Horse muscle types

Muscle Types and their Responses to Exercise and Training

Muscles that are used for locomotion in the horse are predominantly the skeletal muscles. They are made up of long fibres, which contain many tiny myofibrils. These myofibrils are what help the muscle to contract. Myofibrils are gathered together in bundles, which are surrouned in connective tissue, which makes a muscle fibre. Every myofibril have myofilaments, these are parallel and overlap each other, and they contain actin and myosin which help to contract the muscle, when it is stimulated by a nerve impulse. This causes the overlapping of the actin and myosin filaments, which shorten the length of the muscle, pulling the desired leg or part of the body and moving it. This reaction need the help of energy called ATP, which is made by glycogen stores in the muscle, this glycogen is stored in small organs called mitochondria.

The horse has 3 types of muscle fibres, which can be helpful for different uses:

Type I fibres are called slow- twitch fibres, they have a good oxidative capacity and so work aerobically, they are slow at contraction and relaxation and are quiet small fibres, which means they take longer to respond to a stimulus. This muscle type is good for endurance purposes. Slow muscles are more efficient at using oxygen to generate more fuel (ATP) for continuous, extended muscle contractions over a long time.

Type IIa fibres are called High oxidative fast-twitch fibres, they contain a high concentration of myoglobin, this makes the muscle dark red, these muscles are used mainly aerobically. These types of muscle fibres would let the horse work at a moderate pace for a longer period of time.

Type IIb fibres are called low oxidative fast-twitch fibres, they don’t have a lot of myoglobin and are a lot whiter in colour. This is because they do not need a big oxygen supply to generate ATP as they are mainly anaerobic, which are used for powerful acceleration and short bursts of fast energy, which causes fatigue quickly, such as sprint race horses.

The makeup of a horses muscle type fibre can depend on the breed and genetics of the horse and may help to determine what discipline a horse may be good at. For instance a horse that has mainly type I and II a fibres may be better at endurance activities than a horse with more II b fibres which may be more suited to sprinting or fast bursts of energy.

Short-term muscular responses that occur during exercise are:

Oxygen uptake, this increases from about 5ml per minute to 160ml per minute, when high intensity exercise is occurring, in a normal racehorse. This happens because more oxygen needs to be delivered to the muscles that are used for locomotion, so that they can work and make ATP for aerobic respiration. As there is a lag time of around ten to fifteen minutes for the horse to start working aerobically; most racehorses will be working anaerobically for the duration of the race.

Blood flow increases dramatically to allow the oxygen to travel faster to the locomotory muscles needed to produce ATP for aerobic respiration in exercise. In horses the blood flow increases from 4 litres per minute to 200 litres per minute when the body is a maximum cardiac output.

Temperature of the horse rises because the muscles are contracting faster, and warming them up. Because of the horse moving its body faster and the muscles warming up, body temperature rises by 1 degree. Although 80% energy is lost through heat and only 20% used for locomotion.

If horses use all of their energy stores in all muscle fibre types and anaerobically they can convert fat stores into energy, this happens while the horse is exercising, because the horse has a big supply of fatty acid stores they are unlikely to ever be all used up.

Short-term exercise responses are those that return back to the body’s normal levels after exercise has stopped, and oxygen does not need to be rushed around the horse’s body to the muscles. Long term training responses however, are adaptations that happen to the horse’s body because of repeated sessions of exercise, which do not return unless the horse is left and not exercised for a long time, even then they will have a better fitness than a horse that has never trained before.
There are 3 types of adaptations that muscle fibres can develop into, the type of adaptation depends on the type of exercise, and also the time and frequency the exercise is performed for. The three types of adaptations are;

1. Hypertrophy, the myofibres stay the same in structural appearance but they just grow and increase in size.

2. Remodelling without hypertrophy, the myofibres change their structural characteristics, this also can change the microvasculature. This time the myofibers stay the same size.

3. Mixed response, hypertrophy and remodelling happen together.

In training the muscle contracts and relaxes at a higher speed than usual, this means type I slow-twitch fibres are encouraged to adapt. Type II a+b fibres show better training adaptations than the slower twitch fibres. This is because type II a+b fibres are used for higher intensity exercise than type I fibres, so they can adapt to become more efficient in working. Type II b fibres cannot change, but they can, however act more like II a fibres after training, there is also shown that the number of type one fibres decreases after training.

One of the first muscular adaptations to training is increased enzyme and aerobic metabolic activity of for example, the electron transport chain and fat oxidisation. Other responses include more efficient removal of waste products such as CO2 and improved oxygen diffusion. These changes are linked with increased capillary and mitochondria densities.

Training can also increase the amount of glycogen stored in the muscle, this delays onset to fatigue. Increased myoglobin content can lead to a greater oxygen storage capacity, also increased motor skills can occur, this enables the horse to move without using as much effort or energy.

There are also physiological effects of training and exercise to horses, these can be; heightened stress, especially at competition that can cause heightened cortisol levels in the blood. Training also can induce exercise induced ultra-structural damage in equine skeletal muscle.