ISO/DTS 16840-16

• Abstract:

  This document presents approaches to computer modelling and analysis of wheelchair seating systems, their occupants, and their interaction. This document provides a general introduction to biomechanical concepts, computer simulations, and terminology, and their validation, around the virtual evaluation of wheelchairs and wheelchair seating systems as a complement to standardized bench testing. This process of analysis facilitates understanding and sharing of information of how a wheelchair system and humans interact with each other, revealing potential internal effects on the tissues and interface reactions that cannot otherwise be observed or measured. This document presents: • General principles of computer simulations • Introductions to product simulation concepts, including inputs and validation • Introductions to human simulation concepts, including inputs and validation • Interaction of simulated wheelchair systems and human simulation in the virtual world. • How product designs can potentially be iterated to optimize the use conditions • How simulations of bench tests themselves are usable to optimize the methods, or to conduct simulated testing as part of the design verification and iteration process, prior to conducting bench testing • Connection of simulated results with human subject validation in a use environment This document is relevant to people with a disability, occupational therapists, physical therapists, biomedical engineers, medical and para medical personnel, and device manufacturers . NOTE: Data and analyses from standardized bench tests and simulations are not appropriate for ranking or scoring wheelchair seating elements nor for directly matching these characteristics with the requirements of individual users. While the results can aid the clinician in providing care to the patient through selection of physical characteristics that will, in their professional judgment, aid the care, treatment, or recovery of the patient, these pre-clinical analyses are not to be interpreted as prescriptive in and of themselves.

• Status: Not Published
• ICS: 01.040.11 - Health care technology (Vocabularies)
• ICS: 11.180.10 - Aids and adaptation for moving
• Technical Committee: ISO/TC 173/SC 1 - Wheelchairs
• Drafting Committee: ISO/TC 173/SC 1/WG 11 - Wheelchair seating
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 15-Sep-2026
• Completion Date: 15-Sep-2026

Overview

ISO/DTS 16840-16: Wheelchair seating - Part 16: Terminology and concepts related to computer simulations and their applicability to assessing wheelchair seating and systems is an international technical specification from ISO. This document introduces key terminology, concepts, and validated approaches for using computer modelling and simulation in the assessment of wheelchair seating systems and their interaction with human occupants. It serves as an essential resource for professionals involved in wheelchair design, clinical evaluation, and product development.

The standard emphasizes how virtual computer simulations can complement standardized bench testing, bridging the gap between mechanical test rigs and clinical use by simulating the biomechanical interactions between seating systems and users. Its guidance helps to improve understanding of how wheelchair seating affects user health, particularly in ways that are difficult or impossible to measure with physical tests.

Key Topics

• Computer Modelling and Simulation Basics

  • General principles of computer simulations for wheelchair products and occupants
  • Introduction to terminology, such as computational modelling, finite element analysis (FEA), and digital twins

• Product Simulation Concepts

  • Creating virtual models of wheelchair seating systems and cushions
  • Use of computer aided design (CAD) data and material properties
  • Validation against standardized physical indenter tests

• Human Simulation Concepts

  • Building representative models of wheelchair users, including body shape and tissue properties
  • Simulation of internal tissue stresses and interface pressures
  • Validation approaches, including comparisons against empirical and volunteer data

• Interaction of Product and Human Simulations

  • Assessment of interactions between virtual wheelchair systems and human models
  • Capability to experiment with various seating configurations, angles, and user characteristics

• Simulation as a Complement to Bench Testing

  • Use of virtual tests to augment or optimize physical bench tests
  • Pre-clinical evidence generation before human trials
  • Simulation for optimizing design iterations, safety, and performance

• Credibility and Risk in Modelling

  • Emphasis on credible model development using established verification and validation frameworks
  • Highlighting risk assessment procedures for simulation outcomes

Applications

ISO/DTS 16840-16 is highly applicable across multiple domains within wheelchair and seating system design and assessment, including:

• Product Development and Engineering

  • Enables rapid design iteration and virtual prototyping, reducing time and resource needs
  • Early identification of potential weaknesses or user risks in new designs

• Clinical and Biomedical Analysis

  • Provides clinicians and researchers with deeper insights into tissue effects, such as pressure and shear-related risk for pressure injuries
  • Aids in evaluating different seat cushions, posture supports, and system adjustments virtually before clinical trials

• Regulatory and Pre-Clinical Assessment

  • Generates complementary pre-clinical evidence necessary for regulatory submissions
  • Supports compliance and evidence requirements by aligning simulation validation with ISO test methods

• Education and Knowledge Sharing

  • Assists occupational therapists, physical therapists, engineers, and industry personnel in standardizing communication and understanding regarding seating system biomechanics

Note: Virtual evaluation and modelling are not prescribed as direct ranking or matching tools for individual user needs, but serve as valuable aids in professional clinical judgment and product development.

Related Standards

For comprehensive wheelchair seating system assessment, ISO/DTS 16840-16 references and complements several key ISO standards:

• ISO 16840-1: Wheelchair seating - Vocabulary, reference axis convention and measures for body segments, posture and postural support surfaces
• ISO 16840-2: Determination of physical and mechanical characteristics of seat cushions intended to manage tissue integrity
• ISO 7176-26: Wheelchairs - Vocabulary
• ISO 16840-14: Wheelchair seating - Application of computer simulation for cushion analysis

These related standards provide foundational terminology, standardized test methods, and technical references essential for effective use of simulation and physical testing in wheelchair seating evaluation.

FINAL DRAFT
Technical
Specification
ISO/TC 173/SC 1
Wheelchair seating —
Secretariat: SABS
Part 16:
Voting begins on:
2026-09-15
Terminology and concepts related
to computer simulations and
Voting terminates on:
2026-11-10
their applicability to assessing
wheelchair seating and systems
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
Technical
Specification
ISO/TC 173/SC 1
Wheelchair seating —
Secretariat: SABS
Part 16:
Voting begins on:
Terminology and concepts related
to computer simulations and
Voting terminates on:
their applicability to assessing
wheelchair seating and systems
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Use of simulations in assessing wheelchair seating systems . 2
4.1 General principles .2
4.2 Product simulation .3
4.2.1 Overview .3
4.2.2 Inputs .3
4.3 Human simulation .4
4.3.1 Overview .4
4.3.2 Inputs .4
4.4 Interacting simulated wheelchair systems and simulated humans. .5
4.4.1 General .5
4.4.2 Analysis of varying positions of the simulated seated human .5
4.4.3 Analysis of internal tissue effects on the simulated seated human .6
4.5 Complementary roles of ISO standardized bench tests and simulations .6
4.6 Pre-clinical evidence .6
4.7 Credibility and risk of models .7
Annex A (informative) Product simulation process ‒ procedure for the analysis of products
such as wheelchair cushions . 8
Annex B (informative) Human simulation process ‒ introduction to the development of human
body models . 14
Annex C (informative) Cushion simulation case study .18
Annex D (informative) Cushion and simulated human interaction case study .26
Bibliography .31

