Author(s): LI, Y., LI, H., Institution: BEIJING SPORT UNIVERSITY, Country: CHINA, Abstract-ID: 1953

INTRODUCTION:
The quest for sub-2h marathons established curved plates and resilient foams as the hallmark of advanced footwear technology. Although longitudinal bending stiffness (LBS) is a core feature, its impact on running economy (RE) remains equivocal, suggesting a U-shaped relationship where excessive stiffness may negate metabolic savings.This study investigates the biomechanical parameters of transitioning from a stiff threshold to a stiffest configuration. This study quantifies the influence of these two LBS gradients on RE and lower-limb kinetics in elite rearfoot strikers across different speeds. We aim to clarify the mechanical trade-offs between metatarsophalangeal (MTP) efficiency and ankle stability in ultra-stiff footwear.
METHODS:
Nine male rearfoot strikers were recruited. This study evaluated two experimental condition shoe models with different LBS increments(Stiff :Adizero Adios Pro 4, 21.0 N·mm -1 and Stiffest: PLAID 3.0, 28.8 N·mm -1 . RE (Cosmed K4b2) was measured at 16 km/h; biomechanics (Qualisys 200Hz; Kistler 1000Hz) were captured at 12, 14, 16 km/h (±5%). Paired t-tests were used (p < 0.05).
RESULTS:
(1)RE: No significant differences were observed in RE, including VO2/kg, respiratory quotient, and energy expenditure, at 16 km/h (p > 0.05). (2)Spatiotemporal: At 16 km/h, flight time was significantly shorter in PLAID 3.0 (p = 0.014). As speed decreased, this adaptation became more pronounced: at both 14 km/h and 12 km/h, PLAID 3.0 exhibited a significantly prolonged contact time (14 km/h: p = 0.012; 12 km/h:
CONCLUSION:
While RE was comparable at 16 km/h, the ultra-high LBS of PLAID 3.0 significantly optimized MTP efficiency by reducing energy dissipation and enhancing active energy return across all tested speeds. These findings support the lever effect of carbon plates, yet the observed increase in contact time and decrease in flight time suggest a transition toward a rolling gait strategy rather than a bouncing mechanism to accommodate extreme stiffness.Crucially, the elevated ankle eversion velocity and increased negative work indicate that supra-optimal LBS shifts mechanical demands proximally. This mechanical trade-off implies that while ultra-high stiffness footwear facilitates distal energy management, it simultaneously imposes greater stability requirements and eccentric loads on the ankle complex. Consequently, athletes with superior foot-core strength and ankle stability may be better positioned to exploit the propulsive advantages of high-stiffness AFT, whereas others may face an increased risk of fatigue-related compensatory injuries. These insights provide a scientific framework for individualized footwear selection and the structural optimization of future racing shoe designs.