Human Impact Tolerance And the STAR Helmet Rating (Duma).pdf · Human Impact Tolerance And the STAR Helmet Rating ... Each overall injury risk is weighted based ... to assess helmet

1

Human Impact ToleranceAnd the STAR Helmet Rating

Stefan M. Duma and Steven RowsonSchool of Biomedical Engineering and Sciences

Virginia Tech – Wake Forest University

January 17, 2012

Presentation Outline• Part 1: Injury Biomechanics Background

– Reducing injuries in auto-safety, sports, military

• Part 2: STAR Helmet Rating Details– Review of exposure and risk analysis

• Part 3: Youth Football Data

2

• No financial interest in SIMBEX, HITS, or any other helmet related sensor or product

• No financial interest in Riddell, or any helmet manufacturer

• No helmet expert witness consulting

• Speaking fees donated to buy new helmets for youth teams in south-west Virginia.

Financial Disclaimer

Funding Sources

National Institutes of HealthNational Inst. Of Child Health & Human Development, R01HD048638

Department of TransportationNational Highway Traffic Safety Administration

Department of DefenseUS Medical Research and Material Command

Toyota Motor CorporationToyota Central Research and Development Labs

3

Virginia Tech - Wake Forest UniversityCenter for Injury Biomechanics

10 Faculty Members:

11 Research Staff Members:

27 Graduate Students:

12 Collaborating Faculty Members:

CIB‐ICTAS Impact Biomechanics Laboratory CIB‐VTTI Sled Laboratory CIB Testing Laboratory at WFU

Duma Gabler GayzikHardyStitzel VandeVordKemper Rowson Danelson

McNally Shifflett Covey Moreno Strom Kennedy Burke Harris Smith Anthony Tan

Alphonse Beeman Brown Daniel Daniello Donoughe Fievisohn Gregory Howes Johnson Kusano

Kusano Owen Sajja Sandberg Loftis Thompson Tsoi Urban Vaughn Vavalle Weaver

Herring Madigan Porta Cormier Meredith Brolinson Funk Manoogian Martin Martin Schoppe Apel

White

Longton

Virginia Tech Transportation Institute

Golman

Rhyne

Moody

40,000 sq ft of Dedicated CIB Research Facilities:

Untaroiu

Whitlow Powers Maldjian

VT-WFU Research ExperienceHead: FOCUS Headform−Eye Modeling/Experimental−Skull Fracture

Neck−Head Supported Mass−Crash Pulse and Parachuting

Restraint Evaluation−Helicopter Airbags−Upper Limb Injuries

Sponsors: Army Research Office, US Army Aeromedical Research Laboratory, US Army Medical Research and Materiel Command

Chest− Lung Tissue−Rib Fractures

4

FOCUS Head

Denton, nowHumanetics

Military Head Injury Prevention5 year design and development

(2001 - 2006)5 year injury criteria

251 tests on 57 heads(2006 - 2011)

Toy Design

5

Automobile Analogy

Federal Motor Vehicle Safety Standards(FMVSS) are pass / fail

6

New Car Assessment Program(NCAP) NHTSA rates safety on 5 star scale

35 mph

Fixed Barrier

38.5 mph

20 mph

Injury risk to the head, neck, chest, and femur are considered for frontal and side tests (rollover is ratio calculation)

A total injury risk for each testing configuration is computed

Each overall injury risk is weighted based on exposure and summed to compute overall risk

Overall risk = 5/12 * frontal + 4/12 * side + 3/12 * rollover

Total Risk = 1 – (1 – Riskhead)*(1 – Riskneck)…*(1 – Riskchest)*(1 – Riskfemur)

7

Parallel Analogy

• Auto-industry fought FMVSS and NCAP– Will result in more dangerous cars, fatalities

– Stiffening vehicle and adding padding will result in more injuries to occupants

– 4 Decades ago, 1970s

Automobile

FMVSS

NCAPIIHS

Football Helmets

NOCSAE

STAR

Pass/Fail required

Star ratings based on lower risk of injuries

44,525

33,808

~1968 FMVSS 208 (pass/fail)

~1978 NCAP frontal (stars)

~1997 NCAP side

8

Active Research in all Current and Future Body Regions

We do not know 100% about entire body, but know enough to make safety advances

Head injury (HIC)

