Changes in physiological parameters in overtrained Standardbred racehorses.
Authors: Hamlin M J, Shearman J P, Hopkins W G
Journal: Equine veterinary journal
Summary
# Editorial Summary Overtraining remains a poorly understood phenomenon in equine athletics, despite its significant performance and welfare implications; this 34-week investigation tracked 10 Standardbred racehorses through a controlled protocol of baseline training (24 weeks), acute overload (8 weeks), and recovery (2 weeks), with fortnightly assessment via time trials and progressive submaximal exercise testing. The overtrained horses demonstrated clear performance deterioration, with final 1200 m split times slowing by 4.0% and peak velocity declining by 6.9%, alongside elevated blood lactate concentrations at both maximal and submaximal intensities—a pattern suggesting impaired aerobic capacity and accumulated fatigue. Notably, bodyweight loss, reduced plasma cortisol, decreased packed cell volume, and lower velocity at 200 bpm heart rate (V200) all responded substantially to overtraining, whilst resting heart rate, basal cortisol, skeletal muscle damage markers (creatine kinase and aspartate aminotransferase), and blood volume parameters remained relatively unchanged. For practitioners managing elite Standardbreds, monitoring bodyweight trends, V200 derived from submaximal exercise tests, post-exercise blood lactate responses, and dynamic (post-exercise) rather than resting cortisol levels offer practical, objective indicators of overtraining before performance collapse occurs, enabling timely training intervention and recovery strategies.
Read the full abstract on PubMed
Practical Takeaways
- •Monitor bodyweight, V200, post-exercise lactate, and plasma cortisol as practical indicators that a racehorse may be overreaching during heavy training blocks
- •A 4-6% decline in time trial performance and peak velocity signals overtraining has occurred; plan recovery periods when these metrics decline
- •Maximal heart rate, basal cortisol, and muscle damage markers (CK, AST) do not reliably indicate overtraining status, so rely instead on performance metrics and the four identified parameters
Key Findings
- •Final 1200 m time trial performance decreased by 4.0% (95% CI 1.7-5.8%) and peak velocity decreased by 6.9% (4.7-8.9%) following 8 weeks of heavy training overload
- •Acute overtraining coincided with increased blood lactate concentration after time trial and submaximal exercise tests
- •Substantial decreases in bodyweight, plasma cortisol concentration, and velocity at heart rate 200/min (V200) were associated with overtraining state
- •Bodyweight, V200, postexercise blood lactate, and plasma cortisol concentrations are useful biomarkers for detecting acute overtraining in equine athletes