iii
Foreword
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 technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO’s adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 173, Assistive products, Subcommittee SC 01,
Wheelchairs.
A list of all parts in the ISO 16840 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
Wheelchairs and wheelchair seating are intended to provide the wheelchair user with mobility and
independence. Part of this independence includes maintaining health, which can be promoted through
appropriate postural support, and through protection of the skin and soft tissues, which are subject to
potentially deadly wounds, previously referred to as bed sores or decubitus ulcers, and now termed pressure
ulcers in Europe and pressure injuries in much of the rest of the world.
Prolonged sitting puts wheelchair users at particular risk of pressure ulcers/injuries, especially when seated
in abnormal postures. Additionally, other health complications can occur, including decreased respiration,
decreased digestion, swallowing difficulties, decreased communication, bladder issues, isolation, and
depression.
ISO 16840-2, ISO 16840-6, ISO 16840-11, ISO 16840-12 and ISO 16840-13 of the ISO 16840 bench tests series
(ISO 16840 tests) include methods that primarily assess support surfaces for their influences on tissue
integrity and stability characteristics. However, these bench tests are limited to testing in a single position,
typically with the support surface horizontally ‘flat’ with application of a normal/perpendicular load. While
this provides some useful insight for comparison of cushion performance, this assessment does not account
for the varying changes in angles and positions provided by the wheelchair and the user’s movements
and activities. For example, when using tilt and recline wheelchair functions and positions, when actively
ambulating, when reaching and repositioning, and when the wheelchair users merely shifts while ‘sitting in
place’.
Additionally, our contemporary understanding of the aetiology of pressure ulcers/injuries informs us that
both external and internal mechanical loading and risks contribute to the formation of injuries, through
deformation damage to the cells and tissues. External forces and stresses, including pressure, shear forces
and shear stresses, are applied to the individual through gravity, and the support surface interface. The
‘ISO 16840 tests’ can assess how different surfaces respond to the load, using the mechanisms described
in the 2019 NPIAP/EPUAP/PPPIAP clinical practice guidelines, such as immersion/envelopment and/or
redirection and offloading. However, these external loads translate within the body to the deeper tissues,
and risks are compounded by the additional internal stresses and strains that results within the seated
body from the skeletal structure pressing on the skin tissues, providing pressure and shear loads, further
amplified when the wheelchair user moves and the skeleton rotates and further presses.
ISO 16840 tests also have limitations in their similarity to the human body, although these same limitations
also have scientific and research benefits. Conducting testing with human subjects would result in great
variability between people, functions, movements, shapes, sizes, etc. Having standardized rigid test fixtures
allows for the use of simplified body representations, whether rigid thighs and buttocks made with simple
semi sphere and truncated cone geometrics, dual hemispheres to represent buttocks, or simple cylinders to
represent the ischial tuberosities and greater trochanters of the pelvis. However, this does provide a gap to
real-life.
Computer simulations can provide a “bridge” between mechanical bench testing and human subject clinical
trials. This simulation process, “in silico”, reduces the material and labour costs and time associated with
production and testing of numerous rounds of prototypes, thereby creating a more environmentally
sustainable approach to product design and development This document provides an insight into the
important role that computer simulation plays in informing seating system design and use.
This process of analysis facilitates understanding and sharing of information of how a wheelchair system
and humans interact with each other, revealing potential internal effects on the tissues and interface
reactions that cannot otherwise be observed or measured.
Although not offering direct measures, modelling with finite element analysis has been developed as a cost-
effective way to model in three dimensions the friction, shear, and pressure effects of the human body’s
interaction with a cushion (ISO/TS 16840-14:2023). These analyses can give additional information about
the external, interface interactions between the body and the cushion. More importantly, the effects on
internal tissues can be estimated. This is invaluable, as it is of course impossible to measure directly or
empirically these effects on deep tissues. Additionally, computer simulations have the potential to analyse
the effects of the wheelchair user on the cushion, and vice versa, in different angles, configurations, and use

v
scenarios. Examples to illustrate the potential from the simulation process can be found in Annexes C and D
of this document.
NOTE Data and analyses from standardized bench tests and simulations are not appropriate for ranking or scoring
wheelchair seating elements nor for directly matching these characteristics with the requirements of individual users.
While the results can aid the clinician in providing care to the patient through selection of physical characteristics
that will, in their professional judgment, aid the care, treatment, or recovery of the patient, these pre-clinical analyses
are not prescriptive in and of themselves.
Having this additional insight can provide more information to help determine which solution best suits
the individual, depending on their unique needs. It can also aid in product development, in optimizing
technologies and solutions for individual needs, and in developing new testing and analysis approaches.

vi
FINAL DRAFT Technical Specification ISO/DTS 16840-16:2026(en)
Wheelchair seating —
Part 16:
Terminology and concepts related to computer simulations
and their applicability to assessing wheelchair seating and
systems
1 Scope
This document covers how computer simulation can provide complementary information as to the
biomechanical interactions between a person and their seating system in order to understand how product
design and orientation affects human tissues.
This document provides a general introduction to biomechanical concepts, computer simulations, and
terminology, and their validation, around the virtual evaluation of wheelchairs and wheelchair seating
systems as a complement to standardized bench testing.
This document presents:
— general principles of computer simulations;
— introductions to product simulation concepts, including inputs and validation;
— introductions to human simulation concepts, including inputs and validation;
— interaction of simulated wheelchair systems and human simulation in the virtual world;
— how product designs can potentially be iterated to optimize the use conditions;
— how simulations of bench tests themselves are usable to optimize the methods, or to conduct simulated
testing as part of the design verification and iteration process, prior to conducting bench testing;
— connection of simulated results with human subject validation in a use environment.
This document is relevant to people with a disability, occupational therapists, physical therapists, biomedical
engineers, medical and paramedical personnel, and device manufacturers.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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 7176-26, Wheelchairs — Part 26: Vocabulary
ISO 16840-1, Wheelchair seating — Part 1: Vocabulary, reference axis convention and measures for body
segments, posture and postural support surfaces
ISO 16840-2, Wheelchair seating — Part 2: Determination of physical and mechanical characteristics of seat
cushions intended to manage tissue integrity

