military

1. Stress fracture injury in young military men and women
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David W. Armstrong III, John-Paul H. Rue, John H. Wilckens,* and Frank J. Frassica
National Naval Medical Center, Bethesda, MD 20889, USA
United States Naval Academy, Annapolis, MD 21402, USA
Department of Orthopaedic Surgery, Johns Hopkins University/Johns Hopkins Bayview Medical Center, Baltimore, MD 21224, USA
Received 19 December 2003; revised 13 May 2004; accepted 14 May 2004
Available online 8 July 2004
Abstract
Approximately 5% of all military recruits incur stress fracture injuries during intense physical training, predominately in the lower
extremity. We compared young men and women with stress fracture injury (subjects) to a matched group of uninjured volunteers (controls)
during a summer training program at the United States Naval Academy to identify possible risk factors for stress fracture injury. The subject
group was composed of 13 female and 18 male plebes with training-induced stress fracture injury verified by plain radiographs and/or nuclear
bone scan. The control group was composed of 13 female and 18 male plebes who remained without injury during plebe summer training but
who were matched with the 31 injured plebes for the Initial Strength Test (1-mi run time, means: women, 7.9 min; men, 6.4 min) and body
mass index (means: women, 23.4; men, 23.8). We found that the subjects lost significant body weight (mean, 2.63 F 0.54 kg) between Day 1
and the date of their diagnosis of a stress fracture (mean, Day 35) and that they continued to lose weight until the date of their DEXA scan
(mean, Day 49). Among female plebes, there was no evidence of the female athlete triad (eating disorders, menstrual dysfunction, or low
bone density). Thigh girth was significantly smaller in female subjects than in female controls and trended to be lower in male subjects than
in male controls. Total body bone mineral content was significantly lower in the male subjects than in male controls. Bone mineral density of
the distal tibia and femoral neck were not significantly different between the groups. DEXA-derived structural geometric properties were not
different between subjects and controls. Because, on average, tibias were significantly longer in male subjects than in male controls, the mean
bone strength index in male subjects was significantly lower than that of male controls. We conclude that significant, acute weight loss
combined with regular daily physical training among young military recruits may be a significant contributing risk factor for stress fracture
injuries in young military men and women.
D 2004 Elsevier Inc. All rights reserved.
Keywords: Structural geometry; Bone mineral density (BMD); Physical training; Recruits; Weight loss
Introduction
Each July, approximately 1200 new recruits enter the
U.S. Naval Academy in Annapolis, Maryland, as plebes.
Plebe summer is a physically challenging, 2-month indoc-
trination of midshipmen into military life. Physical chal-
lenges include the normal activities associated with military
life (e.g., instruction, formations, marching, and drilling)
and the formal morning physical education program
designed to enhance the physical fitness level of the
incoming midshipmen.
This physical training program, consisting of early morn-
ing runs (approximately 10 mi per week; Fig. 1) and
calisthenics (60–90 min of stretching, push-ups, sit-ups,
pull-ups, sprinting, and agility drills) 5 days a week, initiates
changes in metabolic events that contribute to a loss of body
weight in some recruits. Intense exercise training may also
contribute to muscle fatigue and soreness, dehydration,
muscle structural damage, muscle swelling, central nervous
system fatigue, and increased use of nutrient stores. Speci-
fically, rigorous training may increase metabolism, increase
activation of the hypothalamic–pituitary–adrenal axis, de-
plete muscle glycogen, and contribute to muscle amino acid
loss, hepatic gluconeogenesis, and a negative nitrogen
balance [1–3].
8756-3282/$ - see front matter D 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.bone.2004.05.014
$
The views expressed in this article are those of the authors and do not
reflect the official policy or position of the Department of the Navy,
Department of Defense, nor the U.S. Government.
* Corresponding author. c/o Elaine P. Henze, Medical Editor, Depart-
ment of Orthopaedic Surgery, Johns Hopkins Bayview Medical Center, 4940
Eastern Avenue, #A672, Baltimore, MD 21224-2780. Fax: +1-410-550-
2899.
E-mail address: ehenze1@jhmi.edu (J.H. Wilckens).
www.elsevier.com/locate/bone
Bone 35 (2004) 806–816

2. The physical demands on plebes result in common
medical situations such as heat strain, dehydration, blisters,
sprains, and various musculoskeletal overuse injuries. Of the
overuse injuries among plebes during their first summer
training, stress fractures are a persistent medical problem: It
affects 3% of the men and 10% of the women (personal
communication, Brigade Medical Officer MKD), resulting
in substantial loss of training opportunities and a decrease in
physical performance. These injuries, which occur during a
critical period of training and indoctrination, impact avail-
ability for continued training.
Stress fractures, also known as fatigue or march frac-
tures, have remained a well-recognized medical condition
since Breithaupt [4] initially described them in 1855. Stress
fracture injuries are ubiquitous among military organizations
and disproportionately affect women [5]. Within the U.S.
military, reported rates for stress fracture among new
recruits range from 2% to 12%, contributing to substantial
lost training time [6–9]. Stress fractures in military recruits
commonly occur in the tibia, femur, metatarsals, and pelvis,
and they present with the insidious onset of localized pain
that worsens with activity [5,7,8,10]. A low level of physical
fitness at entry into military recruit training may be a
contributing factor to training-induced musculoskeletal in-
jury. Shaffer et al. [11] have reported that baseline physical
fitness among Marine recruits is poor. Less than 15% of
recruits were in excellent physical condition, and fewer than
half the recruits ran three times per week or averaged a
training distance of more than 2.5 mi.
