A consort‐guided randomized, blinded, controlled clinical trial on the effects of 6 weeks training on heart rate variability in thoroughbred horses

Abstract Background Fitness assessment of horses remains challenging. Heart rate variability (HRV) can be used to monitor human athlete's training, but its value is unknown in horses. Hypothesis The linear domain HRV variables are affected by fitness. Animals Twelve healthy untrained thoroughbreds were randomly split into a training group (6 weeks of incremental racetrack training) and a control group (no training). Methods Linear domain HRV variables were analyzed (high frequency [HF], low frequency [LF], their normalized units [Hfnu, Lfnu], root mean square of successive differences between beats [RMSSD], Poincaré plot features [SD1 and SD2]) while resting overnight before (baseline) and after 2, 4 and 6 weeks of training. V̇O2max and echocardiographic indexes were measured at baseline and after 6 weeks. Changes in HRV variables over time (ANOVA), and correlation with V̇O2max (Pearson's chi‐squared test) were tested (P < .05 significance). Results V̇O2max, LF/HF ratio, and LFnu increased while HFnu decreased in the training group (before and after training mean [SD] values: V̇O2max 134 [12.8]‐146 [16.5] mL/[kg min]; P < .001; LF/HF ratio 0.51 [0.2]‐0.91 [0.3] [P = .02]; LFnu 37.5 [10.1]‐46.8 [7.8] [P = .02]; HFnu 76.2 [7.9]‐53.2 [7.7] [P < .001]). Training did not affect RMSSD, SD1, SD2, resting HR, or echocardiographic indexes. Strong correlations were found between V̇O2max and HRV variables (V̇O2max and LFnu [r = −0.59, P = .04]; ΔV̇O2max and the corresponding ΔLF [r = −0.88, P = .02]). Conclusions and Clinical Importance Six weeks of training affected some frequency domain HRV variables. Further studies are necessary to validate the use of HRV for monitoring horses' responses to training.