3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7176-26, ISO 16840-1 and
ISO 16840-2 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
computational modelling and simulation
CM&S
use of computers to simulate and study complex systems using mathematics, physics, and computer science
Note 1 to entry: A computational model contains numerous variables that characterize the system being studied as
[20]
defined within the National Institute of Biomedical Imaging and Bioengineering .
3.2
finite element analysis
FEA
process of simulating the behaviours of a physical structure to understand how it can behave under certain
physical conditions (e.g. load, displacement, temperature)
Note 1 to entry: To solve a problem, the body of the structure is divided into smaller, simpler parts called finite
elements.
3.3
finite element model
FEM
engineering calculation method used to assess the mechanical behaviour of a structure under applied loads
Note 1 to entry: The model is discretised, by mesh generation, into smaller parts called elements to assess the system
response to the applied loads.
3.4
in silico
conducted by means of computer modelling or computer simulation
3.5
in vitro
conducted in a test tube, petri dish, or elsewhere outside a living organism
3.6
rigid cushion loading indenter
RCLI
cushion loading indenter with a rigid exterior surface contour
Note 1 to entry: This is specified in ISO 16840-2.
4 Use of simulations in assessing wheelchair seating systems
4.1 General principles
Computational modelling and simulation have been used broadly in many industries to analyse structural
components and product designs. While simulations are never identical to, nor a replacement for, the real-
life conditions that wheelchair occupants experience, they do provide extremely informative insights
into product strengths, weaknesses, and the effects upon occupants that cannot easily (or in some cases,
ethically) be analysed in the real world.

4.2 Product simulation
4.2.1 Overview
Computer simulations of products are often created before the products themselves are built, as part of
the design process. Design engineers use sophisticated modelling software to develop concepts, and
analyse the use conditions, inputting the product’s dimensions, material properties, wall thickness, etc.,
to apply simulated use conditions, and analyse whether potential weaknesses or failures could occur. This
sophisticated tool and analysis process allows for rapid design iterations and confidence in safety and
performance. Resulting prototypes are then tested in real world conditions as part of the design verification
and validation process.
Likewise, existing products can be reproduced as simulations, making a ‘digital twin’ that can be analysed,
tested, and optimized in the virtual world (for examples see Annex A).
4.2.2 Inputs
4.2.2.1 Framework
Computer aided design (CAD) files, with geometric data and descriptions of the product, serve as the digital
framework for the finite element models and the resulting simulation.
EXAMPLE Wheelchair frame tube diameters, wall thickness, air cell cushion cell shapes, sizes, locations.
4.2.2.2 Material properties
Quantifiable and reliable properties are critical to building a realistic simulation, and are obtained through
existing standards/specifications, material handbooks, and material data sheets. Mechanical testing of
materials can also be directly conducted in the laboratory, and the measured properties and mechanical
behaviours can be compiled in databases, allowing for the selection and simulation of varying designs, and
effects of material choices.
EXAMPLE 1 Specific steel alloy specifications and associated hardness, compressive, and tensile strengths and
yield points or through experimental data obtained in bench testings.
EXAMPLE 2 Stress/strain curves and hysteresis data of urethane foams under applied load.
For a correct modelling approach the user should ensure the complete model behaviour is captured, e.g. for
soft materials the elastic Young’s modulus is not sufficient.
4.2.2.3 Validation of the model
Bench testing performance data provide validation of the simulations or calibration of the model. The
[1]
American Society of Mechanical Engineers have published an extensive guide for this process.
For these activities, a simulation of the performance test is applied, in the virtual world, to determine if the
virtual results correspond to the real-world results. In the case of the ISO 7176 and ISO 16840 series, this
involves creating virtual indenters and test rigs that will interact with the products in the virtual world in
the same way as the bench tests in the real world.
For cushions, applying the ISO 16840-2 RCLI indenter in laboratory conditions provides some level of
validation with a simplified geometric analogue of one type of body. Comparison of bench test results
with the indenter and a cushion, to the results from a computational analysis of a simulated indenter and
simulated cushion, gives an indication of the appropriateness of the model. Calibration is conducted as
required.
EXAMPLE Double drum testing of wheelchairs, comparison of simulated and experimental foam stress and strain
curves, pressure mapping of wheelchair cushions under the ISO standardized RCLI indenter.

NOTE Since human subjects are not geometrically homogeneous structures, there will of course be variation from
the bench test analogue, but performing such a validation increases confidence in the simulation data or can provide
calibration data.
4.3 Human simulation
4.3.1 Overview
Wheelchair seating systems have significant effect on the health, well-being, and independence of the
people who use them. The population that uses mobility solutions is typically an at-risk population, with
diverse health issues, concerns, life goals, and ambitions. Clinical trials are often not appropriate for these
individuals, or are extremely expensive or time consuming, with potential risk to the individuals. For this
reason, using a simulated human can be appropriate for understanding the effects that wheelchairs and
wheelchair seating can have on individuals.
Additionally, wheelchair users are at great risk of pressure injuries. These injuries occur as the result of
[2]
pressure or pressure and shear over time, among other factors. Although under investigation, at the time
of writing this document, there is not an ISO standardized test or sensing system that can be applied in
the lab or the clinic to measure interface shear forces, stresses, and strains effectively. Internal stresses
and strains resulting in cellular deformation damage of deep tissues can lead to some of the most severe
pressure injuries. Since there is no practical method to measure this directly, simulations provide an
invaluable insight into this internal tissue deformation.
Modelling can also be used to simulate a population. One such methodology is that of Virtual Patient
Population (VPP) where a small group of patients are evaluated. Using statistical techniques, the
characteristics and traits of these patients can be expanded into a large virtual group.
For examples see Annex B.
4.3.2 Inputs
4.3.2.1 Framework
Creating a simulation of a representative human is not as straightforward as reproducing a single product
design in the human world. Numerous decisions, assumptions, and limitations are made, regarding the
size, shape, gender, mass, etc. Once general overall parameters are chosen, there are open source body
‘frameworks’ that can be applied to start the simulation, much like a CAD model.
4.3.2.2 Material properties
As with product simulations, the simulated body also has ‘material properties’ that must be entered, and
these are complex. There are published values for properties of ‘typical’ fat, muscle, bone, and other tissues,
but again, decisions, assumptions, and limitations must be applied regarding muscle mass, fat, atrophy, and
other conditions of the body.
4.3.2.3 Validation against human subjects
The material properties in the literature are often derived by tissue sample tests in animals, e.g. pigs. These
materials are then included within the setup of human body models. For a high level of prognosis, it is
necessary to validate the human body model behaviour in comparison with test data from volunteers.
In general, two levels of validation can be differentiated. In the first level they are external measurements,
such as an interface pressure map. This enables a validation of the global human body behaviour. In the
second step, local behaviour is validated, like the deformation of the different tissue layers in the buttock
while an external load is applied. This kind of validation includes higher levels of measurements like MRI,
enabling a separation of different types of tissue.

