Thoroughbred racehorses often undergo a reduction in training, or detraining, because of locomotor disorders or subsequent rehabilitation, psychological or behavioral issues, or other factors. Various types of detraining programs are used with racehorses, but only a few studies1–4 have evaluated the physiologic effects of reducing or ceasing training. It is important for planning the management of horses undergoing detraining to identify the effects and understand the changes that detraining induces in physiologic systems and subsequent racing performance.
The studies by Knight et al2 and Art and Lekeux5 revealed that the maximal rate of oxygen consumption (o2max) of trained horses rapidly decreases and returns to pretraining values following 2 to 3 weeks of detraining. This is similar to the response of some human athletes in whom o2max declines within days of cessation of training.6,7 However, there are conflicting data to indicate that o2max in horses does not decrease until week 6 of a detraining period after 34 weeks of training8 or does not decrease even after 15 weeks of detraining following 6 months of training.1 Knight et al2 and Art and Lekeux5 studied mainly the effects of detraining after relatively shorter periods of training or submaximal training, compared with training conditions used by Butler et al1 and Tyler et al.8 The training status of horses before detraining may influence changes in cardiovascular function induced by detraining.9 Horses in the studies of Knight et al2 and Tyler et al8 were typically detrained in a yard with almost no physical activity, whereas detraining programs of horses in real-world settings vary according to the horses' physical status.
For the study reported here, our intent was to test whether use of detraining programs of different exercise intensities would result in differential changes in variables of interest (eg, oxygen transport, aerobic capacity, and indices of performance) at the end of detraining. We planned to assess the effects of detraining programs that are broadly applicable to certain types of common injuries sustained by racehorses. The experimental groups were designed to test for differences in the effects of detraining programs on horses that may run at a slower speed after a race because of muscle soreness (a cantering protocol), be walked as part of rehabilitation after tendinitis (a walking protocol), or remain in their stalls at all times because of bone fracture or other severe disease (a stall-rest protocol). The study was designed to provide data with which to better understand how detraining protocols involving different degrees of reduced activity affect racehorses, compared with the effects of detraining by means of stall rest alone.
The purpose of the study reported here was to determine whether Thoroughbred racehorses undergoing regular exercise at 1 of 2 intensities or stall rest during detraining would differentially maintain their cardiopulmonary and oxygen-transport capacities. We were also interested in assessing whether those changes in aerobic and circulatory capacities would differ from those determined in previous detraining studies. We hypothesized that the racehorses' aerobic and circulatory capacities would decrease in proportion to the reduction in exercise intensity during detraining. To test this hypothesis, Thoroughbreds were trained on a treadmill for 18 weeks, and aerobic capacity and oxygen-transport variables were quantified; then, the horses' aerobic capacities and oxygen-transport variables were again measured after a 12-week period of detraining during which they underwent different degrees of reduced exercise intensity.
Supported by the Japan Racing Association.
Presented in abstract form at the 9th International Conference on Equine Exercise Physiology, Chester, United Kingdom, June 2014.
Arterial oxygen concentration
Arterial-mixed-venous oxygen concentration difference
Mixed-venous oxygen concentration
Maximal heart rate
Mixed-venous oxygen partial pressure
Maximal cardiac output
Maximal mass-specific cardiac output
Arterial oxygen saturation
Cardiac stroke volume
Maximal cardiac stroke volume
Maximal mass-specific cardiac stroke volume
Mixed-venous oxygen saturation
Run time to exhaustion after reaching 10 m/s during a standardized treadmill exercise protocol
Speed eliciting maximal heart rate
Speed at which plasma lactate concentration reaches 4mM after 2 minutes during a standardized treadmill exercise protocol
Maximal rate of oxygen consumption
Maximal mass-specific rate of oxygen consumption
Speed eliciting maximal rate of oxygen consumption
Mustang 2200, Kagra, Fahrwangen, Switzerland.
SM-29, Fukuda Denshi, Tokyo, Japan.
DATAQ DI-720, DATAQ, Akron, Ohio.
Windaq Pro+, DATAQ, Akron, Ohio.
LFE-150B, Vise Medical, Chiba, Japan.
TF-5, Vise Medical, Chiba, Japan.
MG-360, Vise Medical, Chiba, Japan.
CR-300, Kofloc, Kyoto, Japan.
Surflow, Terumo, Tokyo, Japan.
MO95H-8.5, Baxter International, Deerfield, Ill.
SP5107U, Becton, Dickinson and Company, Franklin Lakes, NJ.
Statham P23d, Gould Instruments, Valley View, Ohio.
ABL-555, Radiometer, Copenhagen, Denmark.
OSM-3, Radiometer, Copenhagen, Denmark.
YSI 2300 STAT Plus, Yellow Springs Instruments, Yellow Springs, Ohio.
COM-2, Baxter International, Deerfield, Ill.
JMP 6.0.3, SAS Institute Inc, Cary, NC.
1. Butler PJ, Woakes AJ, Anderson LS, et al. The effect of cessation of training on cardiorespiratory variables during exercise In: Persson SGB, Lindholm A, Jeffcott LB, eds. Equine exercise physiology 3. Davis, Calif: ICEEP Publications, 1991;71–76.
2. Knight PK, Sinha AK, Rose RJ. Effects of training intensity on maximum oxygen uptake. In: Persson SGB, Lindholm A, Jeffcott LB, eds. Equine exercise physiology 3. Davis, Calif: ICEEP Publications, 1991;77–82.
