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ARTICLE
Injuries in Canadian Youth Ice Hockey: The Influence
of Relative Age
Nick Wattie, BPHE, BSca, Stephen Cobley, BSc, MAb, Alison Macpherson, PhDa,c, Andrew Howard, MD, MSc, FRCSCd,e,
William J. Montelpare, PhDf, Joseph Baker, PhDa,b
a
AQ: A
Lifespan Health and Performance Laboratory, School of Kinesiology and Health Science, York University, Toronto, Ontario, Canada; bCarnegie Research Institute, Leeds
Metropolitan University, West Yorkshire, United Kingdom; cInstitute for Clinical Evaluative Sciences, Toronto, Ontario, Canada; dDivision of Orthopaedic Surgery, Hospital
for Sick Children, Toronto, Ontario, Canada; ePopulation Health Sciences, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada; fSchool of Kinesiology,
Lakehead University, Thunder Bay, Ontario, Canada
The authors have indicated they have no financial relationships relevant to this article to disclose.
ABSTRACT
AQ: B
OBJECTIVE. The purpose of this study was to investigate the relationship between
relative age and injury prevalence in Canadian youth ice hockey.
METHODS. In study 1, youth ice hockey–related injuries (among children 10 –15 years
of age) collected by the Canadian Hospitals Injury Reporting and Prevention
Program between 1995 and 2002 were analyzed. The relative ages of injured
children were compared across different age groups and injury characteristics
(mechanism of injury and severity of injury). In study 2, injuries reported in the
Hockey Canada Insurance Database were analyzed. The relative ages of injured
children at different levels of play (ie, representative versus house league teams)
were compared.
RESULTS. In study 1, the majority of injured players were of older relative age.
However, relative age was not related to mechanism of injury or severity of injury.
In study 2, 70% to 80% of injured players at the highest level of play were
relatively older, whereas only 50% to 55% of house league injured players were
relatively older.
CONCLUSION. Relatively older children within ice hockey age groups are at increased
risk of injury compared with their younger peers. Furthermore, the risk of injury
for relatively older players is greater at more-competitive levels of play. This study
proposes that the relative age advantage associated with selection to Canadian
youth ice hockey teams is accompanied by an increased risk of injury.
www.pediatrics.org/cgi/doi/10.1542/
peds.2006-2672
doi:10.1542/peds.2006-2672
Key Words
relative age effect, injury, sports, bodychecking
Abbreviations
CHIRPP—Canadian Hospitals Injury
Reporting Prevention Program
HCID—Hockey Canada Insurance
Database
OR— odds ratio
CI— confidence interval
RAE—relative age effect
Accepted for publication Mar 15, 2007
Address correspondence to Nick Wattie, BPHE,
BSc, School of Kinesiology and Health Science,
York University, Toronto, Ontario, Canada M3J
1P3. E-mail: [email protected]
PEDIATRICS (ISSN Numbers: Print, 0031-4005;
Online, 1098-4275). Copyright © 2007 by the
American Academy of Pediatrics
PEDIATRICS Volume 120, Number 1, July 2007
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A
T FIRST GLANCE, annual age-grouping strategies in
both sports and education seem to be a developmentally appropriate way to distribute children. This has
been achieved by establishing “cutoff” date criteria,
which ensure that children have turned a specific age
before being placed in a certain grade for education or
being eligible to participate on certain sports teams. For
example, with a cutoff date of September 1, only children who had turned 6 years of age before September 1
of the current academic year would be allowed to enter
first grade. However, there are problems with such an
age-grouping strategy, as exemplified by relative age
effects (RAEs).1,2 RAEs describe the potential advantages
(or disadvantages) that result from age differences between peers within an annually age-grouped cohort.
From the previous example, someone born shortly after
September 1 would be almost 1 year older than someone
born on August 30, but both would be in the same grade.
In sports, the consequences of such relative age differences suggest that relatively older members of a cohort are more likely to be selected for local junior
teams3,4 and representative teams5,6 and to attain elite
professional status,2,7,8 which highlights participation
and attainment inequality. RAEs have been identified in
many sports (eg, baseball,8 ice hockey,9 swimming,10 and
basketball11) and across cultural contexts (eg, in Belgium, Holland, and France,12 England,13 and Australia,
Brazil, Germany, and Japan14).
