Heart Rate and Velocity Derived from a Global Positioning System Assesses Fitness in Thoroughbred Racehorses

Changes in fitness can be detected with measurements of heart rate and global positioning system (GPS)-derived velocity during field exercise tests in Thoroughbred racehorses in commercial exercise training. This approach enables regular monitoring of fitness with field exercise tests in racehorses without use of strict exercise test protocols. The technique also avoids the need for treadmill exercise tests for monitoring fitness in racehorses.

1. Introduction

Race performance in Thoroughbreds is dependent on energy supply by aerobic and anaerobic ATP resynthesis. Energy supply by the aerobic pathway is as high as 80% during 1600-m races, with a greater dependence in races with greater duration [1]. Furthermore, maximum amount of oxygen consumed (VO2max) is significantly greater in superior Standardbred racehorses [2], and VO2max correlates with running performance over 1600 m in Thoroughbreds [3]. Moreover, VO2max increases in response to intensity and duration of Thoroughbred training [4,5]. An incremental step exercise test to fatigue on a high-speed treadmill is needed to measure VO2max. Most Thoroughbred trainers do not have access to treadmills, and there is very little use of treadmill laboratories for assessment of racehorse fitness. Therefore, a reliable and sensitive index of fitness derived from field exercise tests is desirable.

The linearity of the relationship between velocity and heart rate (HR) occurs from speeds at trotting up to the velocity that results in VO2max. The velocity at which a horse reaches VO2max is the same velocity at which the horse reaches HRmax [6,7]. Therefore VHRmax, the velocity at which a horse reaches HRmax, may be a valid index of fitness. Use of the relationship between velocity and HR for routine assessment of racehorse fitness has not been common. Traditionally, exercise protocols have used multiple steps in speed and measurement equipment such as stop watches in calculating velocity [8-10]. More recently, Kobayashi et al [11] attained a repeatable velocity/HR regression line with an exercise test of freely increasing speeds throughout gaits. However, a stop watch was used in which parallax and human error, additional labor, and dependence on direct vision of the horse were necessary. Fitness assessments reliant on velocity and HR typically depend on the use of standardized exercise tests, but such tests are not appropriate in the field for Thoroughbreds. If fitness tests could be conducted with the normal racetrack exercise routines, there may be greater adoption of this approach to fitness testing in racehorses.

The purpose of this study was to measure and calculate VHRmax and velocity HR measurements of fitness with field measurements of velocity and HR with a global positioning system (GPS) without use of a standardized exercise test. The hypothesis was that VHRmax and velocity that results in a heart rate of 200 bpm (V200) would increase significantly during training that included high-intensity fast gallops.

2. Materials and Methods

The effect of training on VHRmax and V200 were assessed with a field exercise test in weeks 2 and 6 of fast exercise training. Twelve 2-yr-old Thoroughbreds (three colts, four geldings, and five fillies) were used in this study. Horses had previously undergone a 6-wk, 6 days/wk low-intensity trot and canter training before fast training. Horses then underwent fast training, which included 2 days/wk of fast day training sessions and 4 days/wk of trotting and low intensity gallops.

Velocity was recorded with a GPS [a] at 1 Hz, and 5-s averages were calculated. During the racetrack gallop, a Polar HR Monitor S610 [b] was used to measure 5-s averages of HR. Data was accessed using Microsoft Excel, and a velocity/HR plot of simultaneous pairs of data was constructed. A regression line was used to describe the relationship between velocity and sub-maximal HR. This regression line was derived from velocity and HR data during trotting and galloping. VHRmax in each horse was subsequently calculated by substituting HRmax for the exercise into the y value of the corresponding velocity HR regression line equation and expressed in meters per second. The velocity corresponding to a HR of 200 beats/min (V200; m/s) was also calculated. The effect of training was studied by a paired t-test. If data were not normally distributed, the Wilcoxon signed rank test was used. Significance was taken at p < 0.05.

