Wheelchair seating — Part 2: Determination of physical and mechanical characteristics of devices intended to manage tissue integrity — Seat cushions
ISO 16840-2:2007 specifies apparatus, test methods and disclosure requirements for wheelchair seat cushions intended to maintain tissue integrity and prevent tissue trauma. It does not include test methods or requirements for determining the fire resistance of cushions and addresses neither the interface pressure distributing characteristics of seat cushions nor the heat and water vapour dissipation characteristics of seat cushions. ISO 16840-2:2007 can also be applicable to tissue integrity management devices used as other support systems, as well as to cushions used in situations other than a wheelchair.
Sièges de fauteuils roulants — Partie 2: Détermination des caractéristiques physiques et mécaniques des dispositifs de répartition de pression — Coussins d'assise
Standards Content (Sample)
Wheelchair seating —
Determination of physical and mechanical
characteristics of devices intended to
manage tissue integrity — Seat cushions
Sièges de fauteuils roulants —
Partie 2: Détermination des caractéristiques physiques et mécaniques
des dispositifs de répartition de pression — Coussins d'assise
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1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and abbreviated terms . 2
5 Apparatus . 3
6 Test environment . 6
7 Preparation of test cushion . 7
7.1 Choice of cushion . 7
7.2 Preconditioning the cushion . 7
7.3 Setup . 8
8 Sequence of testing . 8
9 Load-deflection and hysteresis test . 8
9.1 Rationale . 8
9.2 Test method . 8
9.3 Method of calculation . 9
9.4 Test report . 10
10 Frictional properties . 10
10.1 Rationale . 10
10.2 Test method . 10
10.3 Test report . 10
11 Impact damping under normal loading conditions . 10
11.1 Rationale . 10
11.2 Test method . 10
11.3 Method of calculation . 12
12 Recovery . 12
12.1 Rationale . 12
12.2 Test method . 12
12.3 Test report . 13
13 Loaded contour depth and overload deflection . 13
13.1 Rationale . 13
13.2 Test method . 13
13.3 Method of calculation . 15
13.4 Test report . 15
14 Water spillage . 15
14.1 Rationale . 15
14.2 Test method . 15
14.3 Test report . 15
15 Biocompatibility . 15
15.1 Rationale . 15
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15.2 Test method . 15
15.3 Test report . 15
16 Test report . 16
17 Disclosure requirement . 16
Annex A (normative) Tapered uniform geometry RCLI . 17
Annex B (informative) Fire resistance considerations . 19
Annex C (informative) Horizontal and lateral stiffness . 20
Bibliography . 25
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ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
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adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 16840-2 was prepared by Technical Committee ISO/TC 173, Assistive products for persons with disability,
Subcommittee SC 1, Wheelchairs.
ISO 16840 consists of the following parts, under the general title Wheelchair seating:
— Part 1: Vocabulary, reference axis convention and measures for body segments, posture and postural
— Part 2: Determination of physical and mechanical characteristics of devices intended to manage tissue
integrity — Seat cushions
— Part 3: Determination of static, impact and repetitive load strengths for postural support devices
— Part 4: Seating systems for use in motor vehicles
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Wheelchair seating is a sub-speciality of rehabilitation services involving the selection and provision of
wheelchair seating products that provide improved body support and injury prevention to the wheelchair user.
Seating products are designed and manufactured to meet the needs of persons with varying types and degrees
of disability. Some products, such as wheelchair cushions, are designed to manage tissue integrity for persons
who are at risk or have pressure ulcers.
The tests described herein are intended to differentiate performance characteristics between cushions and are
not appropriate for ranking or scoring cushions or for directly matching these characteristics with the
requirements of individual users. The link to clinical efficacy, although implied, has not been validated. It is
intended that this part of ISO 16840 will evolve when the evidence of clinical relevance is confirmed. This part
of ISO 16840 specifically describes test methods that characterize the physical and mechanical properties of
seat cushions. Further parts of ISO 16840 are planned that describe test methods for disclosing the pressure
distributing characteristics of seat cushions and their heat and water vapour dissipation characteristics.
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INTERNATIONAL STANDARD ISO 16840-2:2007(E)
Wheelchair seating —
Determination of physical and mechanical characteristics of
devices intended to manage tissue integrity — Seat cushions
This part of ISO 16840 specifies apparatus, test methods and disclosure requirements for wheelchair seat
cushions intended to maintain tissue integrity and prevent tissue trauma. It does not include test methods or
requirements for determining the fire resistance of cushions. Annex B provides guidance on selecting cushions
with appropriate fire resistance characteristics. This part of ISO 16840 does not address the interface pressure
distributing characteristics of seat cushions nor the heat and water vapour dissipation characteristics of seat
cushions that will be addressed in further parts of ISO 16840.
