Body mass index moderates the effects of midsole hardness on metatarsophalangeal joint biomechanics during running in male recreational runners.

Abstract

Whether body mass index (BMI) modifies biomechanical responses to midsole hardness remains unclear. This study tested whether BMI moderates the effects of midsole hardness on lower-limb biomechanics in male recreational runners, with particular focus on metatarsophalangeal (MTP) joint responses at the foot-shoe interface. Thirty male recreational runners were stratified into low-, medium-, and high-BMI groups and ran at 2.7 m/s in shoes with three midsole hardness levels (40C, 53C, and 65C). Joint kinematics, kinetics, stiffness, and ground reaction force variables were analyzed. BMI significantly affected several lower-limb biomechanical variables, whereas midsole hardness showed limited main effects. BMI × hardness interactions were observed primarily in MTP-related outcomes, particularly peak MTP positive power during propulsion, suggesting a load-dependent response at the distal foot-shoe interface. Harder midsoles were associated with greater MTP positive power in high-BMI male runners, whereas softer midsoles were associated with greater MTP positive power in low- and medium-BMI male runners. These findings indicate that BMI may moderate midsole-hardness responses in male recreational runners, particularly in MTP propulsion-related mechanics.