Michelle L. Lange, OTR/L, ABDA, ATP/SMS
Dynamic Seating components typically include an option to change resistance levels. Resistance to force allows the dynamic seating component to return to its starting position. Generally, if resistance is too low, the client will not readily return to a starting position. For example, a client may extend, and a Dynamic Back will move in response. If resistance is too low, the client may remain in a more open seat to back angle for long periods of time. If the resistance is too high, the client may be unable to readily activate the component. For example, the client may extend and yet the Dynamic Back will not respond. The same is true for other dynamic components, such as Dynamic Footrests, and Dynamic Head Support Hardware.
Resistance and Breakage
Many clients use Dynamic Seating specifically to prevent breakage to the wheelchair and seating system. Did you know that if resistance is not set correctly for an individual, that the dynamic component will not function optimally, and breakage can still occur? How?
“Goldilocks figured it out, too hard or too soft didn’t work, it had to be just right.
If the elastomer is too hard it might as well be a solid piece of metal instead. In this case you have removed the ability of the device to absorb energy and the full load is transmitted into the equipment until it arrives at a link weak enough to not withstand the force … and things bend or break.
If the elastomer is too soft, it is much the same except that the client could get a running start with movement and then bottom out with a higher force impact.” – Greg Peek
If the resistance is set too high, forces which are normally dissipated by the dynamic component (elastomers, springs, or hydraulics) will be transferred through the dynamic component to the frame itself. These unrelieved forces may lead to damage to the wheelchair frame. So, with resistance set too high, breakage to the frame is possible, though unlikely. The resistance needs to be changed. In the case of the Seating Dynamics Rocker Back interface (DRBi), the elastomers need to be changed to a lower resistance (i.e. Extra Firm to Firm).
If the resistance is set too low, the dynamic component may be compressed to the point that the hardware on either side touches. For example, on the DRBi, if the elastomer is too soft for an individual, the sides of the Dynamic Back hardware may contact each other above and below the elastomer (see picture). At this point the forces can be transferred through the dynamic component directly to the frame, leading to damage. In a Seating Dynamics dynamic component, the elastomers need to be changed to a higher resistance (i.e. Soft to Medium). It may be that the current elastomer is worn to the point where the hardware touches – in this case the elastomer needs to be replaced, though the resistance may remain the same.
Dynamic Seating components are designed to reduce or prevent damage to the wheelchair frame, seating system, and other hardware. To do so effectively, the proper resistance must be used. In extreme cases, resistance which is too high or too low can result in forces being transferred through the dynamic component rather than being absorbed and diffused.