kSIST-TP FprCEN/TR 16148:2026

• Abstract:

  This document is designed to provide information to convenors that will help in choosing test limits in relation to a particular injury type and severity. It is worth noting that accident investigators use a scale known as the Abbreviated Injury Scale, AIS (AAAM). This was developed (in the USA) so that injury severity could be recorded in databases regardless of the body region and type of injury thus avoiding lengthy medical terms that were unfamiliar and difficult to interpret. ; This document begins by reviewing the AIS scale and its application to head and burn injuries. Thereafter, each measurement type is reviewed and the severity of injury for given values is identified where possible. A section on burn injuries and fatigue related to heat exposure has been included to assist with Standards for equipment to protect firefighters. The Appendix describes the skin structure and the category and consequence of burn injuries.

• Status: Not Published
• Public Enquiry End Date: 15-Dec-2026
• ICS: 01.120 - Standardization. General rules
• ICS: 13.340.20 - Head protective equipment
• Technical Committee: OVP - Personal protecting equipment
• Current Stage: 5520 - Unique Acceptance Procedure (UAP) (Adopted Project)
• Start Date: 28-Oct-2026
• Due Date: 17-Mar-2027
• Ref Project: FprCEN/TR 16148 - Head impact and burn injury criteria - A Guide for CEN helmet standards committees

• Revises: SIST-TP CEN/TR 16148:2011 - Head and neck impact, burn and noise injury criteria - A Guide for CEN helmet standards committees
• Effective Date: 27-Aug-2025

kSIST-TP FprCEN/TR 16148:2026 is the Slovenian publication of CEN FprCEN/TR 16148, issued by SIST. It is a technical guide on head impact and burn injury criteria for helmet testing, written for CEN helmet standards committees and working groups. The document helps them choose test methods and set injury thresholds for different injury types and severity levels.

What does kSIST-TP FprCEN/TR 16148:2026 specify?

kSIST-TP FprCEN/TR 16148:2026 is guidance for helmet standard writers, not a requirements standard. It organizes the technical background needed to relate helmet test results to head injury risk and burn injury risk.

The document is arranged in clauses 4 to 10 plus Annex A. Clause 4 covers injury prevention and testing, clauses 5 to 7 cover linear, rotational and combined injury metrics, clause 8 covers finite element based metrics, clause 9 covers skull crushing and penetration force, and clause 10 covers heat, burns and fatigue.

Annex	What it covers
Annex A	Biomechanics of head injury from impact and heat, including anatomy, injury mechanisms and burn background

What are the key requirements of kSIST-TP FprCEN/TR 16148:2026?

kSIST-TP FprCEN/TR 16148:2026 does not set product requirements, but it gives the technical basis for deciding what a helmet standard should measure and how to interpret the results. The main guidance points are:

• Clause 4 says helmet certification should measure protective performance in a specific load case that represents real accident scenarios, using a test method that is robust and repeatable. In practice, committees need to choose a test that reflects the hazard they want to control, not just any convenient impact.
• Clause 4.2 links each risk type to a suitable loading mode and metric. Blunt impact, crush and penetration risks use different test set-ups and different measurement outputs, so the headform and instrumentation must match the intended injury risk.
• Clause 4.3 explains that injury risk is based on risk functions rather than a hard safe or unsafe line. This matters because human tolerance varies across the population, so a threshold should be chosen as a policy decision tied to the accident scenarios and injury severity the standard is trying to limit.
• Clause 4.4 uses the Abbreviated Injury Scale (AIS) to express injury severity for head and burn injuries. That gives committees a common severity language when they compare candidate thresholds or explain the level of injury a helmet is intended to prevent.
• Clause 5 focuses on linear shock absorption and the metrics PLA, HIC, force transmission and SFC. The practical point is that some metrics respond to peak load, while HIC also reflects duration, so a standard can better match the injury mechanism by choosing the right measure.
• Clause 6 covers rotational shock absorption and the metrics PRA, PRV, BrIC, DAMAGE and tangential force at the helmet surface. This matters because rotational loading is linked to diffuse brain injury and concussion, so helmet standards often need metrics beyond linear acceleration alone.
• Clause 7 presents combined injury metrics, especially HARM and Combined Probability (CP). These combine linear and rotational responses, which is useful when one metric alone does not describe the concussion risk well enough.
• Clause 8 explains finite element based metrics and the coupled experimental-numerical method. Measured six-degree-of-freedom headform motion is applied to a rigid skull model to estimate brain response, and the document discusses the KTH, SUFEHM and GHBMC models. In practice, this gives committees a way to assess tissue-level injury risk rather than only headform kinematics.
• Clauses 9 and 10 add guidance for skull crushing and penetration force, plus heat, burns and fatigue. That broadens the document beyond blunt impact and helps in helmet standards for environments such as firefighting, where thermal exposure and wearer fatigue also affect protective performance.

