Refereed PHYSIOLOGICAL RESPONSE TO THE CROSS-COUNTRY PHASE IN EVENTING HORSES H. Amory, DVM; T. Art, DVM, Phl); A. Linden, DVM, Phi); D. Desmecht, DVM; M. Buchet, DVM; and P. Lekeux, DVM, PhD. SUMMARY 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 Lace and HR during the cross-country were studied using regression analysis. Mean HR during the cross-countrywas linearlyrelated 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 Lace and mean speed during the cross-countrywas exponential (r=0.93, p < 0.001), but blood lactate accumulation occurred at a lower Authors' Address: Laboratory for Functional Investigation, Department of Physiology, Faculty of Veterinary Medicine, University of Liege, Belgium. Correspondence address: Service de Physiologie, Faculte de Medecine Veterinaira, Universite de Liege, Bat. B42, Sart Tilman, B-4000 Liege, Belgique. Acknowledgments:The authors would like to thank the organizers and riders competing in the three-day events for their kind cooperation. Special thanks are due to the riders who took part in the standardized exercise test, i.e.J. Caulier, D. Massart, X. Roland and P. Rose. We are also grateful to M. Leblond for assistance in preparing the manuscript and to C.D.H. Larem for performing blood analysis. Financial assistance was provided by Equine Research Fund (Belgium). speed than during the SET (r=0.94, p < 0.001). The correlation between Lace and HR was stronger when Lace 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 Lace. 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. INTRODUCTION Each year, improvements are noted in performance of human athletes in almost all disciplines. These improvements have been partly attributed to an increase in the knowledge of exercise physiology, which allowed a better scientific ap- proach to athletic training and conditioning. 21 In contrast, several authors pointed out the lack of improvements in racetrack results for many years,l°'ts and this might be partly due to the empirical training methods applied in horses.~ The major objective of training is to cause biologic adaptations to improve performance in specific tasks, de- pending on the discipline in which the athlete will perform.21 This specificity principle is the preliminary principle of training: "Specific training elicits specific adaptations creat- 646 JOURNAL OF EQUINE VETERINARY SCIENCE