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Does Dynamic Seating Increase Shear?

Does Dynamic Seating Increase Shear?

Michelle L. Lange, OTR/L, ABDA, ATP/SMS

What is Shear?

Well, first of all we have Sheer and Shear. Sheer can mean several things including a sheer cliff (perpendicular), sheer curtains (semi-transparent), or sheer delight (used for emphasis). Shear, on the other hand, can mean to shear a sheep (cut the wool off) or, to be specific in our application, “a strain in the structure of a substance produced by pressure, when its layers are laterally shifted in relation to each other” (

In wheelchair seating, shear occurs when the client’s body tissues move in relation to the surfaces of the seating system. This can occur during transfers (i.e., the body tissues sliding across a cushion), movement within the seating system (i.e., the client extending within a static seating system), or movement of the seating system or wheelchair frame (i.e., during recline). Shear forces can lead to tissue deformation, a key pathway to pressure injuries. Specifically, shear is displacement of tissue with friction, leading to bunching of tissue reducing or cutting off blood flow in the epidural layer. Shear can also lead to a loss of client position.

There are two primary causes of shear forces:

  1. When a client reclines in their seating system, using a traditional low pivot reclining back, this leads to shear forces between the client’s body and the support surfaces. Over the years, recline design has improved significantly to reduce this risk. As a matter of fact, the sliding back was invented and patented by Seating Dynamics President, Greg Peek, in 1982 and greatly influenced future design. When a client ‘slides down’ within their seating system, similar shear forces can occur.
  2. Wheelchair users with increased tone and/or movement patterns may move out of alignment with the support surfaces. This also leads to shear forces. This population is not as likely to develop pressure injuries due to frequent movement. Use of recline is also not common in this population, rather tilt is a more typical intervention. However, these shear forces lead to a loss of position in relation to the support surfaces as well as a great deal of friction – this friction can even lead to skin rashes and abrasions.

Does Dynamic Seating Increase Shear?

Dynamic Seating allows coordinated movement within the wheelchair, typically at the hips, knees, and neck. The primary area of concern related to shear is at the hips. A Dynamic Back works by opening the seat to back angle (similar to a recline) and then facilitating a return to upright. We know that recline can lead to shear – is this a similar risk?

Remember the last time you went to the dentist? As the dental chair was reclined back, your body was pulled down and you probably had to reposition yourself to get your head back on the head pad. Recline leads to shear forces as the pivot point typically is not close to the body’s natural pivot point (the ischial tuberosities), leading to a loss of alignment with the seating surfaces. Some power recline systems raise this pivot point in combination with a back which slides down during recline to reduce loss of alignment.


The body loses alignment with the seating system during recline.


shear pivot point

The pivot point is at the top of the cushion portion of this molded seating system.

In Dynamic Back design, two features reduce shear forces. First is the placement of the pivot point. The pivot point must be raised, ideally to a level just above the cushion surface. Second is the degree of movement. Unlike recline, most Dynamic Backs do not move as far. The Seating Dynamics Rocker Back interface (DRBi) only opens the seat to back angle approximately 10-15 degrees. This amount of movement leads to a minimal amount of shear, allowing the client to remain in alignment with the contours of the back.

Dynamic Seating can reduce Shear Forces

By providing movement in response to force, alignment may actually be improved, and these shear forces decreased. Instead of a client extending within a static seating system, leading to a loss of alignment with the seating system itself, friction, and loss of position – the client moves with the seating system through Dynamic Seating, maintaining contact with the seating surfaces for improved alignment and postural support. Friction is also reduced. One of the clients I work with had a friction induced rash covering his back for years. After receiving a Dynamic Back, the rash cleared as a result of reduced friction.

shear diagrams

So Dynamic Seating does not increase shear, but rather can decrease these forces for many clients. Dynamic Seating can be an important part of a plan to promote alignment with the seating support surfaces.

Check out our Quick Class on Shear for more information.

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