What terms does kSIST-TP FprCEN/TR 16148:2026 define?

• Abbreviated Injury Scale (AIS) - A 0 to 6 severity scale, where 0 means uninjured and 6 means unsurvivable, used in the document to describe head and burn injury severity.
• Peak Linear Acceleration (PLA) - The highest linear acceleration measured during an impact test, used as a simple indicator of skull fracture and related injury risk.
• Head Injury Criterion (HIC) - A duration-sensitive measure based on resultant linear head acceleration, used to capture how both load level and pulse length affect injury risk.
• Peak Rotational Acceleration (PRA) - The highest rotational acceleration of the headform, used to assess injury risk from angular loading.
• Peak Rotational Velocity (PRV) - The highest rotational speed reached during impact, used with other rotational measures to estimate concussion risk.
• Brain Injury Criterion (BrIC) - A rotational injury criterion based on the peak rotational velocity components about the three head axes.
• Finite Element (FE) - A numerical modelling approach used to estimate brain tissue loading and deformation from measured impact kinematics.
• Head Acceleration Response Metric (HARM) - A combined metric that brings together HIC and DAMAGE to estimate concussion risk from both linear and rotational loading.

Who uses kSIST-TP FprCEN/TR 16148:2026?

kSIST-TP FprCEN/TR 16148:2026 is used by CEN helmet standards committees, especially CEN/TC 158 working groups, when they develop or revise helmet standards. It is also relevant to test laboratories, certification bodies, rating programs and technical writers who need to choose injury criteria, select headforms and set pass-fail thresholds.

The document is useful across helmet applications discussed in the text, including bicycle, motorcycle, equestrian, ice hockey, construction and firefighting equipment. In practice, these users apply it when they design test programs, interpret injury metrics and justify the severity level that a helmet standard should target.

What changed in kSIST-TP FprCEN/TR 16148:2026 from the previous edition?

kSIST-TP FprCEN/TR 16148:2026 supersedes CEN/TR 16148:2011.

What does the kSIST-TP FprCEN/TR 16148:2026 document contain?

The document contains background material, tables, figures, formulas and model descriptions used to support helmet standard writing. It includes a table that links risk types to injuries, protective properties, test methods and metrics, as well as AIS tables for head injuries and burn injuries.

It also contains burn exposure tables for surface contact, water and radiant heat, plus figures on head anatomy, head injury types, skin structure and injury mechanisms. Clauses 5 to 8 present the technical basis for linear, rotational, combined and FE-based injury assessment, including the HIC, SFC, BrIC, HARM and CP formulas.

Annex A adds detailed biomechanical background on impact injury and heat exposure, including head anatomy, skull fracture, haematoma, concussion, diffuse axonal injury and burn-related fatigue. For readers, that means the report is not a test method itself, but a reference for designing tests and choosing injury criteria for helmet standards.

SLOVENSKI STANDARD
01-december-2026
Merila za določanje poškodb glave, ki nastanejo zaradi udarcev in opeklin - Vodilo
za odbore CEN, ki pripravljajo standarde za čelade
Head impact and burn injury criteria - A Guide for CEN helmet standards committees
Kriterien für Verletzungen durch Einwirkung auf Kopf und Verbrennungen - Leitfaden für
Arbeitsgruppen, die europäische Helmnormen erarbeiten
Critères relatifs au traumatisme crâniens et aux brûlures - Guide destiné aux comités
des normes sur les casques de protection du CEN
Ta slovenski standard je istoveten z: FprCEN/TR 16148
ICS:
01.120 Standardizacija. Splošna Standardization. General
pravila rules
13.340.20 Varovalna oprema za glavo Head protective equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

FINAL DRAFT
TECHNICAL REPORT
RAPPORT TECHNIQUE
TECHNISCHER REPORT
October 2026
ICS 13.340.20 Will supersede CEN/TR 16148:2011
English Version
Head impact and burn injury criteria - A Guide for CEN
helmet standards committees
Critères relatifs au traumatisme crâniens et aux Kriterien für Verletzungen durch Einwirkung auf Kopf
brûlures - Guide destiné aux comités des normes sur und Verbrennungen - Leitfaden für Arbeitsgruppen, die
les casques de protection du CEN europäische Helmnormen erarbeiten

This draft Technical Report is submitted to CEN members for Vote. It has been drawn up by the Technical Committee CEN/TC
158.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
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 supporting documentation.