Neck injury (Nij)

Chest compression

Abdomen

Pelvis

Tibia

Ankle complex

Femur loads



• Part 2: STAR Details– Review of exposure and risk analysis

• Part 3: Child Football Data

9

Basic Helmet Function• There are two primary components of a helmet to protect

from injury

– Helmet Shell

• Deflects to distribute forceover a larger area

– Helmet Liner

• Modulates the energy transferredto the head

Impact response can be tuned to meet design requirements

Helmets can be designed to reduce concussion risk

Football helmets used as an example, but applicable to all sports

• Objectives:– Create a public database

– Independent, no conflict of interest

– Consumer resource and education

– Catalyst for improved design

Adult Football HelmetsThe STAR Rating System for Football Helmets

STAR = Summation of Tests for the Analysis of Risk

• STAR Development Principles:– Do no harm: must pass NOCSAE first

– Data driven, Repeatable, Useful Analysis

– Limitations and reasonable progress

10

STAR Rating System for Football Helmets

STAR: Summation of Tests for the Analysis of Risk

4

1

6

1L H

aRhESTAR

Combines true impact exposure with an unbiased risk analysis using real world biomechanical data

to assess helmet safety for consumers.

HIT System

6 Accelerometers mounted normal to the skull

6DOF Device

12 Accelerometers mounted tangential

Helmet InstrumentationTwo parallel systems for past 10 years

11

HIT System

6 Accelerometers mounted normal to the skull

3 Linear and 2 Rotational Accelerations (5DOF)

~$1,000/helmet

Validated by NFL, ohters

6DOF Device

12 Accelerometers mounted tangential

3 Linear and 3 Rotational Accelerations (6DOF)

~$10,000/helmet

Validated HIT System

Helmet InstrumentationTwo parallel systems for past 10 years

Accelerometers are Spring-Mounted

Remains in contact with the headMeasures head acceleration, not helmet acceleration

12

Helmet vs Head Acceleration

Mounting mechanism decouples sensors from the helmet and insures consistent head/sensor contact

Head Acceleration Data Collection

• Up to 64 Virginia Tech players instrumented for each season

• Data collected for every game and practice

- instrumented helmet

13

Wireless Coverage

Base Unit

120 Impact Onboard Memory

Cumulative HITS Data Collection

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

2003 2004 2005 2006 2007 2008 2009 2010 2011

Tota

l N

um

ber

of

Imp

acts

Rec

ord

ed a

t V

irg

inia

Tec

h

Sch

oo

ls U

sin

g t

he

HIT

Sys

tem

Virginia Tech Virginia Tech

North Carolina

Oklahoma

1 High School

Virginia Tech

North Carolina

Oklahoma

Dartmouth

Arizona State

5 High Schools

Virginia Tech

North Carolina

Oklahoma

Dartmouth

Brown

2 High Schools

Minnesota

Indiana

Virginia Tech

North Carolina

Oklahoma

Dartmouth

Arizona State

5 High Schools

Indiana

Illinois

Virginia Tech

North Carolina

Oklahoma

Dartmouth

Brown

3 High Schools

Indiana

Virginia Tech

North Carolina

Oklahoma

Dartmouth

Brown

4 High Schools

Indiana

Virginia Tech

North Carolina

Oklahoma

Dartmouth

Brown

4 High Schools

Indiana

Virginia Tech

North Carolina

Oklahoma

Dartmouth

Brown

4 High Schools

Indiana

155,000+ impacts recorded at Virginia Tech2,000,000+ impacts recorded at all institutions

14

All head impact data was used to create:

Football Helmet Evaluations

Combines impact exposure with a risk analysis using real world biomechanical data to assess helmet

performance for consumers.



