Skeletal (including bone, tendons, and ligaments)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.
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."
The loss of coordination and fine control that accompanies even minor problems of the central nervous system can seriously limit exercise performance.
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.