The Horse Tutorial
Horse - Scapula
To understand how a horse’s forelimb works we need to grasp how the horse’s Scapula moves. It easy to dissect the motion of the horse slender limbs, there nice and visible, but the scapula is hidden away underneath Muscle, ligaments and cartilage. What knowledge is know about a horse scapula movement isn’t that well documented and i’ve read some conflicting accounts. Know one has been mad enough to x-ray a horse in motion, yet. So from the knowledge we do have from anatomy and from observations, we can make an educated guess as to the scapulas range of movements. The Horse’s forelimbs suspend the weight of its thorax (the chest). Unlike us humans there is no connecting clavicle from the thorax to the scapula. Instead the scapula is positioned via a set of strong muscular attachments. In turn the chest is held in place by a sling of muscles and ligaments, some connect to the medial (inner) side of the scapula, with the two limbs acting like pillars. This allows for greater freedom of movement and any impacts are absorbed by the the soft tissue and not the spine.
Image of skeleton Scapula
Can we see the scapula when looking at a horse. Not entirely. The top part of the scapula is cartilage. When we watch a horse walk or trot it is the cartilaginous extension, not the bone that can been seen on either side of the wither (the top ridge between the shoulder blades). So don’t be confused with this shape as the bone showing through, keep in mind the cartilage around it when animating the breadth of movement.
gif of Scapula bearing weight
gif of Scapula non bearing weight
How does the scapula operate in motion. The Connective tissue on the scapula allows it to slide over the ribs, giving a great deal of movement to the entirety of the forelimb. It is able to translate forward and back and acts like the springs of a car suspension.
As a horse bears weight on the forelimb the withers move down in relation the the top of the scapula (the scapula is the suspension and the soft tissue supporting the thorax absorbs the impact).
As the forelimb is non weight bearing , the withers rise and the of of the scapula moves down in relation to each other. (the scapula no longer bearing weight drops even if the limb is rising)
As the forelimb moves forward, the muscles (thoracic part of the trapezius muscle) pulls the top part of the scapula back and down, in turn the bottom part goes forwards slightly.
When the forelimb retracts, the muscles (cervical part of the trapezius muscle) pulls the top of the scapula forward and up and the bottom part back.
So keep in mind that the movement of the top of the scapula is the opposite direction to the movement of the lower forelimb. The rotations doesn’t occur at the top of the scapula because the main muscle connections are not at the top of the Scapula, rather they are spread out over the upper third of it. So if you are using a rig with the pivot point of the scapula at the top then it’s not rigged anatomically correct. Not to say that there is one correct pivot point on the scapula. The upper third is a general direction that’s more accurate, but as the bone is controlled by muscle the pivoting is fluid and s