We often find that having an excellent horse with a very good mouth, that turns well to both sides, that reins in (stops) perfectly, after a time it begins to change -generally towards the end of the season of activity-; it is no longer the same horse; it hasn't the same mouth; it has trouble turning to one side or the other; or even worse, it starts jumping up and down when it stops.
Although it is true that there are many factors that could originate these problems, one of the most frequent and that often goes unnoticed, is that of pain in one or both hocks.
The intensity of these pains which are very incipient and only slightly evident initially, start increasing and begin to have an effect on mouth sensitivity, diminishing it as a defense mechanism.
There are predisposing factors such as the very genetics, a bad physical structure (above all in the hock area), the type of breaking-in to which the foal was subjected and the degree of bone development. Everything that causes injury to the region we are dealing with is a determining factor resulting from a great variety of accidents such as a sprained joint, bruises, micro-fractures, etc.
During training or during the actual game, the activity of the hocks will become limited if they are not in the best of health, and this will obscure its sporting performance.
In order to discover whether a hock is healthy or not, it is necessary at least to know what the anatomical structures look like, as well as how they move. Fortunately it is an area with very little muscle coverage which makes it possible to compare profiles and detect possible injuries by just looking at them.
Early diagnosis by a suitable professional and the correct treatment are essential if one is to solve the problem of many of these injuries.

Anatomical Base
The osteoarticular structure of the hock is complex and its movement is a determining factor as regards the animal's aptitude, and just as we have "no foot no horse", as the saying goes, one could say likewise, that if there is no hock there is no polo pony.

Bone Base
The hock (tarsus) is made up of a group of short bones, the tarsal bones, which in horses vary in number between five and seven.
The top row has two bones, the astragalus or tibial tarsal bone (ankle bone) and the calcaneus or the peroneal tarsal bone. The first is situated on the inside of the hock and has a throat or trochlea (pulley) to articulate with the tibial bone.
The second is on the outside and is the one that determines the hocks' rear profile.
The central bone is between the two and lastly we have the lower row where there are four bones: the first and second tarsal bones which are often joined, and the third and fourth tarsal bones.

Joints
The tarsus (hock) is a compound joint that is in fact made up of several joints: the joint between the tibial and tarsal bones, the intertarsal joint and the joint between the tarsus and the metatarsus or shank (tarsometatarsal joint).
The most important and mobile joint is the first (between the tibial and tarsal bones): it is precisely the crests of the tibial tarsus (trochlea) that point obliquely forwards and outwards at an angle of 12º to 15º to the middle plane, which is what prevents the horse from hurting its ventral area (belly) when performing at high speed.
The other joints have little or no movement (arthrodial joints).

Mobility
Mobility is given by the flexing and extension movements with sideways limitation which occur in the tibial tarsal joint.
When the horse is in standing position (standing still), the front angle of the joint is about 150º. This angle can vary more or less according to the conformation of each individual animal, extremes being harmful: on the one hand, the hock in standing position with a very open angle, or on the other, a very closed angle, sickle hocked, as the Americans call it.

Cross Section of Hock Conformation (angles)

The hock, as the center of a horse's rear movements, acts as shock absorber and driving force–depending on the case–; receives weight from all sides, and generates forces; it is the main support of that great lever that is originated in the large upper muscle masses and which, through the leg extremities gain momentum on the ground.
When the horse stops and turns, only one hock (the one on which it turns), supports all its own weight plus that of the rider. That is why, when there is lack of training (overweight animals), or when the opposite occurs, i.e. excess-training (fatigue); or when you have heavy horsemen who are rough riders or rough at handling the horse; or when the ground is slippery, hyperflexions or articular hyperextensions occur, which result in lesions that cause pain and hinder normal mobility.
If these lesions are not treated duly and in timely fashion, they evolve into chronic irreversible lesions, and depending on the degree of injury, they very often do not reveal lameness, but show positive in hyperflexion tests (subtle or obscure lamenesses). Mobility is not what it was, and as we saw before, has repercussions on mouth sensitivity, giving rise to our first reaction, which is to apply a stronger bit or snaffle, believing that we are thus solving the problem.

The Most Common Ailments or Unsoundnesses
We can summarize these into soft hereditary defects and hard hereditary defects, according to the parts involved. The former are seated within the hock joint capsule (hydroarthrosis), tendon sheaths (tenosynovitis), and serous sacs (bursa enlargement). In the case of the last of these (serous sacs), there is a very common injury resulting from continual bruising on the calcaneus area (point of hock), in which the serous sac that protects this tuberosity becomes inflamed, resulting in a callosity or capped hock.
This affection is commonly seen in horses that kick stall or trailer walls, hence the importance of having protection.
As regards the hard hereditary defects we have the spavin on the inner-side of the hock, the tumor on the outer hock and lastly the crook-knee at the rear end.
Using two studs on hind leg horseshoes is not advisable, precisely because it affects the health of hocks when turning, especially on hard ground.
Finally, when choosing a future polo pony, it is important that you buy well aligned horses in general; but as regards their hindquarters in particular, with good hock conformation, healthy and with perfect mobility.