Frequency domain analysis of heart rate variability in horses at rest and during exercise.
Authors: Physick-Sheard P W, Marlin D J, Thornhill R, Schroter R C
Journal: Equine veterinary journal
Summary
# Editorial Summary Physick-Sheard and colleagues employed frequency domain analysis of heart rate variability (HRV) to investigate autonomic nervous system modulation during rest and exercise in six Thoroughbreds, examining three frequency bands—very low (VLF: 0–0.01 cycles/beat), low (LF: 0.01–0.07) and high (HF: 0.07–0.5)—to calculate indices of sympathetic and parasympathetic activity across walk, trot and gallop on the treadmill. As exercise intensity increased from rest to trot, power in all frequency bands declined progressively; however, at the gallop, whilst VLF and LF power continued to decrease, HF power rose substantially, indicating a relative shift towards parasympathetic dominance despite heightened physical effort. The sympathetic-to-parasympathetic ratio inverted at higher intensities (lowest at gallop), and notably, horses demonstrated considerable inter-individual variation in these autonomic indicators, with raw spectral power measures showing more consistent patterns than calculated indices across increasing effort levels. A critical threshold emerged at approximately 120–130 beats/min, above which non-neural factors (humoral mechanisms, respiratory-gait entrainment, metabolic demand) appear to override ANS modulation of heart rate. For clinical and conditioning practice, HRV frequency domain analysis offers promise as a non-invasive diagnostic tool for assessing autonomic function and exercise tolerance at submaximal intensities and in response to training interventions or pathology, though its application at high-intensity work remains limited and requires further investigation into the complex interplay between neural and metabolic influences on cardiac rhythm.
Read the full abstract on PubMed
Practical Takeaways
- •Frequency domain HRV analysis may be useful for non-invasively assessing autonomic nervous system function during low-intensity exercise only; it becomes unreliable at intense exercise intensities above 120-130 bpm
- •Considerable individual variation exists between horses in autonomic indicators, so HRV analysis may be more valuable for tracking changes within individual horses than for between-horse comparisons
- •At gallop, parasympathetic activity increases and sympathetic activity decreases, suggesting different autonomic modulation patterns at high-speed gaits that may be influenced by respiratory-gait coordination and breathing mechanics rather than pure nervous system control
Key Findings
- •Spectral power in all frequency bands decreased progressively from rest to trot, but very low and low frequency power continued falling at gallop while high frequency power rose
- •Sympathetic index (LO/HI ratio) rose from rest to walk then fell with increasing exercise intensity, becoming lowest at gallop
- •Parasympathetic index (HI/TOTAL ratio) fell from rest to walk then rose, becoming highest at gallop
- •The relationship between spectral power and heart rate changed at exercise intensities above 120-130 beats/min, suggesting non-neural mechanisms become dominant at high intensities