For a horse to perform well as an athlete, all body systems must be in good working order. When one or more system "breaks down," the horse is no longer able to perform to its potential, and the owner, rider, and trainer will notice a decline in performance during training and competition. In some cases, the reason for the loss of performance is quite obvious--the horse is lame with a swollen joint; he developed exertional rhabdomyolysis (tying-up); or there is an abnormal discharge from both nostrils, suggesting a respiratory problem. In those cases, a thorough clinical examination using traditional diagnostic procedures and tests usually will allow a specific diagnosis to be made.
However, many problems that result in poor performance are quite subtle and, very often, the horse appears to be fit and well when examined at rest. These kinds of problems are only evident during exercise, and some form of exercise testing usually is required to make a diagnosis.
In this article, we discuss the use of exercise testing in the evaluation of performance problems. The aim is to provide you, the horse owner, with an understanding of the types of testing procedures available and how these tests are used to diagnose performance-related problems.
What Type Of Test?
There are several important considerations when developing an exercise test for assessment of fitness and the reasons for poor performance. Ideally, the exercise test should mimic the type of exercise performed by the horse during training and competition. As well, the test must be standardized so that the results can be compared with those obtained from other horses. The test also must be repeatable--when a horse is subjected to the same exercise test on several occasions, we expect the results to be very similar. If not, our ability to detect abnormalities that underlie the performance problem is severely hampered. Finally, and most importantly, we must be able to monitor the horse during and after exercise in order to evaluate function of the various body systems.
There are two main types of exercise tests. First, a horse can be exercised on a track or a similar set course (a field exercise test). Second, the exercise test can be carried out on a treadmill. Each has advantages and disadvantages, and the choice will depend on the aim of the test and the type of measurements that must be collected. The main advantage of a field test is that the horse is exercised in its normal environment (e.g., a racehorse on a track)--the normal level of work can be achieved during the test, and the usual rider (or driver) can work with the horse. On the other hand, the standardization and repeatability of field tests can be hampered by wide fluctuations in environmental conditions, such as air temperature, relative humidity, or ground surface conditions.
Measurements that can be made during exercise in the field include heart rate (by use of a heart rate meter) and electrocardiogram or ECG (a recording of the electrical activity of the heart). However, collection of ECG tracings requires use of specialized telemetric equipment that transmits the ECG information from the horse to a recording device. This equipment is not commonly used in routine practice, and these measurements usually are made during treadmill exercise tests. Blood samples can be collected before and after exercise for measurement of, for example, lactate concentrations and enzyme markers of muscle damage. As well, the respiratory system can be examined after exercise--useful diagnostic procedures include endoscopic examination of the airways (nose, throat, and windpipe) and bronchoalveolar lavage. These procedures are discussed in more detail below.
Use of high-speed treadmills for exercise testing and clinical evaluation of horses with poor performance has increased greatly in the past decade. Although such exercise testing facilities are available only at some universities and larger equine hospitals, it is common practice for veterinarians to refer horses with performance problems to one of these centers. The main advantage of treadmill tests is the ability to monitor closely the horse during exercise. More sophisticated measurements--such as oxygen consumption and gait analysis--can be made. In addition, it is possible to perform upper airway endoscopy while the horse is running, a procedure often crucial to the diagnosis of upper airway obstructions. It also is much easier to standardize the nature of the exercise test and the environmental conditions.
What about disadvantages? One disadvantage already mentioned is that high-speed treadmills are expensive and not widely available. Second, the nature of the exercise undertaken often is very different from that performed on a track or similar setting--it can be difficult to mimic the same workload as that normally undertaken, in part because the horse exercises without a rider (or a sulky and driver). In human athletics, this issue has been shown to be of great importance when evaluating performance--cyclists should cycle, runners should run, and rowers should row.
Despite these limitations, the advent of clinical exercise testing on a high-speed treadmill has greatly improved our ability to diagnose conditions that contribute to poor exercise performance and, at least for now, is the gold standard for examination of horses during exercise.
Several types of treadmill exercise tests are used depending on the focus of the examination, breed of horse, and current state of fitness. One type of standardized exercise test calls for the horse to run several short intervals of increasing velocity (an incremental exercise test). This test is most useful for assessment of overall fitness, including measurement of maximum aerobic capacity (VO2max), heart rate, and blood lactate concentrations. After a warm-up at the trot (treadmill speed around four meters/second), the speed of the treadmill is increased sequentially at 60 to 90 second intervals until the horse tires and is no longer able to keep pace with the treadmill. An example protocol would require the horse to run for three minutes at four m/s, 90 seconds at six m/s, then 60 second intervals at eight, 10, 11, 12, 13, and 14 m/s (the latter speed is equivalent to a sub two-minute mile). This is a fairly strenuous test, but one that is well tolerated by fit racehorses.
A modified incremental test often is used when the primary purpose of the examination is diagnosis of a loss in performance. For endoscopic evaluation of the upper airways, the horse must be examined at or near its maximum capacity. This is because many of the abnormalities of the upper airway--such as laryngeal hemiplegia and displacement of the soft palate--only occur when the ventilation (movement of air in and out of the lung) of the horse is at its maximum. The running speeds necessary to achieve maximum effort will vary between horses.
