If we look at the muscles of the horses sternum it is easy to see that they are specialized for their function. The sternum is attached to the first eight ribs by their costal cartilages which are connected to each other and to the sternocostal ligament, an elastic structure that allows for a slight expansion of the chest during breathing.
This connection to the ribs is what causes the horse to move its sternum forward and upward during breathing. It is also what makes the sternum vulnerable to excessive pressure from tight girths. Often, a horse that is being worked hard in a lunge will start to lift its sternum and girth, this is because it is trying to balance the work of its other front limbs by using these muscles to help the sternum to rise and forward shift.
As well as these muscles that connect to the sternum there are other important muscles in this area which help the horse breathe, chew and swallow. One of these is the hyoid apparatus which is small and sits deep between the horse’s ‘jawbones’ (the mandibular rami). This muscle attaches to muscles that open the pharynx during breathing, as well as to the lower jaw bones (odontoid process) and to the scapula or shoulder blade (sternothyroid and omohyoideus).
The omohyoideus muscle is important for allowing the shoulders to move freely in a backwards and forwards direction. This is essential for the horse to be able to cross the forelimbs over during walking and running. Any change in this muscle’s function or tension would result in an alteration in the position of the scapula and thus changes to the horse’s gait.
A recent study by Wilson et al (2015) examined the extrinsic and intrinsic muscles of the horse’s thoracic limb in order to investigate how they are specialized for their function. They found that the extrinsic muscles were large with long fascicles compared to their aponeurosis length which would equate to them having a high capacity for doing work.
In contrast, the intrinsic muscles had shorter fascicles and a low muscle-to-tendon ratio. This means that the intrinsic muscles had a less capacity for doing work and were more suited to their passive role in thoracic limb movement.
This article is part of an ongoing series on the anatomy of the equine body. Other articles include; The skeletal system of the equine head and neck, Muscles of the horse’s limbs, and The skeletal system of the horse’s forehand. To view the full range of articles click here.