A huge amount of time, effort, and money often are invested in the preparation of horses for various athletic events, including Thoroughbred and Standardbred racing, three-day eventing, steeplechasing, dressage, hunter-jumper events, reining, cutting, and endurance racing, to mention but a few. Regardless of discipline, we expect that a well-trained and properly schooled horse will be competitive. It is therefore not surprising that failure of a horse to perform up to expectations can result in a very high degree of frustration and cause us to ask why. Very often, this situation presents a particularly difficult diagnostic challenge for the veterinarian.

In this article, we'll look at some of the reasons for so-called poor exercise performance. Our focus this month is to understand why some of these conditions can limit exercise ability. Next month, we will delve further into this topic by considering the approach your veterinarian will take in trying to sort out the reason for apparent loss in (or lack of) performance ability.

What Is Poor Performance?

Simply stated, poor performance is the inability to exercise or perform at a level previously observed or at a level that can be reasonably expected based on the horse's physical characteristics and state of training. The veterinarian usually is presented with two main scenarios when dealing with exercise performance problems. The first is a horse which clearly has suffered a loss of proven performance. The second scenario in-volves an unproven horse which is not performing up to the expectations of the owner and/or trainer--in many cases, these expectations are overly lofty and in reality there is nothing medically amiss with the horse.

Let's use a racing Standardbred or Thoroughbred horse as an example. In the first scenario, the horse has been racing well, and has perhaps even won a race or two. During this same campaign, however, the horse might experience a sudden or more gradual loss of form, the end result being the same--racing performance falls off and he finishes down the track.

Contrast that situation with a two- or three-year-old horse which is yet to break its maiden--is this really a case of poor exercise performance or simply a lack of ability?

Unfortunately, it very often is the latter.

This same problem can arise when a racehorse is stepped up in class. Even though the horse is running against better company, the owner often expects continued success. If the horse does not perform up to those expectations, the owner and/or trainer will seek an explanation for the apparent "decline" in exercise performance, and perhaps call in a veterinarian to diagnose and treat the problem.

For Western horses engaged in barrel racing or other timed speed events, such as short-distance racing, an increase in running times clearly indicates a loss of performance. For the endurance horse, an early sign of a fall-off in performance can be a higher than normal heart rate during exercise or a delayed heart rate recovery. As well, there will be a sudden or more gradual slowing of work and race times.

It is not as easy to document poor performance in some of the other disciplines. For the most part, the horse appears to be quite healthy, has a good appetite, and is in good body condition. Nonetheless, owners, trainers, and riders might notice a decline in "zip" during training and a fall-off in performance during competition.

Another term that sometimes is used to describe performance problems is exercise intolerance. Normally, this term is used when a horse experiences obvious difficulty or distress during exercise--he is intolerant to the level of work required. A good example would be a horse with an obstruction of its upper airway that severely restricts his breathing during exercise. At rest the horse appears normal, but during exercise the obstruction causes an abnormal respiratory noise (termed a stridor) and labored breathing, and the horse tires very quickly. Another example is a horse with recurrent tying-up problems--these repeated epi-sodes of muscle soreness can occur with only light exercise and, in effect, the horse is exercise intolerant.

Are All Systems On Go?

A large number of factors contribute to athletic performance. As we all know, some of us are born to run, while others are not as well suited to that activity. In a sense, this is the genetic potential for athletic performance. For superior athletic performance, the horse requires good structural conformation, efficient movement, optimal function of a number of systems within the body, and the will to compete. Just like a well-oiled and fine-tuned motor vehicle, the engine of the horse must be firing on all cylinders for peak performance. Failure of one or more of the cylinders will slow the horse or result in exercise intolerance.

The most important body systems for athletic performance are:

Skeletal (including bone, tendons, and ligaments)
The horse must be sound and pain free.

Muscle
The horse's musculature provides the power for movement. When muscles fatigue prematurely or become painful (as with tying-up), the horse cannot generate the power for running. Similarly, muscle problems will cause the dressage horse to lose the fine control required for top-level performance.

Respiratory (the upper airway--nasal passages and throat--and the lungs)
Any problem that prevents the normal flow of air into or out of the lung or impairs the transfer of oxygen into the body will prevent the horse from generating the energy needed to exercise.

Cardiovascular (the heart and blood vessels together with the volume of blood and number of red blood cells)
The heart must move oxygen-laden blood from the lungs to the working muscle. Problems with heart rhythm, for example, can reduce the effectiveness of the heart as a "blood pump."

