Background: There have been few investigations into the impact of high-intensity interval training on blood rheology. Objectives: This study aimed to evaluate how high-intensity interval training conducted with different duration affects fibrinogen and plasma viscosity in sedentary young college men. Methods: For this study, 36 healthy male participants were selected and grouped based on their individual characteristics. The groups included a control group (n = 9), a 45-second training group (tr-45; n = 9), a 30-second training group (tr-30; n = 9), and a 15-second training group (tr-15; n = 9). The training regimen comprised six sessions over two weeks, with varying sets (4, 5, 6, 6, 7, 4 respectively) and durations of 15, 30, and 45 seconds for different groups. There was a fixed 4-minute rest interval between each set, with a consistent load of 0.6 on the Wingate cycle ergometer. Blood samples were collected 48 hours before and after the final session to analyze fibrinogen and plasma viscosity levels. Results: There were no significant differences between groups in plasma fibrinogen levels (F3, 32 = 2.303, P = 0.96). However, post-test analysis revealed a significant decrease in plasma fibrinogen in the tr-45 group (P = 0.027) compared to pre-test levels. Plasma viscosity did not significantly change between groups (F3, 32 = 0.651, P = 0.058), but there was a significant interaction between time and group (F3, 40 = 4.43, P = 0.009). Post-test analysis showed a significant decrease in plasma viscosity in the tr-45 (P = 0.010), tr-30 (P = 0.002), and tr-15 (P = 0.003) groups, while it significantly increased in the control group (P = 0.004) compared to pre-test levels. Conclusions: The findings suggest that high-intensity interval training can effectively decrease blood rheology and factors such as fibrinogen and plasma viscosity, with the 45-second HIIT being more efficient. Therefore, incorporating this training into Physical Activity Programs could be beneficial for inactive men.
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