Researchers have charted the rise and fall of muscle enzymes and muscle metabolites from Eventing cross-country rides, finding that several were still elevated the next morning.
Researchers, writing in the journal Animals, said Eventing is a demanding discipline that requires horses to jump over solid obstacles at high speed in the cross-country phase, putting them under considerable physical strain.
To reduce the risk of injury and ensure the health and performance of the horses, it is essential to assess musculoskeletal and cardiac function during the recovery period.
Johanna Giers and her fellow researchers set out to examine whether blood-based markers for the skeletal and cardiac muscle function of top-class eventing horses changed after exercise under realistic and updated competition conditions.
Unlike previous studies that overlooked the shift in plasma volume when analyzing blood parameters after exercise, they introduced an adjustment to enhance the interpretation of blood parameters immediately after exercise.
Their approach eliminated the concentration changes caused by shifts in fluid balance arising from spleen contraction, plasma volume shift, and dehydration.
Eight riders with 20 horses participated in the study, which took place at 14 international 2- to 4-star-level Eventing competitions at five different venues in Germany and Poland between March and September 2022.
All horses were sampled at two to five competitions during the season, resulting in a sample count per horse ranging from 4 to 20.
For each competition, the horses went through the mandatory veterinary checks before and after the cross-country test, which declared the horses “fit to compete”.
The horses were aged 7 to 15, with an average age of 11. They comprised 10 mares and 10 geldings, and belonged to nine different warmblood breeds.
The researchers targeted parameters that provided information about fluid balance, muscle enzymes, metabolites and cardiac muscle-specific markers.
Parameters were measured in blood samples taken on the morning of the cross-country (before exercise), 10 minutes and 30 minutes after completion of the course, and the next morning.
Most parameter concentrations were found to have changed in an exercise-dependent manner 30 minutes after completion of the cross country.
Most exercise-dependent markers were back to pre-exercise levels the next morning, but creatine kinase (up 26%) and lactate dehydrogenase (up 15%) were still above the levels obtained before the cross country.
Both were trending down.
Levels of cardiac troponin I increased above the reference range in 40 of the 55 rides (73%). Levels were still elevated in 18 of the 20 horses the morning after exercise.
The increase in cardiac troponin levels indicates cardiac fatigue following the cross-country ride, they said.
“The reasons for this, besides myocardial damage, could be increased cell membrane permeability because of changes in oxygen tension or local pH during exercise.”
To identify myocardial damage, measurements at several time points after exercise are necessary to assess how long the troponin release lasts, they said. A sustained release over several days is indicative of myocardial damage, while rapidly decreasing levels are indicative of a physiological process.
By way of comparison, in Endurance horses after completion of 120 to 160km races, researchers have measured troponin levels more than 20-fold higher than the maximum value measured in the current study.
“The increase in cardiac troponin I levels observed following cross-country exercise reflects changes in the cellular function of cardiomyocytes, which we posit aligns with the concept of cardiac fatigue. Notably, the definition of cardiac fatigue does not inherently necessitate the presence of myocardial necrosis.”
Myocardial damage was not detected in the study.
“Consequently, we interpret the elevated cardiac troponin I levels we measured as an indication of a physiological manifestation of cardiac fatigue.”
The study team comprised Johanna Giers, Alexander Bartel and Heidrun Gehlen, all with the Free University of Berlin; Katharina Kirsch, with the Leibniz Institute for Agricultural Engineering and Bioeconomy; Simon Franz Müller, with Laboklin Veterinary Laboratory Diagnostics; and Stephanie Horstmann, with the German Olympic Committee for Equestrian Sports.
Giers, J.; Bartel, A.; Kirsch, K.; Müller, S.F.; Horstmann, S.; Gehlen, H. Blood-Based Markers for Skeletal and Cardiac Muscle Function in Eventing Horses before and after Cross-Country Rides and How They Are Influenced by Plasma Volume Shift. Animals 2023, 13, 3110. https://doi.org/10.3390/ani13193110
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