This part of ISO 16840 can also be applicable to tissue integrity management devices used as other support
systems, as well as to cushions used in situations other than a wheelchair.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced document
(including any amendments) applies.
ISO 554:1976, Standard atmospheres for conditioning and/or testing — Specifications
ISO 1302:2002, Geometrical Product Specifications (GPS) — Indication of surface texture in technical product
ISO 7176-26, Wheelchairs — Part 26: Vocabulary
ISO 9073-8:1995, Textiles — Test methods for nonwovens — Part 8: Determination of liquid strike-through time
ISO 10993-1:2003, Biological evaluation of medical devices — Part 1: Evaluation and testing
ISO 10993-10:2002, Biological evaluation of medical devices — Part 10: Tests for irritation and delayed-type
ISO 16840-1:2006, Wheelchair seating — Part 1: Vocabulary, reference axis convention and measures for body
segments, posture and postural support surfaces
GUM:1993, Guide to expression of uncertainty in measurement, BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML
BS 3424-10:1987, Testing coated fabrics. Methods 12A and 12B. Determination of surface drag
ECE Regulation 16, Uniform provisions concerning the approval of safety belts and restraint systems for adult
occupants of power-driven vehicles, Revision 3, Amendment 3, 27 February 1996
FMVSS 209, Standard No. 209; Seat Belt Assemblies. Federal Motor Vehicle Safety Standards, 49CFR
part 571.209, 1 October 1992
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3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7176-26, ISO 16840-1 and the
cushion loading indenter
apparatus that is used to apply indentation forces to a seat cushion to determine its support characteristics
NOTE A cushion loading indenter can comprise loading components that are compliant or rigid.
impact damping rigid contoured loading indenter
apparatus that is used to rapidly apply loads to the cushion to determine its capacity to absorb impact energy by
measuring the rate of deceleration of the indenter as its weight is rapidly transferred to the cushion
loaded contour depth
maximum depth of contour resulting from load on the cushion’s surface at the site designed for buttock loading
loaded contour jig
means of loading cushions with an indenter representing the ischial tuberosities and trochanters used to
measure bottoming and ability of seat cushion to contour under load by representing buttock loading
additional deflection imparted by a 33 % overload condition
rigid cushion loading indenter
cushion loading indenter with a rigid exterior surface contour
measure of the propensity for a CLI to slide off a seat cushion
NOTE This quantity is affected by both the frictional properties of the CLI (3.1) and the cushion as well as the cushion
4 Symbols and abbreviated terms
l overall length
t time for the recording period (associated units: s = seconds; d = days)
h cushion thickness
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5.1 Loading rig, a means of applying a vertical load of up to 830 N to a seat cushion and with the ability to
measure displacement to ±1mm to the reference plane surface of the RCLI as specified in Figure 1 so that the
load remains normal to the reference plane throughout the test.
a) The load is applied at the point specified in Table A.1 on the mid-line of the RCLI reference plane surface in
the range 0N to 830 N as shown in Figure A.1.
NOTE The load accuracy required is specified in each test method.
b) The seat cushion is supported on a rigid horizontal surface such that the base of the cushion does not flex
5.2 Rigid cushion loading indenter (RCLI), a means of loading a cushion with a rigid exterior surface
a) be manufactured from a rigid material such as wood or fibreglass;
b) have the dimensions specified in Table A.1.
NOTE Detailed construction/assembly information is found in Annex A.
5.3 Impact damping rigid cushion loading indenter (IDRCLI), a means of loading a cushion using an RCLI
with a uniformly distributed mass of 500 N± 10 N with an accelerometer attached to the reference plane at the
location specified in 5.6 b) to measure the deceleration of the indenter as it suddenly loads the cushion.
0N 180 N±5N
5.4 Force application rig, a means of applying a load in the range of to to the loaded
5.5 Displacement gauge, a means of measuring the displacement of the top surface of the RCLI during
±1mm 0mm 200 mm
loading to an accuracy of in the range to .
