The severe efforts which horses are called upon to make, and often under the most trying circumstances, render them specially liable to overtax the muscles, tendons, ligaments, and joints of the extremities, or in other words unduly stretch or strain them. Sprain is one of the most common ailments affecting the legs of horses. It may consist in a mere overstretching of the fibres of a structure, or some may at the same time be ruptured.
Its origin cannot be considered altogether apart from conformation, for it is found that any considerable departure of the legs from the perpendicular tending to disturb the centre of gravity has the effect of unequally distributing the weight of the body, and predisposing certain parts on which it falls in excess to sprain. Knees in-bowed or out-bowed, knees set unduly backwards, feet turned outward or inward, cow hocks, and various other defects of conformation are conditions favourable to this accident.
Big joints, besides providing a large surface of support to diffuse and minimize concussion, are also furnished with large strong ligaments by which they are able to resist sprain, while small joints, whose connecting-structures are wanting in substance and strength, more readily yield to the force applied to them.
These unfavourable conditions are aggravated when left out of con-sideration in the operation of shoeing. When one part of the crust is unduly lowered beyond another the already unequal distribution of the weight on the structures which bear it may be materially increased.
Narrow, leggy horses, which lack stability owing to their narrow base of support, are rendered liable to sprain by their tendency to slip.
The exciting causes of sprains consist in violent extension of the structures involved; but Williams observes "extension is not always the cause of a strain, as a muscle may be injured by the opposite condition, namely, violent contraction, its fibres and their thecae broken across their long axes, or its tendinous fibres torn from their attachments at either or both of its extremities".
These will vary with the structure affected. Generally they comprise lameness, tenderness of the part with or without swelling, resting of the limb in such a way as to relieve tension and take the weight off the injured organ. These will be dealt with in the articles on sprains to special structures.