Effects of shoe longitudinal bending stiffness and runner level on Achilles tendon loading through musculoskeletal modelling

Achilles tendon (AT) injuries are prevalent among runners, especially in experienced runners. This study investigated how runner level and shoe longitudinal bending stiffness (LBS) interact to influence AT loading and lower-limb biomechanics. Thirty-one male rearfoot strikers (18 experienced, 13 novice) ran at 4.17 m/s in carbon-fiber shoes of three LBS levels (T1: 10.9; T2: 17.5; T3: 33.0 Nm/rad). AT force was estimated in silico from a subject-specific musculoskeletal model with static optimization, computed as the sum of soleus and gastrocnemius muscle forces. Discrete outcomes were analysed using mixed two-way ANOVA, and waveforms using one-dimensional Statistical Parametric Mapping (SPM1D). Experienced runners produced greater peak AT force than novice runners (p=0.038, η_p^2=0.141). LBS independently elevated peak AT force (p=0.004, η_p^2=0.182) and AT impulse (p=0.006, η_p^2=0.189), driven by an LBS-related increase in peak ankle dorsiflexion (p<0.001, η_p^2=0.493) that redirected sagittal excursion proximally. Significant runner level × LBS interactions for peak knee extension moment, peak hip external-rotation angle, and transverse-plane hip ROM showed that novice runners adapted to stiffer shoes with proximal compensations, whereas experienced runners did not. Runner level and LBS independently elevate AT loading through complementary biomechanical pathways, and novice runners rely on proximal compensations to accommodate stiffer footwear. These findings support an individualized, U-shaped view of optimal LBS.