combat

1. 940 Orthopedics & Biomechanics
Knapik J et al. Stress Fracture Risk Factors… Int J Sports Med 2012; 33: 940–946
accepted after revision
February 21, 2012
Bibliography
DOI http://dx.doi.org/
10.1055/s-0032-1311583
Published online:
July 20, 2012
Int J Sports Med 2012; 33:
940–946 © Georg Thieme
Verlag KG Stuttgart · New York
ISSN 0172-4622
Correspondence
Dr. Joseph Knapik
Army Institute of Public Health
(AIPH) Injury Prevention
ATN: MCHB-TS-DI, APG
21010
United States
Tel.: +1/410/436 1328
Fax: +1/410/436 5449
joseph.knapik@us.army.mil
Key words
●
▶ age
●
▶ gender
●
▶ body mass index
●
▶ height
●
▶ weight
●
▶ race/ethnicity
Stress Fracture Risk Factors in Basic Combat Training
Like athletes, recruits in United States (US) Army
Basic Combat Training (BCT) are exposed to a
heavy schedule of physical activity. These activi-
ties include running, calisthenics, drill and cere-
mony, foot marching, obstacle courses, and other
repetitive activities. The cumulative cycles of
bone loading from these activities in a relatively
short period, coupled with the imbalances in the
bone remodeling processes, appear to increase
stress fracture risk in susceptible individuals
[31,67]. Individuals experiencing stress fractures
in BCT are often removed from training and enter
prolonged rehabilitation that lasts an average of
62 days [26].
Previous work in basic training has shown that
factors increasing susceptibility to stress frac-
tures include female gender [7,30,39,40,46],
older age [21,38,40] and race/ethnicity other
than black [7,18,21,33,38,40]. The association
between stress fracture risk and physical charac-
teristics like height, weight, and BMI is not clear
because results were mixed in the various inves-
tigations [2,16,22,23,33,40,55,63].
The purpose of the present investigation was to
examine the association between age, physical
Introduction
▼
A stress fracture is a debilitating injury common
among participants in a number of weight bear-
ing sports including running, ballet, lacrosse,
crew, football, basketball, and soccer [4]. A stress
fracture is a partial or incomplete bone rupture
associated with habitual mechanical deforma-
tion of the bone. It is an overuse-type injury
generally occurring in individuals with other-
wise healthy bones, often in association with
unaccustomed but repetitive physical activity
[31,42,52,66]. The current etiologic hypothesis
is that repetitive mechanical loading results in an
increase in cyclic hydrostatic pressures which are
sensed by osteocytes within the bone matrix.
Through a variety of mechanisms still under
study, the mechanical pressures stimulate osteo-
cyte-directed transcription of osteoclastics
which begin the bone remodeling process. How-
ever, osteoclastic processes which resorb bone
proceed at a faster pace than osteoblastic proc-
esses that form new bone. This results in a vul-
nerable period when the bone is weakened and
susceptible to stress fractures [8,52].
Authors J. Knapik1
, S. J. Montain2
, S. McGraw2
, T. Grier1
, M. Ely2
, B. H. Jones1
Affiliations 1
US Army Institute of Public Health, Aberdeen Proving Ground MD, USA
2
US Army Research Institute of Environmental Medicine, Natick MA, USA
Abstract
▼
This study examined demographic and physical
risk factors for stress fractures in a large cohort
of basic trainees. New recruits participating in US
Army BCT from 1997 through 2007 were identi-
fied, and birth year, race/ethnicity, physical char-
acteristics, body mass index, and injuries were
obtained from electronic databases. Injury cases
were recruits medically diagnosed with inpa-
tient or outpatient stress fractures. There were
475745 men and 107906 women. Stress frac-
tures incidences were 19.3 and 79.9 cases/1000
recruits for men and women, respectively. Fac-
tors that increased stress fracture risk for both
men and women included older age, lower body
weight, lower BMI, and race/ethnicity other than
black. Compared to Asians, those of white race/
ethnicity were at higher stress fractures risk.
In addition, men, but not women, who were
taller or heavier were at increased stress frac-
ture risk. Stress fracture risk generally increased
with age (17–35 year range) at a rate of 2.2 and
3.9 cases/1000 recruits per year for men and
women, respectively. This was the largest sam-
ple of military recruits ever examined for stress
fractures and found that stress fracture risk was
elevated among recruits who were female, older,
had lower body weight, had lower BMI, and/or
were not of black race/ethnicity.