Bending forces have been implicated as the most impor-
tant factor in the pathogenesis of stress fractures [12]. Most
military recruits report the onset of symptoms of stress
fractures between Days 10 and 12 of training [6]. It has
been hypothesized that as the mechanical forces accumulate,
the rate of bone resorption exceeds bone remodeling and
repair, and the effective loading area is decreased. There-
fore, because force remains constant, stress is increased.
This process progresses until the stress exceeds the limit of
the bone, and fracture occurs. Thus, stress fractures occur
when bone strain increases osteoclastic resorption beyond
remodeling by osteoblasts [6]. Most military recruits report
onset of symptoms of stress fractures early in the training
cycle [6], which may represent structural fatigue produced
from repetitive mechanical forces.
A contributing scenario is that of a progressive decline in
the muscular support of the bone, which may be caused by
several factors. These factors may include a smaller muscle
volume [7], and smaller muscles may be weaker and unable
to provide adequate support for the bone. Muscles that are
not adapted to repetitive work and therefore lack endurance
may fatigue, be unable to support the long bones of the
lower extremity, and fail because of large reductions in
muscle glycogen stores [13,14] secondary to repetitive,
high-intensity training programs and because of the exer-
cising individual’s failure to achieve adequate energy bal-
ance and nutritional support from the diet [13,14].
Investigators have attempted to identify risk factors for
stress fracture, including previous aerobic fitness and
activity level, body somatotype, height, weight, body mass
index (BMI), motivation, city versus country origin,
hyperpronation of subtalar joint, decreased muscle
strength, amenorrhea, oligomenorrhea, decreased calcium,
decreased bone density, tibia torsion, narrow tibias, and
increased hip external rotation [6,7,10,15–22]. However,
as indicated by Burr [23] and Jones et al. [24] in their
extensive literature reviews and to our knowledge, no
study has compared subjects sustaining a stress fracture
injury during military recruit training with matched unin-
jured controls based on gender, BMI, and aerobic physical
fitness data that were collected before they entered mili-
tary recruit training.
Therefore, the purpose of our study was to compare
young men and women at the United States Naval Academy
who sustained lower extremity stress fracture during a
military summer training program with a matched group
of uninjured recruits to identify factors that may increase the
risk of a stress fracture.
Materials and methods
There were 1224 midshipman in the class of 2000, of
which 203 were female (17%). During the study period
(July through August 2000), 40 plebes (23 men, 17 women)
were diagnosed with 58 stress fractures. The incidence of
stress fractures for the plebe indoctrination was 3.3%
overall, 2.3% for male midshipmen, and 8.4% for female
midshipmen. Of the 58 stress fractures, 74% (n = 43) were
in the tibia, 9% (n = 5) were in the metatarsals, 5% (n = 3)
were in the femur, 5% (n = 3) were in the fibula, and 5% (n =
4) were in other sites. The left side was involved in 52% of
fractures; the right side, in 48%.
Of the 40 plebes, 9 (4 women, 5 men) were not
included in the analysis. Of those four women, one had
a stress fracture of the pelvic ramus and was excluded; one
Fig. 1. Line graph showing relationship of symptoms, miles run, and stress
fractures by days of training. SFx, stress fracture.
D.W. Armstrong III et al. / Bone 35 (2004) 806–816 807

3. declined, and two did not return to clinic after their
diagnosis. Of those five men, one left the Academy, two
had experienced at least one stress fracture before arrival at
the Academy, one had a questionable fracture of the fibula,
and one had an upper extremity fracture.
The remaining 31 plebes (18 men, 13 women) with
47 lower-extremity stress fractures confirmed by radiog-
raphy and scintigraphy formed the subject group. Subse-
quently, an uninjured plebe who matched the subject in
terms of gender, BMI (body weight in kilograms/height
in meters squared), and initial strength test (IST, de-
scribed below) 1-mi run time (Table 1) was recruited
near the same time for the control group. Controls
remained uninjured for the entire plebe summer. All 31
subjects and 31 matched controls provided voluntary,
written informed consent as approved by the Committee
for the Protection of Human Subjects at the National
Naval Medical Center.
Initial BMI values were determined from measurements
taken by Navy corpsmen on the plebe’s first day (Day 1) at
the Academy. The medical department at the Academy
supplied Day 1 height in inches and weight in pounds,
which were converted to metric equivalents. The standard-
ized IST was conducted by the Naval Academy’s Physical
Education Department during the first week of plebe sum-
mer training and before the beginning of the mandatory
daily morning physical training program. The IST consists
of the time to complete a 1-mi run, the number of push-ups
performed in 2 min, and the number of sit-ups performed in
2 min. A history of each plebe’s activity level and sport
participation before arrival at the Academy was obtained by
interview at the time of the dual energy X-ray absorptiom-
etry (DEXA) scan (see below).
Plebes seeking medical attention for activity-related
musculoskeletal pain were examined by a member of the
medical staff. If the plebe’s medical history suggested a
stress fracture, plain film radiographs were obtained and
interpreted by a staff radiologist (Table 2). If these radio-
graphs were inconclusive, a radionucleotide bone scan was
performed. Ten stress fractures were diagnosed by scinti-
tium bone scan.
Standard-of-care treatment consisted of protected weight-
bearing activity, using crutches if normal walking was
painful, and mandatory alternative non-weight-bearing aero-
bic exercise, such as swimming. Injured plebes are encour-
aged to maintain their normal summer training schedule
(Table 3) during treatment, including attending class and
observing physical training, drilling, and marching when
participation is precluded by the injury.