Regardless of alterations in cutoff dates or climatic
seasonal variations, age-grouping RAEs persist.15,16 To
date, physical maturational,8,17 experiential,18 and psychological16 hypotheses have been presented to account
for RAE prevalence. Specifically, relative age differences,
especially in prematurity stages, may create significant
height and weight differences among youths participating in sports. This may confer specific physical performance advantages for relatively older players,19,20 leading
to identification and selection for higher levels of competition. With the amount of time spent in sport-specific
practice being closely related to attainment21 and there
being variability of lived experience between relatively
older and younger players, the amount of potential practice and game experience may result in a skill acquisition
disadvantage for relatively younger players. Relatively
younger players may not display the equivalent skills
and competencies, compared with their relatively older
peers. The psychological hypothesis suggests that personal perceptions of ability and competence directly influence behavior and participation.22 Positive competence is thought to reinforce and to maintain motivation
for a given behavior (eg, sports participation), whereas
negative perceptions, including affect, emotion, and
stress, may lead to cessation. Therefore, relatively
younger players may develop low perceived competence
when skills or performance are not equivalent to those
of their relatively older counterparts.16 Such perceptions
2
WATTIE et al
may lead to reductions in effort, interest, and investment
and ultimately to termination of sports participation.23
Separately or possibly in combination, these hypotheses suggest how age-grouping systems can lead to selection and attainment discrepancies in sports. To date,
most relative age research has focused on simply identifying selection and attainment advantages associated
with relative age differences. However, these may not be
the only unintentional outcomes of age grouping; other
implications may be evident but as yet not identified. For
instance, in sports (eg, ice hockey) in which physical
factors such as height, weight, speed, power, and coordination underlie performance, it is plausible that such
differences may lead to discrepancies in injury incidence
according to relative age in physically aggressive sports.
Currently, research has not examined intra-age group
(ie, relative age) patterns of injury.
The purpose of the present study was to examine
whether relative age was related to physical injury as a
result of participation in youth ice hockey (a physicalcontact, high-strategy, team sport) and whether the proportions of relative age injuries changed across youth
development phases (ie, inter-age group). Furthermore,
given the physical contact inherent to ice hockey and the
hypothesized influence of physical size and strength on
creating RAEs, a secondary purpose of the current study
was to examine whether players of different relative
ages were more or less likely to be injured as the result
of body contact or body checking and more or less likely
to incur severe injuries. This secondary purpose was of
interest because previous research identified body
checking as being associated with injury severity.24
Previous research on the influence of relative age
found advantages exclusively for relatively older children, compared with their younger peers.16 It has been
hypothesized that relatively younger players are at a
physical disadvantage with respect to size and strength
and that this disadvantage, compared with their older
peers, results in less selection to youth ice hockey teams.
We hypothesized that these maturational physical discrepancies between players of different relative ages
would result in a greater proportion of injured players
being relatively younger. Similarly, because relatively
older players are thought to be larger and stronger than
their younger peers, we hypothesized that relatively
younger injured players would have incurred a greater
proportion of their injuries through physically aggressive
contact (ie, body checks). Because body checks have
been related to more-severe injuries, we also hypothesized that relatively younger players would incur a
greater proportion of severe injuries.
Level of play (or competition level) was identified
previously as an important covariate in relative age research, and this is particularly true of relative age research on youth ice hockey. Barnsley and Thompson17
observed that the RAE in youth hockey was more pro-
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nounced at higher tiers in all age groupings. It was even
observed that the lowest tiers of competition demonstrate a significant excess of relatively younger children.
If level of play can influence the magnitude and existence of RAEs in a sport, then due consideration of this
covariate is necessary. The purpose of the second study
was to examine a secondary hockey-injury data source
that contained information on level of play, in addition
to information on age group and relative age.
METHODS
AQ: C
Study 1
Sample
The study sample consisted of boys 10 to 15 years of age
(n ⫽ 4736) who visited pediatric emergency departments participating in the Canadian Hospitals Injury
Reporting and Prevention Program (CHIRPP). The
CHIRPP is an injury surveillance program conducted in
certain pediatric emergency departments. All participating hospitals in Ontario and Quebec (excluding the Children’s Hospital of Eastern Ontario) were included. All ice
hockey–related injuries reported by the CHIRPP between September 1995 and August 2002 were included.
