Know your horse's heart

At rest a horse's heart rate can be as low as 28-36 beats per minute (bpm).

During maximum exercise, it may be possible to reach a rate of 240 bpm for a very short period of time.

This wide range of possible rates allows a trainer to make maximum use of the potential increase in cardiac output during exercise. A horse's heart pumps approximately 1 litre of blood in each beat (stroke volume). At a resting heart rate of 40 bpm, this equates to a cardiac output of 40 litres of blood per minute. If the heart rate is increased to 200 bpm, there is a resultant increase in cardiac output to 200 litres per minute.

When exercise commences, circulating adrenaline makes the contraction of the heart more forceful so that more blood is forced from the heart chamber each beat. In addition to this, when the large muscle bulks around the body contract, they force blood through the veins back to the heart increasing the amount of blood available. These effects increase the amount of blood leaving the heart i. e. stroke volume.

With increased fitness, an increased stroke volume can be achieved enabling the heart to beat less frequently to maintain the same cardiac output.

As a result of this, it should be possible to see as training progresses and fitness improves the difference in the horse's heart rate at rest before and after training.

At heart rates lower than 140 bpm, the heart should be able to supply the muscles with all the oxygen that they need and adequately remove the waste products. The muscles are able to burn fuel efficiently. This is called aerobic respiration. This type of exercise can be carried out for long periods of time without fatigue. When a horse stops after aerobic work, his heart rate rapidly returns to normal and respiration rates appear normal.

At higher heart rates, (up to 240 bpm), the heart can no longer supply the muscles with all the oxygen that they need. Fuel burning in the muscles is less efficient and a substance known as Lactate builds up in the blood. The heart rate at which the Lactate begins to accumulate in the blood is known as the Lactate Threshold. At this point, any increase in speed or effort results in more lactate entering the blood. This type of exercise is called anaerobic exercise. High heart rates and the build up of lactate rapidly causes fatigue, so anaerobic exercise cannot be sustained for long periods. Galloping, sprinting and hill work all increase heart rate and cause lactate production. Heart rate recovery following anaerobic exercise is much slower, the body is in oxygen debt and the accumulated lactate must be disposed of. This process takes time.

During anaerobic exercise, the muscles also use more fuel and generate more heat. This is not a great problem during a three minute flat race, but in a 100-mile endurance race a horse that spends long periods above it's lactate threshold will deplete its energy reserves and dehydrate long before the event has completed.

The speed at which heart rate recovers after work also reflects cardiovascular fitness. During fast exercise the heart rate can be used to determine when the horse is sufficiently recovered to gallop or canter again. This is the basis behind the concept of interval training.

The table below shows the average heart rate of conditioned Thoroughbreds undergoing treadmill exercise and gradually increasing speeds. It shows how heart rate increases as the workload goes up to the next speed.

When an individual's heart rate is monitored over time and over standard pieces of work, the rider knows exactly how hard the horse is working and performing on any given day. When the workload is standardised individual horses can be compared with each other to assess their fitness and ability.

The majority of horse owners guess at the fitness of their horses by relying on how the horse feels, whether he pulls or needs encouragement or how long he blows once he stops.

These methods cannot provide a reliable or accurate measurement of a horse's fitness. Monitoring the heart rate before, during and after exercise and competition can provide an accurate and easily obtained method of assessing the fitness.

There are three main ways of monitoring a horse's heart rate:

Using a stethoscope.

The instrument is placed on the left side of the horse's chest behind the elbow and the operator counts the heart rate manually against time. There are several grades of stethoscopes available on the market ranging from simple "nurse" type instruments costing less than £10 to cardiac consultant styles more likely to be seen in E.R around the neck of an aspiring cardiac surgeon. A recent newcomer to the market place is an electronic stethoscope which provides the user with a highly amplified sound of the heart beat through stereo headphones. This unit is available at around £35 and is recommended for those users who have difficulty in finding the pulse or heart beat using a conventional stethoscope.

Palpation: Placing the flat of the hand on the horse's chest behind the elbow. You may feel a thump every time that the heart beats. Feeling the pulse with the tips of the fingers on the underside of the jaw is also possible with practice.

Neither of the above methods can be used effectively whilst riding and this is where the use of electronic heart rate monitors is a necessity. This piece of equipment has now become very affordable with basic systems available from around £50 for a static unit and around £140 for discipline specific "ride-with" systems.

The heart rate monitor detects the electrical signals that instruct the heart to beat and transmit this by means of a radio signal to a receiver unit worn like a wrist watch.

The basic design of all of these "ride-with" monitors is very similar, providing two rubber electrode pads, one placed under the girth and the other placed under the saddle, these are connected by wires to a small transmitter unit which is placed at the front of the saddle. The transmitter sends a signal to a watch like receiver which the rider wears and the average heart rate is calculated by the receiver unit and displayed for the rider to see. The heart rate is typically updated on average every five beats. Using this system the rider can obtain an accurate measurement of the horse's heart rate at all times during a ride or at rest.

Care should be taken in choosing the best monitoring set most suitable for the individuals needs.

Most of the sets available today are based upon human heart rate monitors and as such, many functions that are available within the "watch" receiver unit are of no real use in the equine situation.

Choose a system that does not have too many "buttons" that can create confusion and a disinclination to use all its functions.

Receivers are available that provide simply the heart rate, heart rate and stopwatch/real time, high and low heart rate alarms can be set to provide an audible alarm when certain heart rates are achieved (e. g. lactate thresholds). Some units allow the user to store the history of a training session/ride and download it into a PC for archiving and retrieval at a later date for comparison studies during training.

Care should also be taken when choosing the best type of electrode pads. Some have been developed specifically with harness racing in mind and as such may not be fully transferable to other styles of riding. Bear in mind that the electrode pad should be placed below the saddle in constant contact with the horse's back and as such should be as thin as possible.

Experience has shown that t he stronger the contact between horse and electrode, the more accurate the measurement and because of this it is strongly recommended that the optimum position of the pads is below the saddle and under the girth behind the left elbow.

The use of electrode conductivity gel or wetting the coat will aid contact Equipment is also available to use in driving and adaptation of systems for individual requirements such as monitoring the stress experienced by horses whilst being transported.

The heart rate monitor was introduced into the UK in the early 90s for use in endurance riding. At that time it was very expensive and strongly frowned upon by the powers-that-be because it was seen as being an expensive toy that was of no real use and could only be bought by the better-off. The comment was often heard: "I know my own horse and I don't need one of those things." Heart rate monitors are now viewed as an essential piece of kit for anyone contemplating endurance riding at even the basic level and are deemed essential for any prospective national team members.

Gradually, other equine disciplines are realising the benefits that heart rate monitors can provide in the training of horses to obtain their maximum potential with the minimum of leg work.

Fewer miles on the clock can only be good for the horse.

As well as being an invaluable training and monitoring aid whilst racing, the heart rate monitor can be invaluable in identifying potential problems before more serious symptoms appear. An unusually high heart rate may indicate pain and conditions spotted early can result in limiting further deterioration in a condition, thus making a heart rate monitor system an important welfare tool.

A system now exists which is used throughout the Middle East and several European countries which shows the horse's heart rate on a public display during an endurance ride vet gate. This enhances spectator enjoyment as well as providing a totally level playing field for all competitors by removing any potential or presumed human error within the vetting process.

The age of technology has arrived; why not take advantage of it?

References: Dr Lesley Young - Applying the Science

Published: 03/09/2004