ISO/TS 16840-14:2023

Wheelchair seating - Part 14: Concepts related to managing external forces to maintain tissue integrity

This document describes common terms related to forces and their effects as experienced by human bodies and their support surfaces. It provides further information on concepts around how these forces affect the human body's response to postural support systems, and particularly highlights the impact of the interface between tissues and postural support devices (PSD) on the maintenance of tissue integrity. It provides a general introduction to biomechanical concepts, phenomena, and vocabulary. This is intended to facilitate effective understanding and sharing of information between a range of disciplines/stakeholders involved in providing equipment to manage tissue integrity. Representative stakeholders include people with a disability, occupational therapists, physical therapists, biomedical engineers, nurses, medical and para medical personnel, device manufacturers, and other professionals facilitating development, provision, and access to seating and mobility equipment. This document does not provide detailed information that is currently available in physiological text books or scientific literature.

Sièges de fauteuils roulants — Partie 14: Concepts liés à la gestion des forces externes pour maintenir l'intégrité des tissus

General Information

• Status: Published
• Publication Date: 16-May-2023
• 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 - Wheelchairs
• Current Stage: 6060 - International Standard published
• Start Date: 17-May-2023
• Due Date: 08-Jul-2024
• Completion Date: 17-May-2023

Overview

ISO/TS 16840-14:2023 - Wheelchair seating - Part 14: Concepts related to managing external forces to maintain tissue integrity - is a technical specification that defines common biomechanical terms and explains how external forces at the interface between a person and a postural support device (PSD) affect tissue integrity. It provides a clear vocabulary and conceptual guidance about pressure, shear, friction, strain, and microclimate as they relate to wheelchair seating and other support surfaces. This document is intended to improve interdisciplinary communication among clinicians, engineers, manufacturers and users without duplicating physiological detail found in textbooks or scientific literature.

Key Topics

• Core definitions and measurements
  • Perpendicular force, shear force, pressure, shear stress, axial strain, shear strain, shear modulus, friction (static and dynamic).
• Mechanics of tissue–surface interaction
  • How pressure distributions, pressure gradients and tangential forces induce internal tissue deformation and displacement.
  • Relationships between external loading, internal strain, blood/lymphatic flow alteration, and risk to tissue integrity.
• Phenomenological descriptions
  • Conceptual models of the contact surface between the human body and cushions, including the role of clothing, covers and continence products.
• Material and cover implications
  • Clinical considerations for selecting support surface covers and how cover materials influence microclimate, friction and shear at the skin interface.
• Measurement and modelling
  • Overview of pressure mapping, shear sensors, and pressure/shear modelling approaches for assessing support surfaces.
  • Annex on considerations around the use of shear sensors.
• Practical limits
  • Clarifies that the specification provides concepts and vocabulary rather than detailed physiological protocols or treatment guidelines.

Applications

ISO/TS 16840-14:2023 is useful for:

• Occupational therapists, physical therapists, nurses and clinicians assessing seating and pressure injury risk.
• Biomedical engineers and product designers developing cushions, covers and seating systems.
• Manufacturers specifying materials, testing protocols and user information for wheelchair seating.
• Researchers conducting pressure mapping, shear measurement or computational modelling of tissue loads.
• Procurement specialists and policy makers evaluating seating solutions for tissue integrity and pressure injury prevention. By standardizing terminology and concepts, the specification supports safer seating design, clearer clinical communication and consistent reporting of pressure/shear assessment results.

Related Standards

• ISO 16840-1 - Wheelchair seating: vocabulary, reference axis convention and measures for posture
• ISO 7176-26 - Wheelchairs: vocabulary
• Other parts of the ISO 16840 series cover additional measurement and testing methods for wheelchair seating

Keywords: ISO/TS 16840-14:2023, wheelchair seating, tissue integrity, pressure, shear, support surfaces, postural support device, pressure mapping, shear sensors, microclimate, pressure injury.

