Internal Tibial Forces and Moments in Runners with a History of Stress Fractures

Abstract

Gait kinematics and external forces have been associated with stress fracture in runners. These factors may act in concert to increase internal bone forces and moments, key elements of bone stress. PURPOSE: to quantify internal tibial forces and moments in runners with and without a history of tibial stress fracture (fx). METHODS: Thirty-three age and gender matched runners, 11 cases and 22 controls, ran across a force platform at a self-selected 5K race pace ± 5% while three dimensional kinematic and kinetic data were collected. A combination of inverse dynamics and musculoskeletal modeling was used to estimate internal forces and moments acting along a centroid path through the tibia. Group differences for internal forces and moments at 25%, 50%, and 75% of the distance from the proximal tibia were compared using effect sizes (ES) and 95% confidence intervals (CI) of the ES. SUMMARY OF RESULTS: Groups did not differ on experience, weekly mileage, heel strike index, or 5k race pace. Anterioposterior (AP) shear forces differed between groups at the proximal, mid and distal tibia (ES = 0.75, CI95% = 0.01, 1.49; ES = 0.83, CI95% = 0.08, 1.58; ES = 0.82, CI95% = 0.07, 1.57, respectively). On average, the stress fx group experienced AP shear forces that were 8-12% greater than the control group. Additionally, moments about the AP axes, longitudinal and mediolateral (ML) axes of the distal tibia were greater in the stress fx group (ES = 0.98, CI95% = 0.22, 1.74; ES = 3.46, CI95% = 2.35, 4.56; ES = 0.09, CI95% = 0.09, 1.59, respectively). The stress fx group underwent, on average, 33% more medial surface tension, 263% greater internal torsion, and 26% more anterior surface tension at the distal tibia. CONCLUSION: The results of this preliminary study suggest that differences between internal tibial forces and moments may exist between runners with and without a history of stress fx. The stress fx group experienced greater AP shear forces, bending moments and torsional moments at the distal tibia, a common site for stress fx. The CI of the ES associated with these variables was large, likely due to the small sample size, variability inherent to the data, and that data were not stratified by gender. Nonetheless, this study provides some evidence that magnitude of applied loading in runners with stress fx may be related to injury status.