  A fatigue-failure process is hypothesized to govern the development of tibial
stress fractures, where bone damage is highly dependent on the peak strain
magnitude. To date, much of the work examining tibial strains during running
has ignored uphill and downhill running despite the prevalence of this terrain.
This study examined the sensitivity of tibial strains to changes in running
grade and speed using a combined musculoskeletal-finite element modeling
routine. Seventeen participants ran on a treadmill at $\pm$10{\deg},
$\pm$5{\deg}, and 0{\deg}; at each grade, participants ran at 3.33 m/s and a
grade-adjusted speed - 2.50 m/s and 4.17 m/s for uphill and downhill
conditions, respectively. Force and motion data were recorded in each grade and
speed combination. 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. None of the strain
variables (50th and 95th percentile strain and strained volume $\geq$4000
$\mu\varepsilon$) differed as a function of running grade; however, all strain
variables were sensitive to running speed (F(1)$\geq$9.59, p$\leq$0.03). In
particular, a 1 m/s increase in running speed resulted in a 9% ($\approx$ 260
$\mu\varepsilon$) and 155% ($\approx$ 600 mm^3) increase in peak strain and
strained volume, respectively. Overall, these findings suggest that faster
running speed, but not changes in running grade, may be more deleterious to the
tibia.