At entry into the study, the length of the tibia (medial
tibia plateau to medial malleolus) and the maximum cir-
cumference of both thighs were measured for each parti-
cipant. A body weight was also obtained at this time. Tibia
length determined the ‘‘distal tibia’’ as the point on the tibia
0.67% of the tibia length measured distal to the medial tibia
plateau.
We recorded the dates that each subject first reported to
medical clinic with symptoms consistent with a stress
fracture injury, the date of diagnosis of a stress fracture,
and the date the plebe was returned to full duty. Using these
dates, we calculated the duration of symptoms for each
stress fracture. When the plebe had no pain on palpation at
the fracture site and could perform a hop on the affected side
without pain, plain radiographs were obtained. If the radio-
graph showed signs of fracture healing, then the plebe was
returned to full duty.
All participants (dressed in athletic shorts, T-shirts, and
socks) underwent DEXA scans of their nondominant hips
(right hip, 7; left hip, 55), distal tibias of both legs, and
whole body as described below and in other publications
[25,26]. Immediately before the scan, each participant was
weighed on a digital electronic scale (Lafayette Instruments,
Lafayette, IN) to the nearest 0.1 kg, and height was
Table 1
Subject and control demographics on Day 1a
Group/gender N Mean age F
SE (years)
Dominant
hand (R/L)
Mean height F
SE (cm)
Mean weight F
SE (kg)
Mean BMI F
SE
Mean DEXA body
fat % F SE
Fracture subjects
Women 13 18.5 F 0.17 11/2 163.9 F 1.74 62.9 F 2.31 23.0 F 0.63 20.9 F 1.24
Men 18 18.9 F 0.21 18/0 181.6 F 1.97 78.3 F 2.01 23.8 F 0.54 12.2 F 0.90
Controls
Women 13 18.4 F 0.17 12/1 166.2 F 1.74 65.0 F 2.31 23.5 F 0.63 21.2 F 1.24
Men 18 19.3 F 0.21 14/4 177.2 F 1.97 76.8 F 2.01 24.5 F 0.54 10.0 F 0.81
a
There were no statistically significant differences between subjects and controls within gender.
Table 2
Radiographic findings
Finding Pretreatment
(diagnosis)
Normal 12
Periosteal reaction 20
Cortical thickening 8
Lucent line 2
Cortical irregularity 1
Sclerosis 1
Endosteal reaction 1
Combined: periosteal reaction/cortical thickening 1
Combined: periosteal reaction/sclerosis 1
D.W. Armstrong III et al. / Bone 35 (2004) 806–816
808

4. measured to the nearest millimeter using an Accustat
stadiometer (Genentech, Inc., San Francisco, CA).
DEXA scans were obtained using a Norland XR-36 Dual
Energy X-ray Absorptiometer (Norland Medical Systems,
Inc., Ft. Atkinson, WI) that was calibrated daily using a
Norland-supplied lumbar spine phantom. The phantom
contained 40.83 g of hydroxyapatite within an area of
54.59 cm2
. The area density of the phantom was 0.748 g/
cm2
. The coefficient of variation for the phantom was
0.54%, calculated from a moving average of the previous
16 phantom scans. The in vivo coefficient of variation was
2.34% at the femoral neck and 1.84% at the distal tibia. The
same investigator (DWA) performed and analyzed all scans.
Each participant was positioned on the bed of a Norland
XR-36 DEXA as follows. For the nondominant hip, the
Norland hip-positioning device was placed on the supine
plebe. The greater trochanter of the nondominant hip was
located, and a pilot scan sequence was initiated. After the
pilot scan, the positioning cursor was in the center of the
femoral neck. The hip scan sequence was initiated and run
to completion (2 min). Then, with the plebe still supine, the
lengths of the right and left distal tibia (from the tibial
plateau to the center of the medial malleolus) were mea-
sured. A point corresponding to a position 67% of the tibial
length distal to the tibial plateau was marked on the skin
over the tibia. The plebe’s foot was placed in a foot-
positioning device and rotated internally to an angle of
17.5j. The tibia was scanned for a distance of 10 mm on
each side of the point marked on the skin along the tibial
crest. Two minutes were required to complete the scan. For
the whole-body scan, the hands and feet of the still-supine
plebe were held in place by a sheet. The boom was
positioned 1 cm above the top center of the plebe’s head,
and the point on the scanner bed was marked. The boom
was moved into a position lateral to the spine and between
the iliac crest and lowest rib, and marked. The scan
sequence was initiated and required 6 min for completion.
Using software supplied by Dr. Thomas Beck and algo-
rithms described previously, DEXA image data of the distal
tibia were used to derive mediolateral bone widths, cross-
sectional areas, and cross-sectional moments of inertia
(CSMI) [7,10,20,21]. Section modulus was calculated as
the ratio of CSMI to one half the mediolateral bone width
[20,21]. The bone strength index, calculated as the ratio of
section modulus to bone length, is based on the observation
that strength of a bone under bending or torsion is inversely
dependent on bone length and directly related to the section
modulus [27].
Participants completed the Spielberger Self Evaluation
Questionnaire (State-Trait Anxiety Inventory) at the time of
the DEXA appointment. This standardized self-report in-
strument is designed to measure anxiety. Participants are
directed to respond to 20 forced-choice items as they feel
‘‘right now’’ for the State portion and 20 forced-choice
items as they ‘‘generally’’ feel for the Trait portion. Scores
are summed over the 20 responses and an aggregate score,
ranging from 20 to 80 for each measure, is generated.
