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How many HITS are too many? : The use of accelerometers to
study sports-related concussion
Ellen Deibert and Richard Kryscio
Neurology; Published online before print ;
DOI 10.1212/WNL.0b013e31825875f7
This information is current as of May 16, 2012
The online version of this article, along with updated information and services, is
located on the World Wide Web at:
Neurology ® is the official journal of the American Academy of Neurology. Published continuously
since 1951, it is now a weekly with 48 issues per year. Copyright © 2012 by AAN Enterprises, Inc. All
rights reserved. Print ISSN: 0028-3878. Online ISSN: 1526-632X.
Published Ahead of Print on May 16, 2012 as 10.1212/WNL.0b013e31825875f7
How many HITS are too many?
The use of accelerometers to study sports-related concussion
Ellen Deibert, MD
Richard Kryscio, PhD
Correspondence & reprint
requests to Dr. Deibert:
[email protected]
Neurology® 2012;78:1712–1713
Concussion is a common contact sports injury.
There is growing concern that repetitive head trauma
can lead to the pathologic findings associated with
chronic traumatic encephalopathy (CTE), a progressive neurodegenerative process leading to pronounced behavior and cognitive dysfunction.1
Ideally, clinicians would have a way to predict CTE
and prevent it in our athletes.
An important consideration is determining the
number and magnitude of head impacts suffered by
contact sport athletes. Early studies examined film on
concussed National Football League players and
reconstructed the impact velocities using Hybrid III
crash test dummies; the analysis suggested that concussed players suffered an average peak linear acceleration of 98 ⫾ 28 g (force of gravity).2 This method
was greatly improved by the Head Injury Telemetry
System (HITS), which uses helmets with incorporated accelerometers, validated in multiple studies.3
HITS technology allows researchers to use the playing field as a “laboratory” to study real-time head
impacts. For instance, a study of 88 collegiate football players with 104,714 recorded impacts revealed
that concussion symptoms developed with varying
magnitudes of acceleration (range 60 –169 g) and location.4 Further studies suggested that a more accurate prediction of concussion required a combination
of biomechanical forces rather than a single measure.
This led to the development of the HIT severity profile (HITsp), an algorithm that combines forces of
linear acceleration, rotational acceleration, head injury criterion (HIC), and impact location.5 The HIC
is a mathematical measure of the likelihood of head
injury derived from the information produced by accelerometers looking at acceleration over time.
Forces predictive of concussion in high school players include linear acceleration of 96 g, rotational acceleration of 5,500 rad/s, and locations of front, top,
and back of the head.6
The number of impacts per athlete in a season can
be staggering. A football player could sustain 8,000
impacts in a 4-year high school and 4-year college
career combined.7 Despite the large number of impacts in a career, there are relatively few reported
concussions. This raises the concern that athletes underreport injuries and increases the worry that
asymptomatic hits may have cumulative effects.
Hence, the research conducted by McAllister et al.8
published in this issue of Neurology® is important
and timely. This is a prospective cohort study of 214
National Collegiate Athletic Association Division I
athletes stratified by contact (n ⫽ 159 football and
ice hockey players) and noncontact (n ⫽ 45 crew,
Nordic skiing, and track) sports. Impact measurements utilizing HITS were analyzed for 3 different
time intervals: first day of preseason, 1 week prior to
the end of season, and the last day of the season. All
athletes took Immediate Postconcussion Assessment
and Cognitive Test (ImPACT) pre and post season.
A small cohort of athletes also underwent standardized neuropsychological testing pre and post season.
To correct for possible practice effects, the authors
used a regression-based z-score approach to set a
“reasonable threshold” for post season performance.
The authors found that a higher percentage of contact athletes performed more than 1.5 standard deviations below their predicted level on the California
Verbal Learning Test (CVLT) ( p ⫽ 0.006). In addition, head impact exposure during the last week of
play revealed higher magnitude and frequency of impacts. This correlated with poorer performance on
the Reaction Time Composite on ImPACT and
Trails4/B subtest of the formal test battery. The
authors summarized these findings as follows: 1) contact sport athletes performed below expectations on the
CVLT, suggesting that a subgroup suffer potential
problems with learning and memory after multiple impacts; and 2) the increased head impacts of higher magnitudes and frequencies at the end of the season may
affect cognitive performance. A strength of this study is
that it found possible significant correlations in the
group of athletes tested with more formal tests. As the
authors suggest, further research in this area is needed
before any true conclusions can be made.