Standards Content (Sample)

TECHNICAL ISO/TS
SPECIFICATION 16840-14
First edition
2023-05
Wheelchair seating —
Part 14:
Concepts related to managing external
forces to maintain tissue integrity
Sièges de fauteuils roulants —
Partie 14: Concepts liés à la gestion des forces externes pour
maintenir l'intégrité des tissus
Reference number
© ISO 2023
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
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Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Effects on human tissues from interaction with a support surface .4
4.1 General principles . 4
4.2 Phenomenological description . 4
4.2.1 General . 4
4.2.2 Pressure and shear . 5
4.2.3 Friction . 6
4.2.4 Distribution of strains . 6
4.3 Clinical implications of support surface cover selection . 8
4.4 Measurement of pressure distribution and shear . 9
4.4.1 Pressure mapping . 9
4.4.2 Shear sensors. 9
4.4.3 Pressure and shear modelling . 9
Annex A (informative) Considerations around the use of shear sensors .10
Bibliography .12
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 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. 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 www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
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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 1, 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
The influence of pressure on the health of skin tissues has been acknowledged for some time: what
used to be called "bed sores" or "decubitus ulcers" became renamed "pressure ulcers" in Europe and
"pressure injuries" in much of the rest of the world. However, in recent years, greater awareness of
other extrinsic factors affecting the health of skin tissues has led to increased reference to the more
general term of tissue integrity. The current pressure injury definition from the 2019 International
[1]
Guideline Prevention and Treatment of Pressure Ulcers/Injuries highlights this new understanding.
Pressure injury is referred to in Reference [1] as “localized damage to the skin and/or underlying
tissue, as a result of pressure or pressure in combination with shear.” “The tissue damage occurs as the
result of intense and/or prolonged exposure to sustained deformations in compression (perpendicular
to the tissue surface), tension or shear (parallel to the tissue surface), or a combination of these loading
modes. The tolerance of soft tissue for sustained deformations differs by tissue type and may also be
affected by microclimate, perfusion, age, health status (either chronic or acute), comorbidities, and
conditions of the soft tissues”.
[1]
The current NPIAP/EPUAP/PPPIA Guidelines note the changing views on pressure injury staging.
Stages 1 and 2 are described as “partial-thickness tissue loss” and having the strongest connections to
superficial microclimate (temperature, humidity, altered pH due to incontinence), shear, and friction
[2]
effects. Stage 1 and 2 pressure injuries are also described as ‘outside in’ skin damage . Stages 3 and
4, Unstageable, and Suspected Deep Tissue Pressure Injury are categorized in the Guidelines as “full-
thickness skin and tissue loss”. Deep tissue injury is an ‘inside out’ skin damage, which usually originates
in deep soft tissues subjected to external pressure and shear forces and subsequent deformations
[2]
around the bony prominences . Suspected deep tissue injury is one of the most challenging pressure
injuries for accurate identification: it can present as an intact non-blanchable red, maroon, or purple
[3]
discolouration that can quickly evolve to reveal a full tissue loss . Current guidelines advise clinicians
not to think of the numbered stages as linear progression of the wound towards improvement/healing
or worsening, but rather use the stage descriptions to note the maximum depth of a wound at a single
[3]
point in time .
While the characteristics of an ideal microclimate (skin temperature and humidity) are still being
researched, it is recognized that there are strong connections between microclimate and friction, and
hence surface and internal tissue loads. This is relevant for all pressure injuries, not just the superficial
[1]
ones .
It is the materials closest to the skin, be they clothing, continence products, and/or the materials in the
cover of the support surface that the person is sitting or lying on that often have the most impact on
microclimate, friction, and shear effects on the surface of the skin.
Stage 3 and 4, Unstageable and Suspected Deep Tissue, Pressure Injuries affect the deeper layers of the
skin and around bony tissues and are currently thought to derive from the effects of external pressure,
