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

Abstract: 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. This study explores how different levels of rein tension affect the stress response and heart activity in horses, focusing on the differences between younger and older horses. We aimed to understand whether the experience of older horses makes them less sensitive to rein tension compared to younger ones. Using heart rate variability (HRV) as a measure, we found that higher rein tension activated the horses’ stress response but did not overwhelm them, suggesting the tension used was within a safe range. Interestingly, older horses handled the tension better, likely due to their training experience. We also discovered that using different types of equipment, such as a bit or halter, affected how the horses responded, with the bit causing higher stress levels. Our findings suggest that while rein tension can trigger a mild stress response, it may also be part of the learning process for horses, helping them adapt. This research could inform future training methods that take into account the horses’ stress responses, potentially improving their welfare by identifying optimal training conditions.