Cardiac 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.

The effects of conditioning on cardiac function in young horses is still unknown. For this reason, this study evaluated the left ventricular (LV) function of young horses by echocardiography after six weeks of conditioning. Fourteen untrained young purebred Arabian horses were evaluated at rest and after a stress test (ST) before and after a six-week conditioning program. There was an increase in V₄ (p < 0.001) after conditioning, as well as a reduction in both heart rate (HR) at rest and peak HR during the ST (p < 0.001). There was also a reduction in internal diameter, along with an increase in interventricular septal, free wall and mean thicknesses and LV mass (p < 0.05). After the ST, the conditioned animals showed higher values of velocity time integral, stroke volume, systolic and cardiac indices, ejection (ET) and deceleration times (DT), end-diastolic volume, time to onset of radial myocardial velocity during early diastole and time to peak of transmitral flow velocity, in addition to reduced pre-ejection period (PEP), PEP/ET ratio and mean velocity of circumferential fiber shortening (p < 0.05). The conditioning protocol promoted physiological adaptations that indicate an improvement in the animals’ aerobic capacity associated with an enhanced left ventricular function.

The effect of lactate minimum speed (LMS)-guided training on horses’ homeostasis is still unknown. Thus, this study aimed to evaluate the effect of an LMS-guided training program on the fluid, electrolyte and acid-base status of horses. Ten untrained Arabian horses were submitted to an LMS test on a treadmill before and after six weeks of training. The training intensity was 80% of the LMS in the first three sessions and 100% of the LMS in the other sessions. The venous blood was collected before (T-1) and after (T-2) training at rest, during and after the LMS test for lactate, pH, pCO₂, HCO₃⁻, and electrolyte measurements. The LMS and strong ion difference (SID₄) were calculated. A mild increase in the mean values (p > 0.05) was observed at rest in T-2 in comparison with T-1 in the following variables: pH (from 7.436 ± 0.013 to 7.460 ± 0.012), pCO₂ (from 42.95 ± 1.58 to 45.06 ± 0.81 mmHg), HCO₃⁻ (from 27.01 ± 1.02 to 28.91 ± 0.86 mmol/L), and SID₄ (from 33.42 ± 1.45 to 35.06 ± 2.94 mmol/L). During T-2, these variables were more stable than during T-1. Despite the improvement in fitness, the LMS did not indicate a significant difference (from 5.40 ± 0.55 to 5.52 ± 0.20 m/s). The results confirmed that the LMS-guided training program had a positive impact on the horses’ acid-base status, although some adaptations are still required to improve their fitness.