Changes in Elastic Bounce Mechanics Across a Competitive Cross-Country Season

The spring–mass model (SMM) provides a whole-body representation of elastic behavior during running. Effective contact time (tce), effective aerial time (tae), rebound asymmetry (REB), vertical stiffness (kvert), and step frequency (SF) describe complementary temporal and mechanical features of elastic bouncing, yet little is known whether these characteristics change across a competitive distance-running season. Therefore, this study examined pre- to post-season changes in SMM-derived elastic bounce characteristics in university runners tested at the same within-runner velocity. Twelve university runners from the same team completed a six-minute treadmill run with synchronized motion capture before and after a seven-week season. Bilateral kinematics and kinetics were collected and elasticity variables computed. No significant changes were observed in elastic bounce characteristics. SF increased by 1.65 steps/min (p = 0.409, d = 0.25), but not significantly. kvert showed small decreases (left: p = 0.374, d = 0.28; right: p = 0.613, d = 0.16). REB remained unchanged (left: p = 0.904, d = 0.04; right: p = 0.871, d = 0.05). The competitive season had little influence on group-level elastic bounce characteristics tested at a given running velocity, although individual responses varied.