Cardiopulmonary Function in Thoroughbred Horses Running Uphill and Downhill on an Inclined Treadmill

Abstract

Introduction Eccentric muscle contractions (?negative? work, e.g., downhill running) affect human skeletal muscle differently than concentric contractions, increasing muscle size and strength more than concentric exercise training with reduced oxygen consumption (VO2). Better understanding of skeletal muscle and cardiopulmonary responses to downhill training may indicate a role for this in Thoroughbred training. We hypothesised that Thoroughbreds would reduce metabolic energy cost and cardiopulmonary function when they ran at equivalent speed on a declined treadmill compared with running horizontally or uphill. Methods Five well-trained Thoroughbreds ran for 2-min incremental exercise intervals on a treadmill at 1.7, 3.5, 6.0, 8.0 and 10.0?m/s on a +4% incline, 0% incline (horizontal) and ?4% decline in random order on different days. Cardiopulmonary and O2-transport variables were measured and analysed with two-way repeated-measures ANOVA and Holm-?idák pairwise-comparisons. Results Horses ran at all inclines with identical stride frequency and length. At identical uphill speeds they had significantly higher (vs. horizontal) mass-specific VO2 (43% mean increase) and CO2 production (VCO2/Mb), cardiac output (Q/Mb), heart rate, arterial CO2 tension (PaCO2), and reduced arterial O2 tension and saturation; tidal volume tended to be higher (P?=?0.060). Downhill running reduced (vs. horizontal) VO2/Mb (19% mean decrease), VCO2/Mb, Q/Mb and PaCO2. Fractional energy cost increase (incline) and reduction (decline) decreased linearly with increasing speed. Conclusions Thoroughbreds run downhill with lower energy cost than on the level or uphill and the cost changes with speed. It remains to be determined if eccentric training induces skeletal muscle changes in horses. Ethical Animal Research The study was approved by the Animal Welfare and Ethics Committee of the Equine Research Institute, Japan Racing Association. Sources of funding:?none. Competing interests:?none.