The effect of stress fracture interventions in a single elite infantry training unit (1983–2015)

Highlights

• Elite infantry training in a single unit was used as a model to study stress fractures.

• In over three decades only a few stress fracture interventions tried were effective.

• Stress fracture was reduced by restricting training to the authorized protocol.

• But not by 7 hour/night sleep, more comfortable boots & a unit physical therapist

• Reducing marching by 1/3 & training in flat terrain reduced stress fracture severity.

Abstract

Stress fractures can be seen as an undesired byproduct of demanding physical training. The threshold value of stress that places an individual bone at high risk for stress fracture has not been identified. In a prospective study of stress fractures in 1983, a 31% incidence was found during demanding Israeli infantry basic training by bone scan criteria. Within a subgroup of these recruits, an elite infantry unit was found to have a 40% incidence. Since then and until 2015, eight additional induction companies of the same elite infantry unit were prospectively monitored for stress fractures during their basic training. In all of the studies, stress fracture surveillance and the examining orthopedist were the same.

A retrospective review of all nine studies and of eight training changes was performed to look for a temporal trend in stress fracture incidence and to see if these might be related to training changes. There was a statistically significant trend for lower radiological proven stress fractures (p = 0.0001) and radiological proven stress fractures plus clinical stress fractures (p = 0.0013), as well as lower stress fracture severity by radiological criteria (p = 0.0001) between 1983 and 2015.

The only training change that was associated, by multivariate logistic regression, with a decreased incidence of stress fracture was restricting training to the authorized training protocol (odds ratio, 3874; 95% CI, 1.526 to 9.931; p = 0.004). Increased recruit weight was found by multivariate analysis to be associated with lower stress fracture incidence (odds ratio 1.034; 95% CI, 1.00 to 1.070; p = 0.051). Moving the training to a base with flatter terrain and reducing the formal marching distance by 1/3 was associated with a decrease in high grade stress fractures (odds ratio, 10.03; 95% CI, 3.5 to 28.4; p = 0.0001). Neither the combined changes of enforcing a seven hour a night sleep regimen, training in more comfortable boots and adding a physical therapist to the unit nor stopping specific running exercises and adding lower body strengthening exercises were associated with a decrease in stress fracture. 67% of recruits who sustained stress fractures and 69% who did not sustain stress fracture finished their military service as combat soldiers in the unit (p = 0.87).

There are no magic bullets to prevent stress fractures. Stress cannot be lowered beyond the level which compromises the training goals. It is a problem that can be managed by awareness that identifies and treats stress fractures while they are still in the micro stage and not in the more dangerous macro stage.

Introduction

Stress fracture (SF) is a phenomenon common both to biological and non biological structures. In vigorous athletic and military training, it can be an undesired byproduct of the training. Stress fracture represents structural micro-damage which results in a degradation of the modulus of the material [1]. This means that the stress-strain curve shifts, resulting in higher bone strain for a given applied stress. The cause is repetitive loading. Stresses that result in high strains increase the likelihood of micro-damage occurrence.

Biological material, unlike non biological material has the ability to repair micro-damage [2]. In athletic or military training programs stress fracture prevention is based on the concept of not exceeding the loading and repair potential of bone [3]. For a single trainee this can possibly be titrated to their individual biology, but for group training like that of the military, it must be based on a level that does not put the group at risk. Within a group, members may have different bone loading tolerances and different repair potentials. The effect of factors such as sleep and nutrition on bone micro damage repair is not known [4].

Attempts have been made to lower SF incidence both by altering training programs and training gear [5]. The effect of shoe gear and orthotics on stress fracture incidence has been studied in depth [6], [7]. Anatomical risk factors and genetic factors have been identified that place subjects at increased risk [8], [9], [10], [11]. Pre-conditioning activities that can lower stress fracture incidence in subsequent training have been identified [12]. In spite of this increased knowledge, SF remains a problem for a significant number of sportspeople and infantry trainees.

The Israeli infantry recruit has been found to be a good model for the study of SF [13]. Unlike training athletes, infantry recruits in a given training unit, have a hierarchical imposed training program and uniform norms of treatment and surveillance. This framework allows the infantry soldier to be a good venue for SF prevention interventional studies.

Between 1983 and 2015, nine different induction groups of elite infantry recruits serving in the same unit were followed prospectively for SF. A series of changes in training regimens were made during this time period, including interventions that were specifically instituted in an attempt to lower the incidence of stress fractures. Overuse injury surveillance in each study was uniform, with monitoring done by the same team. We report a longitudinal analysis of these training groups, assess the temporal changes in SF incidence and relate various interventions to the temporal changes. We also assess the likelihood of recruits who sustained SF to successfully continue as elite infantry soldiers.