Physiological effects of shortening steeplechase in a 3-day-event

Various methods for modifying the Speed and Endurance portion (Day 2) of the 3-day-event have been proposed to aid horses in dealing with the hot and humid conditions expected during the next Olympic 3-day-events in Atlanta, Georgia USA in 1996. This study was designed to characterise the effects of shortening the distance required during the steeplechase (Phase B) in the face of Atlanta-like hot and humid weather conditions. Eight conditioned Thoroughbred horses (mean +/- s.e.m. age 3.75 years, range 3-5 years) underwent 3 randomised permutations of a standardised exercise test on a high speed treadmill. Each test consisted of trotting at 3.7 m/s for 10 min (Phase A); galloping at 11 m/s (Phase B) for 4 (cool, control laboratory conditions), 3 (hot and humid), or 2 (hot and humid) min; trotting at 3.7 m/s for 30 min (Phase C); and walking at 1.8 m/s for 10 min (Phase X). Subjects had Swan-Ganz catheters inserted into the pulmonary artery (PA) for measuring core temperature (PAT) in mixed venous blood every 2 min. Heart rate (HR) was measured by an on-board HR computer every 2 min. Rectal temperature (RT) was measured at the beginning (RTzero) and end (RT10) of Phase X using a mercury rectal thermometer as under typical field conditions. Pre- and post exercise bodyweights (bwt) were determined on a digital electronic scale. The point on Phase C at which each horse visibly fatigued and drifted toward the back of the treadmill was defined as the point of fatigue. Differences between treatments were tested for significance (P < 0.05) by repeated measures, Student-Neuman-Keul's and Student's tests where appropriate. Heart rate increased (mean 115.7-136.1) with the onset of trotting exercise in Phase A (P < 0.05), increased further with Phase B galloping (mean +/- s.e.m. 187.8-193.7, P < 0.05) and decreased with a return to trotting during Phase C (mean 108-130.5, P < 0.05) for all 3 treatments. Through the end of Phase C, there were no differences in HR between treatments (P > 0.05). From 3-10 min in Phase X (recovery), HR after 2 min B (mean 81.3-91) were lower than after 3 min B (mean +/- s.e.m. 98.4-100.5, P < 0.05) and were no different than 4 min B HR (mean 85.9-94.8, P > 0.05). Pulmonary artery blood temperature increased (mean 38.1-38.7) with trotting in Phase A (P < 0.05), increased further with Phase B galloping (mean 39.4-40.2, P < 0.05) for all 3 treatments and then decreased (mean +/- s.e.m. 39.3-39.9, P < 0.05) during Phase C under cool conditions (4 min B) but plateaued or continued to rise slightly under hot and humid conditions (mean 39.7-40.2). Throughout Phases C and X, PAT was lower for 4 min B than for either hot and humid treatment (P < 0.05). Bodyweight decreased after exercise for all treatments (P < 0.05) with the largest bwt loss (mean 10.9 kg) after 3 min B (P < 0.05) followed by 2 min B (8.3 kg) and then by 4 min cool B (6.5 kg). Point of fatigue was different between the 3 treatments (P < 0.05), with 4 min B the longest (mean +/- s.e. 24.8 min), followed by 2 min B (21.8 min), and then 3 min B (16.3 min). Rectal temperature was not different between the 3 treatments (P > 0.05), but there was a trend for both RTzero and RT10 to be highest after the 3 min B, lower after the 2 min B, and lowest after the 4 min cool B. It was concluded that there was a progressive gain in restoring cool weather performance and recovery by a progressive shortening of Phase B under hot and humid conditions, based on net weight loss, point of fatigue and recovery HR. Shortening Phase B by as much as 50% under hot and humid conditions still did not allow a complete return to cool weather performance and recovery. Further modifications to Phase C will be required in order to aid horses in net heat loss during Phases B and C.