How beds work: what they do, why, and how? PART 2

How beds work: what they do, why, and how? PART 2

November 8, 2017

PART 2: The orange, the table, and the sponge


In the last installment, we looked at an illustration of pressure, and discussed that pressure is force divided by area of contact. So, to reduce pressure, there are two things we can do. We can reduce force, or we can increase area of contact.

Lying in bed, the force is nothing other than your body weight. Reducing this is, I am afraid, not within my area of expertise. Attempts have been made, but I find that starvation runs contrary to instinct. Fortunately, increasing area of contact is a much easier, and generally more agreeable proposition.

A brief illustration of how this works goes like this: imagine an orange, sitting on a hard table top.

You will see that the orange contacts the table top at a single point. Any contact area at all is only the result of the deformability of the orange, and will be very small indeed. Accordingly the pressure (force divided by area) will be very large indeed, at this point.

If we made a map of the pressure on the table, it would look like this:

A small dot of very high pressure.

If this was your body, you would probably experience this as pain, under a pressure point such as the bones in your hips or shoulders. Certainly, the pressure would be high enough to block local circulation to the skin and soft tissues over those bones, which would soon start to cry out for you to move.

Now imagine we put a sponge in between the orange and the table.

Now, the sponge deforms, to accommodate the shape of the orange. The Force is still the same, but the contact area is now bigger. So, the pressure is now smaller.

The pressure map now looks like this:

A larger area of less intense pressure. If this was your body, It would now be less uncomfortable over your pressure points, and would take longer before you had to move.

Now let’s put the orange on a slightly softer sponge.

This sponge deforms more under the weight of the orange, accommodating it further, and further increasing the contact area. This is even better!

Looking at the pressure map, we now have an even smaller, less intense pressure, spread over an even larger area.

By now, a pattern is clearly emerging: the softer the sponge, the more the orange sinks in. The more the orange sinks in, the bigger the contact area. And the bigger the contact area, the smaller the pressure. We’re tempted to think, at this point, that the solution is clear: the Holy Grail is softness, and the softer the better!

Sadly, our quest for the grail resumes, as this theory is dashed by the next test. This time, we put the orange on the softest sponge yet.

See what happens now. Yes, it’s true that the area of contact is now bigger than ever before. However, see also that the orange is now sinking completely through the very soft sponge, and is now effectively making contact with the table top again. This means we now really have two contact areas to think about. The large area of overall contact, which is now not providing enough reaction to take the whole weight of the orange (which is why the orange is sinking through), and the small area of point contact with the table, which is taking the remainder of the weight. This situation is known as ‘bottoming out’, and looks like this on the pressure map:


We knew that the table top was too hard for our orange. We figured out that a hard sponge might not be soft enough. Now, though, it turns out that a very soft sponge can actually be too soft to distribute pressure effectively on our orange. Similarly, a mattress can be too hard, or too soft for your body. How hard is too hard? How soft is too soft? That very much depends on your body.

In the next instalment, we look at the other way: hammocking.


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