The validation via test data from volunteers leads very often to a calibration of the material properties to
get a better correlation. For a higher level of validation the model behaviour should, after the calibration, be
compared with another data set of test results.
In the validation against test data of volunteers, a prerequisite is that the main anthropometric values such
as body height, seated height, mass, waist circumference, and the level of muscle tone should be similar.
Nevertheless, a perfect correlation between a human body model and any person does not exist.
4.4 Interacting simulated wheelchair systems and simulated humans.
4.4.1 General
The following excerpt from ISO/TS 16840-14:2023 describes the interactions between a surface and a user’s
body “When any surface comes into direct contact with a user’s body, then there will be a number of effects:
a) the skin surface is subject to complex pressure distributions;
b) the pressure distribution gradient at the interface surface, and friction, induce internal tissue
deformation;
c) tissue deformation from the skin to the bony prominences produces an overall tissue displacement with
respect to the unloaded tissue condition;
d) tissue deformation comprises internal axial strain and shear strain:
1) axial strain and shear strain induce modification of internal blood and lymphatic circulation;
2) blood vessel occlusion can lead to cellular death due to lack of oxygen and nutrients;
3) high levels of cell strain can lead to cellular death due to disruption of the cytoskeleton;
e) there is modification of thermal exchange;
f) there is modification of moisture exchange;
g) there is influence on the posture adopted by the body.”
4.4.2 Analysis of varying positions of the simulated seated human
The interactions and effects of varying positions of the simulated seated human can occur in a virtual
environment. When the simulation human is applied to the simulated cushion and wheelchair system,
numerous variables are defined, including the cushion adjustment (if applicable), position of the back, tilt
and or recline angles, elevation of the legs, and any other adjustments and interactions. Once the desired
positioning is made in the simulated system, gravity can be applied to the person as they sink into the cushion,
back support, and full chair. Initial calculations can be made in the areas of interest of interface pressure and
shear forces and stresses, as well as within the body, such as the region of interest underneath the ischial
tuberosities or other bony prominences. Once these conditions have been evaluated, the possibilities of
experimentation and simulation are numerous: changing the chair tilt and/or recline angles, repositioning
the back support angle or height, adding or subtracting lateral supports, raising the foot supports, changing
the cushion, modifying the simulated materials, and more. And these are just with one standardized human
assessment.
By holding the simulated human form constant, analyses can be performed by isolating and changing one
variable at a time. Not all varieties of human forms need to be assessed in every condition – the combinations
and time to evaluate would (currently) be time and cost prohibitive. However, much as with the cushion
bench tests in the ISO 16840 tests that have a standardized indenter, holding the human form constant can
reveal useful data as other variables are evaluated. Of course, if it is variability of the human form and the
effects that the seating system has, the human simulation itself can become the variable, and numerous
sizes, weights, shapes, heights, sex, etc. can be evaluated. Coupled with testing the wheelchair seating
variables, these combinations are seemingly endless. For the sake of time and efficiency, it shall be carefully

considered which questions need to be answered with the simulations, which variables should be held
constant, and which should be varied.
4.4.3 Analysis of internal tissue effects on the simulated seated human
It is important to note that simulations provide analyses that cannot be obtained from bench testing or
clinical trials, i.e. the ability to assess potential mechanical effects of seating that occur within the body.
Pressure injuries are the greatest secondary complication of wheelchair users, and the most deadly,
‘deep tissue’, injuries begin within the tissues, un
...

ISO/TR DTS 16840-16
:2026(E) ISO/TC 173/SC 01/WG 111
Secretariat: SABS
Date: 2026-08-31
Wheelchair Seating – seating —
Part 16:
Terminology and concepts related to computer simulations and their
applicability to assessing wheelchair seating and systems

©© ISISOO 20262026 –– AAllll rriighghtsts rreseserervveded
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
E-mail: [e-mail]
Website: www.iso.org
DTS stage
©© ISISOO 20262026 –– AAllll rriighghtsts rreseserervveded

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this
publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical,
including photocopying, or posting on the internet or an intranet, without prior written permission. Permission
can be requested from either ISO at the address below or ISO’s member body in the country of the requester.

ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Fax: [phone]
©© ISISOO 20262026–– AAllll rriighghtsts rreseserervveded 11

Published in Switzerland
ii
Contents
Foreword . iv
Introduction . v
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Use of simulations in assessing wheelchair seating systems . 9
4.1 General principles . 9
4.2 Product simulation . 9
4.3 Human simulation . 10
4.4 Interacting simulated wheelchair systems and simulated humans. . 11
4.5 Complementary roles of ISO standardized bench tests and simulations . 12
4.6 Pre-clinical evidence . 13
4.7 Credibility and risk of models . 13
Annex A (informative) Product simulation process ‒ procedure for the analysis of products
such as wheelchair cushions . 15
Annex B (informative) Human simulation process ‒ introduction to the development of human
body models . 29
Annex C (informative) Cushion simulation case study . 37
Annex D (informative) Cushion and simulated human interaction case study . 52
Bibliography . 3