Warning : This document is not a Technical Report. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a Technical Report.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. FprCEN/TR 16148:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
Introduction . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Injury prevention and testing . 6
4.1 General . 6
4.2 Injury metrics . 7
4.3 Injury risk assessment . 8
4.4 Abbreviated Injury Scale, AIS . 9
5 Linear shock absorption . 10
5.1 General . 10
5.2 Peak Linear Acceleration (PLA) . 11
5.3 Head Injury Criterion (HIC) . 11
5.4 Force transmission . 11
5.5 Skull Fracture Correlate (SFC) . 12
6 Rotational shock absorption . 12
6.1 General . 12
6.2 Peak Rotational Acceleration (PRA) . 12
6.3 Peak Rotational Velocity (PRV) . 12
6.4 Brain Injury Criterion (BrIC) . 13
6.5 DAMAGE . 13
6.6 Tangential force at the helmet surface . 13
7 Combined injury metrics . 14
7.1 General . 14
7.2 HARM . 14
7.3 Combined Probability (CP) . 14
8 Finite Element (FE) based metrics . 14
8.1 Introduction . 14
8.2 Methodology and hypothesis . 15
8.3 KTH head model . 16
8.4 Strasbourg University FE Head Model (SUFEHM) . 16
8.5 GHBMC head model . 17
9 Skull crushing and penetration force . 18
10 Heat: burns and fatigue . 18
10.1 Burns . 18
10.2 Heat fatigue . 19
Annex A (informative) Biomechanics of head injury from impact and heat . 21
A.1 General . 21
A.2 Head injuries . 22
A.2.1 Head anatomy . 22
A.2.2 Skull fracture . 24
A.2.3 Cerebral contusion . 25
A.2.4 Intracranial haematoma . 25
A.2.5 Cerebral concussion . 28
A.2.6 Diffuse axonal injury . 29
A.3 Head injury mechanisms . 30
A.3.1 The dynamics of impact . 30
A.3.2 Skull fractures . 31
A.3.3 Intracranial injury . 33
A.4 Heat: burns and fatigue . 36
A.4.1 Introduction . 36
A.4.2 Anatomy of the skin . 36
A.4.3 Burns and the consequences . 38
A.4.4 Burn severity and heat exposure . 39
A.4.5 Fatigue and heat exposure . 40
A.5 Conclusions . 41
Bibliography . 42
European foreword
This document (FprCEN/TR 16148:2026) has been prepared by Technical Committee CEN/TC 158
'Head protection'
This document is currently submitted to the Vote on TR.
This document will supersede CEN/TR 16148:2011
In comparison with the previous edition, the following technical modificâtions have been made:
— added Clause 1 Scope and Clause 3 Terms and dèfinitions;
— added Table 1 with test methods and metrics correlated to different injury risks;
— updated Table 2 AIS scale with head injury severity;
— differentiating between linear and rotational shock absorption;
— updated metrics for linear shock absorption;
— added metrics for rotational shock absorption;
— added 5.4 Force transmission;
— added Clause 8 on Finite Element (FE) based metrics;
— removed Neck injury as it is not within the scope of head protection;
— removed Noise injury as it is not within the scope of head protection.
Introduction
Members of helmet Standards committees frequently need to choose a test procedure and dèfinè limits
for the test procedure. Such limits should represent test values that indicate the potential for injury
corresponding to the accepted risk level. Over the years, injury criteria, or risk functions have been
developed for different body regions derived from a combination of accident and casualty data, as well
as data from tests on cadavers, animals or human volunteers. It is important to stress that a test response
should always be measured as a risk of injury. The human response to a given dose of applied load varies
across a range of the population and not all humans would respond similarly to the same load.
It should further be noted that the available risk functions all have their limitations. The available data
consist only of subgroups representing a fraction of human data, or they are scaled from animal or
cadaver data, which introduces uncertainties regarding their accuracy. However, such injury criteria
are often used by the industry as pass/fail values without considering the variation in human tolerance
to injurious forces. This sometimes leads to the mistaken belief that any value below the stated limit
implies uninjured and all values above imply a serious or fatal injury. Instead it should be concluded
that the spècifièd limit values are intended to ensure an adequate level of safety with a low risk of injury
for the assessed impact energies.
Another aspect in injury mechanics to the head is that many injury risk functions are developed by the
automotive industry and thus represent typical automotive accidents. The head kinematics in
automotive accidents and helmeted head impacts are different. This means that the kinematic metrics
developed for automotive application as well as their thresholds, is not suitable for assessing helmets.
Many accidents to wearers of helmets, which cover a wide range of activities from sports like horse
riding or downhill skiing to industrial work and firè fighting, result in a closed head injury. This is when
the brain is damaged without any skull or external tissue damage. Historically, the main concern in head
injury prevention has been skull fractures and the test methods have focused on assessing pure
translational loads, mainly connected to skull fracture. This report introduces available metrics related
to brain injuries produced by rotational loading that can be used in helmet standards to assess the risk
of closed head injury as well as skull fractures.
Children and adults have distinct pathophysiology and injury patterns, indicated by a meta-analysis of