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2. 941
Orthopedics & Biomechanics
Knapik J et al. Stress Fracture Risk Factors… Int J Sports Med 2012; 33: 940–946
characteristics, and race/ethnicity in a large sample of US Army
recruits. Included in this investigation were over 580000 recruits
and virtually the entire US Army recruit population over an
11-year period.
Methods
▼
This retrospective cohort study utilized databases at the Armed
Forces Health Surveillance Center (AFHSC). These were 1) the
Defense Manpower Data Center (DMDC) Master Personnel File,
use to identify basic trainees and their age and race/ethnicity, 2)
the Defense Medical Surveillance System (DMSS) used to obtain
injury data and 3) the Military Entrance Processing Station
(MEPS) database for gender and physical characteristics (height
and weight). The study protocol was reviewed and endorsed by
the Human Use Review Board of the US Army Research Institute
of Environmental Medicine. The study met the ethical standards
of the International Journal of Sports Medicine [25].
Basic trainees were identified from the date of their first demo-
graphic record in the DMDC Master Personnel File. To be consid-
ered for the study, the individual must have had a rank of private
(E1) to specialist (E4) and be 17–35 years of age at that first
record. DMDC records from January 1997 to December 2007
(11-year period) were searched and all recruits meeting the cri-
teria within this timeframe were considered for inclusion in the
study. Inpatient and outpatient medical encounters for this
cohort of basic trainees were obtained from the DMSS. The
DMSS included data from the Standard Ambulatory Data Record,
Standard Inpatient Data Record, and Health Care Service Report/
Tricare Encounters. Medical encounters were obtained from the
DMSS for the inclusive time from each recruit’s first DMDC
record to 10 weeks after the first DMDC record. This time block
was selected as it largely covered the period of BCT plus time in
the reception station (where recruits are initially in-processed).
However, it was recognized that some individuals may have
been delayed in the reception station and/or would not have
completed BCT in the time period examined because of recy-
cling (spending extra time to meet training requirements); some
may also have been discharged from service before completing
the 10 week period. A sampling period beyond 10 weeks was not
considered because the majority of soldiers enter specialty
training after BCT and this training presumably has variable
injury risk, depending on the recruit’s military occupational
specialty.
The medical encounters from the DMSS were examined to find
International Classification of Diseases, Version 9, Clinical Modi-
fication (ICD-9) codes for pathological fractures or stress frac-
tures, (ICD-9 codes 733.1–733.19, 733.93–733.98). Cases were
recruits who were diagnosed with inpatient or outpatient path-
ological/stress fractures as indicated by the ICD-9 codes. Only an
individual’s first encounter was considered. The ICD-9 codes
733.1–733.19 (pathological fractures) were included because,
prior to 2001, there was no ICD-9 code specifically for stress
fractures and the 733.1–733.19 series were the codes that clini-
cians in military facilities were instructed to use for this pur-
pose. It was suspected that some clinicians may have continued
to use these codes to record stress fractures after 2001. It is
unlikely that including positive 733.1–733.19 events between
2001 and 2007 confounded the dataset since it is unlikely that
BCT will produce many “authentic” pathological fractures.
Also obtained from the AFHSC databases were DMDC data on
birth year and race/ethnicity. The Military Entrance Processing
Station (MEPS) database was used to obtain gender, height, and
weight. Body mass index (BMI) was calculated as weight/height2
[34]. Age was calculated as year of entry into BCT minus birth
year.
A dataset was developed by combining the individual variables
obtained from the DMSS, DMDC, and MEPS databases, as
described above. Stress fracture incidence (cases/1000 recruits)
was calculated as: recruits with a pathological or stress fracture/
total number of recruits×1000. Univariate logistic regression
was performed to quantify risk of stress/pathological fractures
at different levels (strata) of the variables of interest (gender,
age, height, weight, BMI, race/ethnicity). Age strata generally
involved 5-year age groups, height strata 6-cm groups, and weight
7- or 8-kg groups. BMI was categorized as low (≤18.5kg/m2
),
normal (18.5–24.9kg/m2
), overweight (25.0–29.9kg/m2
), or
obese (≥30kg/m2
) as recommended by the National Institute of
Health [44]. Simple contrasts were used in the logistic regres-
sion, comparing the risk at a baseline (referent) stratum of the
variable (defined with a risk ratio of 1.00) to other strata of that
variable. Odds ratios (OR) and 95% confidence intervals (95%CI)
were calculated comparing each strata with the referent stra-
tum. Multivariate logistic regression was also used to examine
stress/pathological fracture risk with all variables considered
together, although height and weight were not included in this
analysis since they were components of BMI. To more fully
describe the changes in stress/pathological fracture risk with
age, linear regression was performed examining injury risk at
each chronological age from 17 to 35 years. Pearson product
moment correlations were used to examine the magnitude of
the relationship between age and injury incidence. Predictive
Analytic Software (version 18.0) was used to manipulate and
analyze the dataset.
