Explanation of Dynamic Stability
In response to questions about the Dynamic Stability Testing Portion of the new Power Wheelchair and POV codes, this is a layman's summary of the RESNA test procedure. It does not include all of the details specified in the test procedure. Please refer to the ANSI/RESNA publications listed in the Coding Guidelines for complete details. This test procedure is detailed in ANSI/RESNA WC/Vol. 2 - 1998, Section 2 - Determination of Dynamic Stability of Electric Wheelchairs.
Testing is generally done with a driver in the wheelchair or POV instead of a dummy. Weights are added to the driver, or to the driver's seating, to bring the total mass up to the maximum intended user mass in the chair. Weights may not be added to the chair base. Body movements are to be minimized since the testing is potentially hazardous to the human test driver and other test personnel. The use of an overhead harness, mats and spotters is recommended.
Testing is performed on a level surface as well as on slopes of 6 or 10 degrees as specified in the RESNA standard for which the product is being tested. CMS and the SADMERC recognize that the HCPCS code description states a 9-degree slope and the testing is done at 10 degrees. CMS and the SADMERC assume that if the chair is stable at 10 degrees, it will be stable at 9 degrees. No special rearrangement of the test parameters need be done. The product only needs to be tested on the maximum slope specified in the RESNA standard.
The area needed for testing should be approximately 2 by 10 meters. The sloped area needed to conduct the test must meet the coefficient of friction requirement specified in the ANSI/RESNA WC/Vol 1 - 1998, Section 13 - Determination of Coefficient of Friction of Test Surfaces. Generally, brushed concrete, wood or metal surfaces painted with anti-skid deck paint for the topcoat will meet the requirements.
Since Obstacle Climb data is also required, construction of step transitions on the test surface allow for the testing of transitions onto and off of 25, 50, or 75 mm steps as required by the code. For the RESNA standard, the steps are constructed on a flat and level surface and have a 6 mm radius along the top edge. (See publications for details of construction.) The intention of testing at these obstacle heights is to verify dynamic stability at the maximum step transition height that the chair is designed for. Testing need only be done at the maximum step transition height designed for the specific HCPCS code.
The RESNA standard requires that the seat height, if adjustable, be set to the maximum height allowed by the manufacturer for driving at maximum speed. All user-adjustable speed settings are to be set to their maximum settings. All other settings are to be set to the factory recommended settings. In addition, the anti-tip devices are to be set to their least effective position. These requirements mirror those of the current FDA requirements for 510K-product disclosure.
Please note that the ISO version of this test, 7176-2 Determination of Dynamic Stability of Electric Wheelchairs, specifies that the chair be set in its least stable configuration for rearward, forward and lateral testing. Specific guidelines are included in the ISO standard for drive wheel, caster wheels, seat position fore-aft and height, back position fore-aft, tilt and recline and leg rest angle. It is expected by CMS and the SADMERC that the new RESNA standards currently under revision, will include these set up requirements related to the adjustability of the wheelchair. It is strongly recommended that these guidelines be incorporated into the testing being done now.
CMS testing for dynamic stability allows testing in a specific standard set up or configuration as
specified by the manufacturer. After setting up the chair in its standard configuration, all user adjustable features on the chair shall be set for minimum stability to test the worse case scenario in each group of tests. The product may have lockouts or mechanical stops that prevent movement of the seating by the user beyond a certain range while driving.
It is anticipated by the SADMEC that the wheelchair or POV will be constructed such that adjustments do not or cannot result in an unsafe set up where unstable operation may occur. Safety set up procedures, lock-out mechanisms, factory-set automatic stops, or placarded warnings will be required to ensure that unsafe seating configurations are not used by beneficiaries during powered operation of the chair.
A series of driving tests are performed to test the forward, rearward and lateral stability of the wheelchair or POV. Based on the result of each test, the wheelchair is given a score of 0 to 4. To pass for a particular slope, the wheelchair or POV must receive a score of 2, 3 or 4 on all of the tests that apply to the chair. (One test does not apply to POVs-see below). If the wheelchair gets stuck on its anti-tip devices or completely tips over on any one test, it will receive a score of 1 or 0, respectively, for that particular test and does not pass the dynamic stability test for that slope.
Tests that require stopping with maximum deceleration must be performed by each of the following methods: letting go of the speed control input device, reversing the direction of the control input device, and turning off the power to the mobility device.
Rearward Stability Tests
The rearward stability tests include five tests. Stability must be maintained in all these test conditions:
- Starting with full acceleration uphill while on a slope.
- Stopping with maximum deceleration after driving forward uphill at maximum speed.
- Stopping with maximum deceleration after driving in reverse downhill at maximum speed.
- While positioned against a step transition, stability must be maintained while driving at maximum forward acceleration up the maximum step transition height the chair is designed for.
In ISO 7176-2 Determination of Dynamic Stability of Electric Wheelchairs there is an additional test for rearward dynamic stability, traveling backward down a step transition from a standing start. It is strongly recommended that you conduct this test now since it is expected that the new RESNA standards currently under revision will incorporate this test. Drive the wheelchair backwards at minimum speed down the maximum step transition height the chair is designed for.
Forward Stability Tests
The forward stability tests include four tests. Stability must be maintained in all these test conditions:
- Stopping with maximum deceleration after driving downhill at maximum speed.
- Driving at maximum speed forward downhill and then onto a level surface without slowing down.
- Driving the chair at maximum forward speed up the maximum step transition height the chair is designed for.
- Driving the chair at a minimum forward speed down the maximum step transition height the chair is designed for.
Lateral Stability Tests
The lateral stability tests include four tests for standard power base chairs and three tests for POVs. Stability must be maintained in all these test conditions:
- Beginning with the chair pointed downhill, the chair is driven at maximum speed in a minimum radius turn until it is pointed uphill.
- Determine the minimum centerline radius that the chair can turn a circle in, at maximum speed, without lifting a wheel.
- While driving full speed forward on a level surface, a maximum rate turn is commanded to the control input device. (This test does not apply to POVs or any mobility device that has a manual tiller device for steering).
- At a minimum speed on a level surface, run the wheelchair or POV in the forward direction such that the wheels on one side of the chair drive off the edge of the maximum step transition height the chair is designed for at a 10 degree angle from parallel to the step transition.
Published by Palmetto GBA as the SADMERC, May 27, 2005. Republished by Noridian as the PDAC, August 2008. Republished by Palmetto GBA as the PDAC, January 2019.