Simple SummaryHorses can develop “athlete’s heart” in a similar way to humans and dogs after physical training. The purpose of this study was to assess by echocardiography the cardiac changes of young purebred Arabian horses after performing a lactate-guided conditioning program. The conditioning program was established individually, according to the physical fitness of each animal, as determined by an incremental exercise test (IET). The IET and echocardiograms were carried out before and after six weeks of treadmill conditioning. At the end of the conditioning period, the horses showed improved aerobic capacity and structural and functional heart changes associated with “athlete’s heart”.AbstractCardiac adaptation to conditioning in horses was evaluated after empirical training based on trainers’ experience. Twelve purebred Arabian horses, aged (mean ± SD) 28.42 ± 3.75 months, which did not perform any type of exercise prior to the research, were submitted to treadmill conditioning for six weeks. The conditioning program was based on the velocity run by the horse at which the blood lactate concentration, determined in an incremental exercise test (IET), reached 2 mmol/L (V2). The velocity at which the blood lactate concentration reached 4 mmol/L (V4) was also determined. The echocardiograms were performed at rest with pulsed-wave and tissue Doppler imaging in B- and M-modes. All procedures were carried out before and after the conditioning period. The results showed increases in V2(from 5.2 ± 0.3 to 6.7 ± 0.4 m/s) and V4(from 5.8 ± 0.4 to 7.6 ± 0.5 m/s) (p< 0.0001). There were also increases in the left ventricle internal diameter at diastole (LVIDd), left ventricle mass (LV mass), and stroke volume (SV), while no changes were observed in the LV free wall thickness and mean and relative wall thicknesses. The conditioning protocol, which was completed by all horses, proved to be safe and efficient, as it improved the aerobic capacity of the animals. Finally, the cardiac remodeling that occurred was mainly associated with the effect of physical training.