CHIRPP data include demographic variables and numerous variables pertaining to injuries. Specific injury information, such as injury type and severity, were reported
by the treating physician, whereas information on how
the injury was sustained was provided by the patient and
parents.25
Measures
The primary independent measure was the relative age
of the injured ice hockey players, as reported by the
CHIRPP. Consistent with previous relative age research,
players’ birth dates were grouped into monthly quartiles
starting from the selection date criteria (in this case,
starting on January 1 and ending on December 31 of the
same calendar year). For example, quartile 1 included
January, February, and March birth dates and quartile 4
included October, November, and December birth dates.
Subjects born in quartile 1 were therefore relatively
older than their peers born in quartile 4.
Several moderating variables were examined, to ascertain their interactions with relative age and subsequent RAEs. From 1995 to 2002 (7 ice hockey seasons),
the youth ice hockey age groups dictated by Hockey
Canada were Atom (10 –11 years), Peewee (12–13
years), and Bantam (14 –15 years). To determine
whether relative age injury proportions were moderated
by age groups, the proportions of relative age injuries
were compared at different age groups.
Another moderating variable of interest was the
mechanism factor, reported as “how the injury occurred” on the CHIRPP form.25 Injury was dichotomized
as either resulting from a body check or not resulting
from a body check. Hockey Canada defines body checking as the “physical extension of the body toward the
puck carrier moving in an opposite or parallel direction.”26 Methods for classifying injuries as checking related or not checking related were consistent with previous use of this CHIRPP data set for examination of
youth ice hockey–related injuries.24
In light of the previously identified relationship between injuries resulting from body checks and the severity of injuries,24 a measure of injury severity was
included in the analyses. Injuries were categorized as
severe if the athlete was admitted to the hospital and
nonsevere if the athlete was not admitted.
Analyses
Statistical analyses were conducted by using SPSS 15.0
for Windows (SPSS, Chicago, IL). Frequencies of relative
age (ie, quartile of birth) and age group (Atom, Peewee,
or Bantam) were used to describe the study sample; ␹2
statistics (significance of P ⬍ .05, assuming equal distribution) were used to compare the observed versus expected proportions of relative age injuries in each of the
age groups (Atom, Peewee, and Bantam). Logistic regression analyses were used to examine whether relative
age was associated with (1) the risk of being injured in
different age groups, (2) the risk of being injured by a
body check, and (3) the risk of incurring a severe injury.
The aforementioned risks (odds ratios [ORs]) were calculated by comparing the proportion of quartile 1 injured players with that of quartile 4 injured players
(using 95% confidence intervals [CIs]).
Study 2
Sample
The Hockey Canada National Insurance Program provides financial resources to help compensate players and
families that have experienced financial loss as the result
of hockey participation in Canada.27 The study sample
consisted of hockey injuries reported to the Hockey Canada National Insurance Program between September
1998 and August 2003. Each claim made to the Hockey
Canada Insurance Database (HCID) contained information on the date of injury and how the injury occurred,
as well as each player’s date of birth, age group, and level
of play.
Measures
The primary independent measure of this study was the
relative age of the injured athletes. Players’ birth dates
were grouped into monthly quartiles (quartile 1: January to March; quartile 2: April to June; quartile 3: July to
September; quartile 4: October to December).
A moderating variable of interest included in analyses
was the age group of the injured players. The age groups
included in the HCID were Peewee (12–13 years) and
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Bantam (14 –15 years). Within each of the age groups,
there were 10 level-of-play tiers, including AAA (the
highest, competitive-level tier), AA, A, B, C, CC, D, DD,
E, and house league (the lowest tier). For the purpose of
analysis, the “intermediate” B, C, CC, D, DD, and E tiers
were combined into 1 group, to maintain disclosure
confidentiality of players. Therefore, in the analyses, 5
levels of play were examined as potential moderating
variables (AAA, AA, A, intermediate tiers, and house
league). Because the purpose of this study was to examine how the level of play influences the proportion of
relative age injuries, all cases that contained no information on the level of play were excluded from analyses.
