Acute Reactive Strength Responses and Peak Power–Performance Relationships After Flywheel Calf Raises in National‐Level Track‐and‐Field Athletes: A Randomized Crossover Trial

ABSTRACT This study examined reactive strength responses to flywheel eccentric calf raises performed under three inertial loads and the associations between peak power and post‐intervention reactive strength. Thirty‐seven national‐level male track‐and‐field athletes completed three randomized crossover trials (≥ 48 h apart), each involving 3 × 8 calf raises under Light (0.0281 kg·m 2 ), Moderate (0.0711 kg·m 2 ), or High (0.1011 kg·m 2 ) inertia. Reactive strength was assessed pre‐ and 4‐min post‐exercise using a 10/5 repeated‐jump test measuring ground contact time (GCT), jump height (JH), and reactive strength index (RSI). Concentric and eccentric peak power (ConPP, EccPP) were recorded. JH and RSI increased after Light ( p = 0.018 and p < 0.001) and Moderate (both p < 0.001), while GCT decreased after both conditions (both p < 0.001). No statistically clear improvement was detected after High. Moderate produced greater RSI improvements than Light ( p = 0.008) and High ( p < 0.001), and greater JH gains than Light ( p = 0.020) and High ( p = 0.016). Light produced greater RSI gains than High ( p = 0.022), while GCT decreased more after Light and Moderate than High (both p < 0.001). Power–performance associations varied across loads, with significant ConPP × Load interactions for JH ( p = 0.002) and RSI ( p = 0.003), and EccPP × Load interactions for JH ( p = 0.040) and RSI ( p = 0.025). Under High load, ConPP was associated with JH ( β = 1.37 cm, 95% CI 0.57–2.16) and RSI ( β = 0.086, 95% CI 0.020–0.153), while EccPP was associated with JH ( β = 1.27 cm, 95% CI 0.42–2.12; all p ≤ 0.012). Overall, greater improvements were observed 4‐min after light and particularly moderate inertia, whereas high‐load peak power was more strongly related to post‐intervention performance.