Heart rate during exercise

There is a linear relationship between heart rate and speed of submaximal exercise. This pattern is common to all horses during exercise. However, the actual heart rates that any horse records at the trot or slow canter can vary greatly, depending on age, fitness and health of the horse. The heart rate during slow work can also be elevated in horses with lameness or other problems.

For all horses there is a work speed which does not result in an increase in heart rate. A plateau occurs, and no further increases in heart rate occur, regardless of increases in speed. The highest heart rate that can be recorded in such a test is called the maximum heart rate (HR_max). Maximum heart rates in horses are generally in the range 195–240 bpm. Each horse has its own individual HR_max, and the HR_max is not influenced by training. There does not seem to be any relationship between a horse’s maximal heart rate and its fitness. Racehorses with the higher velocities at which maximal heart rate is attained have superior performance.

During swimming, heart rates may be as high as 180–200 bpm when horses are forced to swim at higher intensites. However, heart rates as low as 120–130 bpm have also been recorded. Effects of swim training have been demonstrated.

Heart rates at onset of trot and canter exercise can be erratic, taking 4-5 minutes to achieve a steady rate. This may reflect excitement, or can be related to a physiological phenomenon in horses whereby the heart rate increases quickly at the start of exercise, and then decreases after this initial ‘overshoot’ to a lower rate over the next few minutes. Rapid changes in blood volume may also contribute to HR instability at onset of exercise.

At the start of fast exercise, the increase in heart rate is very rapid. Within 30 s, the heart rate can be greater than 200 bpm. The rate of increase in heart rate at the commencement of exercise depends on the use of a “warm-up” period.

After exercise, heart rates decrease rapidly in the first minute, and then continue to decline at a slower rate. Results of several studies demonstrate that recovery heart rates are lower after a standard exercise test as a result of training. However, routine use of recovery heart rates to assess fitness is not advised. The results can be easily influenced by excitement and the activity of the horse during recovery, and unless the exercise is carefully controlled on a treadmill, differences in intensity of exercise performed will make the results very difficult to interpret. Lack of standardisation of the horse’s activity both during exercise and during the recovery period makes useful interpretation of recovery heart rates very difficult.

Post-race heart rates can be used as a guide to inadequate recovery, as heart rates which remain elevated post-exercise are a warning sign. Veterinarians who are requested to check horses that may have performed poorly in races should routinely check the heart rate and rhythm. Heart rates which remain >130 bpm for 5–10 min after exercise suggest a poor recovery. Such a result could indicate that the horse is not fit enough, or that the horse may have clinical problems, such as atrial fibrillation, respiratory infection or lameness. .

There is a predictable effect of training on heart rate during slow exercise, but little change in either maximal heart rate or resting heart rate. During submaximal exercise heart rate falls by about 20–30 bpm. This means that at a set speed of exercise resulting in heart rate less than HR_max, training will decrease the heart rate at that speed by approximately 20–30 bpm, or more. Therefore the horse can work at faster speeds at the same heart rate. The increase in blood volume is thought to contribute to this physiological response to training. Stroke volume increases, and so the same cardiac output can be maintained with a lower heart rate.

Several other factors can influence heart rate during submaximal exercise. If the horse developed lameness, muscle soreness, respiratory infection or another illness, the heart rates at the same speed could increase. Likewise, if the horse stopped training, the loss of fitness during detraining would be reflected in a higher heart rate during slow exercise.