Female plebes provided a menstrual history. All women
in this study reported having normal and regular menstrual
periods in the previous 12 months.
Female plebes (11 subjects, 13 controls) also completed
Garner’s Eating Disorder Inventory (EDI) at the time of the
DEXA appointment [28]. This is a standardized self-report
measure consisting of 91 forced-choice, Likert format items
organized into 11 subscales measuring 11 psychological
traits commonly associated with eating disorders. Eight of
the subscales are from the original EDI and three provisional
subscales were added to the EDI-2. The EDI-2 does not
yield a specific diagnosis of an eating disorder; rather, it is
designed to measure psychological traits or symptom clus-
ters presumed to have relevance to understanding and
treatment of eating disorders. The psychological profile
provided by the EDI-2 is consistent with the understanding
of eating disorders.
Data analyses were conducted using JMP Software
version 5.0 (SAS Institute, Cary, NC). Paired Student’s t
tests were used to test for differences between measures
acquired on Day 1 and data acquired on the date of the
DEXA scan (e.g., change in body weight, BMI, height,
weight, etc.). Differences between subjects and controls
were determined separately for male and female plebes
using analysis of variance (ANOVA). Means for all bilateral
(i.e., tibia, thigh) measurements were used because statisti-
cal analysis did not reveal significant differences between
right and left sides. We report adjusted means (LSMEANS)
and their standard errors. Significance was set at P V 0.05.
Results
Demographics
Within the same gender, there were no differences in Day 1
age, height, weight, and BMI between fracture subjects and
controls. Comparing genders, male plebes were nearly
8 months older, 15 cm taller, and 13 kg heavier than female
Table 3
Sample plebe summer schedule
Time Activity
06:00–7:30 a.m. Physical training program
07:30–8:45 a.m. Room clean-up/morning meal
08:45–11:45 a.m. Morning training
11:45–12:00 p.m. Administrative activities/squad
leader discussion
12:00–12:50 p.m. Noon meal
01:00–03:00 p.m. Afternoon training
04:00–05:50 p.m. Sports period
06:00–06:50 p.m. Evening meal
07:00–09:00 p.m. Drill period
09:10–09:30 p.m. Personal time
09:30–09:45 p.m. Counsel time
10:00 p.m. Taps/lights out
D.W. Armstrong III et al. / Bone 35 (2004) 806–816 809

5. plebes, but BMI was not different (23.8 F 0.29) between
gender. Twenty of 26 women (77%) and 29 of 36 men (81%)
were Caucasian. In the subjects, stress fractures (confirmed
by radiograph or bone scan) occurred in the tibia (n = 18) in
all 13 women, and in the tibia (n = 13), proximal femur (n =
2), metatarsal (n = 2), and fibula (n = 1) in the men.
Preadmission sports participation/activities
During high school, all participants had played a variety of
sports. Seventeen of the 26 women (65%) and 24 of the 36
men (67%) were members of a varsity team in high school.
There was no association between varsity sport participation
and stress fracture. However, plebes that participated primar-
ily in non-weight-bearing high school sports (e.g., swim-
ming) showed a trend ( P = 0.07; Odds Ratio = 3.2) to stress
fracture; plebes who participated primarily in weight-bearing
sports (e.g., basketball, soccer) did not (Table 4).
Comparing subjects and controls, there were no differ-
ences in self-reported preadmission running miles per week
(8.7 F 1.0 mi*week 1
) or additional exercise training
minutes (66.7 F 5.4 min*day 1
).
Physical strength/readiness
Within gender, there were no significant differences for
the IST 1-mi run time between subjects and controls and no
significant differences in the 1-mi run times between study
participants and the entire plebe class. The mean 1-mi run
time for the entire class of female plebes was 7.54 F 0.07
min and that for the male plebes was 6.37 F 0.02 min.
There were no significant differences between female
subjects and controls for the number of push-ups (45 F 4)
or sit-ups (63 F 3) completed during the IST. Male subjects
performed significantly fewer push-ups than did male con-
trols (means: 59 F 4 and 72 F 4, respectively). There was
no difference between male subjects and controls in terms of
sit-ups (mean: 67 F 4).
Diagnosis and DEXA scanning
Male and female controls underwent DEXA scanning
83 F 5 days after Day 1 at the Academy. Female subjects
reported to the medical clinic with symptoms at 14 F 3
days after Day 1, were diagnosed with a stress fracture at
34 F 4 days, and underwent DEXA scanning at 49 F 5
days. Male subjects first reported to the medical clinic with
symptoms at 16 F 2 days after Day 1, were diagnosed at
38 F 3 days, and underwent DEXA scanning 51 F 4 days.
The mean time from onset of symptoms and return to duty
for the fracture subjects was 61 days.
Body weight
From Day 1 to the date of diagnosis (mean, Day 34), the
weight of female subjects declined 1.05 F 0.69 kg and
continued to decline until the date of their DEXA scans
(mean, Day 49) (Fig. 2). Female subjects showed a mean
loss of 2.03 F 0.74 kg (0.47 kg*week 1
) during the study
period. In contrast, weight loss among female controls was
not significant: 0.67 kg at recruitment to the study and 0.77
kg at Day 85 (Fig. 2).
