Effects of Isometric Priming on the Horizontal Force-Velocity Profile and Multidirectional Speed Performance in Youth Soccer Players.

BACKGROUND: Priming strategies are used to enhance neuromuscular performance before competition, yet the effects of isometric (ISO) priming on sprint mechanical determinants and change of direction (COD) performance remain unclear in youth soccer players. HYPOTHESIS: An ISO priming session may improve COD performance and horizontal force-velocity (FV) profile variables, and participants' strength levels may be associated with individual performance responses. STUDY DESIGN: Randomized crossover trial. LEVEL OF EVIDENCE: Level 2. METHODS: Ten male youth soccer players completed 2 conditions during training performed 3 days before match-play: an ISO condition (isometric midthigh pulls [3 × 3 repetitions of 3 seconds, with 10 seconds between repetitions and 2 minutes between sets]) and a control condition comprising their usual soccer training. Horizontal FV profile variables, 30-meter sprint, and modified 505 COD performance were assessed before and 24 hours after each condition. Exploratory regression analyses examined associations between participants' strength levels and performance responses. RESULTS: Significant time × condition interactions were observed for FV-profile theoretical maximal force (F0) and modified 505 with the nondominant leg (505-ND), both improving after ISO (P ≤ 0.05). However, sensitivity analyses indicated that the 505-ND response was influenced by condition sequence (P = 0.003). In contrast, ISO decreased FV-profile theoretical maximal velocity (V0) and worsened the decrease in force orientation with increasing velocity (both P ≤ 0.05). No significant interactions were found for 30-meter sprint time or modified 505 with the dominant leg, although 30-meter sprint time increased similarly across both conditions. Relative peak force was associated with changes in 505-ND performance (r = 0.633; P = 0.05). CONCLUSION: ISO priming may represent a practical, low-fatigue strategy to enhance selected force-oriented performance in youth soccer players, although effects on velocity-oriented sprint outcomes appear limited. Exploratory analyses also suggest that baseline relative strength may influence individual responsiveness to this strategy, particularly for COD performance. CLINICAL RELEVANCE: ISO priming may help enhance horizontal force production during competitive microcycles in youth soccer players and may provide benefits for selected COD outcomes. However, these benefits may not extend to velocity-dependent outputs, supporting its use as a complementary and individualized priming strategy.