Muscle Problems Can Cause Poor Equine Performance

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Originally published on TheHorse.com

Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.

Many equestrians, at one time or another, have dismounted after a long or strenuous ride and thought, "Boy, are my legs are going to be sore tomorrow!" Your horse has probably experienced a similar sensation, and as one researcher recently explained, a variety of muscle problems can cause a decrease in athletic performance in horses.

At the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La., Erica C. McKenzie, BSc, PhD, Dipl. ACVIM, ACVSMR, associate professor of large animal medicine at Oregon State University, gave an overview of some common muscle problems that affect equine athletic performance.

"Equine athletes have proportionally greater muscle mass than comparably athletic species, with muscle tissue comprising approximately 45% of body weight in most equine breeds, and up to 55% of body weight in the Thoroughbred," she explained. Thus, muscle problems can have a profound impact on performance quality.

Muscle Soreness and Strain

McKenzie explained that the degree of muscle strain and soreness horses experience, and the muscles affected, often depends on the activity in which they compete or train.

"Athletic horses may participate in brief, intense athletic events; prolonged endurance events; events that require a combination of activities (such as jumping and dressage); or events that require the horse to perform athletic activities while sustaining a demanding body position, such as working in a collected frame or sliding stops," she said.

Further, she noted, horses ill-prepared for the work load they're asked to carry out can also develop muscle soreness and/or strains. Additional factors that can contribute to the frequency of muscle soreness or strains include inadequate warm-up time or methods, preexisting conditions, working the horse to the point of fatigue, and partaking in activities the horse isn't accustomed to, she said.

Across the disciplines, common muscle groups affected include those located on the horse's back, hind quarters, shoulders, and legs, McKenzie said.

"In many cases lo- grade muscle strain is subtle enough clinically to go unnoticed," she said. Clinical signs or indicators of muscle soreness or strain range in severity depending on the injury, McKenzie said; these signs include:

• Subtle lameness;
• Reactivity to palpation;
• Unwillingness to engage the hind quarters during work;
• Pain;
• Heat and swelling of the affected muscle;
• Muscle fibrosis;
• Ossification (mineralization); and
• Mechanical lameness, which is most prominent at slower gaits.

McKenzie said that diagnosing these problems often presents a challenge to veterinarians due to the subtleness of clinical signs and potential difficulty locating sources of pain. She recommends using a combination of thorough physical examination, evaluation of serum creatine kinase and aspartate transaminase levels (which indicate damage to muscle cells), and ultrasonography or thermography to identify inflammation, damage, and ossification.

The treatment of muscle soreness and/or strain is based around rest or reduced activity, McKenzie said. Short-term anti-inflammatory medications and icing therapy might be beneficial, as well, she noted. In addition, "accessory modalities"--such as physical therapy, therapeutic ultrasound, cold laser, acupuncture, electromagnetic therapy, massage, and spinal manipulation--can be beneficial in helping affected horses return to work.

"The prognosis for full return to function is usually good for mild injuries, but more guarded for chronic recurrent injuries or in cases where horses will continue to perform an activity that carries a high probability of repeated aggravation," she said.

Back Pain

McKenzie then discussed spinal muscle abnormalities, a "frequent cause of abnormal hind limb motion in performance horses." Back pain caused by spinal muscle pathology is likely frequent in performance horses and Thoroughbred racehorses, she said.

Diagnosis begins with a detailed examination of the spinal region to look for evidence of muscular or osseous pathology, she explained. If evidence is identified, she recommends investigating the area further using ultrasonography, thermography, radiography, or nuclear scintigraphy.

"A remarkable number of rehabilitative modalities are now available, and back pain is one of the most likely phenomena to benefit from their application," McKenzie added.

Tying Up

Finally, McKenzie discussed exertional rhabdomyolysis (commonly known as tying-up), which she describes as "the most important and prevalent muscular cause of poor performance in athletic horses."

A few different groups of horses are known to be affected by exertional rhabdomyolysis, and McKenzie described each group and how their form of tying-up is characterized:

• A genetic disorder associated with abnormal intramuscular calcium regulation was discovered in Thoroughbred horses and has since been implicated as the cause of tying up in these horses. Young fillies in race training are most likely to be affected, McKenzie said, but the condition is prevalent in both genders. Risk factors include anxiety, preexisting lameness, high starch diets, and intermittent training patterns, and most affected Thoroughbreds can be managed by eliminating risk factors.
• Quarter Horses and related breeds develop exertional rhabdomyolysis in association with another genetic disorder, polysaccharide storage myopathy (PSSM, which affects glycogen metabolism), she said. McKenzie said that PSSM-related exertional rhabdomyolysis is highly prevalent in halter horses in the United States. Similar to Thoroughbreds, Quarter Horses and related breeds can be managed with a low-starch, high-fat diet and a regular work schedule.
• Finally, a form of exertional rhabdomyolysis that represents a variant of classic PSSM has been identified in Warmblood horses. However, in Warmbloods, this form of exertional rhabdomyolysis is characterized by an unwillingness to move forward or work in a collected frame and by signs that may appear somewhat similar to the disease "Shivers." Like Quarter Horses, Warmbloods can be managed using similar means as described for Thoroughbreds affected by exertional rhabdomyolysis.

Take-Home Message

Muscular causes of poor performance in equine athletes are relatively common. Understanding what to look for and how to treat the disorders can help a performance horse successfully return to work.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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