iii
Foreword
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 technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawnISO draws attention to the possibility that some of the elementsimplementation of this
document may beinvolve the subjectuse of (a) patent(s). ISO takes no position concerning the evidence,
validity or applicability of any claimed patent rights in respect thereof. As of the date of publication of this
document, ISO had not received notice of (a) patent(s) which may be required to implement this document.
However, implementers are cautioned that this may not represent the latest information, which may be
obtained from the patent database available at www.iso.org/patents. ISO shall not be held responsible for
identifying any or all such patent rights. Details of any patent rights identified during the development of the
document will be in the Introduction and/or on the ISO list of patent declarations received (see ).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO'sISO’s adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 173, Assistive Productsproducts, Subcommittee
SC 01, Wheelchairs.
A list of all parts in the ISO 16840 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
Wheelchairs and wheelchair seating are intended to provide the wheelchair user with mobility and
independence. Part of this independence includes maintaining health, which can be promoted through
appropriate postural support, and through protection of the skin and soft tissues, which are subject to
potentially deadly wounds, previously referred to as bed sores or decubitus ulcers, and now termed pressure
ulcers in Europe and pressure injuries in much of the rest of the world.
Prolonged sitting puts wheelchair users at particular risk of pressure ulcers/injuries, especially when seated
in abnormal postures. Additionally, other health complications can occur, including decreased respiration,
decreased digestion, swallowing difficulties, decreased communication, bladder issues, isolation, and
depression.
Parts ISO 16840-2, ISO 16840-6, ISO 16840-11, ISO 16840-12, and ISO 16840-13 of the ISO 16840 bench tests
series (ISO 16840 tests) include methods that primarily assess support surfaces for their influences on tissue
integrity and stability characteristics. However, these bench tests are limited to testing in a single position,
typically with the support surface horizontally ‘flat’ with application of a normal/perpendicular load. While
this provides some useful insight for comparison of cushion performance, this assessment does not account
for the varying changes in angles and positions provided by the wheelchair and the user’s movements and
activities. For example, when using tilt and recline wheelchair functions and positions, when actively
ambulating, when reaching and repositioning, and when the wheelchair users merely shifts while ‘sitting in
place’.
Additionally, our contemporary understanding of the aetiology of pressure ulcers/injuries informs us that
both external and internal mechanical loading and risks contribute to the formation of injuries, through
deformation damage to the cells and tissues. External forces and stresses, including pressure, shear forces and
shear stresses, are applied to the individual through gravity, and the support surface interface. The ‘ISO 16840
tests’ can assess how different surfaces respond to the load, using the mechanisms described in the 2019
NPIAP/EPUAP/PPPIAP clinical practice guidelines, such as immersion/envelopment and/or redirection and
offloading. However, these external loads translate within the body to the deeper tissues, and risks are
compounded by the additional internal stresses and strains that results within the seated body from the
skeletal structure pressing on the skin tissues, providing pressure and shear loads, further amplified when the
wheelchair user moves and the skeleton rotates and further presses.
‘ISO 16840 tests’tests also have limitations in their similarity to the human body, although these same
limitations also have scientific and research benefits. Conducting testing with human subjects would result in
great variability between people, functions, movements, shapes, sizes, etc. Having standardized rigid test
fixtures allows for the use of simplified body representations, whether rigid thighs and buttocks made with
simple semi sphere and truncated cone geometrics, dual hemispheres to represent buttocks, or simple
cylinders to represent the ischial tuberosities and greater trochanters of the pelvis. However, this does provide
a gap to real-life.
Computer simulations can provide a “bridge” between mechanical bench testing and human subject clinical
trials. This simulation process, “in silico,””, reduces the material and labour costs and time associated with
production and testing of numerous rounds of prototypes, thereby creating a more environmentally
sustainable approach to product design and development This document provides an insight into the
important role that computer simulation plays in informing seating system design and use.
This process of analysis facilitates understanding and sharing of information of how a wheelchair system and
humans interact with each other, revealing potential internal effects on the tissues and interface reactions that
cannot otherwise be observed or measured.
Although not offering direct measures, modelling with finite element analysis has been developed as a cost-
effective way to model in three dimensions the friction, shear, and pressure effects of the human body’s
v
interaction with a cushion (ISO/TS 16840-14:2023). These analyses can give additional information about the
external, interface interactions between the body and the cushion. More importantly, the effects on internal
tissues can be estimated. This is invaluable, as it is of course impossible to measure directly or empirically
these effects on deep tissues. Additionally, computer simulations have the potential to analyse the effects of
the wheelchair user on the cushion, and vice versa, in different angles, configurations, and use scenarios.
Examples to illustrate the potential from the simulation process can be found in Annexes CAnnexes C and DD
of this document.
NOTE: Data and analyses from standardized bench tests and simulations are not appropriate for ranking or scoring
wheelchair seating elements nor for directly matching these characteristics with the requirements of individual users.
While the results can aid the clinician in providing care to the patient through selection of physical characteristics that
will, in their professional judgment, aid the care, treatment, or recovery of the patient, these pre-clinical analyses are not
prescriptive in and of themselves.
Having this additional insight can provide more information to help determine which solution best suits the
individual, depending on their unique needs. It can also aid in product development, in optimizing
technologies and solutions for individual needs, and in developing new testing and analysis approaches.
vi
Wheelchair Seating – seating —
Part 16:
Terminology and concepts related to computer simulations and their
applicability to assessing wheelchair seating and systems
1 Scope
This document covers how computer simulation can provide complementary information as to the biomechanical
interactions between a person and their seating system in order to understand how product design and orientation
affects human tissues.
This document provides a general introduction to biomechanical concepts, computer simulations, and terminology,
and their validation, around the virtual evaluation of wheelchairs and wheelchair seating systems as a complement
to standardized bench testing.
This document presents:
— generalGeneral principles of computer simulations;
— Introductionsintroductions to product simulation concepts, including inputs and validation;
— Introductionsintroductions to human simulation concepts, including inputs and validation;
— Interactioninteraction of simulated wheelchair systems and human simulation in the virtual world.;
— Howhow product designs can potentially be iterated to optimize the use conditions;
— Howhow simulations of bench tests themselves are usable to optimize the methods, or to conduct simulated
testing as part of the design verification and iteration process, prior to conducting bench testing;
— Connectionconnection of simulated results with human subject validation in a use environment.
This document is relevant to people with a disability, occupational therapists, physical therapists, biomedical
engineers, medical and paramedical personnel, and device manufacturers.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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 7176-26, Wheelchairs — Part 26: Vocabulary
ISO 16840-1, Wheelchair seating — Part 1: Vocabulary, reference axis convention and measures for body segments,
posture and postural support surfaces
ISO 16840-2, Wheelchair seating — Part 2: Determination of physical and mechanical characteristics of seat cushions
intended to manage tissue integrity
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7176-26 and, ISO 16840-1 and ISO 16840-
2 and the following apply.
ISO and IEC maintain terminologicalterminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
3.1
computational modelling and simulation
CM&S
use of computers to simulate and study complex systems using mathematics, physics, and computer science.
Note 1 to entry: A computational model contains numerous variables that characterize the system being studied as defined
[20]
within the National Institute of Biomedical Imaging and Bioengineering [20].
3.2
finite element analysis
FEA
process of simulating the behaviours of a physical structure to understand how it can behave under certain physical
conditions (e.g. load, displacement, temperature)
Note 1 to entry: To solve a problem, the body of the structure is divided into smaller, simpler parts called finite elements.
3.3
finite element model
FEM
engineering calculation method used to assess the mechanical behaviour of a structure under applied loads
Note 1 to entry: The model is discretised, by mesh generation, into smaller parts called elements to assess the system response
to the applied loads.
3.4
in silico
conducted by means of computer modelling or computer simulation
3.5
in vitro
conducted in a test tube, petri dish, or elsewhere outside a living organism
3.6
3.6.1.1 in vivo
conducted in a living organism

