Stress fracture injury in young military men and women

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 ± 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.

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 indoctrination of midshipmen into military life. Physical challenges 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 morning 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. Specifically, rigorous training may increase metabolism, increase activation of the hypothalamic–pituitary–adrenal axis, deplete muscle glycogen, and contribute to muscle amino acid loss, hepatic gluconeogenesis, and a negative nitrogen balance [1], [2], [3].

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 availability for continued training.

Stress fractures, also known as fatigue or march fractures, 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], [7], [8], [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 injury. 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 important 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. Therefore, 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 exercising individual's failure to achieve adequate energy balance 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], [16], [17], [18], [19], [20], [21], [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 uninjured controls based on gender, BMI, and aerobic physical fitness data that were collected before they entered military 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.