Thoroughbred racehorse fitness assessment with glowing vials of blood on treadmill.

Does Training Impact Performance? Unveiling the Link Between Exercise and Inflammation in Horses

Lena Voss in Health & Wellness December 2025 • 4 min read.

"Explore how training affects inflammation and fitness in Thoroughbred racehorses, and what it could mean for equine athletes."

For years, exercise has been known to trigger a cascade of changes within the equine body, particularly influencing oxidant and antioxidant balances. Of increasing interest is the role of plasma myeloperoxidase (MPO), a key indicator of neutrophil activity and potential disruption of the delicate oxidant/antioxidant equilibrium. Neutrophils, a type of white blood cell, increase after exercise and subsequently release MPO, making it a marker worth investigating.

Increased plasma MPO concentrations have been observed following exercise in various equine disciplines, including eventing, endurance riding, and even in Standardbred trotters. These observations have led researchers to wonder whether training impacts the magnitude of the MPO response or if the timing of sample collection in relation to exercise influences the results.

To address these knowledge gaps, a recent study delved into the dynamics of plasma MPO concentrations in Thoroughbred racehorses. The goal was to evaluate MPO levels at different time points before and after high-intensity exercise, assess the impact of training on these levels, and explore the relationships between MPO concentrations, physiological measurements, and racing performance.

Key Findings: MPO, Training, and Thoroughbreds

Thoroughbred racehorse fitness assessment with glowing vials of blood on treadmill.

The study tracked 26 Thoroughbred racehorses in active training. Over nine months, the horses underwent exercise tests on a high-speed treadmill at the beginning, middle, or end of the training period. Researchers carefully monitored heart rate, speed, and distance, and collected blood samples at specific times: before exercise (To), at maximal speed (TVmax), 5 minutes post-exercise (T5min), and 4 hours post-exercise (T4h).

The blood samples were analyzed for plasma lactate, serum creatine kinase (CK), plasma MPO, white blood cell count (WBC), neutrophil count (Neut), lymphocyte count, and neutrophil percentage (%Neut). Here's a summary of the key findings:

White Blood Cell Count (WBC): WBC increased above baseline (To) at all sampling points, with no discernible training effect.
Neutrophils (Neut) and %Neut: Both Neut and %Neut increased above baseline at T4h. Notably, values at TVmax, T5min, and T4h increased as training duration progressed.
Plasma MPO Concentration: MPO levels increased above baseline at all sampling points in response to exercise. All values increased as training duration increased.
Training Intensity: Training intensity (number of fast work sessions) did not significantly affect any of the measurements.
Correlations: The study did not identify significant correlations between biological markers (including MPO) and physiological/training measurements. Also, no significant differences were found between elite and non-elite horses.

These results suggest that plasma MPO concentration is indeed affected by exercise and, importantly, by the duration of training. The lack of correlation with training intensity suggests that other factors, such as individual horse characteristics or the specific type of training, might play a more significant role.

The Bottom Line: MPO as a Potential Fitness Indicator

The study concludes that plasma MPO concentration is influenced by exercise and training duration, paving the way for future investigations into MPO as a potential tool for assessing fitness and readiness for competition in horses. Further research could explore the factors that influence MPO responses, helping trainers and veterinarians optimize training programs and monitor the well-being of equine athletes.

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Everything You Need To Know

How does exercise impact the levels of plasma myeloperoxidase (MPO) in Thoroughbred racehorses?

Exercise significantly affects plasma myeloperoxidase (MPO) concentrations in Thoroughbred racehorses. MPO levels increase above baseline at all sampling points following exercise. Additionally, MPO levels tend to increase as the duration of training progresses, indicating a cumulative effect of training on this inflammatory marker. The study did not find significant correlations between MPO and training intensity, suggesting other factors beyond intensity may play a larger role.

What is the significance of myeloperoxidase (MPO) in the context of equine exercise physiology, and why is it important to measure its levels?

Plasma myeloperoxidase (MPO) is a key indicator of neutrophil activity and the oxidant/antioxidant balance in horses. Neutrophils, a type of white blood cell, release MPO after exercise, reflecting the body's inflammatory response. Measuring MPO levels can provide insights into the physiological stress experienced by horses during training and competition. Elevated MPO levels may suggest increased inflammation. While this study shows MPO levels are affected by training duration, further research is needed to determine if MPO can serve as a reliable marker for assessing fitness and readiness for competition in horses. The missing factor here is the impact of MPO on performance, this link wasn't clearly established.

The research indicated that training intensity didn't significantly affect the measured biological markers. What factors might be more influential than intensity in determining a horse's inflammatory response to exercise?

The study's finding that training intensity did not significantly affect the measured biological markers, including plasma myeloperoxidase (MPO), suggests that other factors play a more crucial role. Individual horse characteristics, such as genetics, pre-existing conditions, or baseline fitness levels, could influence their inflammatory response. The specific type of training, variations in recovery periods, nutritional status, and environmental conditions are additional potential factors. The non-significant correlation could be attributed to the limited sample size and lack of controlled experimentation of the factors. Future research should consider and control those other factors to get a better reading.

Did the study find any significant differences in myeloperoxidase (MPO) levels or other markers between elite and non-elite Thoroughbred racehorses?

No, the study did not identify significant differences in plasma myeloperoxidase (MPO) levels or other measured biological markers between elite and non-elite Thoroughbred racehorses. This suggests that baseline MPO levels and their response to exercise do not directly differentiate between higher and lower performing horses, based on the measurements. Further research may reveal differences in MPO kinetics (how quickly MPO levels change), or interactions between MPO and other factors that distinguish elite performers.

Based on the study's findings, how can plasma myeloperoxidase (MPO) monitoring potentially be used to improve training programs and monitor the well-being of equine athletes, and what further research is needed to solidify its role?

While the study shows plasma myeloperoxidase (MPO) concentration is influenced by exercise and training duration, further research is needed to solidify its role in training. If further research can determine the ideal MPO response relative to performance, trainers could use it as an indicator to avoid overtraining or undertraining. Longitudinal studies are needed to track individual horses and identify optimal MPO ranges associated with peak performance and minimal risk of injury. This could help tailor training programs to each horse's specific needs, enhancing both their performance and well-being. The specific type of training and individual horse characteristics needs to be analyzed and determined.