Heart Rates at the Trot on a Mechanical Walker Are Correlated to Cardiovascular Fitness in Thoroughbred Racehorses

The relationship between heart rate (HR) and velocity under saddle or on a treadmill can be used to estimate cardiovascular fitness in horses. Whether HR data gathered during other types of exercise can be used to estimate fitness is unknown. This study hypothesized that HR data collected on a mechanical walker could be used to estimate cardiovascular fitness in Thoroughbred racehorses (TBs), young horses having lower fitness with higher trotting HRs than older horses. Five, 2- and 3-year-old TBs (YOUNG) (2.2 ± 0.5 yr) and 5, 5- to 9-year-old TBs (MATURE) (6.6 ± 1.7 yr) were used. Four of the YOUNG horses had recently been prepped for 2-yearold in-training sales. MATURE horses had been in race training for over a year. During the 4-wk study, horses were exercised on a mechanical walker 3 d/wk. Exercise consisted of a 5-min warm-up walk (1.7 m/s), 15-min slow trot (2.7 m/s), and 5-min cool-down walk (1.7 m/s). HR data were collected from the walker during the final 10 min of the trot (HRtrot). All horses were also exercised on the racetrack 3 d/wk. The HR-velocity relationship during track exercise was determined using a Polar on-board HR monitor and KER ClockIt smartphone fitness app. Fitness was assessed by velocities (V) calculated at different HRs (V160, V180, and V200) by linear regression. Differences between age groups were determined using unpaired t-tests. One walker session and one track session from each week were used from each horse to evaluate the relationship between HRtrot and cardiovascular fitness via simple linear regression. All results are reported as mean ± SD. One YOUNG horse was excluded from analysis due to unsteady pace on the walker resulting in erratic HRtrot. One MATURE horse was excluded from analysis due to excitability under saddle resulting in lack of linearity between HR and speed during track exercise. HRtrot was lower for MATURE (77.3 ± 3.8 bpm) versus YOUNG (87.6 ± 4.5 bpm) (P < 0.01). Speed differed between groups on the track at all HR levels (V160: MATURE: 7.3 ± 0.7 m/s, YOUNG: 6.3 ± 0.8 m/s (P < 0.01); V180: MATURE: 9.2 ± 0.6 m/s, YOUNG: 8.0 ± 0.7 m/s (P < 0.01); V200: MATURE: 11.1 ± 0.7 m/s, YOUNG: 9.7 ± 0.8 m/s (P < 0.01)). HRtrot and V160, V180, and V200 were negatively correlated in both groups (V160 R2 = 0.44 (P < 0.001); V180 R2 = 0.62 (P < 0.001); V200 R2 = 0.66 (P < 0.001)). These results suggest that cardiovascular fitness under saddle may be estimated from HR collected at a trot on a mechanical walker. It is unknown whether differences observed between groups’ cardiovascular fitness were due to age or previous degree of training.

Springer, R.W., and J.D. Pagan. 2021. Heart rates at the trot on a mechanical walker are correlated to cardiovascular fitness in Thoroughbred racehorses. Journal of Equine Veterinary Science 100:103605.