Equine Musculature

There are three types of muscle found within the body:

cardiac, which is specific to the heart and cannot be consciously controlled.
smooth, which is involuntary and plays a part in the circulatory and digestive systems.
skeletal, which produces movement, maintains posture, and stabilizes joints. This muscle type is under conscious control although it will contract involuntarily as a reflex response.

Muscles are generally arranged in pairs, and in the simplest situations one flexes a joint whilst it pair extends it. This allows for very careful control of movement. If there is nerve damage, the control may fail (e.g. in stringhalt, the flexor muscles contract, but the movement is regulated by the extensors, giving over-rapid flexion of the hind limb).

Some muscles have alternative functions e.g. the brachiocephalius runs from the top of the neck to the humerus. It can pull the humerus forward (if the limb is in the air), or alternatively if the leg is weight-bearing and the humerus is fixed, it will pull the head and neck backward.

Still other muscles may act in one way if they act singly, and another if they act in pairs, e.g. the longissimus dorsi acting singly will flex the neck and back to one side. If both left and right act together they will extend the neck and back (pulling the head and neck upwards).

Finally some muscles have different parts which act in different ways. E.g. the trapezius: the cervical portion attaches to the neck and pulls the shoulder forward, whilst the thoracic part attaches to the thoracic spine and pulls the shoulder backward; both parts acting together will pull the shoulder upwards.

Skeletal muscles come in all shapes and sizes. They respond to motor nerve impulses, are highly elastic, and have strong contractile power.

Muscles have a fleshy belly comprising thousands of muscle fibres intertwined with connective tissue called fascia. Muscle fibres decrease towards the ends of a muscle, reducing its circumference until only the longitudinally arranged collagen fibres remain in the form of a tendon. This attaches to the bone via a tough fibrous membrane known as periosteum. Muscles are attached to, and therefore move the skeleton by passing over joints.

The points at which the skeletal muscles attach to the bones via tendons are known as:

the point of origin - nearest to the body centre
the point of insertion - furthest away from the body centre.

Muscles consist of fibres made up of many thousands individual muscle cells that run parallel to each other. The fibres are bound together in bundles, called fascicles, by very thin layers of connective fascia.

Within each fibre are thousands of smaller threads known as myofibrils, which give the muscle the ability to lengthen and shorten. Within the myofibrils are millions of minute bands known as sarcomeres, which comprise of myofilaments made up of proteins. Actin produces thin myofilaments and myosin produces thick ones. These are responsible for muscle contraction. They slide over one another when muscles contract thereby shortening it. They slide back to their original position as the muscle relaxes.

Very simply muscles convert chemical energy into movement. Skeletal muscle fibres come in different types. These are inherited, so although you can train to get the best from your horse, you can not actually change them.

The muscle types are:

slow twitch, that produce energy slowly over a long period. They work anaerobically. requiring oxygen to create energy. Horses with predominance of these types of muscle fibres are less likely to fatigue and are good for endurance.
fast twitch, that are physically larger than slow twitch fibres. They work anaerobically producing small amounts of energy quickly and explosively but they tire out easily. Horses with muscles that have a predominance of fast twitch fibres are good for jumping.

Like humans all horses have a combination of both fibre types. It is the predominance of one fibre type that determines activity.