Let’s get it clear: Pressure, Shear, and Friction

Let’s get it clear: Pressure, Shear, and Friction

Whats the difference, how do they relate, and why should each be handled in their own right?

There has actually been an awareness for a very long time that pressures in the incorrect places can lead to harm to skin tissues, damage usually providing as pressure ulcers.

The influence of pressure on the health of our skin has actually been acknowledged for some time: as a result, what used to be called bed sores or decubitus ulcers ended up being relabelled pressure ulcers in Europe and then pressure injuries in much of the rest of the world. However, in the last few years, there has actually established a greater awareness of the value of other external factors, rather than pressure on its own, on the health of our skin tissues– and this has actually caused an increased reference to the more basic term of tissue stability in order that all the blame does not lie with pressure alone.

In this article we look at what we imply by pressure, friction, and shear, and evaluate the respective impacts each of these have on our skin tissues. Along the way, we hope to dispel a few of the widely-held mistaken beliefs which still exist.

With pressure, other harmful influences enter play, such as shear, microclimate, and friction.

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If you push versus the top of a drawing pin with a certain force, that force is spread out over a broad area and is acceptable, whereas if you apply the exact same force over the point of the drawing pin, your force is over a very little location, and you get an unpleasant hole in your finger! In seating and bed linen, we are attempting to spread out the forces of gravity over as wide an area as possible to reduce the pressure– a domain in which we utilize pressure mapping, to see where the forces are going.


The pressures that we experience on our skin originated from the force of gravity pulling our body mass towards the ground, and the equal and opposite reaction from the surface that we are standing, lying or sitting on. The quantity of pressure we experience depends on the area over which the force is spread.

Therefore, we know that pressure, friction, and shear are very important, but regularly these terms are misused, and for this reason, an ISO standard (ISO 16840-1) in the wheelchair seating series is being developed, to offer definitions for these terms, and discuss the implications of each element on skin health. This short article provides a few of the material from this draft requirement and tips of the different results that each element can have on the skin.

The other external factors that impact tissue integrity include microclimate (heat and moisture), shear forces, and friction. These elements do not have an impact without pressure bringing them into play, however it is these other external factors which presently are believed to be more influential in impacting the health of the outer layers of the skin, and providing rise Grade 1 and 2 pressure ulcers.

Figure 1 Axial Strain

Dynamic friction is generally a bad thing where kinetic energy is being transformed into thermal energy. Friction arises from a mix of inter-surface adhesion, surface roughness, and surface area deformation, so what a cushion or bedspread is made of will have a direct effect on the skin.

Friction is the force that opposes the relative lateral movement of 2 surfaces in contact, and from a seating perspective can be fixed (between non-moving surface areas) or vibrant friction. Static friction is what is stopping us from moving out of our seats, and is generally a good idea.

On the other hand, when the forces withstanding this lateral movement are conquered, and the surfaces start moving over each other, the result is dynamic friction, which can damage the surface of the skin, such damage is often described as friction burns (for instance when peoples feet are dragged across sheets as they are brought to the edge of a bed, to help them rise, or for turning purposes).

Pressure can compress various products, such as our skin tissues, or the materials we are resting on. For the quantity that something is extended or warped, the term stress is used.



Figure 2 Left: Pressure at the contact surface (put in by the cushion on the skin). Right: Shear Stress due to pressure and friction (at right angles to pressure) on the contact surface (exerted by the cushion on the skin).

For the compression that occurs as an outcome of pressure, this is called axial stress (Fig 1), in that the deformation occurs along the axis of the force.

Figure 3 Shear Strain

Shear is the sideways force that occurs at best angles to pressure (Fig. 2). Fixed friction, where skin and assistance surface areas meet, grabs the respective surfaces and misshapes the deeper materials sideways: this sort of distortion is called shear strain (Fig. 3).

Whats happening to, and in the skin?

Figure 4 Example of skin defects arising from contact surface area forces when resting on a cushion

Thus, the attendant impacts of shear strain, alongside the pressures presented by the impacts of gravity developing areas of pressure on the body, have a quicker and more damaging effect on the tissues themselves, than from pressure alone. Ideally, if the materials that a person is sitting or lying on can absorb or rearrange the axial and shear pressures, they ought to lower the stress that the skin has to accommodate.

In conclusion

Usage of a picked mix of cover products and deeper bed/cushion products to redirect the forces to thicker skin locations (e.g. away from the pelvis to the thighs) where the forces can be spread out over a bigger area will be advantageous.

Figure 4 illustrates the defects, or pressures, that the skin tissues experience from the pressure and shear forces illustrated in Figure 2. There is a mix of the compressive forces of pressure creating axial pressure, and the sideways results of shear stress creating shear pressure.

Further products can be found at www.beshealthcare.net. Please contact barend@beshealthcare.net if you are interested in receiving additional information on the topic.

The axial stress from pressure on skin tissues can slow down the rate of circulation in capillary and blood vessels, and over days this can lead to cell death. Shear strain distorts the cells and cell walls, and lead to cell content leak, and imbalance of the cell contents, which in turn can cause cell death even more rapidly, i.e. in less than a day.

Knowing the possibly destructive impacts that pressure, friction, and shear each in their own and in associated ways can have on the skin, and the impacts that various assistance surface areas can have in ameliorating these effects, ought to lead the prescriber to more appropriate choice of seating and bed materials to meet a persons needs.

Dr Barend ter Haar has been associated with seating and movement for over 30 years, consisting of lecturing globally and developing worldwide seating standards.

Click to learn more from the Lets get it clear series from Dr Barend ter Haar

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