The familiarity factor: How experience with spring surfaces influences joint mechanics during hopping

Prior exposure to spring surfaces may shape the biomechanical interactions between the lower limbs and joints during movement. This study investigates how surface-specific training influences joint stiffness and mechanical energy expenditure (MEE) during hopping tasks. Thirty male athletes with relevant training backgrounds participated in controlled trials on both elastic and rigid surfaces, conducted before and after a structured familiarization period. Kinematic and kinetic analyses were employed to evaluate alterations in leg and joint stiffness, alongside changes in MEE. The results indicate that while leg stiffness remained consistent overall, surface-specific training prompted distinct joint-level adaptations. Notably, knee stiffness increased, and hip stiffness decreased across both surface conditions post-training, suggesting a redistribution of mechanical load. Additionally, ankle MEE rose following spring-surface training, signifying heightened energy demands, whereas hip MEE declined, pointing to improved energy efficiency. These outcomes emphasize the importance of familiarity in refining neuromechanical strategies for surface adaptation and highlight the dominant roles of the ankle and knee in regulating stiffness. The findings also underscore the critical need to consider prior exposure when evaluating the biomechanical effects of surface properties on lower limb function.