Rein tension and heart rate variability in horses: an experiment on experience

Studies on stress and emotions are fundamental to assessing welfare in wild and domestic animals. In this study, we experimentally investigated how different levels of rein tension affect autonomic nervous system activation and heart rate variability (HRV) in horses. We hypothesized that increased rein tension, through a learning process, would elevate sympathetic nervous system activity and HRV, and that adult horses would be less affected by stimulus administration (EXP) than young horses due to their experience. The magnitude of rein tension significantly altered the sympatho-vagal balance in horses. Both the type of equipment (bit versus halter) and the age of the horses influenced their responses. HRV frequency domain analysis showed a significant shift in sympatho-vagal balance during EXP, with values returning to baseline during the recovery period (R) (P < 0.001). Notably, during EXP, the observed increase in LF power alongside a decrease in HF power suggests a shift toward sympathetic dominance and a reduction in vagal modulation. Interestingly, the maximum rein tension did not affect the parameters used to measure these activities, suggesting the tension levels were within an acceptable range for the horses. The balance between sympathetic and parasympathetic activities improved in R with the low-frequency/high-frequency power ratio decreasing by 49.4% from EXP to the baseline condition (B) and by 37.5% from B to R, indicating stress release and increased vagal activity (P = 0.002). During EXP, we recorded an elevated heart rate, indicating heightened arousal, particularly in young horses (P = 0.005) and when using a bridle with a bit (P = 0.024). Our findings suggest that rein tension can induce mild stress, potentially enhancing the learning process. A better understanding of these effects could improve training practices for equids' welfare.