48,074 child cyclist head injuries, suggesting their helmets should not be tested the same way [1]. Two
factors are expected to affect the protection level of helmets, the characteristic of the head impact and
the head injury tolerance. The real-world head impact biomechanics of children are poorly
characterised. Unlike in adults [2], we have less or no qûântifièd accounts of common head impact
speeds, angles, or locations in child cyclists, so it is unclear how closely the conditions used in current
child helmet test methods represent real-world incidents. Nevertheless, given differences in travel
speeds, fall heights, and vehicle wrap-around geometry, child impact characteristics are likely to be
distinct from those of adults [3]. The second factor is that children are expected to have a lower head
injury tolerance than adults. Their proportionally larger heads and weaker cervical musculature amplify
head excursion and angular kinematics under a given loading [4], [5], their thinner and more deformable
skulls alter contact mechanics at equivalent loads [6], and their lower-mass heads rotate more under a
given load, producing greater brain tissue strain[7]. Together, these factors suggest the need for helmet
test conditions and pass/fail limits tailored to children, but further work is needed to determine child-
representative impact conditions and injury-driven pass thresholds.
The intention of this document is to supply the members of helmet standards committees with a
summary of available injury criteria and metrics for a range of injuries and risk levels, corresponding
to available test methods. This revised version of the report has increased the focus on the relation
between test method and injury metrics, included metrics related to closed head injury and has removed
the chapter on neck injury to focus on assessment of head protection.
1 Scope
This document outlines criteria related to head injury risk and burn injury risk in the context of helmet
testing. It is intended to support the work of CEN/TC 158 working groups by providing guidance on
selecting appropriate test methods and dèfining injury thresholds based on injury type and severity.
This document does not include requirements or test methods.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and dèfinitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
• ISO Online browsing platform: available athttp://www.iso.org/obp
• IEC Electropedia: available athttp://www.electropedia.org/
4 Injury prevention and testing
4.1 General
A head injury caused by an impact is usually referred to as a traumatic head injury. The severity of the
injury depends on the load case and what protection was used. The purpose of the helmet cèrtificâtion
is to measure the protective performance of the helmet in a spècific load case representing a set of
accident scenarios. The test method also needs to be robust and repeatable. For each chosen test method
there is a corresponding injury metric that can be measured and analysed if the helmet performs
according within given threshold values. If it exceeds the thresholds the helmet does not fûlfil protection
requirements according to standard. The injury metrics are chosen to cover the most relevant types of
head injuries.
Head injury is a broad term that covers a wide range of injuries to the scalp, skull, brain and underlying
tissues and blood vessels in the head. Broadly, these injuries can be categorised as sûpèrficiâl injury,
skull fracture or brain injury as dèfinèd by what tissues are injured, see Figure 1. It can also be a
combination of any of the three where e.g. a fracture causes damage to the brain. A more detailed
description of head anatomy and different types of injuries is found in the Annex A.
Figure 1 — Head injury types
4.2 Injury metrics
Helmet standards are designed to ensure that helmets provide sûfficiènt protection in relevant accident
scenarios and help reduce the risk of associated injuries. Since the injury mechanism is different for
different tissues so e.g. the skull and brain are sensitive to different types of loading, helmets needs to
be tested in various test set-ups and assessed with injury spècific metrics.
Several studies have shown that for helmeted riders, cerebral concussion is the most common head
injury [8][9]. Also, previous studies have shown a larger reduction in skull fractures compared to brain
injuries when wearing a helmet compared to not wearing a helmet [10][11]. However, both skull
fractures and brain injuries occur in helmeted bicycle accidents [8]. Also, within construction it has been
shown that concussion and internal injuries are more prevalent than fractures among reported head
injuries [12]. Previous research has shown that many of the brain injuries correlate better to rotational
kinematics [13][14][15] whereas skull fractures better correlate with linear kinematics [16][17].
Many different injury metrics have been proposed over the years. Some are solely based on linear
kinematics such as Head Injury Criterion (HIC) 5.3, some solely on angular kinematics such as Brain
Injury Criterion (BrIC) 6.4, whereas some have a combination of linear and angular kinematics, e.g. Head
Acceleration Response Metric (HARM) 7.2. There are also injury metrics that are based on the response
from Finite Element (FE) models, such as maximum principal strain and von Mises stress 8.1. At the
moment there is no consensus about the most suitable injury metrics to predict different types of head
injuries but many are used in parallel.