external shear forces, and the resulting internal shear stresses and strains.
Both short-term high pressure and long-term moderate pressures can be harmful for soft tissues.
Internal muscular, adipose, and dermal tissue deformations are linked to a multitude of damaging
effects: partial or total occlusions of microvascular and lymphatic network, tissue ischaemia, direct cell
deformations with cytoskeleton distortions and breakdown, cellular DNA damage, tissue inflammation
and necrosis, pH changes in interstitial fluids, altered orientation of collagen fibres, and subepidermal
[2][4][5][6]
separation .
Combined effects of shear and pressure can be more damaging than effects of pressure and gravity
forces alone. These effects can be ameliorated or exacerbated by the materials and construction of
support surfaces.
The accompanying effects of shear strain alongside the pressures introduced by the effects of gravity
creating areas of pressure on the body, have a more damaging effect on the tissues themselves, than
pressure alone.
v
Frequently the terms discussed in this document are misused or confused in general usage. This
document has been created to aid in understanding the differences between the defined extrinsic
elements and their respective effects on human tissues.
vi
TECHNICAL SPECIFICATION ISO/TS 16840-14:2023(E)
Wheelchair seating —
Part 14:
Concepts related to managing external forces to maintain
tissue integrity
1 Scope
This document describes common terms related to forces and their effects as experienced by human
bodies and their support surfaces. It provides further information on concepts around how these forces
affect the human body's response to postural support systems, and particularly highlights the impact
of the interface between tissues and postural support devices (PSD) on the maintenance of tissue
integrity. It provides a general introduction to biomechanical concepts, phenomena, and vocabulary.
This is intended to facilitate effective understanding and sharing of information between a range of
disciplines/stakeholders involved in providing equipment to manage tissue integrity.
Representative stakeholders include people with a disability, occupational therapists, physical
therapists, biomedical engineers, nurses, medical and para medical personnel, device manufacturers,
and other professionals facilitating development, provision, and access to seating and mobility
equipment.
This document does not provide detailed information that is currently available in physiological text
books or scientific literature.
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
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7176-26 and ISO 16840-1 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 https:// www .electropedia .org/
3.1
perpendicular force
force occurring at 90° to an element’s surface
Note 1 to entry: It is measured in newtons (N).
3.2
shear force
F
s
force occurring parallel with an element’s surface
Note 1 to entry: It is measured in newtons (N).
3.3
pressure
p
force per unit area in a direction perpendicular to the surface
p = X / A
where
p is the pressure (MPa);
X is the perpendicular force (N);
A is the area (mm );
1 MPa = 1 000 kPa.
Note 1 to entry: It is measured in pascals (Pa) or equivalent units.
3.4
shear stress
τ
shear force divided by the area of the element’s surface to which the shear force is applied, parallel with
the slope or plane in which it lies
τ = F / A
s
where
τ is the shear stress (MPa);
F is the shear force (N);
s
A is the area (mm );
1 MPa = 1 000 kPa.
Note 1 to entry: It is measured in pascals (Pa) or equivalent units.
3.5
axial strain
normal strain
ε
change of dimension due to the action of pressure
Note 1 to entry: Axial strain is dimensionless.
EXAMPLE Compressive effects from pressure are illustrated in Figure 1.

Figure 1 — Axial strain effects from the compressive effect of pressure
3.6
shear strain
γ
change in the shape of an element due to the action of shear stress
Note 1 to entry: Shear strain is dimensionless.
Note 2 to entry: See Figure 2.
Figure 2 — Shear strain effects
3.7
shear modulus
G
ratio of shear stress to shear strain
Note 1 to entry: It is measured in pascals (Pa).
3.8
friction
force resisting the relative motion of two objects with surfaces in contact
3.8.1
static friction
friction force resisting the initial relative motion
3.8.2
dynamic friction
friction force resisting the motion while motion is in process
4 Effects on human tissues from interaction with a support surface
4.1 General principles
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 forces, 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
...