Response of patella and fibula during simulated falls: Body mass effects and differential efficacy of polyurethane damping

Sports-related falls are a common injury mechanism, but how impact materials affect different bones is not well understood. This study used finite element analysis to examine how polyurethane (PU) foam under the foot influences patellar and fibular stress during simulated 5-m falls across body masses from 50 to 110 kg. Three main findings emerged. First, PU foam’s effectiveness depends on body weight. For 50–80 kg subjects, fibular peak stress dropped by 23%–41%. Energy dissipation in the PU foam ranged from 13.65 J (for 50 kg) to 1.08 J (for 100–110 kg), representing only 0.56% and 0.04% of the total impact energy (2452 and 5394 J, respectively). Therefore, while the foam shows weight-dependent performance, its absolute energy absorption is modest relative to the total energy requiring dissipation. Second, PU foam had opposite effects on nearby bones: while fibular stress decreased, patellar stress increased by 15%–28% in the same simulations. Third, this divergence stems from the elastic mismatch between the patella (510 MPa) and fibula (around 18 GPa), which altered bending moments when the foam deformed plastically. These findings challenge the assumption that impact-absorbing materials offer uniform protection. For athletes under 80 kg, PU foam reduces fibular stress and provides a net benefit. However, for heavier individuals and for patellar protection, alternative materials or positioning is needed. Biomechanical evaluation of protective gear must therefore account for bone-specific responses, not just global stress reduction.