Background: It has been shown that unilateral balance training (BT) enhances balance and neuromuscular performance in the trained and untrained leg and thus has the potential to reduce side-to-side differences. However, it is unresolved how effective unilateral single-mode BT compared to unilateral combined balance and plyometric training (BT + PT) is in reducing interlimb asymmetry. Thus, we examined the impact of both training modalities on soccer players` interlimb asymmetry in balance and neuromuscular performance. Methods: Sixty male elite youth soccer players were randomly assigned to a unilateral single-mode BT group (n = 20), a unilateral combined BT + PT group (n = 22) or an active control group (n = 22). All groups trained for nine weeks (2 sessions/week) with their non-dominant leg. The interventions included balance exercises for the single-mode BT group, balance and plyometric exercises for the combined BT + PT group and stretching exercises for the active control group. Training progression was achieved by manipulating sensory input (balance exercises), the number of floor contacts (plyometric exercises), or the duration of the stretching exercises. Before and after the intervention period, performance in the Y Balance Test - Lower Quarter (YBT-LQ) and the single leg drop landing (SLDL) test as well as leg muscle activity were assessed. The magnitude of interlimb differences was quantified by calculating the limb symmetry index (LSI) and compared using repeated measures analysis of variance (significance level: p < .05). Results: The analysis revealed significant main effects of test (pre vs. post) for the LSI values of the YBT-LQ (anterior reach: np2 = .07; posterolateral reach: np2 = .11) and activity of the gastrocnemius muscle (np2 = .33) but not for the SLDL test. Further, a significant test×group interaction was detected for the LSI value of the anterior reach (np2 = .10) and post-hoc analyses yielded a significant LSI reduction for the unilateral single-mode BT group (d = .79) only. Conclusion: Nine weeks of unilateral single-mode BT and combined BT + PT proved to be effective and feasible (attendance rates of ˜ 99%-100%). In particular, unilateral single-mode BT appears to be suitable for reducing the LSI value of the YBT-LQ anterior reach, which is a predictor for time loss non-contact lower limb injuries.
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