Muscle Fatigue and Stress Fractures

Stress fracture is a common musculoskeletal problem affecting athletes and soldiers. Repetitive high bone strains and strain rates are considered to be its etiology. The strain level necessary to cause fatigue failure of bone ex vivo is higher than the strains recorded in humans during vigorous physical activity.

A study was performed to prove that during fatiguing exercises, bone strains may increase and reach levels exceeding those measured in the non-fatigued state. To test this hypothesis, this study was performed in 4 subjects to measure tibial strains, the maximum gastrocnemius isokinetic torque and ground reaction forces before and after two fatiguing levels of exercise: a 2km run and a 30km desert march. Strains were measured using strain-gauged staples inserted percutaneously in the medial aspect of their mid-tibial diaphysis. There was a significant decrease in the peak gastrocnemius isokinetic torque of all four subjects’ post-march as compared to pre-run indicating the presence of gastrocnemius muscle fatigue. Tension strains increased 26% post-run and 29% post-march as compared to the pre-run phase. Tension strain rates increased 13% post-run and 11% post-march and the compression strain rates increased 9% post-run and 17% post-march. The fatigue state increases bone strains well above those recorded in rested individuals and may be a major factor in the stress fracture etiology. (Milgrom C. Radeva-Petrova DR. Finestone A. Nyska M. Mendelson S. Benjuya N. Simkin A. Burr D. The effect of muscle fatigue on in vivo tibial strains. Journal of Biomechanics. 40(4):845-50, 2007.

Consider stress fracture as a diagnosis in adolescent athletes complaining of worsening vague pain without a clear mechanism of injury. Remember that initial radiographs may be normal, especially early in the clinical course. If the fracture is of low risk for delayed or non-union, conservative management is indicated, with repeat radiographs 2 weeks after initiation of treatment. If a high-risk fracture is suspected, early diagnosis with MRI, bone scan, and, in some cases, CT is important for surgical decision making. (Logan K. Stress fractures in the adolescent athlete. Pediatric Annals. 36(11):738-9, 742, 744-5, 2007)