Shifting joint regulation: The influence of hard spring surfaces on lower limb mechanics during hopping

The interaction between surface and leg stiffness is crucial in sports biomechanics, influencing leg mechanics and energy expenditure during activities such as hopping. While previous studies have examined leg stiffness adjustments in response to surface compliance, limited research has investigated these dynamics on hard spring surfaces commonly used in athletic settings. This study aimed to determine the influence of hard spring surfaces on lower limb mechanics during hopping. Thirty male participants engaged in hopping exercises on surfaces with different stiffness levels (300, 400, and 500 kN/m) as well as a force plate, following a repeated measures design. Kinematic and kinetic analyses were conducted to measure leg and joint stiffness, maximum VGRF, and mechanical energy expenditure. The statistical analysis was conducted using ANOVA, Linear Mixed Models, and Principal Component Analysis. The results indicated that leg stiffness, VGRF, ankle joint angle and stiffness, and ankle joint MEE were greater during hopping on the force plate compared to the hard spring surface. Conversely, hip joint stiffness and MEE were higher on the spring surface. Results also showed that the ankle joint served as the primary regulator on the force plate, while the hip joint became the main regulator on the spring surface. The findings suggest that hopping on hard surfaces requires a distinct locomotion strategy, characterized by a reduced reliance on ankle stiffness and increased hip joint involvement. These surfaces not only enhance athletic performance through efficient energy use but may also lower injury risks in sports environments. This study contributes to the understanding of leg-surface interactions and provides insights for optimized surface design in athletic facilities.