There are a number of different test methods described in various standards. These are used
interchangeably or in combination when they measure different properties of the helmet. This report
includes a description of the injury metrics related to the most common test methods in the European
helmet standards.
Different test methods evaluate different loading conditions, and the metrics represent risks for
different injuries. See Table 1. It should further be noted that the test methods use different headforms
depending on what injury risk or metric it is intended to represent. The headform properties are spècific
for the requirements of the test and cannot be interchanged with any other test headform without
affecting the results negatively.
Table 1 — Test methods and injury metrics associated with risk
Risk type Blunt impact from Crush Penetration
— Falling objects — Sharp impact
— Falls — Projectile
Typical associated Skull fracture Concussion Skull crushing Skull penetrating
injury
Contusion Focal injury e.g.
subdural
Epidural
haematoma
haematoma
Diffuse axonal
injury
Protective Linear shock Rotational shock Structural rigidity Penetration protection
property absorption absorption
Test methods Dropped headform Dropped headform Lateral deformation Penetration
Striking object
Measured metric Force or Linear Rotational Magnitude of perforation or not
acceleration acceleration or deformation
Rotational velocity
Global injury PLA PRA, PRV, N/A N/A
criteria
HIC BrIC
Peak transmitted Combined metrics
force
SFC
Tissue based Internal energy (FE) Strain (FE)
injury criteria
Stress (FE)
4.3 Injury risk assessment
When assessing injury risk in biological tissues, variations in human anatomy and tissue properties
across the population are to be considered. Individuals differ in their sensitivity to impact forces,
resulting in varying probabilities of injury under the same loading conditions. While most people may
tolerate a certain level of strain, some may sustain injuries at that same level due to lower tolerance.
Consequently, there is no absolute threshold that dèfinitivèly separates injury from non-injury. Instead,
injury risk is represented by risk functions - a dose response curve - which express the probability of
injury as a function of a load parameter, such as force or strain. Risk functions are typically derived from
injury statistics and experimental data and expressed with statistical distributions.
The dose response curve tends to be "S" (sigmoid) shaped such that as the magnitude of the injurious
parameter increases so does the percent of the population that sustains an injury of a given severity.
Thus, a family of "S" curves can be generated for a range of injury severity such as Abbreviated Injury
Scale (AIS) 4.4 and a measurement or criterion such as HIC 5.3. Unfortunately, the data needed for
generating risk curves is generally difficûlt to obtain. Injury statistics does not give the exact level of
load and measurements generated by test apparatus do not relate directly to injury severity since
surrogate headforms and instrumentations does not respond like a human head in an impact. Hence, it
is necessary to find a relationship between these test measurements and injury severity.
Risk functions usually represent a subgroup of the entire population, such as American football players,
and thus the risk is biased based on that subgroup’s characteristics. Many values used for injury risk
th
analysis represents a 50 percentile male, which means that male of average stature would sustain an
injury of a given AIS severity for the corresponding range of acceleration.
Furthermore, in real life, the helmet impact scenario will vary from minor impacts to very high loading
conditions as well as cover many different surface conditions and impact angles. In a helmet standard
the number of tests performed are limited due to cost. Thus, for the standard test requirement, both
impact severity and injury risk threshold are chosen as a representation of typical accident scenarios
and injury mechanisms. The cèrtificâtion testing of head protection use pass/fail criteria to ensure that
the helmet has a minimum level of protective capabilities. However it is also taking into account that it
should be possible to design a helmet with reasonable dimensions, making it acceptable for the user to
wear. Note that historically, risk values have been set which correspond to AIS 5 severe and that this
has resulted in helmets that have given reasonable protection.
The risk and severity of the injury that may be tolerated in different helmet standards may vary
depending on the chosen scenarios. This is up to the standard to dèfinè but could consider e.g.:
a) Minimising the risk of death and disability following a head impact - allowing moderate injury.
b) Reducing the risk of long-term brain injury, e.g. Chronic Traumatic Encephalopathy (CTE), from
repetitive head impacts - protecting also for minor injury.
c) Lowering the likelihood of incapacitation following a head impact, thereby reducing use-case
spècific hazards such as drowning or post-crash firès - protecting against minor injury such as
concussion.
4.4 Abbreviated Injury Scale, AIS
The Abbreviated Injury Scale (AIS) is a scale that extends from 0 to 6 where 0 is uninjured and 6 is
unsurvivable [18]. Each level can be applied to anybody region according to a coding manual developed
by the Association for the Advancement of Automotive Medicine (AAAM). Table 2 gives the AIS level for