rigid cushion loading indenter
RCLI
cushion loading indenter with a rigid exterior surface contour
Note 1 to entry: This is specified in ISO 16840-2.
4 The useUse of simulations in assessing wheelchair seating systems
4.1 General principles
Computational modelling and simulation have been used broadly in many industries to analyse structural
components and product designs. While simulations are never identical to, nor a replacement for, the real-life
conditions that wheelchair occupants experience, they do provide extremely informative insights into product
strengths, weaknesses, and the effects upon occupants that cannot easily (or in some cases, ethically) be analysed in
the real world.
4.2 Product simulation
4.2.1 Overview
Computer simulations of products are often created before the products themselves are built, as part of the design
process. Design engineers use sophisticated modelling software to develop concepts, and analyse the use conditions,
inputting the product’s dimensions, material properties, wall thickness, etc,., to apply simulated use conditions, and
analyse whether potential weaknesses or failures could occur. This sophisticated tool and analysis process allows
for rapid design iterations and confidence in safety and performance. Resulting prototypes are then tested in real
world conditions as part of the design verification and validation process.
Likewise, existing products are able tocan be reproduced as simulations, making a ‘digital twin’ that can be analysed,
tested, and optimized in the virtual world (for examples see Annex AAnnex A).).
4.2.2 Inputs
4.2.21.1.1 Inputs
4.2.2.1 Framework
Computer aided design (CAD) files, with geometric data and descriptions of the product, serve as the digital
framework for the finite element models and the resulting simulation.
Examples:EXAMPLE Wheelchair frame tube diameters, wall thickness;, air cell cushion cell shapes, sizes, locations.
4.2.2.2 Material properties
Quantifiable and reliable properties are critical to building a realistic simulation, and are obtained through existing
standards/specifications, material handbooks, and material data sheets. Mechanical testing of materials are ablecan
also to be directly conducted in the laboratory, and the measured properties and mechanical behaviours are able
tocan be compiled in databases, allowing for the selection and simulation of varying designs, and effects of material
choices.
Example 1: specificEXAMPLE 1 Specific steel alloy specifications and associated hardness, compressive, and
tensile strengths and yield points or through experimental data obtained in bench testings.
Example EXAMPLE 2: stress Stress/strain curves and hysteresis data of urethane foams under applied
load.
Note: For a correct modelling approach the user should ensure the complete model behaviour is captured, e.g. for
soft materials the elastic Young’s modulus is not sufficient.
4.2.2.3 Validation of the model
Bench testing performance data provide validation of the simulations or calibration of the model. The American
[1]
Society of Mechanical Engineers have published an extensive guide for this process [1].
For these activities, a simulation of the performance test is applied, in the virtual world, to determine if the virtual
results correspond to the real-world results. In the case of the ISO 7176 and ISO 16840 series, this involves creating
virtual indenters and test rigs that will interact with the products in the virtual world in the same way as the bench
tests in the real world.
For cushions, applying the ISO 16840-2 RCLI indenter in laboratory conditions provides some level of validation
with a simplified geometric analogue of one type of body. Comparison of bench test results with the indenter and a
cushion, to the results from a computational analysis of a simulated indenter and simulated cushion, gives an
indication of the appropriateness of the model. Calibration is conducted as required.
Example: EXAMPLE Double drum testing of wheelchairs, comparison of simulated and experimental foam stress
and strain curves, pressure mapping of wheelchair cushions under the ISO standardized RCLI indenter.
Note: NOTE Since human subjects are not geometrically homogeneous structures, there will of course be variation from the
bench test analogue, but performing such a validation increases confidence in the simulation data or can provide calibration
data.
4.3 Human Simulationsimulation
4.3.1 Overview
Wheelchair seating systems have significant effect on the health, well-being, and independence of the people who
use them. The population that uses mobility solutions is typically an at-risk population, with diverse health issues,
concerns, life goals, and ambitions. Clinical trials are often not appropriate for these individuals, or are extremely
expensive or time consuming, with potential risk to the individuals. For this reason, using a simulated human can be
appropriate for understanding the effects that wheelchairs and wheelchair seating can have on individuals.
Additionally, wheelchair users are at great risk of pressure injuries. These injuries occur as the result of pressure or
[2]
pressure and shear over time, among other factors [2]. Although under investigation, at the time of writing this
document, there is not an ISO standardized test or sensing system that can be applied in the lab or the clinic to
measure interface shear forces, stresses, and strains effectively. Internal stresses and strains resulting in cellular
deformation damage of deep tissues can lead to some of the most severe pressure injuries. Since there is no practical
method to measure this directly, simulations provide an invaluable insight into this internal tissue deformation.
Modelling can also be used to simulate a population. One such methodology is that of Virtual Patient Population
(VPP) where a small group of patients are evaluated. Using statistical techniques, the characteristics and traits of
these patients can be expanded into a large virtual group.
For examples see Annex BAnnex B.

