During exercises of varying intensity, the different systems of the horse’s body adapt their function. In this article, we will look at these changes through the respiratory, cardiovascular and muscular systems.
What are the effects of exercises of different intensities on the horse’s body?
Effects of low-intensity exercise
To start, let’s take the example of a horse trotting at 16km/h, which is equivalent to 75-80 strides per minute. His heart rate is an average of 140 beats per minute (BPM). This is therefore what we would call a low level of effort, which is usually done in preparation for the training that will follow.
The respiratory system
The horse’s respiratory system is adapting to the change from walking to trotting. The demand for oxygen has therefore increased so that the muscles can respond to the effort required:
- Ventilation has increased by a factor of 5 to support this increased oxygen supply.
- The airways expand with each breath: oxygen uptake responds to the demand by gradually increasing.
The cardiovascular system
On the cardiovascular system side, there is an increase in heart rate, which becomes possible thanks to the increased oxygen supply:
- The heart rate has been increased by a factor of 3, which makes it reach more than half its maximum heart rate.
- Most of this blood flow goes directly to the muscles and skin.
- The other blood tissues in the horse’s body continue to receive the same rate of blood flow.
Maximum heart rate or FCmax: measures the maximum number of beats per minute that a horse can achieve during exercise. The maximum heart rate varies between horses and their predispositions. On average it is between 204 and 241 BPM (Evans, 2007).
The muscular system
The muscles also adapt to this light effort:
- The body starts to use aerobic metabolism.
- They burn the fatty acids contained in the glucose.
- The glucose still comes mainly from the liver, so the muscle reserves are still preserved.
Aerobic metabolism: An energy-producing mechanism where the horse’s body mainly uses the degradation of fats by oxygen. Aerobic has the advantage of being highly efficient, an almost inexhaustible source and does not produce lactic acid, which is a source of pain.
→ The role of the respiratory system is starting to increase slightly, but we are not yet seeing any major changes, in general, taking place at this intensity of exercise.
Effects of moderate-intensity exercise
Let’s look at the example of a horse doing a canter at about 30km/h, which is equivalent to 100 strides per minute. His heart rate is now at its maximum.
The respiratory system
As a result of the increased effort required, the demand for oxygen has also increased. The respiratory system is slowly reaching its limit and will start to become a limiting factor in prolonging exercise over time:
- Ventilation has increased from a 5-fold increase to a 13-fold increase.
- The work of breathing, in general, has increased 240 times.
- The airways are fully dilated.
- Oxygen (O2) uptake begins to fail and there is a decrease in the ability of the lungs to take in sufficient oxygen, despite the 13-fold increase in ventilation.
The cardiovascular system
On the cardiovascular side, the horse’s body finds itself having to make choices in order to continue to increase the level of blood flow:
- A struggle begins over the direction of blood flow, either to the diaphragm, which is needed for breathing, or to the limbs, which are needed to increase the horse’s speed.
- The blood flow stops in the gut, in order to increase the blood supply to the diaphragm and the working muscles.
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The muscular system
The horse’s muscles are reaching their maximum activity:
- Aerobic metabolism has already been reached during the low-intensity exercise, and the horse begins to enter anaerobic metabolism.
- The horse’s body begins to draw energy from muscle fibres that do not consume oxygen, yet produce lactic acid.
- The body abandons the use of fatty acids, which are too slow in creating energy, and uses more of the glucose that is already in the muscle. However, this resource remains limited and the effort cannot last forever.
Anaerobic metabolism: an energy production mechanism where the horse’s body breaks down the glucose and glycogen (sugar) reserves that are stored directly in his muscles, without using oxygen. Glycogen, unlike fat, is in limited supply in the body, but its breakdown is rapid and produces more energy.
→ The horse’s body is beginning to reach its peak, all of its systems are heavily stressed and the horse must draw on its energy reserves to sustain the exercise.
Effects of high-intensity exercise
Finally, let’s look at the situation of a horse running a race. He is at an average speed of 50km/h, which is equivalent to 140 strides per minute. His body temperature is now 41°C. The horse is at this stage able to contract his muscles in ¼ of a second and relax them in ¼ of a second to produce a movement. It is particularly at this level of exercise intensity that it may be interesting to perform a standardised test to establish a measure of the horse’s maximum HR.
➡️ To find out more about the standardised test, we recommend this article.
The respiratory system
The respiratory system is now at its maximum capacity:
- It is no longer possible to increase the ventilation rate.
- All the CO2 produced by the muscles can no longer be exhaled.
- The horse could breathe in larger volumes of air, but this would mean that he would divert so much blood from the muscles to the diaphragm that it would be forced to slow down.
The cardiovascular system
The blood flow is also reaching saturation point:
- It is now concentrated only on the most important organs: the muscles, the heart and the brain.
- The skin is no longer able to dissipate the heat, so the heat remains stored instead of being dissipated by perspiration.
The muscular system
The same applies to the muscles:
- All muscle fibres are fully engaged in the effort.
- The inside of the muscle cells is at a very high pH and temperature. We observe a pH of 6.2 and a temperature of 44°C.
→ The horse’s body is reaching its maximum capacity. The muscles no longer receive sufficient oxygen to support this activity and temperature regulation no longer takes place. The horse is therefore forced to gradually stop exercising so that his body can return to normal function.
Conclusion
The horse is an athlete capable of adapting to the different efforts that are required of him. Training is an excellent way to increase the capacity of each of these systems, allowing the horse to better tolerate the workload and to establish a real progression over a whole season.
Each intensity of exercise has its benefits. It is therefore important to monitor the horse’s cardiac parameters at each of these stages, in order to verify the horse’s fitness level and to objectify his overall fitness level.
Key words : exercise intensities, horse body, training follow-up