Results
▼
The entire database included 614606 recruits. Of these, 30955
recruits (5.0%) were missing either birth year, gender, height,
and/or weight and were not considered further in the analysis.
The final database contained 583651 recruits, with 475745 men
and 107906 women. The mean±SD age, height, weight, and BMI
of the men were 21±3 years, 175.6±6.9cm, 76.1±13.3kg and
24.6±3.8kg/m2
, respectively; for the women these values were
21±4 years, 162.7±6.4cm, 61.8±9.5kg and 23.3±3.0kg/m2
,
respectively. Recruits that were 24 years of age or younger
included 88% of the men and 86% of the women. A variety of
race/ethnicities were in the dataset but 69% and 52% of the men
and women, respectively, were listed as white, whilst 15% and
29% of men and women, respectively, were listed as black.
●
▶ Table 1 shows the overall incidence of the outcome variables.
Compared to the men, women were at much higher risk of stress
fractures, pathological fractures, and combined stress/patho-
logical fractures. ●
▶ Fig. 1 shows that use of the ICD-9 codes for
pathological fractures declined shortly after the ICD-9 codes
designated specifically for stress fractures were introduced, such
that by 2007 there were very few cases with the pathological
fracture codes. This supported the concept that clinicians were
using the pathological fracture codes for stress fractures prior to
having a specific ICD-9 code for this injury.
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3. 942 Orthopedics & Biomechanics
Knapik J et al. Stress Fracture Risk Factors… Int J Sports Med 2012; 33: 940–946
●
▶ Table 2 shows the results of the univariate logistic regression.
For both men and women, older age, lower body weight, lower
BMI, and race/ethnicity other than black increased the injury
risk. Men 180cm or taller were at higher risk compared to men
in the reference height range; men 78kg or heavier were at
higher risk than men in the reference weight range. Women
180cm or taller (n=734) were at little elevated risk compared to
women in their reference height range (OR (≥180cm/162–
166.9cm)=1.07, 95%CI=0.82–1.39). Women 78kg or heavier
(n=6206) were at somewhat lower risk compared to women in
the reference weight range (OR (≥78kg/59–63.9kg)=0.93,
95%CI=0.83–1.04). Whites were at higher risk than Asians
among both men (OR (white/Asian)=1.24, 95%CI=1.09–1.50)
and women (OR (white/Asian)=1.20, 95%CI=1.05–1.36).
●
▶ Fig. 2 shows that risk generally increased as recruit chrono-
logical age increased. The correlation between age and injury
incidence was 0.98 for men and 0.92 for women. The slope of the
linear regression of age on injury incidence indicated that risk
increased 2.2 and 3.9 cases/1000 recruits per year for men and
women, respectively. The linear regression equation for men
was: injury risk=2.20*age−26.01; for women this equation
was: injury risk=3.90*age+1.86.
●
▶ Table 3 shows the results of the multivariate logistic regres-
sion. Again, older age, lower BMI and race/ethnicity other than
black were independent injury risk factors for both men and
women. Women with higher BMI were at lower risk than those
of normal BMI.
Discussion
▼
This is the largest sample of military basic training recruits ever
examined for stress fracture risk, including almost the entire US
Army recruit population over an 11-year period. The medically
diagnosed stress fracture incidence in previous US Army BCT
studies have ranged from 0.8–5.1% for men and 1.1–18.0%
among women [5,7,30,45,48,68]. The incidences of 1.9% for
men and 8.0% for women in the present study were well within
these ranges. In agreement with previous literature [7,18,21,
30,33,38,40], the present investigation found that female gen-
der, older age, and race/ethnicity other than black increased
stress fracture risk in basic training. The present study also indi-
cated that the association between stress fracture risk and
height, weight, and BMI differed in men and women, at least
partly explaining some of the differences in the literature [2,16,
22,23,33,40,55,63]. While both men and women with lower
body weight and/or lower BMI had higher stress fracture risk,
risk was also elevated in men, but not women, with higher body
weight or higher BMI, although in the multivariate analysis the
association between stress fracture risk and higher BMI was no
longer present among the men. Taller men were at increased
injury risk while this was only very marginally the case with
women.
