A consort-guided randomized, blinded, controlled clinical trial on the effects of 6 weeks training on heart rate variability in thoroughbred horses.
Authors: Santosuosso Emma, Léguillette Renaud, Shoemaker Sierra, Baumwart Ryan, Temple Sierra, Hemmerling Kaneesha, Kell Tessa, Bayly Warwick
Journal: Journal of veterinary internal medicine
Summary
# Heart Rate Variability as a Fitness Marker in Thoroughbreds: Early Evidence from a Controlled Training Trial Whilst heart rate variability has proven useful for monitoring training adaptation in human athletes, its application to equine fitness assessment remains largely unexplored. Santosuosso and colleagues conducted a six-week randomised controlled trial with twelve untrained thoroughbreds, comparing HRV measurements in trained and control groups at baseline and fortnightly intervals, alongside maximal oxygen uptake (V̇O₂max) and cardiac ultrasound assessments. The training cohort demonstrated significant shifts in frequency domain HRV variables: the LF/HF ratio increased from 0.51 to 0.91, whilst normalised high-frequency power (HFnu) declined substantially from 76.2 to 53.2 units, reflecting increased sympathetic nervous system dominance with fitness gains. Most notably, improvements in V̇O₂max (134 to 146 mL/kg·min) correlated strongly with changes in low-frequency normalised power (r = -0.88), suggesting aerobic conditioning produces measurable autonomic shifts. However, time-domain variables such as RMSSD and parasympathetic markers (SD1, SD2) showed no training effect, and resting heart rate and echocardiographic indices remained unchanged, indicating that HRV responses to training are selective rather than global. Whilst these findings provide proof-of-concept that frequency domain HRV variables respond to training intensity in horses, validation through larger longitudinal studies will be necessary before practitioners can confidently integrate HRV monitoring into routine fitness assessment protocols.
Read the full abstract on PubMed
Practical Takeaways
- •Heart rate variability frequency domain variables (LF/HF ratio, LFnu, HFnu) may offer a non-invasive method to monitor fitness development during training, but require further validation before routine clinical use
- •The correlation between changes in VO₂max and LF variables suggests HRV could potentially be used as a field-friendly alternative to maximal exercise testing for fitness assessment in racehorses
- •Current evidence is insufficient to recommend HRV as a standalone fitness monitoring tool; additional studies across different horse populations and training intensities are needed
Key Findings
- •Six weeks of incremental racetrack training increased VO₂max from 134 to 146 mL/(kg·min) (P<0.001) in trained thoroughbreds
- •Training increased LF/HF ratio (0.51–0.91, P=0.02) and LFnu (37.5–46.8, P=0.02) while decreasing HFnu (76.2–53.2, P<0.001)
- •Strong negative correlation found between VO₂max and LFnu (r=−0.59, P=0.04) and between ΔVO₂max and ΔLF (r=−0.88, P=0.02)
- •Training did not significantly affect RMSSD, SD1, SD2, resting heart rate, or echocardiographic indexes