4

1

6

1L H

aRhESTAR

Summates impact exposure and injury risk for multiple drop heights and impact locations to

generate a generalized injury incidence

Exposure x Risk = Incidence

15

Helmet Testing Protocol

1. Impact response of the NOCSAE head form has reasonable biofidelity

2. Manufacturers and testing labs already have this equipment

3. Testing protocol will be relatively simple to implement, for those who chose to replicate these tests

NOCSAE-style tests chosen for several reasons

Front Impact Location

0 5 10 15 20 25 30 35 40-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Time (ms)

No

rma

lize

d A

ccel

era

tio

n

On-field data average

NOCSAE drop test

Impact responses are very similar

16



4

1

6

1L H

aRhESTAR

Summates impact exposure and injury risk for multiple drop heights and impact locations to

generate a generalized injury incidence

Exposure x Risk = Incidence



4

1

6

1L H

aRhESTAR

L: Impact LocationHelmet location, of which, 4 will be tested: front, rear, side, and top

17

Impact Location

All impact data are grouped into general locations:Front, Rear, Left, Right, and Top

Helmet Testing Protocol

Test 4 impact locations:

Front Rear Side Top

Test 5 impact energies: 12 in, 24 in, 36 in, 48 in, 60 in

18

Impact Location• Analysis used to determine the weighting of each impact

location to better represent what athletes experience

• Subset of in situ head acceleration collected at Virginia Tech– 62,974 impacts collected throughout 2009 and 2010

VT Data Mihalik et al. 2007

Front 34.7% 35.9%

Rear 31.9% 30.9%

Side 16.3% 14.4%

Top 17.1% 18.8%

VT Data

Front 34.7%

Rear 31.9%

Side 16.3%

Top 17.1%

References:Crisco, J. J., Fiore, R., Beckwith, J. G., Chu, J. J., Brolinson, P. G., Duma, S., Mcallister, T. W., Duhaime, A.

C., and Greenwald, R. M., 2010, "Frequency and Location of Head Impact Exposures in Individual Collegiate Football Players," J Athl Train, 45(6), pp. 549-59.

Mihalik, J. P., Bell, D. R., Marshall, S. W., and Guskiewicz, K. M., 2007, "Measurement of Head Impacts in Collegiate Football Players: An Investigation of Positional and Event-Type Differences," Neurosurgery, 61(6), pp. 1229-35; discussion 1235.

VT Data

Front 34.7%

Rear 31.9%

Side 16.3%

Top 17.1%



4

1

6

1L H

aRhESTAR

E(h): Impact ExposureImpact exposure as a function of drop height that 1 player will experience throughout the course of 1 season

19

Head Impact Exposure

References:

Comparison to published head impact exposure data:

Study Impacts per Season Subjects

VT Data (Crisco et al. 2010) 1000 Collegiate

Guskiewicz et al. 2007 950 Collegiate

Schnebel et al. 2007 1115 Collegiate and High School

Broglio et al. 2009 565 High School

Study Impacts per Season Subjects

VT Data (Crisco et al. 2010) 1000 Collegiate

Guskiewicz et al. 2007 950 Collegiate

Schnebel et al. 2007 1115 Collegiate and High School

Broglio et al. 2009 565 High School

Broglio, S. P., Sosnoff, J. J., Shin, S., He, X., Alcaraz, C., and Zimmerman, J., 2009, "Head Impacts During High School Football: A Biomechanical Assessment," J Athl Train, 44(4), pp. 342-9.

Crisco, J. J., Fiore, R., Beckwith, J. G., Chu, J. J., Brolinson, P. G., Duma, S., Mcallister, T. W., Duhaime, A. C., and Greenwald, R. M., 2010, "Frequency and Location of Head Impact Exposures in Individual Collegiate Football Players," J Athl Train, 45(6), pp. 549-59.

Guskiewicz, K. M., Mihalik, J. P., Shankar, V., Marshall, S. W., Crowell, D. H., Oliaro, S. M., Ciocca, M. F., and Hooker, D. N., 2007, "Measurement of Head Impacts in Collegiate Football Players: Relationship between Head Impact Biomechanics and Acute Clinical Outcome after Concussion," Neurosurgery, 61(6), pp. 1244-53.

Schnebel, B., Gwin, J. T., Anderson, S., and Gatlin, R., 2007, "In Vivo Study of Head Impacts in Football: A Comparison of National Collegiate Athletic Association Division I Versus High School Impacts," Neurosurgery, 60(3), pp. 490-5; discussion 495-6.