Information on how the injury occurred was used to
create a “mechanism of injury” variable. Twelve descriptors were used throughout the HCID to express how
injuries occurred, of which 10 were used to create the
mechanism of injury variable. Injuries that had no information on how the injured occurred, as well as injuries that were the result of a fight, were not included in
the analyses. The descriptors of “blindsiding,” “checked
from behind,” “collision with opponent,” and “collision
on open ice” were categorized as injury resulting from a
body check. Conversely, the descriptors of “collision
with boards/net,” “fell on ice,” “hit by puck/stick,” and
“noncontact injury” were categorized as injury that did
not result from a body check.
Analyses
Statistical analyses were conducted by using SPSS 15.0
for Windows. Frequencies and percentages of injuries
according to relative age category (quartile of birth), age
group (Peewee or Bantam), and level of play were used
to describe the study sample. To test for injury asymmetries according to quartile of birth, ␹2 analyses were
conducted; ␹2 tests (significance of P ⬍ .05, assuming
equal distribution) were conducted for all levels of play
and both age groups. Logistic regression analyses were
used to examine (1) the risk of injury for those of different relative ages at different levels of play and (2) the
risk associated with mechanism of injury (body checked
versus not body checked) for those of different relative
ages. For all logistic regression analyses, risks (ORs) were
calculated by comparing the proportion of quartile 1
injured players with that of quartile 4 injured players
(using 95% CIs).
RESULTS
Study 1
There were 4736 ice hockey injuries reported by the
CHIRPP between 1995 and 2002. Overall, the proportion of injuries varied significantly according to age
group. Of the overall sample, 18% were children in the
Atom (10 –11 years) age group, 37% in the Peewee
(12–13 years) age group, and 46% in the Bantam
4
WATTIE et al
(14 –15 years) age group (␹2 ⫽ 591.13; P ⬍ .001). The
proportions of injuries according to relative age category
are presented in Table 1. A RAE was found within each
of the 3 age groups. Birth dates (by quartile) were distributed unevenly, with a greater-than-expected number of injuries sustained by quartile 1 players and a
less-than-expected number of injuries sustained by
quartile 4 players (Atom: ␹2 ⫽ 11.20; P ⫽ .001; Peewee:
␹2 ⫽ 32.57; P ⬍ .001; Bantam: ␹2 ⫽ 42.04; P ⬍ .001).
Quartiles 2 and 3 did not differ significantly from each
other or from expected values and were excluded from
analyses; however, parallel analyses were conducted
with those quartiles included, and the direction and
significance of the observed RAEs were not affected.
Although a RAE was observed within each of the 3
age groups, regression analyses suggested that the risk of
injuries according to relative age did not change from
one age group to another. More specifically, on the basis
of a comparison of the risk of injury for relatively older
players versus relatively younger players in the Atom
age group, the risk of injury for relatively older players
(quartile 1) did not increase or decrease in the Peewee
(OR: 1.05; 95% CI: 0.82–1.34) and Bantam (OR: 1.05;
95% CI: 0.83–1.33) age groups.
Neither older (quartile 1) nor younger (quartile 4)
relative age was associated with a disproportionate risk
of being injured as the result of a body check in any of
the age groups (Atom: OR: 0.83; 95% CI: 0.53–1.31;
Peewee: OR: 0.95; 95% CI: 0.71–1.25; Bantam: OR:
1.05; 95% CI: 0.82–1.34). Furthermore, neither older
nor younger relative age was found to be a significant
risk factor for incurring severe injuries in any age group
(Atom: OR: 1.79; 95% CI: 0.47– 6.80; Peewee: OR: 1.08;
95% CI: 0.43–2.72; Bantam: OR: 0.73; 95% CI: 0.37–
1.45).
Study 2
Of the 6864 (Peewee and Bantam) injuries reported by
the HCID, 5681 had information on the variables of
interest in this study and were retained for additional
analyses. Of the final study sample, a greater proportion
of injured players were in the Bantam age group (58%)
than in the Peewee age group (␹2 ⫽ 144.16; P ⬍ .001).
The proportions of relative age injuries (quartile 1
versus quartile 4) for the Peewee and Bantam age groups
are presented in Figs 1 and 2. The ␹2 analyses revealed
TABLE 1 Frequency and Proportion of Injuries According to
Relative Age Category
Age Group
Atom (10–11 y)
Peewee (12–13 y)
Bantam (14–15 y)
a Birth
No. (%)
Quartile 1a
Quartile 4
221 (26.6)
506 (29.1)
646 (29.8)
156 (18.8)
340 (19.6)
433 (20.0)
quartile 1 was January to March; quartile 4 was October to December.
