The Effectiveness of Unresisted Versus Heavy Resisted Sprints on Sprint Speed, Strength Deficit, and Force–Time Measures in Youth Rugby Players

PURPOSE: Although both unresisted sprint training (UST) and resisted sprint training (RST) are widely used in rugby and other team sports, evidence comparing their specific effects on neuromuscular adaptations and strength-related measures is still limited. This study aimed to analyze the effects of two sprint training programs-UST and RST, using 50% of body mass-on secondary outcome measures and force-time characteristics obtained from the squat one-repetition maximum (squat 1RM-strength), associated strength deficits (SDef from 40% to 90% 1RM), isometric squat, Nordic hamstring exercise (NHE), and jump performance in youth rugby players. METHODS: Thirty-five under-19 male rugby players (age: 17.3 ± 0.9 y) volunteered to participate in the study. Players completed the following assessments: (1) UST during a 30-m sprint; (2) isometric squat and squat 1RM-strength; and (3) hamstring eccentric strength via the NHE and countermovement jump tests. RESULTS: Only the UST group showed moderate improvements in 30-m sprint speed (ES = -0.77, 95% CI, -1.43 to 0.07, P < .05). In the isometric squat, both groups demonstrated small-to-moderate increases in force at 200 milliseconds (ES = -0.83 to -0.64, P < .05). In the NHE, the RST group showed large increases in torque and average force across both legs (P < .001, ES = -1.73 to -1.28). Similarly, in the UST group, large increases were observed in NHE average force across both legs (P ≤ .008, ES = -0.99 to -0.92), while NHE impulse was moderately reduced only in the right leg (ES = 0.71, 95% CI, 0.06 to 1.33, P = .031). CONCLUSIONS: Our findings indicate that both UST and RST can be effectively implemented during short in-season periods to maintain sprint performance and strength-related capacities in youth rugby players, especially when time for resistance training is limited or cannot be prioritized. Coaches may therefore select either method according to the specific context, time constraints, and player needs.