Tibial Strains are Sensitive to Speed, but not Grade, Perturbations During Running

Abstract

Tibial stress fractures are thought to result from a fatigue-failure process where bone failure is highly dependent on peak strain magnitude. Little is known regarding the mechanical loading environment of the tibia during graded running despite the prevalence of this terrain. To probe the sensitivity of the mechanical loading environment of the tibia to running grade, tibial strains were quantified using a combined musculoskeletal-finite element modeling routine during graded and level running. Seventeen participants ran on a treadmill at $\pm$10{\deg}, $\pm$5{\deg}, and 0{\deg} while force and motion data were captured. At each grade, participants ran at 3.33 m/s and a grade-adjusted speed, that was 2.20 m/s and 4.17 m/s for uphill and downhill conditions, respectively. Muscle and joint contact forces were estimated using inverse-dynamics-based static optimization. These forces were applied to a participant-informed finite element model of the tibia. 50th percentile pressure-modified von Mises strain was lower ($\leq$-130 $\mu\varepsilon$) during downhill running compared to level and uphill running at 3.33 m/s. However, neither 95th percentile strain (peak strain) nor the volume of bone experiencing strains $\geq$4000 $\mu\varepsilon$ (strained volume) were different between grades (F(4)$\leq$3.28, p$\geq$0.01). In contrast, peak strain and strained volume were highly sensitive to running speed (F(1)$\geq$10.61, p$\leq$0.001), where a 1 m/s increase in speed resulting in a 9 % and 155 % increase in peak strain and strained volume, respectively. Overall, these findings suggest that faster running speeds, but not changes in running grade, may increase the risk of developing a tibial stress fracture