A previous study in Finnish basic training showed that male
stress fracture cases were taller [63], but 2 other studies showed
little association between height and stress fracture incidence in
female Marine recruits [55] or in a mixed population of Finnish
male conscripts and female volunteer recruits [40]. There may
be gender differences in the relation between bone length and
stress fracture risk. The length of long bones in the lower body
(i.e., femur, tibia, fibula) is highly related to height, with correla-
tions in the range of 0.9 [57,62]. Taller individuals with presum-
ably longer bones may experience more bending and strain on
their lower body long bones during physical activity. Beck et al.
[3] showed that men with stress fractures had longer femora
compared to those without stress fractures and the longer bones
contributed to lower bone strength. However, in women there
was little difference between stress fracture cases and controls
in femur length. In the present study, the tallest women did not
have elevated risk. It is reasonable to hypothesize that one pos-
sible explanation for the gender discrepancy in the association
between height and stress fracture risk might be that taller stat-
ure is reflecting longer bones which are associated with lower
bone strength among men but not women.
In consonance with the findings of the present study, several
other investigations have shown that stress fracture cases in
basic training have lower body weight and/or lower BMIs than
individuals without stress fractures [2, 15, 23], or that those
with lower BMI are at higher risk than those in the “normal”
(20–25kg/m2
) BMI range [40]. 2 studies that reported no asso-
ciation between BMI and stress fractures actually showed a
strong trend such that women with lower BMI were at higher
stress fracture risk [47,55]. It is possible that lower body weight
or lower BMI may reflect a paucity of bone mass, fat mass, and/
or fat-free mass. Low BMI may make recruits more susceptible
to injury if they lack the muscle mass or bone strength required
Table 1 Injury incidence by gender.
Injury (ICD-9 Codes) Injury Incidence (cases/1000 recruits) Odds Ratio- Women/Men (95%CI)
Men (n=475745) Women (n=107906)
stress fracture (733.93–733.98) 6.9 26.1 3.85 (3.66–4.05)
pathological fracture (733.1–733.19) 13.4 59.9 4.71 (4.54–4.88)
stress & pathological Fracture (733.1–733.19,733.93–733.98) 19.3 79.9 4.41 (4.28–4.54)
50
40
30
20
10
0
9
7
9
8
9
9
0
0
0
1
0
2
0
3
0
4
0
5
0
6
0
7
Injury
Incidence
(cases/1
000
recruits)
Year
Path Fx Stress Fx Stress/Path Fx
Fig. 1 Stress fractures, pathological fractures and combined stress/
pathological for all basic combat training recruits by year (Path=patho-
logical, Fx=fracture).
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4. 943
Orthopedics & Biomechanics
Knapik J et al. Stress Fracture Risk Factors… Int J Sports Med 2012; 33: 940–946
to adequately perform certain physical tasks and/or if they over-
exert or overuse the available muscle mass or supportive struc-
tures.
Many studies examining the association between BMI and stress
fractures had a very narrow range of BMIs [40,47,55], mostly
skewed to lower values. The present investigation had a broader
BMI range with a relatively large number of individuals in the
“obese” category (≥30kg/m2
). Generally, BMI shows a close rela-
tionship with body fat in military and civilian samples, demon-
strating correlations on the order of 0.7 [34,35,51]. However,
this means that only about 50% of the variance in BMI is
accounted for by body fat. The relationship between BMI and
injury in basic training is likely to be complex because individu-
als can have a high BMI either because of higher body fat or
because of higher fat-free mass. In the univariate analysis, men
with higher BMI were at higher stress fracture risk. However,
once age and race/ethnicity were included in the multivariate
model, there was no increased risk associated with higher BMI.
On the other hand, women with higher BMI tended to be at
lower stress fracture risk in the univariate model and once age
and race were included in the multivariate model stress fracture
risk was further reduced among those in the overweight and
obese category.●
▶ Fig. 3 shows the interaction of BMI and age on
stress fracture risk among the women. The risk decreased in all
age groups as BMI increased but the greatest decreases were in
the youngest and oldest age groups. The reasons for this are not
clear. It may be that younger women can carry more fat at lower
risk while older women with higher BMI have self-selected for
their lower injury risk into BCT. Considerably more research is
required here.