From Day 1 to the date of diagnosis (mean, Day 38), the
weight of male subjects declined 3.69 F 0.57 kg and
continued to decline until the date of their DEXA scans
(mean, Day 51) (Fig. 2). Male subjects showed a mean loss
of body weight 4.03 F 0.78 kg (0.73 kg*week 1
) over the
study period. Male controls did not lose significant weight
(0.50 kg) by recruitment to the study, but their weight loss
was significant by the day of their DEXA scans (mean, Day
85): mean, 1.89 F 0.78 kg (0.17 kg*week 1
) during the
study period. The difference in weight loss between male
subjects and controls at diagnosis was significant but
showed only a trend to significance ( P = 0.062) by the
date of the DEXA.
Overall, fracture subjects lost more than four times as much
weight as the controls by the date of diagnosis of fracture
(2.37 F 0.48 vs. 0.58 F 0.48 kg, respectively). Male controls
continued to lose weight until the date of their DEXA, but the
difference in their weight loss was one half that of male
subjects (1.42 F 0.56 vs. 3.19 F 0.56 kg, respectively).
Female parameters
Eating disorders
Female subjects (n = 11) showed significantly lower
scores on three subscales of the EDI (Drive for Thinness,
Table 4
Preadmission sports participation among 62 midshipmen subjects
Sport Number
participatinga
Number with
stress fracture
Cross-country/track 19 8
Swimming 14 9
No sport 14 7
Baseball/softball 7 2
Soccer 7 1
Wrestling 7 2
Football 4 1
Lacrosse 4 2
Tennis 4 4
Basketball 3 1
Golf 2 1
Volleyball 2 2
Water polo 2 2
Rugby 2 2
Hockey 2 0
Badminton 1 1
Band 1 0
Cheerleading 1 0
Crew 1 1
Marksmanship (rifle/pistol) 1 1
Sailing 1 1
Total 99 48
a
Several midshipmen participated in more than one sport.
D.W. Armstrong III et al. / Bone 35 (2004) 806–816
810

6. Bulimia, and Body Dissatisfaction) than female controls (n
= 11): 4.5 F 2.1 vs. 11.2 F 2.3, respectively. No female
participant scored above the 80th percentile on any EDI
subscale and none were receiving treatment for an eating
disorder at the time of this study or previously. Women not
being treated for an eating disorder who have scores above
the 80th percentile on the EDI Drive for Thinness, Bulimia,
and Body Dissatisfaction subscales (a combined score of z
29 for women) are considered at risk for developing an
eating disorder [28]. The use of the EDI-2, by itself, does
not provide the definitive diagnosis of an eating disorder;
however, we did not conduct follow-up clinical interviews.
Menstrual status
Female participants reported achieving menarche in their
12th year (12.7 F 0.25 years). There were no differences
between groups for self-reported number of menstrual
periods (11.5 F 0.35 cycles*year 1
) in the previous 12
months. At the time of their DEXA appointments, we did
not collect the date of the plebes’ last menstrual periods.
Assuming that the menstrual cycle self-reports were accu-
rate and assuming a normal 28-day menstrual cycle, subjects
would have been 21 days into their second cycle on the date
of their DEXA scans (49 F 5 days from Day 1) and controls
would have been on their third cycle (85 F 5 days from Day
1). Extensive reviews have reported no effects of menstrual
cycle on body weight change and muscle strength and
endurance [29–31].
Seven of the 26 females in this study reported using oral
contraceptives before their arrival at the Naval Academy;
five of the seven reported beginning their use of oral
contraceptives 3 months before entering the Academy. All
seven women reported ending the use of oral contraceptives
shortly after completing plebe summer training.
Anxiety inventory
There were no significant differences between female
subjects and controls concerning response to the Spielberger
State-Trait Anxiety Inventory instruments (means: State,
35.9 F 2.2; Trait, 32.3 F 2.3). Between male subjects and
controls, there was no significant difference in State scores
(means: 39.4 F 2.7 and 34.4 F 2.6, respectively), but the
Trait scores were significantly higher (although not abnor-
mally so) in male subjects than in controls (mean: 38.3 F
2.1 and 32.1 F 2.1, respectively).
Tibia length
Mean tibia length (36.3 F 2.03 cm) was not different in
female subjects and controls, but subjects had significantly
smaller thigh girth (means: 47.6 F 1.04 and 51.9 F 1.04
cm, respectively). Male subjects had significantly longer
tibias than did controls (means: 41.5 F 0.62 and 39.0 F
0.62 cm, respectively) and a trend ( P = 0.075) toward
smaller thigh girth than controls (means: 49.3 F 1.06 and
52.1 F 1.06 cm, respectively).
Total body mineral content and bone mineral density
Female subjects and controls had no significant differ-
ences in total body bone mineral content (TBBMC) or bone
mineral density (BMD) in the nondominant hip or distal tibia.
Male subjects showed significantly lower total body mineral
content, exhibited a trend to lower values in the nondominant
hip BMD ( P = 0.053), and did not have significantly different
tibia BMD than male controls (Table 5).
