Heart rate variability responses of horses and veterans with post-traumatic stress disorder to ground-based adaptive horsemanship lessons: a pilot study.

Heart rate variability (HRV) can be measured as an indicator of autonomic nervous system (ANS) balance and thus, stress and affective arousal. Mixed results have been reported in the limited literature addressing the effects of equine-assisted services (EAS) on HRV in the human and horse participants. The aims of the present study were to determine the effects of ground-based adaptive horsemanship (AH) lessons on veterans' and horses' HRV during weekly lessons as well as veterans' resting HRV outside of lessons. Veterans with post-traumatic stress disorder (PTSD) were randomly assigned to 8 wk of AH or control (CON, no changes in treatments or activities) conditions. Horses were assigned to AH or control (CON, stall in arena) conditions based on previous experience (EAS or recreational riding). Electrocardiogram traces from the veteran and AH and CON horses were recorded during the 30-min lessons. Traces were recorded during resting conditions before (PRE) and after (POST) the 8-wk period from AH and CON veterans and healthy, non-veterans. Heart rate (HR), root mean square of successive differences (RMSSD) and low frequency to high frequency ratios (LF/HF) were calculated after identification of peaks with the Pan-Tompkins algorithm and manual correction during 5-min epochs. Data were analyzed with repeated measures, mixed model ANOVAs (SAS v9.4). HR was lowest (p ≤ 0.0323) in the horses and veterans during the first 10 min of the lessons. Veterans' RMSSD was higher (p ≤ 0.0496) in weeks 4 and 6 than week 2. LF/HF was greater in veterans with PTSD than healthy, non-veterans across PRE and POST time points. Increased HR later in the lessons is likely a result of increased movement in the horses and veterans. An interval of decreased stress and increased resiliency might be present in veterans participating in multi-day AH sessions. Increased sympathetic arousal measured via LF/HF was not mitigated by 8 wk of AH. Horses appeared unstressed by the interactions as no changes in HRV were observed.