Basics Of Equine Exercise Physiology
The horse is justifiably considered to be an elite athlete and is capable of undertaking a wide range of tasks, such as jumping, sprinting and endurance. This is even more remarkable when it is considered that in the wild, horses do not undertake regular exercise and rarely jump or run long distances.
A horse is born a sprinter. Its muscles are highly adapted for sprinting at birth. However, with training it can achieve distances of over 100 miles at average speeds of around 10-12 miles/hour. Not only can a horse be trained to undertake such a wide variety of activities, it is also able to do this with the encumbrance of a rider!
At rest a horse will have a heart rate of around 25 b.p.m. and with each heart beat, around 1 litre of blood will be pumped out. The horse at rest will have a respiratory rate of around 12 breaths per minute and move around 5 litres of air in and out of the lungs with each breath, moving around 60 litres in and out each minute. Itâ€™s body temperature will be close to 38°C and it will be using mainly fat to provide energy for all bodily functions. At the onset of exercise the heart rate will increase rapidly. How much the heart rate increases will depend on what speed the rider sets off at and whether the horse has been warmed up. The increase in heart rate means that blood circulates around the body more rapidly. Whereas at rest, the majority of blood will be circulating through organs such the gut, the liver and the kidneys, at the onset of exercise, the blood vessels in the muscles open up to allow an increase in blood flow. At the same time, the blood flow to organs such as the gut and kidney is reduced. The increase in blood flow though the muscles is necessary for sustained exercise as oxygen must be delivered to them. At the same time, products of energy metabolism within the muscles (such as carbon dioxide) must be taken to the lung to be breathed out. Another important role of the blood during exercise is to pick up heat being produced in the muscles and take it to the skin where is can be dissipated.
In addition to the increase in heart rate, depending on the speed of exercise, some degree of contraction of the spleen may take place. The spleen stores red blood cells (which carry oxygen in the blood) and an increase in the blood allows more oxygen to be carried. A big heart, high blood volume and the ability to increase the amount of oxygen that can be carried are some of the reasons why the horse is an elite athlete.
At rest the horse is getting the majority of its energy from fats rather than from carbohydrates (glucose - in the blood, or glycogen - in the muscle and liver). At the onset of exercise, it takes a while for the mitochondria (the site within the muscle cells at which energy is transformed into a form that muscles can use) within the muscle to get up to speed. The mitochondria require a steady supply of oxygen and are highly efficient at converting both glycogen and fat into energy for muscular contraction. Their limitation is that they work relatively slowly and cannot respond to rapid changes in speed. So at the onset of exercise, the horse has to switch to using carbohydrates. Why not use carbohydrates all the time? The limitation is that the body has relatively low stores of carbohydrates and would run out early in an endurance event if this was the only fuel used.
Muscles cannot transform energy into movement with 100% efficiency, much the same way as a car engine is not totally efficient. As a result, some of the energy is lost in the from of heat. The rate of heat production by working muscles is proportional to how hard the muscles work. Therefore the faster a horse goes, the more heat it produces. The amount of heat a horse produces in a 160 km endurance race would be enough to boil around 170 gallons of water or around 700 kettles!!!! Fortunately for the horse, it is able to dissipate around 97% of the heat it produces during an endurance race in cool-warm conditions!! If not, its body temperature would increase by around 15°C/h.
Keeping cool has a price though. The horse must use energy to cool itself down (as heart rate must be increased to help deal with the heat being produced). Most of the heat is lost at the body surface, although around 15% may be lost during breathing. The horses main way of getting rid of excess heat is by sweating. Horses can sweat more than any other animal and 3-4 times the rate we can sweat. Horses may lose as much as 10-15 litres of sweat an hour in hot or hot humid conditions and therefore could lose around 100 litres in a ten hour race. No animal could lose this much and survive. Overall losses of 50 litres in 500 kg animals have been reported, probably indicating that the horse was able to replace around 50% of the fluid it lost. Even so, this would represent a very large deficit (10% dehydration).
Because horse sweat is hyper-tonic (the electrolyte content of the sweat is more concentrated than the electrolyte content of the other body fluids, such as plasma) large amounts of electrolytes are lost in the sweat, particularly sodium (~40%) and chloride (~40%) and some potassium (~10%) and calcium (2-3%). In hot conditions, horses could lose up to 100g of electrolytes in 1 hour.
The most efficient speed and gait for a horse in terms of the least amount of energy used to travel 1 metre along the ground is the canter, between a speed of around 5 to 11 m/s (18-40 km/h). However, in terms of strategies for endurance racing, this does not allow for the fact that with increased speed, there is increased heat production, but also a reduced ability to dissipate heat. Thus, the advantages of increased efficiency must be balanced against the disadvantages of increased heat production. This may be important in thermally stressful conditions.
Whilst not every rider needs to become an exercise physiologist, a knowledge of how factors such as speed and environmental conditions affect the horse may help in selecting appropriate strategies for competition.
By Dr David Marlin, Centre for Equine Studies, Animal Health Trust, Newmarket, UK.
The Animal Health Trust is a charity that has been helping dogs, cats and horses for more than half a century. We provide specialist veterinary clinical, diagnostic and surgical services and our successes in research have ranged from major breakthroughs in anaesthesia and surgical techniques to the development of vaccines against diseases such as canine distemper and equine influenza. Our scientists and veterinarians, many of whom are world leaders in their field, work alongside bringing together a wide range of expertise for a co-ordinated attack on animal diseases and injuries. By publishing scientific papers, speaking at conferences and talking to other veterinary surgeons about the cases dealt with, this knowledge is passed on to benefit the maximum number of animals.