A characterization of response variables to sub-maximal standardized exercise tests in recreational horses

This study aimed to investigate the use of maximum eye surface temperature (ESTmax), recorded by infrared thermography, as an indicator of physical fitness in ranch horses. Data collection was carried out with 16 horses by recording six physical fitness biomarkers (ESTmax, respiratory rate, creatine kinase activity [CK], and concentrations of total serum protein, plasmatic cortisol, and lactate) before and after a regular working day. We also recorded heart rate during the exercise, exercise duration, average velocity, distance traveled, and air temperature and humidity. The variation between the sampling times was compared by using repeated measures ANOVA or Wilcoxon paired test. The t-test was used to compare ESTmax means of dehydrated and nondehydrated horses. The differences (DIF-) between each variable's sampling times were calculated, and the Spearman's correlation coefficients between DIF-ESTmax and DIF- of other variables were estimated. A hierarchical cluster analysis was used to classify the horses according to their physical fitness. All biomarkers increased after the workday, and the ESTmax means of dehydrated and nondehydrated horses did not differ (P < .05). DIF-CK and DIF-ESTmax were positively correlated (rs = 0.56, P < .05). The cluster analysis defined two groups of horses (LOW and HIGH), with DIF-ESTmax and DIF-CK differing statistically between them. HIGH horses showed CK higher than 350 U/L, indicating some degree of muscle damage. These results suggest that ESTmax has potential to predict changes in CK and, thus could be a useful indicator of physical fitness in ranch horses; although, its effectiveness has to be confirmed in future studies.