See page 1777
From the Department of Neurology (E.D.), University of Massachusetts, Amherst; and Statistics and Biostatistics (R.K.), University of Kentucky, Lexington.
Go to for full disclosures. Disclosures deemed relevant by the authors, if any, are provided at the end of this editorial.
Copyright © 2012 by AAN Enterprises, Inc.
Several aspects of this study require further consideration. First, care needs to be taken when group
biomechanical data are translated to the individual.
Each person may have individual risks for symptoms
resulting in a protracted course of concussion, such
as youth, prior concussion history, and learning disabilities, none of which are included in this study.4,9
Impact risks vary within a given sport by position,
playing time, and skill set. For instance, in football,
lineman have the highest number of hits overall, but
other skilled positions (quarterback) can have a
higher number of more severe impacts.3 The number
of hits per contact sport participant in the study varied widely, diluting the comparison between sport
types. We would expect to observe a significantly different change in mean scores between pre and post
season in the sports contact group, but this did not
happen except for the auditory addition task. In addition, there is no adjustment for testing multiple
Using tools such as ImPACT, Automated Neuropsychological Assessment Metrics, or the Standardized Assessment of Concussion to measure cognitive
change in patients without a clinical diagnosis of
concussion is questionable. ImPACT was designed
to evaluate athletes with a possible concussion and
has 81.9% sensitivity and 89.4% specificity when
used in the appropriate setting. It should be used
within 72 hours of a suspected concussion.10 Outside
of that setting, its validity is unknown. McAllister et
al. performed neuropsychological assessment 27 ⫾
24 days after the athletes’ last contact sport exposure.
So “how many hits are too many” is as yet unknown. Collaborative efforts need to continue
among biomechanical and clinical researchers. The
magnitude and frequency of hits will not be the only
answer, as the many clinical variables involved in this
injury may ultimately outweigh the biomechanics.
Dr. Deibert reports no disclosures relevant to the manuscript. Dr. Kryscio
serves as an Associate Editor for Neurology®. Go to for
full disclosures.
1. Mckee AC, Cantu RC, Nowinski CJ, et al. Chronic traumatic encephalopathy in athletes: progressive tauopathy after repetitive head injury. J Neuropathol Exp Neurol 2009;
68:709 –735.
2. Pellman EJ, Viano DC, Tucker AM, et al. Concussion in
professional football: reconstruction of game impacts and
injuries. Neurosurgery 2003;53:799 – 814.
3. Funk JR, Rowson S, Daniel RW, et al. Validation of concussion risk curves for collegiate football players derived
from HITS data. Ann Biomed Eng 2012;40:79 – 89.
4. Guskiewicz KM, Mihalik JP, Shankar V, et al. Measurement of head impacts in collegiate football players: relationship between head impact biomechanics and acute
clinical outcome after collision. Neurosurgery 2007;61:
1244 –1253.
5. Greenwald RM, Gwin JT, Chu JJ, et al. Head impact severity measures for evaluating mild traumatic brain injury
risk exposure. Neurosurgery 2008;62:789 –798.
6. Broglio SP, Schnebel B, Sosnoff JJ, et al. Biomechanical
properties of high school football. Med Sci Sports Exerc
2010;42:2064 –2071.
7. Broglio SP, Eckner JT, Martini D, et al. Cumulative head
impact burden in high school football. J Neurotrauma
2011;28:2069 –2078.
8. McAllister TW, Flashman LA, Maerlender A, et al. Cognitive effects of one season of head impacts in a cohort of
collegiate contact sport athletes. Neurology 2012;78:
9. Guskiewicz KM, Mihalik JP. Biomechanics of sport concussion: quest for the elusive injury threshold. Exerc Sport
Sci Rev 2011;39:4 –11.
10. Schatz P, Pardini JE, Lovell MR, et al. Sensitivity and specificity of the ImPACT test battery for concussion in athletes. Arch Clin Neuropsychol 2005;1–9.
Neurology 78
May 29, 2012
How many HITS are too many? : The use of accelerometers to study sports-related
Ellen Deibert and Richard Kryscio
Neurology; Published online before print ;
DOI 10.1212/WNL.0b013e31825875f7
This information is current as of May 16, 2012
Updated Information &
including high resolution figures, can be found at:
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Information about reproducing this article in parts (figures,
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Quiz: Concussions (Grades Grades 3 to 5)
Quiz: Concussions (Grades Grades 3 to 5)