Nervous system
The loss of coordination and fine control that accompanies even minor problems of the central nervous system can seriously limit exercise performance.

Problems involving one or more body systems can result in impaired performance ability. However, it is important to understand that whether a particular problem impairs the horse's exercise ability depends on two factors. First, the level of exercise required by the horse, and second, the severity of the problem. Light exercise activities such as pleasure trail riding clearly are much less taxing than, for example, an endurance race or a three-day event. Minor ailments might not impact upon exercise ability during light exercise tasks. Conversely, the same seemingly minor ailment could pose a problem during harder exercise when all cylinders must be firing!

Causes Of Poor Performance

Let's now look more closely at a few of the conditions that limit exercise performance. It is not possible to discuss all potential causes of poor performance. As shown in the table on page 84, a large array of problems can adversely affect horses during exercise. Instead, we will focus on conditions of the musculoskeletal and respiratory systems, which are by far the most important cause of an apparent loss of performance and an interruption of a horse's training program.

I think most owners will agree that a sound horse (i.e., one which is free from major problems involving bone, tendon, ligament, and muscle) is important for athletic performance. In my experience, a large number of horses examined because of performance problems are, for a variety of reasons, lame. On occasion, the owner or trainer has not noticed the lameness or has disregarded the lameness as a potential cause of the horse's performance problem. While it is true that some horses compete successfully while carrying low-grade lameness problems, the pain associated with more severe lameness will contribute to poor performance, in part by altering the horse's attitude toward exercise. Indeed, treatment of the lameness often results in a rapid return to the former level of performance.

There are several other reasons why lameness can limit athletic capacity. First, persistent lameness often limits the amount of training that can be undertaken. For example, horses with chronic tendon or suspensory ligament problems often cannot withstand the rigors of hard training, and a lighter schedule is necessary to prevent further deterioration of the tendon or ligament. As a result, the horse might not be able to achieve the level of fitness required to compete successfully.

We also know that even low-grade lameness can contribute to development of other performance-limiting problems. Back pain is an important cause of poor performance in the equine athlete. Although there are numerous reasons for a sore back, many affected horses have a primary leg lameness that contributes to development of the back problem. Recent studies also have demonstrated that some racehorses with chronic tying-up have concurrent lameness. Again, it is likely that the presence of lameness increases the likelihood for development of tying-up episodes. In short, it is imperative that the horse's musculoskeletal system is in sound working order.

A recent article (June 2000 Sports Medicine column) provided an in-depth discussion of the various causes of muscle problems. Suffice it to say that muscle disorders, particularly tying-up (termed exertional rhabdomyolysis), are important causes of poor performance. To recap, episodes of tying-up can be placed in one of two broad categories: 1) Sporadic exertional rhabdomyolysis--this classification applies to horses which, on rare occasion, experience an episode of generalized tying-up; 2) Chronic exertional rhabdomyolysis--when a horse experiences repeated episodes of tying-up, with the first episode usually occurring at a young age. It is this latter category that is of greatest concern. Affected horses are prone to repeated episodes of tying-up, a circumstance that severely hampers preparation of the horse for competition and, in many cases, limits their athletic careers.

Recent studies have suggested that a more subtle form of muscle injury or tying-up might be a common cause of performance problems. This form of muscle damage has been termed subclinical myopathy because signs of muscle pain and soreness are not evident, yet affected horses have increased blood activities of creatine kinase (CK), a muscle protein that "leaks" into the blood when muscle damage occurs during exercise. In a recent study, 53 of 348 horses (15%) examined for poor performance had this form of muscle injury in response to a standardized exercise test (Martin et al. 2000).

Problems of the respiratory system are the next most common cause of a loss of performance (see table on page 84). These problems can be divided into those involving the upper airways (nasal passages and throat region) or lower airways (lungs). A large number of upper airway conditions cause performance problems. These include laryngeal hemiplegia (paralysis usually of the left side of the larynx), displacement of the soft palate, epiglottic entrapment, and dynamic collapse of the pharynx (when the airway just in front of the throat collapses during exercise). Quarter Horses with hyperkalemic periodic paralysis (HYPP) can suffer dynamic collapse of the airway when exercising.

These upper airway problems have a single common denominator--they result in a restriction to airflow. As a result, less air reaches the lung and the normal exchange of oxygen and carbon dioxide is impaired. In short, the horse's body becomes starved for oxygen during exercise, and he tires easily.

