About The Horse's Scapula
The Scapula The function of the horse’s forelimb is to suspend the weight of the thorax. There is no joint or bony attachment of the forelimb to the rest of the skeleton. Instead, the chest cavity is held in a sling of muscles and ligaments rather like the two pillars of a suspension bridge. This allows greater freedom of movement. Unlike us, the horse has no collar bone. Concussion is absorbed by soft tissue which would otherwise be absorbed by the spine.
The scapula is a large triangular flat bone which lies at an angle, partly covering the last cervical vertebrae, the first seven thoracic vertebrae and the heads of the first six or seven ribs.
The inner (medial) side of the bone is smooth and slightly concave, enabling it to fit smoothly over the first few ribs, and to provide traction for the many muscles and ligaments which attach to it and comprise the sling which suspends the weight of the thorax.
The spine of the scapula is a long, prominent ridge which runs longitudinally down the centre of the outer (lateral) side of the bone; the function of which again, is to provide an area for muscle attachment. This can be felt on all horses.
The top part of the scapula is cartilage and is larger and harder than many people realise. When riding, it is this cartilaginous extension, not bone that can be seen on either side of the withers as the horse moves. Although technically flexible, the cartilage feels as rigid as bone.
Connective tissue allows the scapula to slide over the ribs as the horse moves, thus facilitating a greater movement. As the forelimb moves forward, into the protraction phase of the stride, the muscles (especially the thoracic part of the trapezius muscle) actually pull the top part of the scapula back. When the forelimb retracts, the muscles (especially the cervical part of the trapezius muscle) pull the top of the scapula forward.
The movement of the scapula is opposite to the movement of the forelimb. This is what is meant by the rotation of the scapula. When riding rising trot, it is the rotation of the scapula that riders sometimes check to ensure they are on the correct diagonal. Rotation of the scapula is exaggerated in extended trot and when jumping.
A greater anatomical angle to the scapula allows enhanced rotation, resulting in a greater range of movement in the shoulder joint. This is considered to be a conformational asset. A horse with a good sloping shoulder, (about 45 degrees) will have a greater range of movement compared to a horse with a more upright shoulder (approximately 50-55 degrees).
There are a number of factors affecting this angle (or degree of obliquity). A longer scapula must necessarily be inclined back, while a short one will be more upright. As the scapula at the upper end is attached to the spinous processes by muscles and ligaments, the longer they are, the further back the point of attachment will be, thus the more obliquely the scapula will lie. Restriction of the scapula, or any attempt to mould or influence it, will affect the horse’s movement patterns.