Bone Geometry, Strength and Muscle Mass in Runners with and without a History of Stress Fracture

Abstract

Stress fractures are common among distance runners. While low areal bone mineral density has been associated with stress fractures, little is known about the role of bone structure and muscle mass in the pathophysiology of stress fractures. PURPOSE: To determine differences in bone geometry, estimates of bone strength, and muscle size in female runners with and without a history of stress fracture. METHODS: A total of 39 competitive female distance runners ages 18-35, with (SFX, n = 19) or without (NSFX, n = 20) a history of lower limb stress fracture were recruited for this cross-sectional study. Peripheral Quantitative Computed Tomography (pQCT, Orthometrix XCT 3000) was used to assess volumetric bone mineral density (vBMD, mm3), bone area (ToA, mm2), and estimated compressive bone strength (bone strength index; BSI= ToA * ToD2) at the distal tibia (4%). Total (ToA, mm2) and cortical (CoA, mm2= ToA * ToD2) bone area, cortical density, and estimated bone bending strength (stress strain index; SSIp, mm3) were measured at the 15%, 25%, 33%, 45%, 50% and 66% sites. Muscle cross sectional area (MCSA) was measured at the 50% and 66% sites. Questionnaires were used to assess health and training history. RESULTS: Participants in the SFX group had significantly smaller (7 - 8%) CoA at the 45%, 50%, and 66% sites in both the right and left legs (p < 0.05 for all). SSIp was also significantly lower (9 - 10%) at the 50% and 66% sites. Runners with a history of stress fracture also had a smaller MCSA (7 - 8%) at the 66% site in both the legs. The remaining bone parameters, including vBMD were not significantly different between groups. After adjusting for MCSA there were no differences between groups for any measured bone outcomes. CONCLUSION: These findings suggest that bone CoA, strength and MCSA are all lower in runners with a history of stress fracture. However, the lower bone CoA and strength was adapted to the lower muscle size, suggesting interventions to reduce stress fracture risk might be aimed at improving muscle size and strength.