1.1.1 Inputs
4.3.2 Inputs
4.3.1.14.3.2.1 Framework
Creating a simulation of a representative human is not as straight forwardstraightforward as reproducing a single
product design in the human world. Numerous decisions, assumptions, and limitations are made, regarding the size,
shape, gender, mass, etc. Once general overall parameters are chosen, there are open source body ‘frameworks’ that
can be applied to start the simulation, much like a CAD model.
4.3.1.24.3.2.2 Material properties
As with product simulations, the simulated body also has ‘material properties’ that must be entered, and these are
complex. There are published values for properties of ‘typical’ fat, muscle, bone, and other tissues, but again,
decisions, assumptions, and limitations mustlmust be applied regarding muscle mass, fat, atrophy, and other
conditions of the body.
4.3.1.34.3.2.3 Validation against human subjects
The material properties in the literature are often derived by tissue sample tests in animals, e.g. pigs. These materials
are then included within the setup of human body models. For a high level of prognosis, it is necessary to validate
the human body model behaviour in comparison with test data from volunteers.
In general, two levels of validation can be differentiated. In the first level they are external measurements, such as
an interface pressure map. This enables a validation of the global human body behaviour. In the second step, local
behaviour is validated, like the deformation of the different tissue layers in the buttock while an external load is
applied. This kind of validation includes higher levels of measurements like MRI, enabling a separation of different
types of tissue.
The validation via test data from volunteers leads very often to a calibration of the material properties to get a better
correlation. For a higher level of validation the model behaviour should, after the calibration, be compared with
another data set of test results.
In the validation against test data of volunteers, a prerequisite is, that the main anthropometric values such as body
height, seated height, mass, waist circumference, and the level of muscle tone should be similar. Nevertheless, a
perfect correlation between a human body model and any person does not exist.
4.4 Interacting simulated wheelchair systems and simulated humans.
4.4.1 General
The following excerpt from ISO/TS 16840-14:2023 describes the interactions between a surface and a user’s body
“When any surface comes into direct contact with a user’s body, then there will be a number of effects:
a) the skin surface is subject to complex pressure distributions;
b) the pressure distribution gradient at the interface surface, and friction, induce internal tissue deformation;
c) tissue deformation from the skin to the bony prominences produces an overall tissue displacement with respect
to the unloaded tissue condition;
d) tissue deformation comprises internal axial strain and shear strain:
1) axial strain and shear strain induce modification of internal blood and lymphatic circulation;
2) blood vessel occlusion can lead to cellular death due to lack of oxygen and nutrients;
3) high levels of cell strain can lead to cellular death due to disruption of the cytoskeleton;
e) there is modification of thermal exchange;
f) there is modification of moisture exchange;
g) there is influence on the posture adopted by the body”.”
4.4.2 Analysis of varying positions of the simulated seated human
The interactions and effects of varying positions of the simulated seated human can occur in a virtual environment.
When the simulation human is applied to the simulated cushion and wheelchair system, numerous variables are
defined, including the cushion adjustment (if applicable), position of the back, tilt and or recline angles, elevation of
the legs, and any other adjustments and interactions. Once the desired positioning is made in the simulated system,
gravity can be applied to the person as they sink into the cushion, back support, and full chair. Initial calculations
can be made in the areas of interest of interface pressure and shear forces and stresses, as well as within the body,
such as the region of interest underneath the ischial tuberosities or other bony prominences. Once these conditions
have been evaluated, the possibilities of experimentation and simulation are numerous: changing the chair tilt
and/or recline angles, repositioning the back support angle or height, adding or subtracting lateral supports, raising
the foot supports, changing the cushion, modifying the simulated materials, and more. And these are just with one
standardized human assessment.
By holding the simulated human form constant, analyses can be performed by isolating and changing one variable
at a time. Not all varieties of human forms need to be assessed in every condition – the combinations and time to
evaluate would (currently) be time and cost prohibitive. However, much as with the cushion bench tests in the
ISO 16840 tests that have a standardized indenter, holding the human form constant can reveal useful data as other
variables are evaluated. Of course, if it is variability of the human form and the effects that the seating system has,
the human simulation itself can become the variable, and numerous sizes, weights, shapes, heights, genderssex, etc.
can be evaluated. Coupled with testing the wheelchair seating variables, these combinations are seemingly endless.
For the sake of time and efficiency, theit shall be carefully considered which questions that need to be answered
with the simulations, which variables should be held constant, and which should be varied, shall be considered.
carefully.
4.4.3 Analysis of internal tissue effects on the simulated seated human
It is important to note that simulations provide analyses that cannot be obtained from bench testing or clinical trials,
i.e. the ability to assess potential mechanical effects of seating that occur within the body. Pressure injuries are the
greatest secondary complication of wheelchair users, and the most deadly, ‘deep tissue’, injuries begin within the
tissues, under bony prominences, where cells and tissues undergo mechanical stress and strain that lead to cellular
and tissue death. Direct measurements cannot be taken of these internal conditions. However, estimates can be
made of the effects on internal tissues, when external loads and conditions are applied to the simulated person. By
having this objective computer analysis, the simulated person can then be ‘seated’ in multiple positions, on multiple
cushions or wheelchair configurations, to compare and contrast the different effects on the body, externally and
internally. Of course, the simulations are never identical to the real world, but they are able to provide more
[8]
information to the clinician to aid their decision making [8].
4.5 Complementary roles of ISO standardized bench tests and simulations
Bench testing based on existing ISO standards are able tocan provide validation data for simulations, whether
through cushion performance indicators or mechanical failures. Additionally, the use of simulation is able tocan
reduce the number of prototypes and real-world build simulations that are needed in the design and development
process. Simulations is ablecan also to evaluate potential effects on the human body, without user risk, and while
also drastically reducing the time to obtain information about how a body is able to interact with the wheelchair and
its seating, potentially under a variety of positions and conditions, with a variety of simulated body types.
Additional insights can be gained regarding the product performance from simulations with a simulated indenter.
While a physical indenter test can give insight into external results like the force, the simulation is able to reveal
additional product behaviour, characteristics, and operational state. This analysis allows for deeper understanding
of the product design and performance, and provides a baseline measurement against which future models are able
to becanbe measured, and improvements quantified.
4.6 Pre-clinical evidence
Both standardised bench tests and computer modelling represent ‘pre-clinical’ forms of evidence. These types of
data and analysis are able tocan quantify and characterize seating systems, providing a level of confidence in design
and in the potential match of a particular solution to the needs of a wheelchair user. While the bench testing perhaps
provides data that is furthest from the user’s experience, it still gives objective characterisation data. Modelling takes
the analysis one step closer to the human, by use of a ‘phantom’ that is closer to the human, with properties that are
more typical of a person. Of course, these conditions are still not precisely the same as the human, but the data are
useful in providing further understanding of the wheelchair system. In many cases, this pre-clinical evidence is
useful as the basis for progressing to clinical use trials, with human subjects.
Note: NOTE In the United States, medical device safety and efficacy can be demonstrated and approved through research
and analysis gained from four types of evidence: animal testing, bench testing, clinical trials, and digital evidence. Within the
regulatory science department of this government agency (the FDA), extensive work has been undertaken both to promote the
use of modelling for medical device approval, and to provide guidelines for ensuring models are appropriately developed,
verified, and validated.
4.7 Credibility and risk of models