Structural geometry
There were no significant differences between female
subjects and controls in terms of distal tibia structural
geometry characteristics (Table 6), e.g., mediolateral tibia
width, cortical bone area, CSMI, section modulus, or bone
strength. Comparing male subjects and controls, there were
no significant differences in distal tibia mediolateral bone
Table 5
Bone characteristics and confidence interval data
Group/gender
and CI
N Mean
TBBMC F
SE (g)
Mean femoral
neck BMD F
SE (g*cm 2
)
Mean distal
tibia BMD F
SE (g*cm 2
)
Fracture subjects
Women 13 2623 F 75 0.973 F 0.03 1.343 F 0.03
Men 18 3073 F 78a
1.076 F 0.035 1.455 F 0.03
Controls
Women 13 2808 F 75 1.037 F 0.03 1.374 F 0.03
Men 18 3447 F 78a
1.176 F 0.035 1.531 F 0.03
95% CIb
Women 165 2628–2717 1.029–1.065 1.349–1.389
Men 86 3085–3250 1.121–1.188 1.501–1.566
CI, confidence interval.
a
Significant difference between subjects and controls within gender.
b
95% CI data for female midshipmen data from Ref. [26]. 95% CI data for
male midshipmen data from Ref. [25].
Fig. 2. Line graph showing weight change for participants at time of
diagnosis and DEXA scan. SFX, stress fracture.
D.W. Armstrong III et al. / Bone 35 (2004) 806–816 811

7. width, CSMI, or section modulus (Table 6), but the former
exhibited a trend ( P = 0.052) to lower cortical bone area.
Because the tibias of male subjects were, on average, 2.5 cm
longer than those of male controls, male subjects showed a
significantly smaller distal tibia bone strength index than did
male controls (Table 6).
Discussion
To our knowledge, our study is the first to compare 18-
year-old military men and women with stress fracture to
uninjured controls matched by gender, age, BMI, and pread-
mission aerobic physical performance in terms of demo-
graphic, injury, and outcome data for the purpose of
identifying possible risk factors for stress fracture injury.
Unlike other military recruits [11], plebes enter the Academy
with a high level of physical fitness: 85% of this incoming
class (of whom 15% were women) had a varsity letter in at
least one high school sport and 50% of the female plebes
were recruited to play a division I sport. Our study supports
the consistent finding in other reports that the most common
site of stress fracture in military recruits is the tibia [32,33].
In contrast to controls, most of our fracture subjects
reported participating primarily in a nonimpact sport, e.g.,
swimming, before arrival at the Naval Academy (Table 4).
This finding may indicate that a lack of sufficient previous
strain-generating, impact-loading exercise may increase sus-
ceptibility to stress fracture injury during subsequent intense
physical training periods [20,34–36]. Milgrom et al. [37]
reported that participation in ball sports, such as basketball
(n = 3 in our study), for at least 2 years before starting basic
training significantly reduced the incidence of stress frac-
tures during basic training.
Like other investigators [5,8,9,38], we found that female
recruits experienced a higher relative incidence of stress
fractures at the Naval Academy than did male recruits.
Previous studies examining risk factors have suggested that
this higher incidence of stress fractures in young women
may be secondary to decreased BMD associated with eating
disorders and irregular menses [5,8,38]. However, we found
no significant differences between female subjects and
female controls in terms of age at menarche or the number
of reported menstrual periods in the previous 12 months. We
recognize that some women may experience luteal phase
disruption, anovulation, and subclinical menstrual disorders,
which may contribute to a hypoestrogenic state and there-
fore may contribute to loss of bone [39]. However, all the
women in this study reported normal cyclic menstrual
function. The women in our study showed tibia BMD and
TBBMC values that were close to or within the Naval
Academy 95% confidence interval for the particular bone
site measured [26] (Table 5); there were no indications that
any of the women in our study were at risk for an eating
disorder, and we found no evidence of the Female Athlete
Triad [40] in any of our female subjects. Therefore, findings
from our study do not support an association between
female plebes with stress fracture and altered menstrual
function or eating disorders.
In our study, TBBMC was significantly lower in men
with stress fractures, but not abnormally low. The mean
TBBMC of the fracture subjects was very close to the lower
limit contained within the 95% confidence interval for 87
men [25] and 165 women [26] derived from previous
reports of normal men and women at the Naval Academy
who did not experience a bone injury. The mean BMD for
the distal tibia was not significantly lower in male or female
subjects than in their respective controls. This finding is
supported by Giladi et al. [38], who showed no difference in
BMD at a point on the tibia located 8 cm above the ankle
mortise. The 95% confidence interval derived from our
previous work with midshipmen contains the distal tibia
BMD values for our female subjects [26] (Table 5) but not,
by a small margin, those for male subjects [25] (Table 5).
Beck et al. [7] reported findings similar to ours for tibia
BMD of male and female Marine Corps recruits with and
without stress fracture. However, they reported that tibia
BMD in women (but not in men) with stress fracture was
significantly lower than that in their respective uninjured
recruits [7]. In our study, the distal tibia measurement was
approximately 11.4 cm above the medial malleolus in female
participants and 12.4 cm above the medial malleolus in male
participants. For male subjects in the current study, this distal
tibia site was very close to the level 1 site (13.7 cm) on the
Table 6
Distal tibia structural geometry from DEXA scans
Group/gender N Mean width (mm) Mean cortical
area F SE
(mm2
)
Mean CSMI F
SE (mm4
)
Mean section
modulus F SE
(mm3
)
Mean tibia
length F SE
(cm)
Mean bone strength
(modulus/tibia length)
Fracture subjects
Women 13 20.10 F 0.34 291.3 F 7.5 10,021 F 573 991 F 40 36.6 F 0.57 27.2 F 1.02
Men 18 22.54 F 0.33 355.7 F 9.6 15,618 F 785 1378 F 53 41.5 F 0.62a
33.2 F 1.37a
Controls
Women 13 20.66 F 0.34 302.0 F 7.5 10,938 F 573 1051 F 40 36.0 F 0.57 29.1 F 1.02
Men 18 22.93 F 0.33 383.7 F 9.9 17,109 F 809 1482 F 54 39.0 F 0.62a
38.1 F 1.41a
CSMI, cross-sectional moment of inertia.
a
Significant difference between subjects and controls within gender.