3. Results

One horse was excluded because of a lower respiratory tract disease, and one horse was excluded because it did not show a HRmax in the second week of fast training field exercise test (FET). There were significant increases in VHRmax and V200 between the second and sixth week of fast training. Relative increases were 8% and 8.3%, respectively. The increases in VHRmax and V200 were not associated with significant differences in the intercept or slope of the regression of sub-maximal HR on velocity.

4. Discussion

This study is the first to show the effect of training on VHRmax in a commercial Thoroughbred training establishment. VHRmax and V200 both increased concurrently with an increase in training intensity and duration in 2 yr olds that had previously completed low-intensity training for 6 wk and 1 wk of fast training. No previous studies have examined the effect of such training on VHRmax in the field or on the treadmill. However, the effect of training on V200 has been studied in field studies [10-14] and in treadmill tests [15-17]. Ohmura et al [13] found that conventional low-intensity Thoroughbred training increased the V200 in yearlings. Furthermore, these same horses tested as 2 yr olds in the field with a standardized exercise test increased V200 in response to an increase in training intensity and duration.

The index of fitness used in this study, VHRmax, was a sensitive measurement. The study detected an 8% increase in VHRmax, and this approximates a 7% increase in VO2max. 18 However, Hiraga et al. reported that a 7% increase in VO2max was not associated with obvious changes in the velocity/HR relationships. Treadmill studies may have limitations when using velocity and HR measurements because excitement during treadmill tests may limit the ability to detect small changes in HRs. In field exercise tests, horses exercise in their natural environment, and HRs may therefore be less subject to the effects of excitement. Detection of small changes in fitness may be easier with field tests.

The increase in VHRmax in this study was not associated with significant decreases in the gradient (B) or y-intercept (A) of the velocity HR regression line. HRmax was also unchanged. The increase in VHRmax occurred secondary to a decrease in the slope and the y-intercept of the velocity HR regression line. However, there were notable differences between individual horses in their response to training. In some horses, the regression line shifted to the right without a notable change in slope of the line, whereas in other horses, a change in the slope of the regression line was more obvious. Kobayashi et al [11] also found that the mean gradient and y-intercept decreased as the velocity HR regression line shifted to the right during training. Subsequently, V200 increased in 2-yr-old Thoroughbreds undergoing race training in Japan.

The increase in VHRmax could be caused by cardiac adaptations and increased VO2max and /or improved economy of locomotion with training. Furthermore, an increased VHRmax may reflect the effect of growth of the horses. However, this is unlikely in this study because of its short duration. Also, the body weight of the horses did not change significantly. Previous studies have shown that both stroke volume (SV) and VO2max increases occurred in training in 2 yr olds and older Thoroughbreds [5,14]. Therefore, VHRmax increases in this study may partly be a result of increases in SV and a concurrent decrease in HRs during sub-maximal exercise. Improved economy of locomotion could also contribute to an increase in VHRmax after training.

In conclusion, the fitness of Thoroughbred racehorses expressed as VHRmax or V200 can be measured in a commercial Thoroughbred training establishment with a combination of HR and velocity measurements obtained with an on-board GPS. The exercise protocol was typical of fast day training, and no strict exercise test protocol was used. Furthermore, VHRmax increased in 2-yr-old Thoroughbreds after 4 wk of fast training. These results are important because the methodology enables easy measurement of fitness in field exercise tests that are not dependent on a standardized exercise test. Future studies should investigate relationships between VHRmax and racing performance in Thoroughbreds and other athletic horses.

This study was supported by the following contributors: Rural Industries Research and Development Corporation, Nature Vet Pty Ltd., and Equitronics Technologies Pty Ltd.The support of horse trainer C. Connors is also gratefully acknowledged. A.K.S. has a financial interest in the sales of software and hardware used in this project. A.D.V. is employed by Nature Vet Pty Ltd., which is involved in sales and marketing of products derived from research described in this manuscript. Nature Vet Pty Ltd. has financially supported the attendance of D.L.E. at this and other scientific conferences to speak on this topic.

Footnotes

[a] Equitronics Technologies Pty Ltd., Sydney NSW 2006, Australia.
[b] Polar Electro Oy, FIN-90440 Kempele, Finland.