A consistent finding in the literature is that women have a higher
stress fractures incidence than men in basic training [7,30,
39,40,45,67] and this was robustly supported in the present
investigation. The gender difference in injury risk may be related
to fitness levels, bone characteristics, anatomy, and/or other fac-
tors. Women have lower average fitness levels than men and
lower fitness is associated with higher overall injury rates in
basic training [36,37] as well as a higher incidence of stress frac-
tures [3,30,47,55,63]. Men and women train side-by-side in US
Army BCT, and women’s lower fitness results in a larger relative
effort (e.g., greater %VO2max or % maximal strength) leading to
a higher perception of effort, more rapid fatigue, and changes in
movement mechanics (e.g., gait) that could affect injury rates
[12,20,27,32,41]. In the bones of the lower body, where most
120
100
80
60
40
20
0
Stress/Pathological
Fracture
Incidence
(cases/1000)
1
7
1
8
1
9
2
0
2
1
2
2
2
3
2
4
2
5
2
6
2
7
2
8
2
9
3
0
3
1
3
2
3
3
3
4
3
5
Age (years)
Men Women
Fig. 2 Stress/pathological fracture incidence by age.
Table 2 Univariate associations between stress/pathological fracture and age, height, weight, bmi, and race/ethnicity.
Variablea
Men Women
Strata n Injury
Incidence
(Case/1000)
Odds Ratio
(95%CI)
p-value Strata n Injury
Incidence
(Case/1000)
Odds Ratio
(95%CI)
p-value
age <20 years 206392 14.0 1.00 Referent <20 year 49698 64.0 1.00 Referent
20–24 210853 20.2 1.45 (1.39–1.52) <0.01 20–24 42715 86.5 1.39 (1.32–1.46) <0.01
25–29 44652 31.4 2.28 (2.14–2.43) <0.01 25–30 10879 105.6 1.73 (1.61–1.85) <0.01
≥30 13848 45.5 3.35 (3.07–3.66) <0.01 >30 4614 129.2 2.18 (1.98–2.38) <0.01
height <170 cm 77349 18.1 0.95 (0.89–1.02) 0.14 <157cm 16681 83.4 1.05 (0.98–1.13) 0.13
170–174.9 117726 18.3 0.96 (0.91–1.02) 0.21 157–161.9 28586 80.9 1.03 (0.96–1.08) 0.47
175–179.9 136770 19.0 1.00 Referent 162–166.9 32495 79.4 1.00 Referent
180–184.9 94239 20.6 1.09 (1.03–1.16) <0.01 167–171.9 20692 79.1 1.00 (0.93–1.06) 0.90
≥185 49661 22.2 1.18 (1.10–1.26) <0.01 ≥172 9452 74.3 0.93 (0.85–1.02) 0.10
weight <64.0kg 92926 20.7 1.15 (1.07–1.22) <0.01 <53.0kg 19274 89.2 1.16 (1.08–1.24) <0.01
64.0–70.9 95250 16.9 0.93 (0.87–1.00) 0.05 53.0–58.9 25950 81.5 1.05 (0.98–1.12) 0.15
71.0–77.9 91651 18.1 1.00 Referent 59.0–63.9 22158 77.9 1.00 Referent
78.0–86.9 95132 19.7 1.09 (1.02–1.17) <0.01 64.0–69.9 20341 77.8 1.00 (0.93–1.07) 0.96
≥87 100780 21.1 1.17 (1.10–1.25) <0.01 ≥70 20153 73.3 0.94 (0.87–1.01) 0.07
BMI <18.5kg/m2
11917 30.9 1.72 (1.54–1.92) <0.01 <18.5kg/m2
4683 100.4 1.27 (1.15–1.41) <0.01
18.5–24.9 259845 18.2 1.00 Referent 18.5–24.9 72572 80.5 1.00 Referent
25.0–29.9 156300 19.9 1.10 (1.05–1.15) <0.01 25.0–29.9 29058 75.2 0.93 (0.88–0.98) 0.01
≥30 47683 20.6 1.13 (1.06–1.21) <0.01 >29.9 1593 77.2 0.96 (0.79–1.15) 0.63
race/ White 324089 21.0 1.73 (1.61–1.86) <0.01 White 55580 90.5 1.54 (1.45–1.62) <0.01
ethnicity Black 72155 12.2 1.00 Referent Black 31661 60.9 1.00 Referent
Hispanic 52684 19.3 1.59 (1.45–1.74) <0.01 Hispanic 13388 81.5 1.37 (1.27–1.48) <0.01
Asian 15439 17.0 1.40 (1.22–1.61) <0.01 Asian 3608 76.8 1.28 (1.13–1.46) <0.01
Am. Indian 4812 21.4 1.77 (1.44–2.17) <0.01 Am. Indian 1784 81.3 1.37 (1.15–1.63) <0.01
Other 1393 25.8 2.14 (1.53–3.00) <0.01 Other 428 102.8 1.77 (1.29–2.42) <0.01
Unknown 5173 16.0 1.32 (1.05–1.65) 0.02 Unknown 1457 74.1 1.24 (1.01–1.51) 0.04
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5. 944 Orthopedics & Biomechanics
Knapik J et al. Stress Fracture Risk Factors… Int J Sports Med 2012; 33: 940–946
basic training stress fractures occur [2,7,55], women have a
lower bone section modulus and a lower bone strength index
(ratio of section modulus to bone length), compared to men. In
addition, female bones have a thinner cortical area (which pro-