different types of head injuries based on AIS2015. Table 3 gives examples of typical head injuries for
each severity level. Table 4 gives similar information for burn injuries by degree, surface area and region
of the body.
Table 2 — AIS level of different types of head injuries based on AIS2015
AIS1 AIS2 AIS3 AIS4 AIS5 AIS6
Scalp x x x
Artery x x x x
Vein x x
Brain stem  x x
Cerebellum x x x x
Cerebrum x x x x
Brain swelling x x x x
Pituitary x
Cerebral
x x
concussion
Diffuse Axonal
x x
Injury
Base fracture x x
Vault fracture x x x
Headache x
Crush injury   x
Avulsive injury   x
Penetrating injury x x x
Table 3 — Examples of head injuries in the different AIS levels
AIS level Examples of head injury
AIS1 Headache, scalp abrasion or minor laceration
AIS2 Cerebellum tiny subdural haematoma, mild concussion with 0 - <1 hours loss of consciousness,
closed vault fracture
AIS3 Total scalp loss, cerebellum small to moderate subdural haematoma, mild brain swelling/
oedema, cerebrum tiny subdural haematoma, severe cerebral concussion with loss of
consciousness 1-6 hours, closed base fracture, comminuted vault fracture
AIS4 Cerebrum small to moderate subdural haematoma, mild diffuse axonal injury with coma 6-24
hours, complex base fracture, complex or massive vault fracture
AIS5 Brain stem contusion, cerebrum large subdural haematoma, diffuse axonal injury with coma
more than 24 hours
AIS6 Brain stem laceration, crush injury or massive destruction, avulsion injury
Table 4 — AIS scale with burn injury severity
AIS 1 AIS 2 AIS 3 AIS 4 AIS 5 AIS 6
minor slight moderat serious severe unsurvivable
e
1st degree X X
ûnspècifièd X
2nd degree X
a
< 10 % TBS X
3rd degree X X X
a
X
< 10 % TBS
a
< 10 % TBS with face, hand or
X
genitalia involvement
2nd or 3rd degree
a
X
10 % to 19 % TBS
a
10 % to 19 % TBS with face,
X
hand or genitalia involvement
a
20% to 29% TBS X
a
20% to 29% TBS with face, hand
X
or genitalia involvement
a
X
30 % to 39 % TBS
a
30 % to 39 % TBS with face,
X
hand or genitalia involvement
a
40 % to 89 % TBS  X
a
≥ 90 % TBS   X
a
Total body surface
5 Linear shock absorption
5.1 General
Linear shock absorption is primarily measuring the risk of skull fracture, contusions and epidural
haematoma. The most common test methods are to drop a headform on an anvil and the inverted, to
drop a striker onto a rigid headform. The linear shock absorption can be measured either in straight or
angled impacts. Below are the most commonly used injury criteria and risk levels described.
5.2 Peak Linear Acceleration (PLA)
This is the most frequently used parameter in helmet testing and is derived usually from a triaxial
accelerometer mounted in the headform unless the headform is rigidly supported and then the source
is a single axis accelerometer. In both types, the helmet is mounted onto the headform and then the
apparatus allowed to fall unimpeded onto a rigid anvil. One study used Post Mortem Human Subject
(PMHS) tests, FE modelling and drop tests using HIII headform to construct a risk function for skull
fractures based on PLA [17]. This study suggests a 50 % risk of skull fractures for PLA = 220 g. This
study used old subjects. A recent study reconstructed collisions between cars and vulnerable road users
aged 60+ and constructed a PLA risk function for head injuries of AIS4+ [19]. They found 50 % risk of
AIS4+ head injuries is PLA = 200 g. The injury threshold for the normal population is likely to be higher
than these values. Probably PLA = 250 g, suggested by [20], provides a better estimation of 50 % risk
of AIS4+.
A Finite Element (FE) study by Li et al. [21] evaluated risk of skull fracture for child head with varying
age (1,5; 3; 6 and 18 years). They suggested that younger individuals should have a lower threshold for
peak linear acceleration because they are more sensitive due to the fontanelles. They propose to have
a critical value of 150 g for helmets for 1,5 years old or younger. This study is until now one of few
studies evaluating the age-dependency in helmet testing and more data to set spècific criteria for
children are required, but the increased risk for children should be considered in standard writing.
5.3 Head Injury Criterion (HIC)
The Head Injury Criterion (HIC) [22] was developed from the Gadd Severity Index (GSI) and is based
on the Wayne State tolerance curve [23]. The Wayne State Tolerance Curve was derived from a body of
experimental data, including both skull fracture and brain injury data, where the short duration impacts
were from skull fracture data. It evolved from the work of Lissner et al. [23], Gurdjian et al. [24][25] and
Patrick et al. [26], and gives the tolerable average acceleration in A-P direction (Anterior-Posterior) as
function of the pulse duration. HIC is calculated from the resultant linear acceleration of the head’s
centre of gravity over time [27]. Unlike peak linear acceleration, HIC accounts for the duration of the
load. This is important because impacts with the same peak acceleration, but different pulse durations
can lead to different injury outcomes. The HIC Formula (1) is shown below:
2,5
t
1 2
HIC = · a dt · t −t
(1)
t res 2 1
t −t
2 1
max
Where a is the resultant head acceleration as a function of time and t and t dèfinè the time interval
res 1 2
over which the integral produces its maximum value. The duration t - t is set to either 15 ms or 36 ms,
2 1
yielding HIC or HIC , respectively. In helmet impacts, the impact duration is typically less than 36 ms,
15 36
making HIC the more appropriate metric.
