High-Intensity Interval Training Improves Cardiovascular Fitness and Induces Left-Ventricular Hypertrophy During Off-Season

<b>Background</b>: Well-designed endurance training leads to improved cardiovascular fitness and sports performance in prolonged exercise tasks, with the adaptations depending on multiple factors, including the training modality and the population in question. It is still disputable how the type of training affects myocardial remodeling, and the information on myocardial remodeling by high-intensity interval training (HIIT) is particularly scarce. <b>Methods</b>: The current study investigated changes in cardiac structure after volume-progressive HIIT in running mode. As part of their conditioning program, amateur athletes (mean ± SD age of 18.2 ± 1.0 years) exclusively conducted HIIT in a volume-progressive fashion over 7 weeks (a total of 21 sessions). Peak oxygen uptake as well as 200 m and 2000 m running performance were measured, and transthoracic two-dimensional echocardiography was conducted before and after the intervention. <b>Results</b>: Training improved running performance, increased the peak oxygen uptake and left atrium diameter (from 32.0 ± 2.5 to 33.5 ± 2.3 mm; <i>p</i> = 0.01), and induced ~11% thickening of the left-ventricular posterior wall (7.5 ± 0.7 to 8.2 ± 0.4 mm; <i>p</i> = 0.01) and interventricular septum (7.6 ± 0.7 to 8.6 ± 0.9 mm; <i>p</i> = 0.02), but not the dilation of left-ventricular, right-ventricular, or right atrium chambers. <b>Conclusions</b>: HIIT of just 127 km of running per 8.5 h during 7 weeks was sufficient to improve aerobic capacity and running performance, and induce left-ventricular wall hypertrophy and left atrium dilation, in young healthy athletes.