vides less bone strength) and are narrower [3]. With regard to
anatomical differences, women have wider pelves [24] that
results in a varus tilt of the hip and a larger bicondylar angle [58]
which places greater stresses on the hips and lateral aspects of
the knee and lower leg during physical activity. Both men and
women with wider pelves have higher stress fracture incidence
suggesting that a wider pelvis alone (regardless of gender)
increases injury risk [3].
Another relatively consistent finding in the literature is that
older age is associated with higher stress fracture risk in basic
training [7,21,40]. In the present investigation there was a clear
dose-response relationship between advancing age and stress
fracture risk, even within the relatively young group of individu-
als examined here. Total bone mass declines in young adulthood
in both men and women and it is primarily the trabecular com-
partment where this early bone loss occurs. Losses in trabecular
mass in the distal tibia (the anatomical location of many basic
training stress fractures [7,45,55]) amounted to 0.24%/year and
0.40%/year for men and women, respectively, in one study [49].
This loss may be due to a reduction in the number of osteoblastic
stem cells or a reduction in the lifespan of the osteoblasts them-
selves. Most of bone remodeling occurs on surfaces and trabecu-
lar bone has a greater surface area that than cortical bone, so it is
trabecular bone that is more likely to be remodeled [6,59]. The
overall loss of bone mass with age would affect bone strength
and age-related effects on osteoblasts would result in slower
bone remodeling thus increasing susceptibility to activity-
induced stress fractures. With regard to cortical bone (where
most stress fractures manifest [52]), there are age-related
changes in the mechanical properties of the collagen network
(upon which bone minerals deposit), which reduces strength,
modulus, and ability to absorb energy, while increases in bone
porosity reduces bone stiffness and strength [60,65]. These
changes in cortical bone were also likely to contribute to the
higher age-related stress fracture risk in the present study.
A number of studies have indicated that those of black race/eth-
nicity are less likely to develop stress fractures in basic training,
and that those of white race/ethnicity appear to be at the highest
risk [7,19,21,33,38,55]. The lower risk among blacks could be
partly related to the higher bone mineral density of blacks com-
pared to other racial/ethnic groups [1,43,61]. The racial differ-
ence in bone mineral density between whites and blacks persists
after adjustments for body composition, dietary history, sun
exposure, biochemical bone markers, lifestyle characteristics,
and other factors [13]. In addition to higher bone mineral den-
sity, studies of the female femur have shown that there are dif-
ferences in bone geometric properties between black and white
women, as black women have longer and narrower femora with
thicker cortical areas and smaller medullary area. These factors
increase mechanical strength by contributing to lower bending
stresses in the cortical areas during physical activity [43]. The
higher bone mineral density and the manner in which the bone
architecture is arranged may contribute to the lower stress frac-
ture incidence in blacks.
An interesting finding was that men and women of Asian race/
ethnicity had a lower stress fracture risk than whites. This has
not been reported previously, likely because basic training stud-
ies examining racial/ethnic differences lacked a sufficient
number of Asians for adequate statistical power [38,55] or
grouped Asians with other racial/ethnic groups [7,19,21]. Fur-
ther, the category “Asian” may include individuals from a wide
Table 3 Multivariate logistic regression for pathological/stress fractures.