3. Sinha AK, Ray SP, Rose RJ. Effect of training intensity and detraining on adaptations in different skeletal muscles. In: Persson SGB, Lindholm A, Jeffcott LB, eds. Equine exercise physiology 3. Davis, Calif: ICEEP Publications, 1991;223–230.
4. Mukai K, Ohmura H, Hiraga A, et al. Effect of detraining on cardiorespiratory variables in young Thoroughbred horses. Equine Vet J Suppl 2006; 36: 210–213.
5. Art T, Lekeux P. Training-induced modifications in cardiorespiratory and ventilatory measurements in Thoroughbred horses. Equine Vet J 1993; 25: 532–536.
6. Mujika I, Padilla S. Detraining: loss of training-induced physiological and performance adaptations. Part I: short term insufficient training stimulus. Sports Med 2000; 30: 79–87.
7. Neufer PD. The effect of detraining and reduced training on the physiological adaptations to aerobic exercise training. Sports Med 1989; 8: 302–320.
8. Tyler CM, Golland LC, Evans DL, et al. Changes in maximum oxygen uptake during prolonged training, overtraining, and detraining in horses. J Appl Physiol 1996; 81: 2244–2249.
9. Coyle EF, Martin WH III, Sinacore DR, et al. Time course of loss of adaptations after stopping prolonged intense endurance training. J Appl Physiol 1984; 57: 1857–1864.
10. Pascoe JR, Hiraga A, Hobo S, et al. Cardiac output measurements using sonomicrometer crystals on the left ventricle at rest and exercise. Equine Vet J Suppl 1999; 30: 148–152.
11. Birks EK, Jones JH, Berry JD. Plasma lactate kinetics in exercising horses. In: Persson SGB, Lindholm A, Jeffcott LB, eds. Equine exercise physiology 3. Davis, Calif: ICEEP Publications, 1991;179–187.
12. Fick A. Uber die Messung des Blutquantums in den Herzventrikeln. In: Sitzungs Berichteder Physiologishe-Medizinishe Gesselshaft Wurzburg 1870; 16: 16–17.
13. Fedak MA, Rome L, Seeherman HJ. One-step N2-dilution technique for calibrating open-circuit VO2 measuring systems. J Appl Physiol 1981; 51: 772–776.
14. Mujika I, Padilla S. Cardiorespiratory and metabolic characteristics of detraining in humans. Med Sci Sports Exerc 2001; 33: 413–421.
15. Coyle EF, Hemmert MK, Coggan AR. Effects of detraining on cardiovascular responses to exercise: role of blood volume. J Appl Physiol 1986; 60: 95–99.
16. Zavorsky GS. Evidence and possible mechanisms of altered maximum heart rate with endurance training and tapering. Sports Med 2000; 29: 13–26.
18. Hiraga A, Kai M, Kubo K, et al. The effect of training intensity on cardiopulmonary function in 2 year-old Thoroughbred horses. J Equine Sci 1997; 8: 75–80.
19. Evans DL, Rose RJ. Cardiovascular and respiratory responses to submaximal exercise training in the Thoroughbred horse. Pflugers Arch 1988; 411: 316–321.
20. Thomas DP, Fregin GF. Cardiorespiratory and metabolic responses to treadmill exercise in the horse. J Appl Physiol 1981; 50: 864–868.
21. Roca J, Hogan MC, Story D, et al. Evidence for tissue diffusion limitation of VO2max in normal humans. J Appl Physiol 1989; 67: 291–299.
22. Henderson KK, Wagner H, Favret F, et al. Determinants of maximal O2 uptake in rats selectively bred for endurance running capacity. J Appl Physiol 2002; 93: 1265–1274.
23. Crocker GH, Toth B, Jones JH. Combined effects of inspired oxygen, carbon dioxide, and carbon monoxide on oxygen transport and aerobic capacity. J Appl Physiol 2013; 115: 643–652.
24. Jones JH. Circulatory function during exercise: integration of convection and diffusion In: Jones JH, ed. Comparative vertebrate exercise physiology. San Diego: Academic Press, 1994;217–251.
25. Jones JH. Optimization of the mammalian respiratory system: symmorphosis versus single species adaptation. Comp Biochem Physiol B Biochem Mol Biol 1998; 120: 125–138.
26. Hoppeler H, Jones JH, Lindstedt SL, et al. Relating maximal oxygen consumption to skeletal muscle mitochondria in horses. In: Gillespie JR, Robinson NE, eds. Equine exercise physiology 2. Davis, Calif: ICEEP Publications, 1987;278–289.
27. Tyler CM, Golland LC, Evans DL, et al. Skeletal muscle adaptations to prolonged training, overtraining and detraining in horses. Pflugers Arch 1998; 436: 391–397.
28. Mujika I, Padilla S. Detraining: loss of training-induced physiological and performance adaptations. Part II: Long term insufficient training stimulus. Sports Med 2000; 30: 145–154.
30. Schroter RC, Marlin DJ. Modelling the oxygen cost of transport in competitions over ground of variable slope. Equine Vet J Suppl 2002; 34: 397–401.
31. Hickson RC, Kanakis C Jr, Davis JR, et al. Reduced training duration effects on aerobic power, endurance, and cardiac growth. J Appl Physiol 1982; 53: 225–229.
32. Hickson RC, Foster C, Pollock ML, et al. Reduced training intensities and loss of aerobic power, endurance, and cardiac growth. J Appl Physiol 1985; 58: 492–499.
33. Dressendorfer RH, Smith JL, Amsterdam EA, et al. Reduction of submaximal exercise myocardial oxygen demand postwalk training program in coronary patients due to improved physical work efficiency. Am Heart J 1982; 103: 358–362.