Cardiac Cost During Submaximal Exercise as a Practical Monitoring Tool in French Standardbred Trotters: Short-Term...

Routine monitoring in racehorses requires indicators that are reproducible and practical under real training conditions. This observational study evaluated the short-term reproducibility of cardiovascular and speed indicators in French Standardbred trotters, with a particular focus on cardiac cost (CC), defined as the ratio of heart rate to speed (beats·m^-1). The full dataset comprised 483 sessions from 60 trotters and was used to describe age-related patterns. For reproducibility analyses, consecutive monitored sessions within the same horse were grouped into follow-up blocks when the interval between two successive sessions did not exceed 7 days. Only follow-up blocks containing at least three sessions were retained, resulting in 36 blocks, 126 sessions, and 18 horses. Each session included a warm-up, two 2000 m work blocks at increasing intensity, and recovery periods, while heart rate and speed were recorded using a Polar Team Pro system. Adjusted intraclass correlation coefficients indicated moderate reproducibility for CC during the first work block (CC B1: 0.67, 95% CI 0.48-0.78), heart rate recovery (HRR) after B1 (0.60, 0.40-0.73) and B2 (0.66, 0.47-0.78), and V150 (0.59, 0.39-0.73), whereas V180, recovery speed, and CC during B2 showed poor reproducibility. Reproducibility of CC B1 and HRR was preserved after adjustment for ambient temperature. In the full dataset, V200 increased with age, consistent with previous field-test literature. The minimal detectable change was 0.04 beats·m^-1 for CC B1 and 26 bpm for HRR after B1. These findings suggest that CC B1, HRR, and V150 may be useful indicators for short-term monitoring, although results should be interpreted considering the single-yard design.