D.W. Armstrong III et al. / Bone 35 (2004) 806–816
812

8. tibia reported by Milgrom et al. [35], which corresponds to
the narrowest width in the mediolateral plane with the tibia
rotated 15j internally. Because we used DEXA methodology
in one plane with the foot rotated 17.5j medially, we have
reported only mediolateral structural characteristics for our
subjects. We report CSMI values in the mediolateral orien-
tation for both male subjects and controls, but our values do
not appear to be different from the values reported by
Milgrom et al. [35]. Our distal tibia section modulus values
are not significantly different nor are they predictive of stress
fracture risk. However, when we calculated the bone strength
indices (Table 6), we found that the indices were significantly
lower in our subjects compared to controls due to the
significantly longer tibias of the subjects.
Bending strength is proportional to the fourth power of
the radius for long tubular bones such as the tibia. Thus, a
slight decrease in width of the tibia results in a large
decrease in resistance to bending. Because the size of
bones throughout the skeleton is proportional, a finding
of decreased tibia bone width should correlate to overall
smaller bones throughout the skeleton, and, hence, smaller
bending strengths in all bones throughout the skeleton
[7,12,21,27,35]. Although previous studies indicated that
geometric parameters were in large part responsible for
bone strength differences [7,21,35,38], we found no signi-
ficant differences between subjects and controls for section
modulus. Therefore, we suggest that the difference in bone
strength may be secondary to composition rather than to
cross-sectional geometry.
To determine the structural geometry of the distal tibias
of our subjects, we used the algorithms of Beck et al. [7,21]
to derive the mechanical properties of each subject via the
DEXA data. DEXA technology is widely used to evaluate
the skeleton and its mineral status. Its measurements are
precise and accurate, it has low radiation exposures, and it
affords short examination time. However, DEXA technolo-
gy is limited by its planar nature, providing only area density
or area content [7,12,20,21]. Although structural geometric
characteristics may be derived from DEXA images, these
measures are accurate only in the plane of the image, and
they only partially describe the geometric properties govern-
ing bone strength [7,20,21,35]. The mechanical properties of
bones show the same characteristics as man-made load-
bearing structures, but the skeleton adapts to stress and strain
by altering its shape (Wolff’s Law) and bone mineral
content. Bone factors that affect the risk of stress fracture
include porosity, mineralization, density, trabecular and
cortical architecture, and fatigue microdamage, which
reduces the elastic modulus [16,23,41]. Our study compar-
ing subjects with stress fracture to matched controls showed
no differences in BMD and structural geometry of the distal
tibia.
The muscles of the body provide the mechanism for
movement of skeletal structures through the joints, supply a
substantial degree of protection to the skeleton from forces
that can cause skeletal injury, help maintain posture and
locomotion, and protect bone from bending under impact
loading. Complex neuromuscular reflexes manage forces
applied to the axial skeleton from activities such as lifting,
throwing, running, and jumping. Muscular fatigue engen-
dered by strenuous exercise and unpreparedness (cognitive
fatigue) secondary to physical exhaustion and lack of sleep
are likely major contributors to skeletal injury [42].
Stress fractures are a common overuse skeletal injury in
young military recruits [4,6–8,38,43], and there appears to
be a correlation between the development of such fractures
and the level and pattern of activity [12,42–44]. In our
subjects, stress fracture incidence increased with the cu-
mulative number of miles run during the morning exercise
training periods (Fig. 1). In the tibias of susceptible
individuals, increased weight-bearing physical activity
(e.g., approximately 104
load cycles, or approximately 4
weeks of physical training) is likely to create localized
peak strains that can result in a stress fracture secondary to
muscular fatigue [7,10,18,23,39,42,45]. Most of the stress
fractures in our study were confirmed in the tibia after < 4
weeks of plebe summer physical training.
Muscle fatigue is likely a contributor to stress fracture
in plebes [7,14,18,21,23,42,45,46]. The combination of
significantly smaller thigh girth in both male and female
subjects and the significant loss of body weight, indicat-
ing a hypocaloric state that would lead to increased
muscular fatigue and reduced muscular function during
the summer training program, provides evidence of re-
duced muscular support and protection for the bones of
the lower extremity. Other studies have reported that men
undergoing a simulated march on a treadmill had signi-
ficant fatigue in muscles that support the lower leg and
an increased strain rate on the tibia [16,19,23,45]. A
recent study of men performing a 21K hill walk on low-
energy or high-energy food ration showed that reaction
time, balance, and the ability to maintain body tempera-
ture was significantly impaired during the low-energy
trial. The low-energy trial also resulted in greater fatigue
and a significant number of lower-extremity musculoskel-
etal injuries [47]. In 1993, Yoshikawa et al. [46] reported
a similar finding in a dog model. Muscle fatigue and the
resulting increased bone strain may contribute to stress
fracture injury after daily strenuous exercise. Thus, fa-
tigue in the musculature of the lower leg is consistent
with the observed incidence of stress fracture and ankle
sprain injury in military recruits undergoing rigorous
basic training [7,19,44].
