Purpose Literature examining left ventricular (LV) structural adaptations to combined strength and endurance training is inconsistent. Rugby is a sport that combines these two exercise modalities, both during training and match play. This study aimed to explore differences in LV structure between high-level rugby players and untrained controls. Body composition analysis was performed to determine the most appropriate indexing variable for LV mass (LVM) and understand if increases in LV represent either a training-related physiological adaptation or reflect the groups` anthropometric differences. Methods A cross-sectional design compared 10 rugby players and 10 untrained age-matched, male controls. Body composition was obtained by bioelectrical impedance. M-mode echocardiographic imaging was performed on the LV from the parasternal long axis view. Results Significantly greater end-diastolic interventricular septum, LV internal diameter, posterior wall thickness, LVM and LVM/fat-free mass (FFM) (p < 0.05) were found in rugby players compared to age-matched controls. Moreover, Pearson`s correlation tests revealed FFM to be the body composition variable with the strongest correlation to LVM (r = 0.775, p < 0.001). Conclusion The differences in LV structure between groups suggest that the combined endurance and strength training that rugby athletes are subjected to, induce a process of concentric and eccentric enlargement of the LV structure. Furthermore, the association found with FFM, suggests it to be the most appropriate body scaling variable to index to LVM and, thus, should be considered when describing increases in LVM. The present research suggests that increased LVM in the athletes group represents true physiological adaptations to training.
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