Efectos del curl nórdico a diferentes velocidades en la fuerza excéntrica de futbolistas jóvenes

Introduction: Hamstring injuries are common in soccer and are associated with eccentric strength deficits and inter-limb functional asymmetries. The Nordic Hamstring Curl (NHC) is effective for prevention; however, the influence of execution velocity on functional adaptations in young athletes remains unclear. Objective: To determine the effects of two execution velocities of the Nordic Hamstring Curl on eccentric knee flexor strength and functional asymmetry in young soccer players. Methods: A quasi-experimental study with random assignment was conducted in male U-17 soccer players (n=20), divided into a fast-velocity group (FG) and a slow-velocity group (SG). The intervention lasted 4 weeks (3 sessions per week) with progressive volume. Bilateral eccentric strength was recorded in real time during the Nordic Hamstring Curl using a validated load-cell device; inter-limb asymmetry was calculated. Results: Improvements in eccentric strength were observed in both groups. Statistical significance was reached only in the non-dominant limb of the FG (p=0.0157). No between-group differences were found bilaterally. Functional asymmetry was significantly reduced in the FG (p=0.0119), while it remained stable in the SG. Discussion: A short Nordic Curl training cycle improved eccentric strength and reduced inter-limb asymmetry, especially with a fast tempo as a key modulator, showing both agreements and discrepancies with previous studies. Conclusions: The execution velocity of the Nordic Curl emerges as a key factor in short-term adaptations: both tempos improve eccentric strength, and the fast, braking-emphasized variant additionally promotes the correction of asymmetries.