solve pressure relief issues they will often create new issues and may not solve the the issue they are trying to "fix". This is also why describing a mattress as a whole as either soft or firm can be very misleading because all mattresses need some of both.

This is also where different types of mattress design can be chosen because every layer will interact with other layers and in real life they don' act independently of each other. Upper layers used more "exclusively" for conforming pressure relief (softer/thicker comfort layers) need firmer middle and lower layers to "stop" further compression while a similar effect can be achieved with a combination of thinner upper layers and slightly softer middle or lower layers which can create the same depth of pressure relieving cradle.

These two methods can take advantage of the different characteristics of different types of foam and other components to create a similar depth of cradle for pressure relief but allow more or less "extra" compression of the "bony prominences" into the deeper layers to control alignment. The first method (which I call differential construction) leads to sinking into the comfort layers with a more sudden "stop" using firmer materials while the second method (which I call progressive construction) leads to more sinking of the "bony prominences" into the deeper layers with a more gradual "stop". In either case the pressure relief can be the same, what changes is the alignment and the ability of a mattress to adapt to changing positions in terms of both pressure relief and alignment.

Each sleeping position (and the many combinations and variations of each) along with the many different variations of body shape and weight and the natural variations in the range of pressure relief and alignment tolerances for different people (based on their individual physiology, health, sensitivity, flexibility, and other differences) leads to different types of layering working better or worse for different people.

All of this of course is "rough math" and only touches the surface of mattress construction and how different types of mattress materials and components interact both with each other and with different people. Research is also incomplete and ongoing and there is no "exact science" or formula that can define exactly what is best for different people. The "rough math" however can at least give a clear indication of why the sometimes conflicting needs of both pressure relief and alignment are possible for almost all people in the same mattress for those who are in the "normal and reasonably healthy" percentile of the population. For those with special medical or physiological needs or who are in some other way outside this "normal" range ... then there are many other solutions that are available as part of medical treatment or with the advice and help of a medical professional. An example of special needs (in this case pressure ulcers that can develop for those who spend long periods of time on a sleeping surface or who have unusual mobility or sensitivity issues) is in this very interesting article which also includes more technical information for those who want even more detailed information.

And now ... back to some of the simpler issues of "real life" outside of "rough math" :)

Phoenix
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