Physiological response to the cross country phase in eventing horses

Blood samples were collected for lactate concentration determination (Lacc) at the arrival of the cross-country phase from 40 horses of various class levels competing in 6 different three-day events. Moreover, in 8 horses, heart rate (HR) was recorded every 5 seconds during the cross-country. Among them, 5 horses were submitted, at the beginning of the competition's season to a standardized exercise test (SET) on a track which allowed us to study the relationship between HR, blood lactate concentration and running speed on flat ground in these horses. The relationship between mean HR and mean speed, between Lacc and mean speed and between Lacc and HR during the cross-country were studied using regression analysis. Mean HR during the cross-country was linearly related to mean speed during this phase (r=0.87, p ≤ 0.01), but this relationship was shifted upwards in comparison with the corresponding curve obtained during the SET (r=0.97, p ≤ 0.001). In the same way, the relationship between Lacc and mean speed during the cross-country was exponential (r=0.93, p ≤ 0.001), but blood lactate accumulation occurred at a lower speed than during the SET (r=0.94, p ≤ 0.001). The correlation between Lacc and HR was stronger when Lacc was related to HR during the 15 last minutes of the cross (r = 0.84) than to mean HR during the whole cross (r = 0.75) or to HR during the 2 last (r = 0.74) or the last (r = 0.80) minute of the course, which indicated that continuous recording of HR allowed a more accurate estimation of the severity of exercise performed during the cross-country than Lacc. Heart rate ranged from 170 to 190 beats/min during the first part of the cross-country and reached or exceeded values of 190 to 200 beats/min at the end of the course. Therefore, the anaerobic threshold is reached during cross-country. Recruitment of anaerobic me tabolism at the end of the course was otherwise demonstrated by the high values of Lacc. These aerobic-anaerobic meta bolic requirements should be taken into account to design the training program of an eventing horse.