Heart rates during exercise
Aerobic fitness refers to the total capacity of a horse to:
● Transport oxygen from the air, through the lungs to the blood
● Pump the oxygenated blood to the exercising muscles via the heart
● Deliver oxygen to the muscle cells to support aerobic (oxygen-dependent) metabolism.
There are many factors which influence racing performance, but aerobic fitness is fundamental.
The best way to simply assess fitness is to use heart rate (HR) measurements during exercise with a simultaneous measurement of speed. A fit horse will be more efficient at oxygen delivery and hence will require lower heart rate at a given level of exercise intensity, and its HR will fall more rapidly towards normal after exercise has ceased. Measurement of simultaneous speed (a reflection of exercise intensity) and HR is the fundamental concept of the ClockIt EQ System.
Interpretation of Equine Heart Rate Data
Trot Heart Rate
A fundamental measurement is heart rate at a steady trotting speed. Steady speed means a speed that is constant, varying no more than 1-2 kph, for at least 30 seconds. This is easily achieved in relaxed horses after a suitable warm-up of at least 5 minutes trotting.
HR at the start of trotting should be ignored for a few minutes because there is a period of unstable HR, due to excitement and the sudden disturbances to the cardiovascular system. In these first few minutes, trot HR is usually higher than in the true steady state HR. After a few minutes, the HR will settle down, and a graph of the HR and speed shows two lines both at a reasonably steady state (see Graph A below).
In Graph A, the horse is in a fast warm up trot before a gallop. Speed is reasonably steady at 15-17 km/h and heart rate is maintained between 120 and 125 beats per minute (BPM). At the end of the period of trotting the heart rate increases to 145 BPM as the horse anticipates joining the main training track and starting fast work.
An average trot heart rate for this horse was determined at 122 BPM. This value is typically fairly constant for an individual horse. If the trot heart rate is persistently elevated, allowing for excitement or unexpected events in the horse’s immediate environment, then this should be investigated.
A trot heart rate is very consistent for an individual horse, and so typically is any variation in pattern. For example the horse in Graph A gets excited at the prospect of entering the main track, resulting in an consistent elevation in HR at the end of each trot period every day.
Trot HR on a day after the horse has not been exercised is often a little higher and more variable, due to excitement. These emotional disturbances are what make fitness testing of horses so challenging, but it is possible to measure fitness reliably if these factors are accounted for. It is always important to watch the horse carefully during the trotting HR test. Is it excited, nervous, or is there a new jockey on board?
If the HR does not settle down to a steady value after 4-5 minutes, it probably means that the horse was excited. The test needs to be repeated to confirm the true trotting HR.
In Graph B, shown above, a young horse is trotting on a circle on the inside of the main track. As the horse completes each lap of the trot circle it comes close to other horses on the main track and there is a consequent excitement elevation in the trot HR, shown as two peaks in trot HR. These detailed environmental factors are important in assessing the significance of a trot HR measurement.
A trainer can generate a valuable record of a horse’s fitness by recording the trot HR every day and plotting the trend over time. In general, as the horse gets fitter, the HR during trotting will show a small, but reducing trend. However, not all horses respond to training equally, this difference in response to training is probably genetically controlled.
Measuring the trot HR in the days after racing is also a valuable method of assessing the recovery after the race. Trot HR should be the same as before the event or race. Elevated HR during trotting would suggest that full recovery is not yet complete, or there is a clinical problem that might require veterinary diagnosis. Reintroduction of fast work should be cautious in such cases.
Once a baseline trot heart rate has been determined for an individual horse then any unexpected elevation in trot heart rate should be investigated. Marked elevation of trot HR during the warm-up period usually reflects pain, illness or impending injury.
Graph C below shows a horse trotting before work with a heart rate peaking at 200 BPM, having previously had a consistent trot HR each day of 125 BPM. This marked elevation in heart rate at trot was caused by pain and the horse subsequently sustained a condylar fracture of the front fetlock during this training gallop.
Once peak intensity of exercised has passed and a horse starts to reduce speed, the heart rate will fall rapidly. Typically peak exercise intensity is reached for example at the end of a gallop, and then the horse is eased up. Once peak intensity has passed, the heart rate will fall rapidly. This is the Recovery Period, and rapidly falling HR at this time usually reflects a fit horse. As a horse’s fitness increases, for a given intensity of exercise (ie. A known speed for a set distance or time) the Recovery HR should decline more rapidly.
Heart rates during recovery are lower after training, and are lower in horses with superior race performance. ClockIt uses a standard measurement of Recovery Time which is determined by the time taken from the point at which peak heart rate was achieved until the heart rate falls to below 100 BPM. Trending changes in the Recovery Time over a series of sessions will indicate improving fitness.
Recovery HR after exercise will also be elevated after exercise in horses with poor fitness or a clinical problem. Factors such as horse excitement or environmental events can make interpretation of recovery HR difficult. HR during recovery tend to fall very rapidly as illustrated in Graph D.
Maximum HR (HRmax)
Maximum heart rate, such as the peak HR during a gallop is not reflective of fitness. HRmax is genetically controlled, and typically vary from approximately 200-230 bpm. HRmax does not change significantly during training, and tends to be fixed for an individual horse irrespective of the level of training. Drawing generalised conclusions on a horse’s ability in relation to HRmax is inaccurate and can result in poor management decisions. A balance of factors including HRmax, trot HR, recovery times and trends in recovery time over a training period, HR at canter and HR response to Standardised Exercise Tests are required to formulate a meaningful view on a horse’s overall aerobic ability.
HR during trot, canter and slow gallops are affected by training, and so it is those HRs that are most relevant to fitness assessment.