5.6 Impact damping rig, a means of measuring the dissipation of impact loading to the seat cushion.
a) Such that a shell is formed representing the outer contour of a RCLI. Metal spheres are poured into the
bottom of the RCLI of uniform diameter then glued or melted to form a total mass of the IDRCLI of
500 N± 10 N.
NOTE Lead shot can be used for this purpose.
b) Capable of applying an impact load to the cushion using the IDRCLI as shown in Figure 2.
c) Capable of recording acceleration in at least one axis, oriented to measure normal to the surface of the
IDRCLI in the range 0ms to 10 ms with a frequency response in the range 0Hz to at least 200 Hz
incorporating an appropriate anti-aliasing filter, fixed to the top surface of the IDRCLI, on the centre line,
127 mm± 25 mm forward of the rear edge of the IDRCLI.
d)Including a rigid plate (plywood or equivalent) measuring
500 mm± 10 mm× 500 mm± 10 mm× 15 mm±1mm and hinged at one edge providing a means of
supportting the cushion and IDRCLI at an angle of 10 ± 1 .
e) Including two 25 mm±5mm diameter hard rubber cylindrical stops located with their centres at the
corners of the rigid plate, 25 mm from the front and lateral edges of the plate, with a hardness of Shore A
60± 0 supporting the edge of the plate opposite the hinge such that it is horizontal when resting on the
f) Including a block to support the rigid plate at an angle of 10 ± 1 to the horizontal which can be removed
in less than 0,5 s resulting in the plate falling to horizontal.
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Dimensions in millimetres
1 actuator to apply load
4 point of application of load
5 displacement gauge
6 reference plane
7 Velcro method of restraint under cushion
Figure 1 — Loading rig showing the reference plane on the top surface of the RCLI in plan view and a
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5 support block
Direction of fall.
Direction of pull.
Boards are parallel.
Figure 2 — Impact damping rig
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5.7 Loaded contour jig (LCJ), a means of supporting an RCLI at the end of a rigid shaft allowing the RCLI to
move in the lateral and forward directions on the seat cushion in one plane and with the following.
a) A mounting system to accept the RCLI, as defined in Annex A.
b) The capability of using a pivoting rigid member capable of swinging in an arc with a radius of 750 mm and
free to move vertically in a linear bearing as shown in Figure 3.
c) A restraint system on a rigid base with a means of constraining the cushion.
NOTE 1 A hook and loop fastening strip or a restraint bar along the edge of the cushion base are effective means of
constraining the cushion on the test base.
500 N± 10 N
d) The capability of applying vertical load to the RCLI.
e) The capability of applying a force perpendicular to the vertical member, acting in the plane of the cushion in
both the forward and lateral directions and generating an RCLI displacement of 10 mm±2mm at a rate of
2 mm/s± 1 mm/s.
f) Two 50 mm±2mm diameter indenters, centres spaced 120 mm±5mm apart, representing ischial
g) Two 25 mm±1mm diameter indenters, centres spaced 380 mm± 10 mm apart, representing the
h) A rigid bar 25 mm±1mm wide, 400 mm± 20 mm long with a thickness of 10 mm± 0,2 mm.
i) A 50 mm±2mm wide webbing as specified in ECE Reg. 16 and in FMVSS 209 attached to the bar at
395 mm± 10 mm centres using threaded mounting bolts to sandwich the belt between the 25 mm±1mm
diameter indenters and the bar. The webbing is secured to the bar so that it runs over the 50 mm indenters
and under the 25 mm indenters.
j) A force application rig.
k) A displacement gauge.
NOTE 2 Dimensions have a tolerance of ±5% unless specified otherwise.
l) The capability of applying a displacement perpendicular to the vertical member, acting in the plane of the
cushion at a rate of 5 mm/s.
m) The capability of recording (200 Hz minimum sampling rate) the force applied to the indenter.
n) Employing a diameter circular platen, attached to the displacement gauge mounted on the
loading rig with a rigid coupling.
o) Allowing vertical displacement of the circular platen.
p) Capable of applying 3N±1N vertical load to the cushion.
q) Positioned over the test cushion located 125 mm±2mm forward of the rear edge of the seat cushion and
55 mm±2mm lateral to the midline.
NOTE 3 It may be desirable to design this rig so that the circular platen can be placed at other points on the top surface
of the seat cushion.
6 Test environment
An environment with ambient temperature of 23 C± 2 C and relative humidity 50 %±5% which can be
determined as specified in ISO 554.
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1 50 mm wide mesh webbing
2 50 mm× 50 mm indenters
3 10 mm× 25 mm diameter trochanter fasteners
Figure 3 — Components comprising the loaded contour, seat cushion thickness measurement rig, a
means of supporting an RCLI at the end of a rigid shaft allowing the RCLI to move in the forward
direction on the seat cushion in one plane [see 5.7 a) to d) and l) to q)]
7 Preparation of test cushion
7.1 Choice of cushion
Obtain an unused sample seat cushion for testing. If a cover is provided, ensure that it is fitted to the cushion in
the orientation specified by the manufacturer.