Head Impact Exposure• While we have the overall exposure that 1 player experiences

throughout 1 season, we must define exposure on an impact location basis

Percent of Impacts

Front 34.7%

Rear 31.9%

Side 16.3%

Top 17.1%

Total 100%

Percent of Impacts Number of Impacts

Front 34.7% 347

Rear 31.9% 319

Side 16.3% 163

Top 17.1% 171

Total 100% 1000

Must determine impact severity exposure for each location so that each can be weighted to reflect what a player

actually experiences throughout a season

20

Head Impact Exposure: Front Location

Drop Height: none0 in to 6 in 164 Impacts

Drop Height: 12 in6 in to 18 in 138 Impacts




Drop Height: 60 in54 in and above1 Impact

This analysis is done for each impact location distribution

Front Rear Side Top

< 19 g 164 139 81 63

12 in 138 165 75 85

24 in 31 11 4 14

36 in 10 2 1 5

48 in 3 1 1 2

60 in 1 1 1 2

Front Rear Side Top

< 19 g 164 139 81 63

12 in 138 165 75 85

24 in 31 11 4 14

36 in 10 2 1 5

48 in 3 1 1 2

60 in 1 1 1 2

Total 347 319 163 171

Head Impact ExposureExposure is defined as a function of drop height

and impact location

1000 impacts per season

21

Drop Tests Mapped to Impact Exposure

Front Impact – 36 in DropExposure = 10 Impacts

Side Impact – 36 in DropExposure = 1 Impact



4

1

6

1L H

aRhESTAR

R(a): Injury RiskInjury risk as a function of peak linear acceleration

22

Injury Data Comparison

• NFL study used Hybrid III reconstructions of injurious game impacts (Pellman, 2003)

• 31 impacts, 25 concussions– Striking player (non-injury)– Struck Player (injury)

• Injury risk curves based on logistic regression

(King, 2003)

Injury Data Comparison

Dummy Reconstruction Data (25 Concussions):Average Concussion: 98 g ± 27 g

In Situ Head Acceleration Data (32 Concussions):Average Concussion: 105 g ± 27 g



4

1

6

1L H

aRhESTAR

Combines true impact exposure with an unbiased risk analysis using real world biomechanical data

to assess helmet safety for consumers.

23

STAR Testing Process

Impact Location Drop Height Peak G Risk of InjuryExposure per

SeasonIncidence per

SeasonFront Impacts < 19 g ----- 0.0000 164 0.00

Front 12 in Average of 2 Tests From Risk(a) 138 Exp * Risk





Side Impacts < 19 g ----- 0.0000 81 0.00

Side 12 in Average of 2 Tests From Risk(a) 75 Exp * Risk





Rear Impacts < 19 g ----- 0.0000 139 0.00

Rear 12 in Average of 2 Tests From Risk(a) 165 Exp * Risk





Top Impacts < 19 g ----- 0.0000 63 0.00

Top 12 in Average of 2 Tests From Risk(a) 85 Exp * Risk





1000 Sum Incidence

STAR Value

Total Head Impacts In One Season:

STAR Data Worksheet

24

1. Rowson S, Brolinson PG, Goforth M, Dietter D, and Duma SM, “Linear and angular head accelerations measurements in collegiate football,” Journal of Biomechanical Engineering, vol. 131(6) pp. 061016, 2009.

2. Takhounts EG, Ridella SA, Hasija V, Tannous RE, Campbell JQ, Malone D, Danelson K, Stitzel J, Rowson S, and Duma S, "Investigation of traumatic brain injuries using the next generation of simulated injury monitor (SIMon) finite element head model," Stapp Car Crash Journal, vol. 52, pp. 1-31, 2008.

3. Crisco, J.J., Fiore, R., Beckwith, J.G., Chu, J.J., Brolinson, P.G., Duma, S., McAllister, T.W., Duhaime, A.C., Greenwald, R.M., Head Impact Exposures in Individual Collegiate Football Players. Journal of Athletic Training. 45(6):549-559, 2010.

4. Crisco JJ, Therrien B, Machan JT, McAllister TW, Duhaime AC, Duma S, Rowson S, Beckwith JG, Chu JJ, Greenwald RM, “Head Impact Severity Measures in Collegiate Football Players,” Journal of Applied Biomechanics, 2011.

5. Duma, S.M., Rowson, S., Past, present, and future of head injury research. Exercise and Sport Sciences Reviews. 39(1):2-3, 2011.

6. Rowson S, Goforth MW, Dietter D, Brolinson PG, and Duma SM, “Correlating cumulative sub-concussive impacts in football with player performance,” Biomedical Sciences Instrumentation, Vol. 45, pp. 113-118, 2009.

