Heart rate variability as index of emotional stress in horses during therapeutic riding exercise with children affected by autism spectrum disorder

INTRODUCTION: Few data are available on the level of stress that horses can experience during therapeutic riding (TR) especially from a welfare perspective. Since TR programs involve riders with physical and/or psychological problems, horses can undergo to some type of stress or frustration (Kaiser et al., 2004) that jeopardize their welfare and, last but not least, influence the TR effects. In fact, the successful of the TR is strongly influenced by the horse physical and mental fitness. This is particularly important in the case of programs addressed to children with Autism Spectrum Disorder (ASD). Heart Rate Variability (HRV) represents a significant tool to evaluate stress in horses, since it is indicative of the autonomic nervous system response and may reveal the balance between the sympathetic and parasympathetic branch of the heart (von Borell et al., 2007). The hypothesis of the study is that horses involved in TR might be enforced to suppress emotional reactions, a condition which could be considered a work stressor. MATERIALS&METHODS: A multicentre study was conducted on seven healthy horses (mean age 18 years, mean BCS 6). TR sessions were carried out following a standardized protocol (Borgi et al., 2016) with phases: grooming, mounting, riding and stationary exercises. Fourteen sessions (ASD vs a session with a typically developed child, TD) were conducted. Horses were equiped with Polar Equine® RS800G3; data were analyzed by Polar software. The root mean square of successive differences of heartbeats (RMSSD, ms), standard Deviation of all R-R intervals (SDNN, ms), low frequencies (LF, ms2), and high frequency (HF, ms2) in each phase were calculated. Measures taking into account the influences of different levels of physical activity were compared. Ethical approval from the Italian Ministry of Health was obtained. Non parametric Wilcoxon signed-rank test was used to test the equality of matched pairs of observations (Stata 12.1). RESULTS: Lower values of SDNN and RMSSD in TD riding were observed compared to ASD; a decrease of those parameters has been associated to increased levels of stress load (von Borell et al., 2007). HF as well is index of vagal activity; low vagal tone indicates that animals can potentially being more vulnerable to stress (von Borell et al., 2007). The horses in our study during TD sessions had lower levels of HF during mounting and riding. Even a decrease of LF is a sign of stress for the increased activity of the sympathetic activity. However, this parameter is more complex to evaluate. Results suggest that riders with different abilities may cause distinct autonomic responses in horses. Being ridden by a TD child causes lower levels of RMSSD, as well as SDNN. These decreases are associated with increasing levels of stress load; moreover higher sympathetic activity is associated with mental task (Taelman et al. 2008). TAKE HOME MESSAGE: According to this preliminary data, horses show increased emotional reactions during activities with TD children, and a reduced emotional arousal during activities with ASD children, as measured through HRV. Since therapeutic riding programs involve riders with physical and/or psychological issues, horses can experience some kind of stress or frustration that put at risk the horse welfare and influence the therapeutic riding effect. The results of the present research suggest that for horses involved in therapeutic riding show increased emotional reactions during activities with TD children, and a reduced emotional arousal during activities with ASD children, as measured through HRV. The results provide interesting issues to optimize therapeutic riding sessions.