7.2 Preconditioning the cushion
Perform the following:
a) condition the cushion, unloaded in the test environment for at least 12 h at ambient temperature
(23 C± 2 C) and 50 %±5% relative humidity;
830 N± 10 N
b) if indicated by the manufacturer, adjust the cushion to accommodate an load applied using
c) apply 830 N± 10 N using the RCLI for a minimum of 120 s to a maximum of 180 s;
d) unload and reload within 120 s;
e) remove load after 120 s and before 180 s;
f) allow cushion to recover for a minimum 5 min to maximum 60 min.
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Perform the following, prior to performing a test method on a cushion.
NOTE It is not necessary to set up the cushion between repetitions of any one test method on a cushion.
a) If indicated by the manufacturer, adjust cushion to accommodate a 500 N± 10 N load applied using the
b) If the cushion contains a material that remains displaced after loading, reset the cushion by flattening.
c) If the manufacturer specifies adjusting the cushion to the shape of the user, adjust cushion using the
intended indenter to accommodate the intended test load.
d) Allow the cushion to recover 5 min to 60 min.
8 Sequence of testing
Conduct the tests specified in Clauses 9 to 17 in any sequence.
9 Load-deflection and hysteresis test
The load-deflection test provides information about the resilience of seat cushions. As a person sits upon a
cushion, it deflects or displaces. Resilience describes how much the cushion tries to return to its undeformed
shape. In this situation, resilience is not necessarily a positive characteristic as the cushion pushes against the
tissues. In the case when a user leans to the side to perform a task, a resilient cushion will facilitate this person
in returning to an erect posture as the material seeks to recover from the leaning position. A less resilient
cushion might facilitate an oblique posture as the material will not equalise after the person's trunk returns
upright. In this case resilience is a positive feature.
The load deflection test also provides information about the hysteresis characteristics of a seat cushion.
Hysteresis is a measure of the energy lost to the cushion during a cycle of loading and unloading. Hysteresis is
related to impact damping (Clause 11). Cushions with larger hysteresis values will tend to absorb energy when
used on rough surfaces or when dropping down steps, rather than transfer the impact energy to the user’s
9.2 Test method
a) Precondition and adjust the cushion as specified in 7.2 and 7.3.
b) Bring the RCLI into contact with the test surface used to support the seat cushion; zero the height gauge or
otherwise compensate for the height of the indenter portion of the fixture.
c) Raise the RCLI so that the cushion can be placed on the base of the rig.
d) Place the RCLI in contact with the cushion so that the ischial tuberosities of the indenter are
125 mm± 25 mm forward of the back edge of the cushion or are aligned with the analogous part of the
e) Apply a starting vertical load of 8N±3N for 120 s± 10 s.
f) Record height of the cushion at the RCLI to cushion interface h .
g) Increase the load within a 10 s loading period so that the total load is 250 N±5N.
h) Wait 120 s± 10 s and record the height of the cushion at the RCLI to cushion h .
i) Increase the total load to 500 N± 10 N within a 10 s loading period.
j) Wait 120 s± 10 s and record the height of the cushion at the RCLI to cushion h .
k) Increase the load to 750 N± 15 N within a 10 s loading period.
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l) Wait 120 s± 10 s and record the height of the cushion at the RCLI to cushion h .
m) Remove the last applied loading increment within a 10 s unloading period, so that the total load on the
cushion is 500 N± 10 N.
n) Wait 120 s± 10 s and record the height of the cushion at the RCLI to cushion interface h .
o) Remove load within a 10 s unloading period so that the total load on the cushion is 250 N±5N.
p) Wait 120 s± 10 s and record the cushion height at the RCLI to cushion interface h .
q) Remove load within a 10 s unloading period so that the total load on the cushion is 8N±3N.
r) Wait 120 s± 10 s and record the cushion height at the RCLI to cushion interface h .
s) Allow 300 s± 10 s for cushion recovery.
t) Repeat steps e) to s) two more times to generate three total data sets consisting of both increasing and
decreasing loads on the cushion.
NOTE The load deflection and hysteresis test may better define the characteristics of cushions that respond rapidly to load
changes when performed in a continuous loading and unloading manner. For example, where load application and cushion
height measurements are performed simultaneously in an automated test apparatus. See reporting requirements for
deviations from described test methods.
9.3 Method of calculation
Determine the following:
a) The average compressive thicknesses from the three data sets:
— average compressive thickness at 8N h =
— average compressive thickness at 250 N h =
— average compressive thickness at 500 N h =
— average compressive thickness at 750 N h =
b) The average unloading thicknesses:
— average unloading thickness at 500 N h =
— average unloading thickness at 250 N h =