Heart rate response and plasma lactate concentrations of horses competing in the cross-country phase of combined training events.

The exercise intensity of horses (n = 175) competing in the cross-country phase of 4 combined training events at 5 levels of competition [novice (N), training (T), preliminary (P), intermediate (I) and advanced (A)] was investigated. Environmental conditions varied from cool and humid (mean +/- s.d. temperature and humidity = 12.1 degrees C +/- 0.6 and 88.4% +/- 8.9 RH) to moderately hot and humid (27.8 degrees C +/- 2.1 and 62.5% +/- 7.8 RH). Heart rates (beats/min) were recorded continuously at 5 s intervals with a heart monitor. Mean +/- s.d. heart rates (beats/min) for each level of competition were: 154.4 +/- 19.2 beats/min (n = 45) (N), 160.4 +/- 28.1 beats/min (n = 47) (T), 160.6 +/- 33 beats/min (n = 53) (P), 181 +/- 26.6 beats/min (n = 29) (I) and 194.6 +/- 4.2 beats/min (n = 4) (A). Plasma lactate concentration was determined at rest in a random sampling of horses (n = 36) from all competitions and within 60 s of completion of the cross-country phase in all horses. Mean +/- s.d. lactate concentration at rest was 0.98 +/- 0.3 mmol/l. Mean +/- s.d. lactate concentrations (mmol/l) of horses after exercise were: N = 8.1 +/- 7, T = 11.5 +/- 4.7, P = 17.6 +/- 6.1, I = 19 +/- 6.7 and A = 21.3 +/- 7.3. Mean plasma lactate increased significantly (P < 0.001) as levels of competition increased, although there were few significant differences in heart rate between levels. Rectal temperatures were taken immediately after completion of the cross-country phase and 10 min later. Mean +/- s.d. rectal temperatures were significantly higher 10 min after completion of the cross-country than those taken immediately after cross-country (P < 0.001). Rectal temperatures 10 min after completion of exercise were significantly higher as the level of competition increased for N through I (P < 0.001). Rectal temperatures 10 min after completion of exercise were correlated with ambient temperatures (r = 0.22, P = 0.01). There was no significant correlation between either pulse or respiratory rate taken immediately after completion of exercise with heart rate during the course.