7. Duma S and Rowson S, “Every Newton hertz: a micro to macro approach investigating brain injury,” Conf Proc IEEE Eng Med Biol Soc, 2009, pp. 1123-6, 2009.

8. Rowson S, McNeely DE, and Duma SM, "Force transmission to the mandible by chin straps during head impacts in football," Biomedical Sciences Instrumentation, Vol. 44, pp. 195-200, 2008.

9. Rowson S, McNeely DE, Brolinson PG, and Duma SM, "Biomechanical analysis of football neck collars," Clinical Journal of Sports Medicine, vol. 18, pp. 316-21, 2008.

10. Shain KS, Madigan ML, Rowson S, and Duma SM, “Analysis of the ability of catcher’s masks to attenuate head accelerations upon impact with a baseball,” Clinical Journal of Sports Medicine, 2010. (Accepted)

11. Rowson S, McNeely DE, and Duma SM, "Lateral bending biomechanical analysis of neck protection devices used in football," Biomedical Sciences Instrumentation, Vol. 43, pp. 200-205, 2007.

12. Funk JR, Duma SM, Manoogian SJ, and Rowson S, "Biomechanical risk estimates for mild traumatic brain injury," Association for the Advancement of Automotive Medicine, vol. 51, pp. 343-61, 2007.

Peer Reviewed Publications: Head and Neck Data in Football

All Data and Reports Online

www.SBES.vt.edu

25

STAR Ratings of Current Helmets15 helmet models were evaluated with the STAR system

26

Fundamental Contributions

• Using on-field impact exposure to evaluate helmet performance is good science

• Lowering head acceleration lowers concussion risk

• Some helmets perform better than others

• If you remove HITS and STAR, the ratings simply show which helmets lower acceleration

• Do you want a helmet that lowers head acceleration?

Riddell Revolution Speed

Schutt ION 4D

Xenith X1

Riddell Revolution

Riddell Revolution IQ

Schutt DNA Pro +

5 and 4 Star Helmets

1 and NR Star Helmets

Adams A2000 Pro Elite

Riddell VSR4

Protection?

=><

27

• Collins et al. (2006)• Studied over 2000 high school players• Riddell Revolution reduced risk of concussion by 31%

• Virginia Tech Clinical Data (2005 - 2010)• Studied over 250 college football players• Riddell Revolution reduced risk of concussion by 85%

compared to Riddell VSR4

• STAR Evaluation System– Developed from data on over 100,000 head impacts– Predicts Riddell Revolution reduces risk of concussion

by 54% compared to Riddell VSR4

Clinical Validation of STAR

Clinical Validation of STAR

Riddell Revolution

STAR Value0.362

Riddell VSR4

STAR Value0.791

3 different studies show differences between helmets in ability to reduce concussion risk with Revolution

Collins31%

STAR54%

VT85%

>

28

What About Rotational Acceleration?• Pure rotational impacts do not occur

• Helmets are smooth, round• Helmets do not catch and rotate

the head, like animal tests did

• Linear component of the impact drives the rotational component

• Rotational acceleration is highly correlated to linear acceleration Gennarelli et al. (1982)

Fixed to the HeadIncluded both Linear and

rotational

If linear acceleration is reduced by a helmet,rotational acceleration is also reduced

Tom Foster:

29

0 20 40 60 80 100 120 140 160 180 2000

2000

4000

6000

8000

10000

12000

Linear Acceleration (g)

Ro

tati

on

al A

cce

lera

tio

n (

rad

/s2)

Sagittal RotationCoronal RotationTop Impacts

Concussive Impact Direction Comparison

Mode Concussions (g) (rad/s2) (rad/s)

Sagittal 33 102.7 ± 33.6 4986 ± 1909 22.1 ± 8.5

Coronal 7 105.8 ± 16.6 5192 ± 1166 23.0 ± 5.2

Axial 17 100.6 ± 37.1 2192 ± 1790 9.7 ± 7.9

Top Impacts

Sagittal + CoronalRotation

Future: We Include both Linear and Rotational Acceleration

• We published rotational acceleration risk function (Rowson 2011); similar to NFL