Our finding that thigh girth was significantly (approxi-
mately 7.5%) smaller in subjects than in controls supports
the work of Beck et al. [7,21], who reported a similar
finding in Marine Corps recruits. Thus, smaller thigh girth
in subjects than in controls provides evidence that the leg
muscles in fracture subjects were less likely to generate
enough force to protect bone from unnecessary bending
[7,18,19,42,46]. This finding is supported in part by the fact
that male subjects performed 25 fewer push-ups than did
D.W. Armstrong III et al. / Bone 35 (2004) 806–816 813

9. male controls, an indication of lower whole body muscular
strength and endurance in the injured male plebes at entry
into the Naval Academy.
Additionally, an important finding of this study was
the significant loss of body mass in subjects compared
with their matched controls at the time of the diagnosis
of a stress fracture. Male subjects lost 3.69 F 0.57 kg
and female subjects lost 1.05 F 0.69 kg between Day 1
and the date of diagnosis. Controls also lost weight, but
the loss was not statistically significant. The subjects’
significant weight loss indicates a sustained negative
energy balance during the preinjury training period.
This hypocaloric state among midshipmen may be the
result of several factors. At the Naval Academy, plebes
have approximately 20 min to consume a meal. Three
meals per day are designed to deliver approximately 5000
kcal/day consisting of 40% carbohydrate, 40% fat, and
20% protein (personal communication, Naval Academy
dietician). Fresh fruits and vegetables, salads, and dairy
products are freely available. During plebe summer, be-
tween-meal snacks are extremely limited because plebes
may not remove food from the mess hall and because
access to food and beverage vending machines is not
permitted. If a plebe is not hungry at mealtime or cannot
finish the meal, the calories consumed will likely be
inadequate to replace calories expended. The evening meal
is served at 6:00 pm, and no access to snacks results in
plebes going without food for nearly 13 h between that
meal and breakfast at 7:30 am (Table 3). In addition, 90
min of morning training occurs before the plebe has
breakfast. As shown by the work of Costill et al. [13],
muscle glycogen is depleted over 3 days of intense training
when a diet consisting of just 40% carbohydrate is
consumed, a diet similar to that served at the Academy.
This 40% carbohydrate diet, coupled with the lack of food
for more than 12 h before intense physical training, may
result in significant glycogen depletion in the leg muscles
of plebes within days of beginning the mandatory plebe
summer physical training program. The physical, mental,
and emotional stress of plebe summer, elevated tempera-
ture and humidity, and lack of residence hall air condi-
tioning (heat stress) may also contribute to a plebe’s
reduced caloric intake [13,14,48].
A sustained negative energy balance also may nega-
tively affect the muscle’s ability to recover from physical
exercise [13] and may result in reduced bone collagen
synthesis [41]. Therefore, the leg muscles of the subjects
in our study were likely chronically fatigued and unable
to provide support to the bones of the lower extremity
[7,14,16,18,19,42,45,46]. It also is likely that bone col-
lagen synthesis was impaired, which may have contri-
buted to an increased risk of stress fracture in plebes with
acute negative energy balance [41].
A limitation of our study may be that, in contrast to
the work of Beck et al. [7,21], we did not perform
DEXA scans on the entire plebe complement at the
beginning of summer training. Although obtaining DEXA
scans for 1200 recruits would provide much information,
such scans would have required more than 600 h to
complete, and we do not believe that the data would
substantially improve the study. In contrast to the work
of Beck et al. [7,21], we studied one entire cohort of
recruits during a single training cycle rather than recruit-
ing subjects over a 15-month period from several training
cycles. We assumed that there would be no substantive
change in bone mass, bone density, and structural geo-
metry in the few weeks from onset of the training period
and the time of the DEXA scan. In fact, subjects showed
BMD measures that were within the ‘‘normal’’ reference
population for the Norland XR36 and close to or within
the 95% confidence interval [25,26] for BMD and
TBBMC of several hundred uninjured midshipmen pre-
viously measured during plebe summer training.
Another study limitation is that plebes were not weighed
regularly (i.e., weekly) and therefore it was not possible to
document precisely the change in body weight during the
summer training program. However, our data clearly show
that plebes sustaining a stress fracture injury lost significant
body weight by the time of their stress fracture diagnosis
compared with uninjured matched controls.
We believe the strength of our study, and the major
difference from other studies of stress fracture injury, lies
in the careful matching of injured young men and women
to uninjured controls based on gender, age, BMI (height
and weight), and aerobic physical performance at the
beginning of an intensive summer training period.
We conclude that an acute negative energy balance
contributes to significant weight loss in some young
military recruits. Sustained negative energy balance in
young recruits undergoing basic military training may
pose a risk of increased muscular fatigue, reduced bone
collagen synthesis, and reduced muscular support of the
long bones of the lower extremity. Therefore, a sustained
negative energy balance during basic training may con-
tribute to stress fracture injuries in some young men and
women. The etiology of stress fractures continues to be a
multifactorial conundrum. Additional research may con-
firm our preliminary data, which suggests that some
factors that contribute to these injuries may be modified
and therefore may lead to a reduction in stress fracture
injury in young military recruits.
Acknowledgments
The authors gratefully acknowledge Thomas Beck, PhD,
Department of Radiology, School of Medicine, The Johns
Hopkins University, Baltimore, MD, for providing the
software for the analysis of the distal tibia DEXA scans.
The Chief, Navy Bureau of Medicine and Surgery,
Washington, D.C., Clinical Investigation Program, spon-
sored this study.
D.W. Armstrong III et al. / Bone 35 (2004) 806–816
814

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