Page 118 - March_2023

VETERINARY VIEWS
ASYMMETRIC MUSCLING
by Nancy S. Loving, DVM
Not all horses have symmetrical muscling of their back, haunches, thighs, or along the topline. Sometimes this is not a result of a pathological condition but rather relates to training and use. However, it is good to understand what might cause muscle asymmetry in the first place, and how certain conditions might tip you off to a solvable and treatable problem.
MUSCULOSKELETAL PROBLEMS
A common cause of asymmetric muscling is a result of musculoskeletal pain and discomfort that causes a horse to protect a painful limb
by bearing less weight on it than on the opposite, normal limb. The result is less tone and/or muscle development of the painful limb. An example is seen with a horse that
has degenerative arthritis of the hock or stifle causing him to be reluctant to advance the limb as far forward as normal or to be reluctant to leave his weight on it for as long as the normal leg. Over time, muscles over the croup and
hip may atrophy slightly to create asymmetric
muscling; this is referred to as disuse atrophy. This situation might also occur if a horse has incurred an injury to the pelvis or hip.
If you’ve ever had a broken arm or leg encased in a cast, when the cast is removed after 5–6 weeks, your muscles may seem to have withered in place. Such disuse atrophy results because the cast did all the work of muscular support plus the limb was rested while the broken bone was mending. This is more extreme than what a horse experiences when limiting use of an uncomfortable or painful limb, but the principle is the same.
Musculoskeletal pain and lameness are best evaluated by a skilled and competent equine veterinarian who has a keen eye for gait asymmetry and is able to identify subtle and obscure lameness concerns. The vet should be familiar with performing diagnostic nerve blocks and imaging techniques such
as radiography, diagnostic ultrasound, and thermography. In some instances, nuclear scintigraphy provides an ancillary diagnostic tool to identify or confirm an elusive problem.
MRI units are more commonly available in referral hospitals, so greater accuracy in imaging is possible.
In contrast to disuse atrophy, a horse that has been kicked or suffered trauma or infection may have swelling and edema of the affected muscle group, yielding a relative asymmetrical appearance in the musculature. The horse is likely lame as well, in these cases. Some sign
of trauma is usually evident thereby providing an easily explainable source rather than a mysterious cause of muscle asymmetry. In some cases of muscle trauma, the muscle may contract or scar in to create an asymmetric appearance that usually doesn’t affect performance.
Another cause, and a common one at that, of asymmetric muscling is related to overuse of one side of the body relative to the other
as induced by training or as a horse’s effort to protect other areas of discomfort. A horse that is ridden on only one posting diagonal may develop muscles on that side that seem slightly more developed than the weaker side. A horse that is forced to tolerate a poorly fitting saddle
A common cause of asymmetric muscling is a result of musculoskeletal pain and discomfort that causes a horse to protect a painful limb by putting less weight on it than on the opposite, normal limb.
Musculoskeletal pain and lameness are best evaluated by a skilled and competent equine veterinarian with a keen eye for gait asymmetry and is able to identify subtle lameness concerns.
116 SPEEDHORSE March 2023