Symmetry in some athletes is both critical and useful in determining the site of injury. In some cases such as human racket players (squash, tennis, badminton etc) the racket hand can have significantly more development than the opposing limb, this is normal. In athletes that are runners, as in the case of a racehorse, symmetry is important in sustaining momentum and enabling in quadrupeds a change of leading pair of legs which gives the other limbs some respite and reduces fatigue.
If a horse sustains an injury and that injury becomes chronic, the horse will adapt its gait to compensate for that injury. Such affected horses often appear sound at the trot-up in hand and even during exercise. In the clinical examination a critical assessment of a horses’ symmetry is useful in determining the site of chronic injury.
Image: Dr Tim Roberts illustrating asymmetry of the hindquarters (pelvis)
There are several types of asymmetries; conformational, skeletal, and muscular.
Conformational asymmetry is genetic and there is not much that can be done about this. Horses do adapt to conformational issues with varing success. Horses can perform at the elite level with some types of conformational asymmetry. This type of asymmetry is most commonly an angular limb deformity, the effect of these is accumulative. For example, many horses have valgus (lateral deviation) of the cannon bone and perform at the elite level. If this is combined with other angular limb issues such as, back at knee, toe out, off set knees, straight forelimb then the affect is accumulative and so the ability to cope on the racetrack may be significantly affected depending on the number and degree of such deformities.
This is a complex discussion so we will leave that for another day.
Skeletal asymmetry (injury associated) is not uncommon in the racing thoroughbred. The most common example and easily recognised skeletal asymmetry occurs in the pelvis. The sacroiliac (where the pelvis attaches to the spine) can be asymmetrical in a sound horse. The sacroiliac ligaments (“joint”) can be injured which causes a structural shift in the position of the ilium. The pelvis of horses’ is like a square box, one side alone cannot alone be affected, there needs to be two points of injury. If one ligament is affected worst the shift is generally through the left / right axis. In more severe cases the shift is through the ventral / dorsal axis and in extreme cases both axis can be affected.
During routine examination of a horse a veterinarian can determine what type of asymmetry is present and if it is currently affecting the horses’ gait and function. There are many instances of pelvic asymmetry where the original injury has healed leaving asymmetry that is visual but not currently affecting the individual.
Muscular asymmetry is very useful in helping veterinarians determine the site of chronic injury in an equine athlete. In chronic injury the compensation of gait causes both wasting and development in different muscle groups. An example is a horse that sustains a chronic back injury can develop larger gluteal muscles on one side when compared to the other. Riders’ feedback can also be very useful in determining the origin of an injury. As the injury becomes more chronic it becomes more difficult for the rider but often more obvious to the veterinarian conducting an examination. A second example is an injury / disease in the shoulder joint where a horse might present with muscle wasting (disuse atrophy) of the shoulder muscles. In this case the shoulder is not directly affected, as such, but the muscle wasting indicates the horse is not using the forelimbs uniformly pointing to the underlying cause in the shoulder joint.