Anatomy Part 3

Skeletal muscles

There are, in reality, three types of muscle in the horse’s body. These are skeletal muscle, which is what most people typically think of when hearing the word muscle; smooth muscle which is not under conscious control and plays a role in blood vessels, the respiratory system and the gut of the horse; and finally cardiac muscle which makes up the heart. In this article, we will look at skeletal muscle which is used to move the horse’s body and is under conscious control.

As the name suggests, skeletal muscles connect bone to bone. They generate movement, but they also stabilise the joints of the limbs, allowing the horse to remain stable and even to sleep standing up.

ATTACHMENTS

Each skeletal muscle attaches to a stable part of the skeleton at its point of origin, with the other end connecting via a tendon (a strong cord composed of collagen fibres) to the bone to be moved.

ANTAGONISTIC PAIRS

Most muscles operate in pairs, which have the opposite action to each other. These pairs are known as antagonistic pairs. Working together, they can bend, pull forwards, pull backwards or straighten a joint. An example of an antagonistic pair in the human is the triceps and biceps in the upper arm. The triceps straighten the arm and the biceps bend it, creating an antagonistic relationship. They are, therefore, an antagonistic pair of muscles.

In order to operate the joint, one muscle must, therefore, contract to pull on the bone whilst its partner relaxes and lengthens to allow movement. When the horse is resting, the antagonistic pairs of muscles and tendons act together to stabilise the joint.

The muscles which bend the limbs are known as flexors and those which extend the limbs are known as extensors. The extensor muscles and tendons are situated at the front of the legs and the flexors at the back.

A NOTE ON TENDONS

Many of the horse’s muscles, particularly those in the limbs, are attached to the bone at their lower end by tendons. These are strong fibrous cords that are composed of many fibres running parallel to each other. The fibres are made of collagen. These fibres are not particularly elastic but can stretch up to 4% of their original length without damage. If overstretching occurs, the fibres don’t spring back into shape, resulting in a sprained or ‘bowed’ tendon. If the injury is severe, several collagen fibres may rupture. The tendon then repairs itself by laying down scar tissue, which is much less elastic than the original collagen. This leaves the structure weak and susceptible to injury.

COMPOSITION

Muscles are made up of thousands of muscle fibres arranged into bundles held together by connective tissue; these bundles are arranged in overlapping sheets. It is the contraction and relaxation of the muscle fibres that activate movement in the muscle.

Muscle fibres can be divided into two types: slow-twitch (Type 1) and fast-twitch (Type 2), and every muscle will contain a combination of both. These fibres have different functions; one type or the other will be used by the muscle depending on the exercise being performed.

Slow-twitch fibres, as the name suggests, contract and relax slowly. These fibres are utilised when the horse is working at a fairly slow rate, i.e. when stamina is more important than speed. Activities such as hacking, long-distance riding and dressage will use predominantly slow-twitch fibres.

Fast-twitch fibres contract much more quickly than slow-twitch. They are used for activities needing acceleration and speed like showjumping and eventing.

Although both fibre types will be present in every horse, with different distributions and proportions in different muscles, the overall proportion of one in relation to the other decides in which particular discipline the horse is most likely to thrive. Thoroughbreds have a very high proportion of fast-twitch fibres, so they will have speed and power rather than stamina. Warmbloods and other horses with a higher proportion of slow-twitch fibres will have excellent stamina but reduced speed.

FINAL THOUGHTS

To give our horses the best chance of performing well, we need to help them to have healthy and trained muscles for the role they need to perform. If your horse is a top showjumper, going on endless hacks is not going to get him to get his fast twitch muscle fibres primed for show day. Similarly, taking your draft horse for a gallop on the track every other day is not going to be overly beneficial if you are looking for the muscles to perform slow and controlled movements in pulling a carriage. As owners and trainers we need to tailor our training to suit our horse’s job, and their muscle fibre composition, so that their muscles can be optimally primed and ready for action!

Similarly, we need to understand that unworked and cold muscles are more likely to experience injury as their attachments are fixed. Making sure that we adequately warm-up our horses before we exercise them, in order to get their muscles firing optimally and moving well, is thus a crucial part of training to reduce the risk of injury.

So, whilst anatomy may seem a little dry as an isolated topic, there are real practical applications for us in the learnings we can derive from it!