Note:
COU = Context = context of Useuse
Figure 1. — Verification and Validation Workflowvalidation workflow
Computational modelling and simulation (CM&S) is seeing increased use in support of designing medical devices
(even Class III devices). Before the release in 2018 of the American Society of Medical Engineers verification and
[1]
validation (V&V) document, ASME V&V 40 [1] ,, there was a lack of specific guidance on how to assess/evaluate
the credibility of all the CM&S activity. ASME V&V 40, supported by the FDA and the EU, provides a framework that
allows its users to follow a consistent work flow (Figure 1[Figure 1]) to determine the appropriate level of credibility
for their CM&S activities and processes. This non-prescriptive approach grants individual organizations the
[10]
authority and responsibility to assign a certain level of risk (Figure 2[Figure 2] [10]) with a specific set of V&V
activities, and the obligation to justify these assignments to internal and external stakeholders (including the
regulatory agencies).
Figure 2 — Model risk
After the release of ASME V&V 40, its adoption has been swift and broad. Several implementations to actual medical
devices have been published as end-to-end examples to provide users with step-by-step guidance. One such study
was published for the evaluation of heart valves and how CM&S has been used to demonstrate credibly the safety,
[10]
performance, and durability of the said devices [10].
The ASME V&V 40 committee has continued work to identify gaps pertaining to patient specific applications and to
expand the framework to address any gaps. With such advanced developments in CM&S technology to Class III
devices, it is expected that assistive devices and solutions, such as wheelchairs (manual and power) and seating and
positioning systems, will expand the use of CM&S activities towards the design and evaluation of such assistive
devices.
4.8 Annex A
Annex A(informative)
Product simulation process -‒ procedure for the analysis of products such as
wheelchair cushions
Note: NOTE Images in Annex AAnnex A are courtesy ofreproduced with permission from Simuserv GmbH.
A.1 Simulation principle
One goal of introducing computer simulations for the analysis and evaluation of wheelchair cushions is to compute
similar results to existing ISO tests by using digital prototypes. This approach iscan often able to reduce costs of
testing and accelerate analysis of design iterations. It iscan also able to provide deeper insight than mechanical bench
testing alone.
Finite element analysis (FEA) is the appropriate approach to simulate RCLI tests and evaluate displacement and
forces (strains, stress) on the product (Figure A.1(Figure A.1).). Applying a human body model to the cushion allows
for computation of tissue loads due to the interaction of the simulated model with the wheelchair cushion, providing
an analysis that cannot be obtained from human trials.

Figure A.1 — Simulation of RCLI with the resulting pressure distribution
A.2 Analysis Processprocess
A.2.1 General
Figure A.2 to investigate the response for a structure regarding applied loads,
FEA follows the process in Figure A.2
such as forces, pressures, temperature.
Figure A.2 — FEA analysis process
A.2.2 Study Objectiveobjective
Before setting up and starting an FEA, the objective shall be defined. Based on the analysis type, the set-up of the
model and other conditions are specified. A prerequisite for a practical usage of FEA is to get realistic results within
an acceptable simulation time ‘as detailed as necessary and as simply as possible!’
The objective can be divided into the steps summarised in Figure A.3Figure A.3.
Study Objective
Method Model Abstraction
Analysis Requests
• Aim of the study • Numerical approach • Relevant model parts
• Mechanical • Analysis type • Model representation

Figure A.3 — Study objective steps
A.2.3 Pre-Processingprocessing
Pre-processing covers the geometrical modelling of the analysed structure (Figure A.4(Figure A.4).). The input data
are sketches or 3D representation (CAD) of the structure.
The properties of the structures, like material behaviour, are defined using mechanical quantities like Young’s
modulus (stiffness) or Poisson ratio (compressibility) for standard materials. For highly deformable materials like
rubber or foam, stress strain curves from tests such as ISO 3386 [16]-1 should be considered.
Pre-Processing
Polyurethane Foam
Material
Nominal Strain
Figure A.4 — Pre-Processingprocessing geometry and material requirements
Next, the model is discretised by finite elements (Figure A.5(Figure A.5).). On each node of the generated mesh, force
and displacements are computed. Accordingly, smaller elements lead to better result resolution, but are
accompanied by longer analysis times.
Finally, fixations, supports, or interactions are defined via boundary conditions on contact.

Figure A.5 — Pre-Processingprocessing modelling steps
A.2.4 Solver
The solver covers the following parts of the analysis (Figure A.6(Figure A.6):):
— Definedefine the load scenario for the analysis: applied forces, displacements, etc.;
— Definedefine the type of analysis: static or dynamic;
— Outputoutput request: mechanical quantities and resolution of output.

Analysis Type
Output
Figure A.6 — Simulation scenario definitions
There are two main types of solver: linear and nonlinear. The appropriate approach depends on the type of analysis
required.
Non-linearities occur due to changing structural response within an analysis, the main reasons being:
— Largelarge deformations;
— Materialmaterial behaviours;
— Contactcontact between objects.
All these effects are relevant for the analysis of wheelchair cushions. Therefore, select software based on their
capabilities to cover nonlinearities.
A.2.5 Simulation post-processing
The post-processing phase covers the evaluation of the analysis and the generation of results (Figure A.7(Figure
A.7).).
The first step is the ‘plausibility check’ of the model behaviour. This should be carried out by a visual check and by
simple results values like reaction forces (e.g. if an indenter is applied with 800 N, the reaction force must equal
800 N)).
If the results are not realistic, the following checks should be made:
— Double checkdouble checking input: material properties, dimensions, units;
— Double checkdouble checking load: boundary conditions;
— Double checkdouble checking numerical effects: increments, stability effects, calculation accuracy, etc.
If everything is correct, the user should double check their expectations. Are there relevant effects that have not
been considered? (i.e. knowledge increase)).
After the plausibility check, the analysis results are generated. There are two types of result values: Field output (e.g.
pressure distribution, image) or History output (X-Y plot).
Key
X time (s)
Y force (N)
Figure A.7 — Results from processing
A.3 Verification and Validationvalidation
A.3.1 General
Verification and validation are prerequisites for using the analysis.
A.3.2 Verification
Verification makes sure that the defined equations are solved correctly (Figure A.8(Figure A.8).).

4.8.1.1.1 Verification
Code - Solver
4.8.1.1.2 Calculation
• Calculation Accuracy
• General setup
• Nonlinearities
• Mesh quality, convergence
Figure A.8 — Verification approaches
Codes are normally verified by the manufacturers via standard examples with known results. The mesh size and
quality shall be checked to minimize its influence on the results. This is normally carried out via sensitivity studies
using different element sizes.
A.3.3 Validation
A.3.3.1 General
Validation makes sure that the right equations are solved (Figure A.9(Figure A.9).).
4.8.1.1.3 Validation
Material Definition
4.8.1.1.4 Model Definition
• Applied parameters
• Setup of the Model
• Material test
• Experimental Results
Figure A.9 — Validation approaches
Validation aims to confirm experimentally the calculated results and simulation approach, and to check the
predictability of the model. The model shall be validated to compare the results with observations.
A.3.3.2 Validation example
The validation process of FEA models starts with materials and ends with a completed component test
(Figure A.10(Figure A.10).). If the setup is complex, in-between validation steps should be applied.
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