When you think of human fitness tracking, likely the first measure that comes to mind is heart rate. Heart rate is commonly incorporated into fitness wearable devices, and many smartphones now also have the ability to provide the user with real-time access to this information. Armed with knowledge regarding heart rate, a typical user is able to cite their resting heart rate and describe how their heart rate changes based on the type of exercise they perform. Adept users know that their heart rate may fluctuate outside of normal values due to anxiety, fatigue, or other underlying factors. These insights enable a user to determine when their body is functioning optimally, and to assess how their body is responding to exercise over time (e.g. are they becoming more efficient at running 5 km as signaled by a lower heart rate during exercise).
The importance and popularity of heart rate is due to the relative ease at which the information can be collected as well as the valuable inferences that can be made from the data. Heart rate is typically reported as the number of heart contractions per minute (beats per minute; bpm). The heart contracts to pump oxygen rich blood and nutrients throughout the body while simultaneously removing metabolic byproducts. An increase in heart rate signifies a greater need by the body to access oxygen rich blood, with higher bpm allowing for an increased volume of blood to circulate through the body. Importantly, heart rate is proportional to the energy demands during exercise, with an increase in heart rate signaling greater energetic requirements from the athlete.
Just as heart rate can provide insightful information in humans, there is a lot to learn from this measurement in horses. In 1952, Detweiler et al. were the first researchers to publish on resting heart rate in horses. Using state of the art devices at the time (which were quite cumbersome by today’s standards), this study provided important baseline characteristics about the horse’s heart rate. This study reported the average resting heart rate of a horse to be 35 bpm, and this value was the basis of many textbooks and articles describing physiological characteristics of the healthy horse. Since then, we have learned a great deal more about how the horse’s cardiovascular system and how the heart responds to environmental and task constraints.
In 1968, Dr. Marsland examined heart rate in Standardbred racehorses. His data indicated that horses with faster race times (more often winners) had lower resting heart rates than their slower stablemates. He also demonstrated that previously untrained horses, following targeted endurance training, saw reductions in heart rate at rest, during submaximal exercise, and during recovery following exercise. This study was important for highlighting the ability of heart rate to provide insights into sport performance potential, with a lower heart rate correlated with better racing performance.
Building on this work, in 1976 Hall et al. furthered our understanding of heart rate in the athletic horse by examining exercise-induced results in Australian Thoroughbreds. Again, these researchers found heart rate was correlated with performance level in racing horses. Additionally, the researchers demonstrated that as the Thoroughbreds approached the track (moved from their home stalls to the preparation barn), their heart rate rose on average by 20 beats/min. This research was important as it quantified the phenomenon of ‘pre-exercise excitation’ for racehorses, demonstrating that a horse’s location and mental state also plays a large role on the observed heart rate. Since this time, numerous other studies have been performed which have demonstrated the sensitivity of heart rate to exercise in a number of different equestrian disciplines.