Hodgson and Thomas [28] suggested that the critical HIC interval is less than 15 ms, even if the HIC
value exceeded the threshold of 1 000 over a longer interval. His finding was based on examination of
events where the concussive outcomes were known or could be determined.
Historically the threshold for injury "dangerous to life" was set as 1 000 for both GSI and HIC [22]. HIC
1000 was confirmèd to correlate with the occurrence of bone fracture in a study by Hodgson [16] and
reports by Mertz and Prasad et al [29][30] correlated HIC with the risk of skull fracture where HIC 700
corresponded to a 9-16 % risk of skull fracture, HIC 1 000 corresponded to 19-40 % , HIC 1 500 48-82
% and 2 000 66-90 % risk of skull fracture, the variation depending on the data set studied. A European
project looking at motorcycle accident data [31] provided tentative values where an injury severity of
AIS 2 was predicted for HIC 1 000 and AIS 3 at HIC 1 500, although, they did not correlate well. The limit
presented by NHTSA for occupant safety is 700 for HIC [32].
5.4 Force transmission
A number of test methods use the transmitted force as a metric. That is the force transmitted from a
striker impacting the helmet as measured in a force plate below the headform. Direct force can produce
skull fractures and focal brain injuries. Tolerance to skull fracture depends on the location, contact area,
and rate of the applied force, with tolerance ranging from less than 5,000 N to over 10 000 N [33].
5.5 Skull Fracture Correlate (SFC)
SFC was developed by Vander Vorst et al. [34] as an improved skull fracture predictor based on
kinematics. It is dèfinèd according to Formula (2) where ΔV is the change in velocity over the time
HIC
interval and ΔT is the time interval (t and t ) that maximizes the HIC value, usefully either maximum
HIC 1 2
15 ms or 36 ms known as HIC or HIC .
15 36
t
∫ a d t
ΔV
HIC t
SFC   =   ΔT = 1  (2)
HIC
t −t
2 1
With logistic regression of experimental data of postmortem human subjects a risk curve was developed
combining both frontal and lateral impacts [35]. According to that risk curve, the 15 % probability of
skull fracture with SFC15 occurs at 124 g.
6 Rotational shock absorption
6.1 General
Rotational shock absorption asses the helmets protective performance regarding rotational motions.
Injuries related to rotational motion are primarily diffuse brain injuries like concussion and DAI but
also vascular injuries like subdural haematoma. To evaluate the rotational shock absorption the test
method needs to allow for the headform to rotate, either freely or connected to a biofidèlic neck.
6.2 Peak Rotational Acceleration (PRA)
Peak Rotational Acceleration parameter is known to contribute substantially to brain injury but the
relationship with injury is difficûlt to quantify and this relationship depends on the application. It is
possible to measure rotational kinematics with instrumentations like a nine-accelerometer or rate
sensors in the headform.
Depreitere et al. [36] evaluated the risk of acute subdural haematomas from bridging vein rupture. They
performed experiments with post mortem human subjects by impacting them at the back of the head
with a pendulum with varying energy and impact duration (4-15 ms). They concluded from the study
a tolerance curve at around 10,000 rad/s for pulses with a duration shorter than 10 ms.
Davidsson et al. [37] performed animal experiments with rats to evaluate the injury threshold for diffuse
axonal injury. The loading was in the sagittal plane rearward. By scaling the rat experiments they found
a threshold for DAI of 10 000 rad/s with a duration of 4 ms.
Anderson et al. [38] presented a risk curve based on reconstruction of American football reconstructions
with hybrid III dummies [39] [40]. They found a 50 % risk of sustaining a concussion at around 5 700
rad/s . Rowson et al. [41] also used data from American football but the data were taken from helmet
mounted sensors. They found a 50 % risk of concussion at 6 383 rad/ s .
6.3 Peak Rotational Velocity (PRV)
Peak rotational velocity is another metric used for evaluating rotational induced brain injuries.
Anderson et al. [38] presented an injury risk functions based on reconstructions from the national
football league [39] [40]. They found a 50 % risk of mild traumatic brain injury at 38 rad/s. Meanwhile,
Rowson et al. [41] presented a 50 % risk of a mild TBI (Traumatic Brain Injury) at 28 rad/s. Their risk
function was based on mouthguard measurements from collegiate football players. Another study
proposing a risk function for peak rotational velocity [42], where they used reconstructions of impacts
in Australian football and rugby. They found a 50 % risk of concussion at 22 rad/s. Wu et al. [43]
performed accident reconstructions of vulnerable road users (cyclists, pedestrians, electrical two
wheelers) and found the 50 % risk of an AIS4 injury to be 27,8 rad/s. However, this risk function was
developed from individuals that were 60 years or older, which could inflûèncè the risk since they can
be more vulnerable to certain types of injuries such as subdural haematomas.
Margulies and Thibault [44] focused on diffuse axonal injury associated with prolonged coma and
structural damage to axons. They used data from experiments on baboons (53 animals), analytical
modelling and physical models of the human and baboon heads fillèd with a brain like material. The
determined asymptotes for peak rotational acceleration and peak change in rotational velocity were 16
krad/s and 46,5 rad/s for DAI AIS4+.