AQ: E
T1
AQ: F
F1-2
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45.0%
Q1
Q4
40.0%
35.0%
30.0%
25.0%
20.0%
15.0%
10.0%
5.0%
0.0%
AAA
AQ: M
T2
AQ: G
AA
A
Intermediate
House
FIGURE 1
Percentages of injuries in older (quartile 1) and younger (quartile 4) relative age categories for different levels of play in the Peewee age group. Q1 indicates quartile 1; Q4, quartile 4.
that, within both the Peewee and Bantam age groups,
RAEs existed at various levels of competition. Within the
Peewee age group (Fig 1), greater proportions of injuries
were incurred by players born in quartile 1 than in
quartile 4 for the AAA, AA, and A levels of play (␹2 ⫽
21.05–36.16; P ⬍ .001) but not the for the intermediate
level of play (␹2 ⫽ 1.01; P ⫽ .31) or the house league
level of play (␹2 ⫽ 2.05; P ⫽ .15). The same was true
within the Bantam age group (Fig 2); greater proportions of injured players were born in quartile 1 than in
quartile 4 for the AAA, AA, and A levels of play (␹2 ⫽
10.65–73.59; P ⬍ .01) but not for the intermediate level
of play (␹2 ⫽ 1.45; P ⫽ .70) or the house league level of
play (␹2 ⫽ 0.01; P ⫽ .89). As in study 1, quartile 2 and
quartile 3 were excluded from analyses because they did
not differ significantly from each other or from expected
values.
Results from the logistic regression analysis showed
that there were intra-level-of-play variations in the proportions of relative age injuries in both the Peewee and
Bantam age groups (Table 2). More specifically, the magnitude of the injury RAE varied according to the level of
play. For example, Bantam AAA injured players were
3.21 times more likely to be relatively older (ie, born in
quartile 1, rather than in quartile 4) than were Bantam
house league injured players. In both age groups, the
risk of injury for relatively older players increased as the
level of play increased from least competitive to most
competitive. Logistic regression analyses revealed that
differences in relative age (in both age groups and all
levels of play) did not result in disproportionate risk of
being injured as the result of a body check.
DISCUSSION
Study 1
Results of this study suggest that there are both inter-age
group patterns of injury, with overall injury prevalence
increasing progressively from Atom to Bantam age
groups, and intra-age group patterns of injury. Within
the Atom, Peewee, and Bantam age groups, greater proportions of children (26%–29%) were born in quartile 1,
compared with quartile 4 (18%–20%), and thus were of
older relative age. Therefore, whereas relatively older
children are more likely to be selected for youth hockey
teams,17 they are also more likely to present to hospital
emergency departments with hockey-related injuries.
Within the scope of relative age research, these findings
are decidedly atypical; previous research in both education28 and sports16 found consistent advantages in attainment for relatively older children, compared with their
relatively younger peers. Interestingly, these results suggest that the advantage in selection and attainment for
relatively older players is accompanied by the disadvantage of increased proportion of injuries. The overall
prevalence of injury increased progressively from Atom
to Bantam age groups but the risk of injury for relatively
older players did not change, which suggests that the
injury disadvantage for relatively older players is stable.
Although it showed surprising results, this study did
have limitations. First, the study was not population
based. Although the CHIRPP has been found to be a
valid indicator of overall youth injury patterns in Canada,29 ideally the relative age of injured children would
have been compared with that of children who were not
injured. Second, the nature of the data collected by the
CHIRPP did not provide information on the children’s
level of play (ie, competitive versus recreational/house
leagues) within each age grouping. Previous research
found the RAE in youth hockey to be more pronounced
at higher levels of play (ie, more-competitive hockey)
and nonexistent, or even reversed, at lower levels of
play.17 Because level of play has been identified as a
major covariate of the RAE, failure to control for level of
play represents a major limitation of this study and may
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45.0%
Q1
40.0%
Q4
35.0%
30.0%
25.0%
20.0%
15.0%
10.0%
5.0%
0.0%
AAA
AA
A
Intermediate
House
FIGURE 2
Percentages of injuries in older (quartile 1) and younger (quartile 4) relative age categories for different levels of play in the Bantam age group. Q1 indicates quartile 1; Q4, quartile 4.
even account for the atypical direction of the RAE observed for youth ice hockey injuries. We focused on this
limitation in study 2.
