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I always chuckle at my fellow boaters who fret at leaving their boats in the water all season long, preferring to keep them on a trailer or storage rack because they believe that it's better for the boat. In fact, the absolute best place for a boat's hull is simply floating in the water, where its weight is so evenly supported that there's virtually no force on the hull. By contrast, a trailer or storage rack creates serious pressure points, which, if not addressed correctly over time, can stress a boat's hull, deforming it at best, cracking it at worst.

In many ways, the physical consequences from pressure points on stored boats are very similar to those experienced by the human body. For example, when lying on an old, hard mattress, we feel every lump and bump – that is, pressure points stress our bodies. Yet, when we float in a swimming pool, there are no perceivable pressure points on our bodies, where we feel all but weightless.

Interestingly, modern pressure management “air” cushions for wheelchairs are based around this principle of “immersion” and “floating” – scientifically called the sub-field of Fluid Statics – and understanding how they work truly explains why air cushions prove so successful at preventing decubitus ulcers (pressure sores).

The True Innovators of Pressure Management
The scientific theory behind pressure management air cushions not only dates back to French mathematician and philosopher, Blaise Pascal, in the 17 century, but is actually rooted as far back as Archimedes, circa 250 BC. See, Archimedes and Pascal both contributed to the field of Fluid Statics that formulates the fact that a fluid – which can be either a liquid or a gas, including “air” – cannot stay in place when contacted by a surface (as with a boat hull immersing into the water), and that fluids can exert a buoyancy force on the contacting surface (which is why a boat of appropriate displacement floats).

Now, where this scientific talk comes into play with pressure management air cushions is that to minimize pressure, the critical areas of one's buttocks – such as the ischial tuberosity bones – should immerse into a cushion, under as little pressure as possible, suspended from contacting firm surfaces. The objective, then, is to “float” the buttocks without hitting the hard seat bottom, not unlike a boat hull floating in a lake, supported by the water, with remarkably little pressure on any one point.

How Fluid Statics Serve Air Cushions
Air is used as the ultimate “fluid” in today's pressure relief cushions – namely for two reasons. Firstly, when air is contained in a cushion made up of segmented cells, it demonstrates remarkable immersion qualities, allowing one to sink right into it; yet, the air can't totally disperse, as it's contained within the cushion's cells, creating “buoyancy” of the user, one might say. And, secondly, air can be easily added or subtracted via a pump to achieve the correct air pressure, providing the perfect balance of immersion with buoyancy force to prevent the user from bottoming out. In very simple terms, then, an air cushion with segmented cells allows one's buttocks to ideally sink in and float – under the principals of Fluid Statics, that is – dramatically reducing seated pressure.

In pressure management cushions, gel is also often used as a “fluid” to create immersion qualities, and while it works well, it isn't as “responsive” as air (gel moves slowly upon surface contact, whereas air moves all but immediately), so “air” remains the most optimal “fluid” for exceptional pressure management.

Surface Matters
Under the principle that maximum immersion into the air cushion is key, the outer surface of the air cushion is critical. If you've ever looked at a range of air cushions, you may have noticed that most are made up of many small, individual cells, resembling an “egg crate.” There's a very specific reason for this. Again, air, as a fluid, naturally moves, allowing immersion – which is what one wishes for a pressure management cushion. Yet, the air can't move too completely or one simply bottoms out. As a result, the air must be contained. If the air is placed in a single bladder, immersion of individual pressure points is limited – a hammock effect occurs on the cushion, where one ends up sitting on a “dish-shaped” surface, where it's limited to how much it can conform. However, with an egg-crate-shaped surface of individual air cells, pressure points can dramatically immerse where needed, while surrounding areas remain supported – the surface literally conforms to every bony protuberance, allowing them to individually immerse for optimal pressure relief, while the cushion naturally equalizes pressure throughout the cells.

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When Fluid Is Too Fluid
With the nature of fluid moving upon contact by a surface, “stability” naturally becomes an issue with air cushions. Much as when one steps on the side of a small boat, the weight displaces the water, and the boat tips, the same instability can occur on air cushions. On a traditional air cushion, if one leans to one side, or sits on the front edge of the cushion for a transfer, the air naturally shifts away from the user, creating instability.

To prevent instability, modern air cushions feature several design strategies. Some air cushions feature chambers, with locking valves, so that the air can't freely flow, making them more stable overall (but this technique does diminish some immersion qualities). Increasingly popular are cushions that have a surrounding foam base, with an air cell insert under the ischial tuberosity region of the user – this offers stable support surfaces, while still providing an optimal immersion “well” for the area of bony protuberances. (It is important to note that such foam-air cushions are intended for those with aligned posture, not those who have complex, asymmetrical seated positioning who may not properly align within the immersion well.)

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But, A Notch Above Others
Although properly-designed air cushions offer optimal immersion characteristics for pressure management, they don't discount the value of other cushion technologies like gel and foam. Many with disabilities aren't at high risk of pressure sores, and the stability of a gel or foam cushion may serve them very well. Also, air cushions are among the highest maintenance, needing routine, proper inflation, and risk punctures – so they're not the fail-safe solution that some wish. Therefore, while air cushions are optimal for pressure management, they're not an end-all solution.

Immersed In It
On the surface, pressure management air cushions seem simple – that is, a cushion filled with air. Yet, where the subject becomes fascinating is in the science to them, where they're actually based in Fluid Statics, resembling boating principals as much as clinical ones. However, beyond the science, what truly makes air cushions remarkable isn't so much how they work, but that they work, maintaining the health of many users.

Published 1/2010, Copyright 2010,