The Effects of Shoe Sole Thickness on Running Style and Stability During Downhill Running at Different Speeds

Advanced footwear technologies (AFT) like carbon plates and thick, lightweight soles were developed to enhance running performance. Previous research on sole thickness focused on level running; however, downhill running, with different biomechanical demands, remains underexplored. This study investigates how running shoe sole thickness affects running style and stability during downhill running at different speeds. Seventeen experienced male runners ran at 10 and 15 km/h on a -10% slope in three shoe conditions: a traditional control shoe (CON27, 27 mm), a thinner AFT-shoe (AFT35, 35 mm), and a thicker AFT-shoe (AFT50, 50 mm). Running style was analyzed using step frequency normalized to leg length, duty factor, vertical center of mass oscillation, vertical stiffness, leg stiffness, and lower limb angles in the sagittal and frontal planes. Increased stability was assessed using both nonlinear (lower maximum Lyapunov exponent for local stability and lower detrended fluctuation analysis for global stability) and linear methods (reduced ankle eversion for ankle stability). Both AFT35 and AFT50 altered running style via changes in ankle and knee kinematics (p = 0.001) and improved global stability (p = 0.004) compared to CON27 but did not affect spatiotemporal variables or local stability. Within AFT design, AFT50 affected ankle kinematics in both the sagittal and frontal planes, with differences of up to ∼4° (p < 0.001). These effects were consistent across running speeds. In conclusion, AFT-shoes characterized with thicker soles influence joint kinematics and global stability during downhill running, whereas sole thickness within AFT designs primarily affects ankle stability and sagittal kinematics.