6.4 Brain Injury Criterion (BrIC)
BrIC is a criterion proposed by Takhounts et al. [45] [46]. The first version, referred to the abbreviation
of BrIC, includes a combination of peak resultant rotational acceleration and peak resultant rotational
velocity [45]. In 2013 [46], the criterion was updated, and the abbreviation for the updated criterion is
BrIC. The updated criterion includes a combination of peak rotational velocity of the three main
components (x,y,z) Formula (3). The critical values (ω , ω , ω ) were dèfinèd based on computer
cr,x cr,y cr,z
simulations using two different FE models [SIMon (Simulated Injury Monitor) and GHBMC (Global
Human Body Model Consortium)]. The pulses that were applied to the models were pulses from vehicle
crashes (NCAP tests and frontal offsets tests). There has also been alternative critical values proposed
of different research groups that is dependent on other FE models [47] [48] [49].
2 2 2
ω
ω y ω
x z
(3)
BrIC   = + +
ω ω ω
cr,x cr,y cr,z
Several studies have developed risk functions for BrIC. Takhounts et al. [46] developed risk functions
for AIS1 to AIS5 based on scaled experiments of animal experiments. The risk was dependent on the
critical values used. When the critical values where based on the cumulative strain density measures
(CSDM), the 50 % risk for an AIS2 injury was 0,78 and an AIS4 injury 1,04. Wu et al. [50] also used animal
experiments together with human volunteers and dummies to develop risk functions for mild and severe
Traumatic Brain Injury (TBI). They found a 50 % risk for mild TBI (mTBI) of 0,77 and 1,19 for severe
TBI (sTBI).
6.5 DAMAGE
Gabler at al. [48] developed DAMAGE (Diffuse Axonal, Multi-Axis, General Evaluation) to predict diffuse
brain injuries with a second-order system of equations. The model use a mass-spring-damper system
to predict the brain tissue strain due to rotational motion around the three principal axes of the head.
The parameters in the second-order system of equations were determined by uniaxial and multiaxial
loadings of a FE model, namely the Global Human Body Model Consortium (GHBMC). The advantages
of this metric compared to many other kinematics-based metrics are that it considers the kinematics
over time and includes all three principal axes of the head. The metric can be seen as a simplifièd FE
model, which is less computationally expensive than a regular FE model.
Wu et al. [50] developed risk functions for DAMAGE for mild and severe traumatic brain injuries based
on animal, human and dummy experiments. They found a 50 % risk of mTBI to be 0,359 for mTBI and
0,617 for severe TBI.
6.6 Tangential force at the helmet surface
An indirect way to assess rotational motion transferred to the head during an oblique impact is to
measure the normal and tangential forces acting on the oblique anvil and compute the friction between
helmet shell and anvil. This value of friction is then used as injury risk estimate. This method is used in
the motorcycle helmet rating method SHARP [51]. The method has been criticised to simplify the helmet
to ground interaction and that the correlation between tangential force and rotational acceleration is
low [52]. The reason to use the indirect method was to avoid the necessity of instrumentation in the
headform capable of measuring rotational acceleration. Today, instrumentations that measure either
rotational motion by a nine accelerometer array or rate sensors, are easily accessible to incorporate in
the test headforms.
7 Combined injury metrics
7.1 General
To assess the risk of concussion, alternative metrics that combine the linear and rotational kinematics
has been presented.
7.2 HARM
The metric HARM (Head Acceleration Response Metric) was developed to evaluate American helmet
performance in relation to risk of concussion by combining DAMAGE 6.5 and HIC 5.3 Formula (4), with
the constants C1=0,0148 and C2=15,6 [53]. The constants were determined based on data from hybrid
III reconstructions of American football impacts [39] [40] [54].
HARM   = C HIC+C DAMAGE
(4)
1 2
7.3 Combined Probability (CP)
The combined probability risk function for concussion [55] has been developed based on data collected
from the HITS (Head Impact Telemetry System). The data consisted of peak linear acceleration and peak
rotational acceleration based on 62 974 sub-concussive impacts and 37 diagnosed concussive impacts
for American football. The formula is presented in Formula (5), where a stands for PLA and α for PRA.
The metric has been used to evaluate different helmet segment performance, e.g. equestrian, American
football and ice hockey helmets. An alternative metrics based on peak linear acceleration and peak
angular velocity (Formula (6)) has also been developed based on the same methodology, where ω is
PRV [56].
CP =
(5)
− 10.2+0.0433a+0.00873α −0.000000920aα
1+e
R a,ω   =
(6)
− −10.2+0.0433a+0.16986ω −0.002075aω
1+e
8 Finite Element (FE) based metrics
8.1 Introduction
Many of the metrics mentioned above rely on risk functions derived from Finite Element (FE)
simulations. In addition, several approaches incorporate FE models directly into the risk assessment
process. The use of FE models allows for representation of the complex geometry and unique tissue
properties of the human head, as dèfinèd by constitutive laws for the brain, skull, and surrounding
tissues. These methods also make it possible to integrate the full six-degree-of-freedom (6D) kinematic
characterist
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