Men Women
Variablea
Strata N Odds Ratio (95%CI) p-value N Odds Ratio (95%CI) p-value
age <20 years 206392 1.00 Referent 49698 1.00 Referent
20–24 210853 1.47 (1.40–1.54) <0.01 42715 1.41 (1.34–1.48) <0.01
25–29 44652 2.33 (2.19–2.49) <0.01 10879 1.80 (1.67–1.93) <0.01
≥30 13848 3.50 (3.20–3.82) <0.01 4614 2.29 (2.09–2.51) <0.01
BMI <18.5kg/m2
11917 1.78 (1.60–1.98) <0.01 4683 1.31 (1.19–1.45) <0.01
18.5–24.9 259845 1.00 Referent 72572 1.00 Referent
25.0–29.9 156300 0.98 (0.94–1.03) 0.39 29058 0.87 (0.83–0.92) <0.01
≥30.0 47683 0.98 (0.92–1.05) 0.63 1593 0.82 (0.68–0.99) 0.04
race/ethnicity White 324089 1.74 (1.62–1.87) <0.01 55580 1.54 (1.46–1.63) <0.01
Black 72155 1.00 Referent 31661 1.00 Referent
Hispanic 52684 1.58 (1.44–1.73) <0.01 13388 1.40 (1.30–1.52) <0.01
Asian 15439 1.29 (1.12–1.48) <0.01 3608 1.23 (1.08–1.41) <0.01
Am. Indian 4812 1.80 (1.46–2.21) <0.01 1784 1.39 (1.16–1.65) <0.01
Other 1393 2.08 (1.48–2.92) <0.01 428 1.78 (1.30–2.44) <0.01
Unknown 5173 1.24 (0.99–1.55) 0.07 1457 1.20 (0.98–1.46) <0.08
200
150
100
50
0
BMI (kg/m2)
≤18.5 ≥30.0
18.5–24.9 25.0–29.9
Stress/Pathological
Fracture
Incidence
(cases/1,000
recruits)
≤19 yrs ≥30 yrs
20–24 yrs 25–29 yrs 17–35 yrs
Fig. 3 Interaction of age and bmi on stress/pathological fracture risk
(women).
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6. 945
Orthopedics & Biomechanics
Knapik J et al. Stress Fracture Risk Factors… Int J Sports Med 2012; 33: 940–946
variety of antecedent cultures and diverse geographic locations
that make the classification somewhat problematic [28,50].
Among older individuals, hip fracture rates are lower among
Asians compared to whites [54,56], but the unadjusted whole
body bone mineral density of Asians is generally lower than that
of whites [1,69]. This lower bone mineral density may be a func-
tion of the smaller size of Asians (see ●
▶ Table 4 for values in the
present study) because when adjustments are made for bone
size and body weight, Asians actually have similar or somewhat
higher bone mineral density [17,53]. Further, a composite bone
strength index of the femoral neck (that include bone mineral
density along with measures of the long and short axis of femo-
ral neck) is higher in Asians than in whites [29], possibly reduc-
ing incidence of femoral neck stress fractures. It has also been
shown that a longer hip axis length (distance from the greater
trochanter to the pelvic brim near the acetabulum) is related to
hip fractures [14] and the hip axis length of Asians have been
shown to be shorter in most [9,11,64] but not all [10] studies.
Thus, it is possible that differences in body size and geometric
properties relating to bone strength (at least at the femoral neck)
may partly account for the lower stress fracture rate in Asians
compared to whites.
There were strengths and limitations to this study. Having
almost the entire US Army BCT population over the period sam-
pled virtually eliminated selection bias and resulted in high sta-
tistical power. Limitations included the fact that we only
examined associations between stress fracture incidence and
the other variables; associations do not necessarily imply a cause
and effect relationship. We had no specific measures of recruit
bone characteristics (e.g., density, geometry, composition) and
so mechanistic explanations for the results were hypothesized
from finding in the literature rather than direct measurements
of the recruits.
Acknowledgements
▼
We would like to thank Dr Angie Eick from the Armed Forces
Health Surveillance Center for providing us with the data for
analyses. The views, opinions, and/or findings contained in this
report are those of the authors and should not be construed as
official Department of the Army position, policy or decision,
unless so designated by other official documentation. Approved
for public release; distribution is unlimited.
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