Standardized exercise test and daily heart rate responses of Thoroughbreds undergoing conventional race training and detraining

Ten healthy sedentary male Thoroughbreds with previous race training experience were studied for 14 weeks. Horses were trained for 9 weeks, using a program designed after those used commonly in the United States. Horses were trained conventionally by slow trotting (250 m/min) for 2 weeks and galloping (390 to 450 m/min) for 4 weeks, followed by 3 weeks of galloping (440 to 480 m/min) and intermittent sprinting exercises (breezes) at distances between 600 and 1,000 m (900 to 950 m/min). The horses were then pasture rested for 5 weeks. A standardized exercise test (SET) involving an 800-m gallop at 800 m/min was administered before and after the 9-week training period and after the 5-week detraining period. Heart rate (HR) was monitored during exercise and at standardized intervals after exercise for 60 minutes. Venous blood for determination of plasma lactate concentration was obtained at 5 minutes after exercise. Heart rate was monitored daily at rest, during exercise, and through the first 60 minutes of recovery. Venous plasma samples (for lactate determination) were obtained 5 minutes after the sprinting exercises. Horses were observed daily before exercise for signs of lameness and were not allowed to train if lame. Differences after 9 weeks' training were seen in the SET recovery HR at 0.5 through 5 minutes after exercise (P < 0.05 to P < 0.01). Differences after detraining were seen in the SET recovery HR at 40 and 60 minutes after exercise (P < 0.05 to P < 0.01). Neither training nor detraining resulted in differences in plasma lactate concentration after the SET gallop. A training-induced resting bradycardia was not observed. The mean maximal HR (HRmax) during workouts was 238 +/- 3.4 beats/min (n = 9). When exercise HR was expressed as a function of HRmax, 22% of trotting, 89% of galloping, and 100% of sprinting workouts were performed at the greater than or equalto 60% HRmax value characterized by the onset of blood lactate accumulation. Plasma lactate concentration further documented that all the sprinting exercises were performed with concentration above the point of onset of blood lactate accumulation. Mean postsprinting lactate concentration was not different over time and ranged from 13.4 +/- 0.9 to 15.6 +/- 0.6 mmol/l. As training progressed, some of the horses had days on which they were lame after exercise. Some lameness was judged sufficient to warrant phenylbutazone (PBZ) administration. Retrospective analysis of the daily HR data indicated that there were no differences in HR during workouts for lame horses given PBZ, compared with those not given PBZ. Using analysis of variance, HR for horses that were lame during workouts was significantly higher than that for horses that were sound during workouts, during and 0.5 minutes after trotting; 0.5, 1, 2, 20, 40, and 60 minutes after galloping; and 0.5 and 20 minutes after sprinting (P < 0.05 to P < 0.01).