The impact of midsole hardness on joint angles and plantar loading during running at multiple running velocities

Introduction Suitable midsoles of running shoes provide better protection for the feet. However, previous studies on the effect of midsole hardness on running biomechanics have ignored the important factor of running velocity and have not reached consistent results. This study set a running velocity with six gradients and aimed to investigate whether the midsole hardness would have a different impact on lower limb joint angles and plantar loading in different velocity ranges. Methods Eight male runners ran on a treadmill under 12 conditions (six velocities: self-selected velocity (SSV), 120% SSV, 140% SSV, 160% SSV, 180% SSV, and 200% SSV; two midsole hardness levels: soft and hard). The Noraxon Ultium ® insole and Motion IMUs were used to collect data on joint angles and plantar loading. Two-way repeated measures (6 velocities × 2 hardnesses) and paired t-tests were used. Results The study showed that at SSV and 120% SSV, the maximum ankle inversion angle in soft midsoles is significantly smaller than in hard midsoles. At 180 and 200% SSV, the maximum hip abduction angle in the swing phase and the maximum force in the metatarsal region in soft midsoles are significantly greater than in hard midsoles, which might lead to a loss of stability and an increased risk of injury in the forefoot. Midsole hardness could have a nonlinear effect on joints and plantar loading as running velocity changed. Conclusion The study provided useful information for reducing the potential incidence of running-related injuries based on midsole hardness and running velocity conditions. When considering the impact of midsole hardness on running injury, researchers should pay particular attention to ankle joint motion during 100–120% SSV and hip, ankle joint motion, and metatarsal and arch loading during 180–200% SSV.