Objective: Tabata training has the potential to induce favorable adaptations in elite athletes. However, the underlying mechanisms remain incompletely understood. Furthermore, varying the training surface can contribute to improvements in athletes\\\' fitness by introducing diversity into the training program. This study compared the effects of Tabata training conducted on an indoor hardcourt vs. sand on maximum oxygen consumption (VO2max), left ventricular morphology, and the expression of microRNA-1 (miR-1) and Hand2 in elite beach soccer players. Methods: Sixteen players were randomly assigned to either a soft (SS) or hard (HS) surface group. The training variables were similar between the groups and only the type of training surface was different. The protocol (4-min sets of 20 s of exercise, 10 s of rest) was performed three sessions/week for six weeks. Shuttle run test and two-dimensional echocardiography were performed 48 h and 72 h, respectively, before and after the intervention. Additionally, venous blood samples were collected before and 6 h after the first and last session. Findings: Repeated measures ANOVA revealed significant time × group interactions for increases in stroke volume (SV; P= 0.039), left ventricular end-diastolic volume (p = 0.013) and dimension (p = 0.029), ejection fraction (p = 0.017), and reductions in relative posterior wall thickness (p = 0.036). The SS group exhibited greater post-training increases in miR-1 and Hand2 protein (p < 0.0001). Moreover, both groups experienced similar increases in VO2max values (p < 0.0001). Conclusion: Tabata training on sand can produce greater improvements in SV values compared to indoor hardcourt, despite similar improvements in relative VO2max. These cardiac improvements may partly relate to enhanced venous return, and the molecular findings suggest a potential contribution of miR-1 upregulation, whereas the role of Hand2 remains less clear due to limitations in serum-based assessment.

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