30

Concussion Incidence Minimization

RuleChanges

ProperTechnique

Better Equipment

MostEffective

3 Strategies: • Reduce exposure to head impact• Rule changes• Proper technique

+• Reduce concussion risk

for remaining head impacts• Improve helmet

design

Fewest Concussions



• Part 2: STAR Details– Review of exposure and risk analysis

• Part 3: Child Football Data

31

• Over 1.5 million head impacts• Over 100 concussive data points

• Has led to:– Improved injury criteria– Improved modeling of injury– Helmet safety evaluation

Adult

Human Data

Brain Biomechanics

≠

Injury criteria, dummies, and models based on adult data

Children are not scaled down adults

No Data

Child

3,500,000 Players6 to 13 years old

1,300,000 PlayersHigh School

College 100,000 Players

NFL 2,000 Players

5,000,000 Football Players in US

Majority of football players are between 6 and 13 years old

32

Youth Football Helmets

Adult Helmet Youth Helmet

There are very few differences between adult football helmet and youth football helmets

Data has not previously been available to design youth-specific helmets

VT-WFU KIDS Study

159 players instrumented on 7 teams in 2012

33

VT and WFU IRB Approved

Consent Obtained from Both Parents and Children

IRB ApprovalIRB Process

Youth Helmet Instrumentation• Youth helmets instrumented with:

– Standard HIT System

– 6DOF head acceleration measurement device

• Adult and youth helmets arevirtually identical

Instrumented VT Helmet Youth Helmet

34

Child Head Acceleration MeasurementData collected wirelessly for every game and practice

Exemplar Acceleration Traces

0 10 20 30 40-20

-10

0

10

20

30

40

50

60

70

Time (ms)

Lin

ear

Acc

eler

atio

n (

g)

X-AxisY-AxisZ-AxisResultant

0 10 20 30 40-2000

-1500

-1000

-500

0

500

1000

1500

2000

2500

Time (ms)

Ro

tati

on

al A

ccel

erat

ion

(ra

d/s

2 )

Max: 68 g Max: 2120 rad/s2

35

0 20 40 60 80 1000

50

100

150

200

250

300

350

400

Peak Resultant Linear Acceleration (g)

Num

ber

of Im

pact

s

Linear Acceleration Distribution

Median = 15 g

95th Percentile = 40 g

Peak = 100 g

21 impacts over 50 g

6 impacts over 80 g

Pediatric Head Impact Data

Age 06 07 08 09 10 11 12 13 14 15 16 17 18

6 – 8 Year OldsAuburn Mites

12 Instrumented Players

9 – 11 Year OldsBlacksburg Juniors


12 – 14 Year OldsBlacksburg Middle School12 Instrumented Players

9 – 11 Year OldsSouth Fork Jr Pee Wee22 Instrumented Players

10 – 12 Year OldsSouth Fork Pee Wee


15 – 18 Year OldsRonald Wilson Reagan High School


Injuries

95th

50th

Players

Impacts

0

38 g

16 g

12

1,959

2

50 g

19 g

60

13,110

2

58 g

22 g

40

14,422

2

55 g

20 g

12

2,726

124 instrumented players under 18 years old

32,217 head impacts recorded

6 players sustained concussions

36

Neurocognitive Testing

• Neurocognitive testing performed to evaluate change in visual and verbal memory, processing speed, and reaction time

• Players under 12.5 years old took Pediatric ImPACT

• Players over 12.5 years old took ImPACT

• Testing performed:– Pre-season– With injury– Post Season

MRI

MEG

Neuroimaging (MRI)

37

Motor (blue), posterior cingulate (green), and visual (red) seed-based MEG networks from an individual youth football player.

Neuroimaging (MEG)

VT-WFU KIDS Study

159 players instrumented on 7 teams in 2012

38

Acknowledgements

Center for Injury Biomechanics

COLLEGE of ENGINEERING

National Highway Safety Traffic Administration

Virginia Tech

Wake Forest University

Human Impact ToleranceAnd the STAR Helmet Rating

Stefan M. Duma and Steven RowsonSchool of Biomedical Engineering and Sciences

Virginia Tech – Wake Forest University

January 17, 2012

Documents

Human Impact Tolerance And the STAR Helmet Rating (Duma).pdf · Human Impact Tolerance And the STAR Helmet Rating ... Each overall injury risk is weighted based ... to assess helmet