Background: The occurrence and clinical significance of asymptomatic bone stress injuries is unknown. Hypothesis: To evaluate by clinical and magnetic resonance imaging follow-up the occurrence of asymptomatic bone stress injuries, their clinical significance, and whether they all progress to stress fractures in subjects undergoing intensive physical training. Study Design: Cohort study (prognosis); Level of evidence, 1. Methods: Twenty-one male elite-unit military recruits voluntarily underwent clinical examination and magnetic resonance imaging before their intensive training period, 6 weeks into it, and on completion of the 5-month training program. Results: Based on magnetic resonance imaging, a total of 75 bone stress injuries were detected. Only 40% (30/75) of the bone stress injuries had been symptomatic. Symptoms depended on location and magnetic resonance imaging grade of injury, with higher grades usually more symptomatic. Repeated clinical and magnetic resonance imaging assessment indicated that asymptomatic grade I bone stress injuries healed (21/25, 84%) or remained grade I and asymptomatic (3/25, 12%). The numbers of bone stress injuries, symptomatic cases, and recruits with bone stress injury increased toward the end of the intensive training period. Conclusions: Asymptomatic grade I bone stress injuries seem common in subjects undergoing intensive physical training. Such bone stress injuries heal or remain asymptomatic grade I bone stress injuries even if intensive physical activity continues. They are therefore of no clinical significance. Only subjects who exhibit symptoms need undergo imaging studies. Subjects with an asymptomatic grade I bone stress injury may continue training but should be clinically monitored for symptoms.
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