Just how common are these upper airway problems? Well, in two reports of studies that evaluated poor performance problems, more than 40% of the horses examined had some type of upper airway obstruction during exercise (Morris and Seeherman 1991; Martin et al. 2000). Most affected horses will produce an abnormal respiratory noise or stridor during exercise and, upon hearing this noisy breathing, experienced riders and trainers often will be suspicious of an airway obstruction. However, not all affected horses produce these characteristic noises during breathing, and direct examination of the horse's throat region during exercise often is necessary to make the diagnosis and determine the best course of treatment (see next month's Sports Medicine column).

Exercise-induced pulmonary hemorrhage (EIPH) and chronic obstructive airway disease (COPD) are the conditions of the lower respiratory tract (the lung) that have been most commonly associated with performance problems. Acute viral and bacterial infections of the lung also can impair performance, although providing the horse is allowed an adequate rest period during recuperation from this type of illness, there usually will be complete recovery of athletic ability.

EIPH is a condition most often associated with racing Thoroughbreds and Standardbreds. However, it can occur in horses performing other types of strenuous exercise, such as barrel racing and the cross-country phase of a three-day event. Although it is widely held that EIPH causes poor performance, there actually is little evidence to support this belief. However, some believe that repeated episodes of bleeding can cause permanent damage to the lung--this scarring eventually could limit exercise capacity.

Of much greater concern for all types of sport horses is COPD, also commonly known as heaves. COPD is a disease of the lung that is very similar to asthma in a person--a severe inflammation develops when susceptible horses are exposed to certain molds and dusts that are present in hay and straw bedding. Affected horses are thought to be allergic to these molds and remain susceptible for the rest of their lives. Disease episodes recur after they are re-exposed to the offending dusts and molds. Viral infections of the lung also are thought to trigger COPD in some horses.

True COPD is uncommon in young horses. However, young performance horses can be afflicted by a less-severe inflammatory airway condition (termed inflammatory airway disease, or IAD) that is exacerbated by poor housing conditions and exposure to molds and dusts in hay. Both conditions can be an important cause of poor performance and exercise intolerance. The clinical signs of COPD can vary greatly--severely affected horses have labored breathing at rest and are clearly exercise intolerant. On the other hand, horses with mild COPD or IAD might have a normal breathing pattern at rest, but often cough repeatedly, particularly during exercise. In addition, a cream- to yellow-colored nasal discharge might be seen after exercise. As with upper airway obstructions, performance problems probably arise because of impairment to the normal exchange of oxygen and carbon dioxide in the lung.

Heart function disturbances can cause poor performance and exercise intolerance in equine athletes. In a study by Dr. Ben Martin and his colleagues at the University of Pennsylvania (Martin et al. 2000), 55 of 348 horses which undertook a treadmill exercise test had evidence of disturbance to heart rhythm (a cardiac arrhythmia).

In addition, on the basis of ultrasound examination immediately after the completion of exercise, 19 horses had poor cardiac contractility (i.e., the strength of heart contraction was below normal). Together, those findings suggest that heart problems are a much more important cause of poor performance than previously recognized.

Twenty-two horses in the study had a combination of cardiac arrhythmia and upper airway obstruction during exercise.

This emphasizes two points regarding diagnosis of poor performance problems: First, veterinary examination of the horse during exercise often is necessary; and second, a thorough and comprehensive work-up that examines the musculoskeletal, respiratory, and cardiovascular systems is required. (We will continue that discussion next month.)


FURTHER READING

Martin, B.B., Reef, V.B., Parente, E.J., Sage, A.D. Causes of poor performance of horses during training, racing, or showing: 348 cases (1992-1996). Journal of the American Veterinary Medical Association 216: 554-558, 2000.

Mitten, L.A. Cardiovascular causes of exercise intolerance. Veterinary Clinics of North America: Equine Practice 12: 473-494, 1996.

Moore, B.R. Lower respiratory tract disease. Veterinary Clinics of North America: Equine Practice 12: 457-472, 1996.

Morris, E.H., Seeherman, H.J. Clinical evaluation of poor performance in the racehorse: the results of 275 evaluations. Equine Veterinary Journal 23: 169-174, 1991.

Valberg, S.J. Muscular causes of exercise intolerance in horses. Veterinary Clinics of North America: Equine Practice 12: 495-515, 1996.

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

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