Study 2
The results of study 2 suggest that the prevalence of
injuries increases from younger age groups to older age
groups (ie, from Peewee to Bantam) and that, within
each age group, relatively older players experience a
greater proportion of injuries than do their relatively
younger peers. Like study 1, study 2 presented both
inter-age group patterns of injury and intra-age group
patterns of injury. Furthermore, as in study 1, relative
age was not related to mechanism of injury (ie, being
injured by a body check or not). However, the addition
of level of play as a covariate in study 2 demonstrated
that the relationship between relative age and youth
hockey injury was more dynamic than revealed previously. The addition of level of play as a covariate suggested that the proportion and risk of injury increased
for relatively older players as the level of play became
more competitive (Table 2).
Like study 1, this study was limited by the fact that it
was not population based. Future research would benefit from examination of both injured and noninjured
players when injury RAEs are considered. Although this
study did control for level of play, the HCID did not
provide a measure of injury severity. Although there
was no relationship between relative age and mechanism of injury, as in study 1, it cannot be assumed that
level of play does not interact with relative age and
severity of injury.
General Observations
Both study 1 and study 2 support the notion that relatively older players at the highest levels of competition
6
WATTIE et al
are at increased risk of injury. Although at first it might
seem counterintuitive that these children, with more
experience and skill, would be at greater injury risk, it is
possible that they are actually playing more, playing at
higher competitive levels, or both and thus are more
likely to be injured because of greater exposure. Research investigating the RAE in soccer not only found a
relative age advantage for selection to soccer teams but
also found that relatively older players had more playing
time in games, compared with their younger peers.30
Because relatively older participants are selected because
of their greater physical maturity, skill, and experience,16
it seems reasonable that these individuals would play
longer and more often. If the same disproportionate
amount of playing time is present for relatively older
children in Canadian youth ice hockey, then these players would have increased exposure to the inherent risks
associated with playing the game. Researchers should
TABLE 2 Odds of Injury for Relatively Older Players (Quartile 1),
Compared With Relatively Younger Players (Quartile 4), at
Different Levels of Play Within the Peewee and Bantam
Age Groups
Age Group
Level of Play
OR (95% CI)
Peewee (12–13 y)
AAA
AA
A
B/C/D/E
House league
AAA
AA
A
B/C/D/E
House league
3.53 (1.90–6.56)
1.78 (1.24–2.56)
1.42 (1.00–2.01)
0.93 (0.66–1.32)
1
3.21 (2.18–4.73)
2.23 (1.58–3.14)
1.45 (1.03–2.06)
1.06 (0.75–1.50)
1
Bantam (14–15 y)
The risk of injury for relatively older players (quartile 1) at the different levels of play was based
on comparison with the lowest level of play (house league).
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consider differences in exposure time as a possible explanatory factor in this unusual disadvantage for relatively older players.
Our results suggest that the profile of RAEs is more
complex than considered previously. In addition to the
selection and attainment advantages noted consistently
for relatively older players in a range of sports, these
participants are at increased risk of injury.
10.
11.
12.
13.
Implications for Coaches and Athletes
The nature of the data in study 2 did not make it possible
to examine the interaction between relative age, level of
play, and injury severity. Such research would benefit
from consideration of injury severity when relative age
and level of play are examined, because this might provide a more-compelling view of the implications of relative age injuries. Similarly, information regarding how
relative age differences affect the length of time an injured athlete is away from play or participation may be
more tangible to coaches, parents, and athletes. It is
noteworthy that increased injuries have been identified
as one of the reasons why, as a result of decreased fun
and performance, youths drop out of sports.31
There has been a great deal of debate regarding strategies and rule changes aimed at decreasing the risk of
injury in Canadian youth ice hockey.24,32 The results
from this study suggest that, regardless of the strategy
adopted, coaches, parents, and athletes should be cognizant of the potential factors leading to increased injury
risk for relatively older ice hockey players, particularly at
higher levels of competition.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
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2. Barnsley RH, Thompson AH, Barnsley PE. Hockey success and
birthdate: the relative age effect. Can Assoc Health Phys Educ
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