Typically for this test, the horse will undergo a thorough warm-up after which the endoscope will be positioned in one nostril. Then, the treadmill speed is increased in a stepwise fashion--the highest speed (often 12 to 14 m/s) will be maintained for a distance of about one mile. As mentioned, problems with upper airway function often occur when the horse tires and the endoscopic examination will be continued as the treadmill speed gradually is reduced at the end of the test.
Frequently, the treadmill is set at a moderate incline in an attempt to mimic the work effort when exercising over ground. Studies have shown that for horses ridden on a track then exercised unmounted on a treadmill at the same speed, a moderate slope (3.0%-3.5% grade) is necessary to result in an equivalent workload. Another consideration is one of safety--very high treadmill belt speeds (over 35 mph) can be necessary to elicit maximum effort. This is particularly true of Thoroughbred racehorses. Such high speeds can be unsafe or, for some treadmills, are above the machine's maximum capacity. Use of an incline allows the horse to reach maximum effort at a safe speed.
A common question by owners and trainers is, "Is it safe for my horse to run on a treadmill?" By and large, the answer to this question is yes. Horses readily adapt to running on a treadmill--for most horses, only one or two training or acclimation sessions are required for the horse to appear comfortable running at different speeds on the treadmill. These training sessions can be performed the day before the actual exercise test, but in many instances are done the same day (see papers by King et al. 1995 and Sloet and Clayton 1999 for further discussion about habituation of the horse to treadmill exercise).
Another consideration for treadmill exercise tests is the tack worn by the horse. Standardbred pacers that race in hobbles should wear those during treadmill exercise. If upper airway endoscopy is to be performed, the horse should be outfitted with the head gear normally worn during performance.
Back To Basics
The key to diagnosis of the cause of poor exercise performance is the thoroughness of the examination. Your veterinarian, and the doctor at a referral center who might examine the horse, will begin by asking you a number of questions about the horse. We term this the medical history. This questioning initially will cover general health history, then will focus on the performance problem. He or she will want to know the length of time the poor performance has been observed and the time during competition when performance falls off, and whether the horse has shown a gradual decline in performance as opposed to a very sudden drop-off in form.
The history also might point to a problem involving a particular body system. For example, a lameness problem might be signaled by reluctance to change leads or go in one direction, gait unevenness, or the tendency to pull to one side. A problem with heart function might be suspected if the horse has become intolerant to exercise. An abnormal noise during breathing is often caused by an obstruction to the horse's upper airway.
Frequently, there is a strong suspicion of a problem involving one area, such as a lameness. However, experience has shown that up to 50% of horses which are examined because of suspicion of an abnormality localized to a specific system actually have problems involving other body systems. As well, it is common for there to be multiple problems (see the September 2000 Sports Medicine article). Therefore, the examination, both at rest and during exercise, must be comprehensive. We also know that poor exercise performance most often is associated with problems involving the musculoskeletal, respiratory, and cardiovascular systems--these systems must be closely evaluated.
There is an array of diagnostic tests that can be done before, during, and after an exercise test. First and foremost, a lameness examination must be performed before any exercise test. Obvious lameness might require further evaluation by use of flexion tests, nerve blocks, and various imaging techniques (radiography, ultrasound, scintigraphy). The respiratory and cardiovascular systems will be examined--the veterinarian will use a stethoscope to listen to the heart and lungs, pass an endoscope to examine the throat (pharynx and larynx) and windpipe (the trachea), and record an ECG. Blood samples to evaluate, for example, the number of red blood cells and the activities of protein markers of muscle damage also are collected before the exercise test.
During treadmill exercise, the horse will be examined for lameness. The gait of some horses might appear uneven at slow speeds (trot), but not at higher speeds. In those cases, lameness probably is not the reason for poor performance. Conversely, many horses with a smooth gait at low speed become progressively more lame at high speeds. The source of this lameness will be investigated further after completion of the exercise test.
Second to musculoskeletal problems, abnormalities of the upper airway are the most common reason for poor performance. The endoscopic examination performed at rest can be helpful in the diagnosis of upper airway problems. However, examination during exercise will give a more accurate assessment and guide decisions regarding the need for surgical correction of the problem. For example, low-grade laryngeal hemiparesis or hemiplegia (this condition is sometimes termed roaring) might be evident at rest. However, in 50% or more of these horses, laryngeal function is normal during exercise and surgical treatment is not warranted. Conversely, the absence of an abnormality at rest does not rule out the possibility of an obstruction during exercise. It therefore is imperative that the upper airway be examined during hard exercise. This is best done with an endoscope that has videotaping capabilities to record the procedure. The videos can be reviewed after completion of the test.
Another common cause of airway obstruction is dorsal displacement of the soft palate (DDSP). This occurs when the tissue on the floor of the airway just in front of the larynx moves up and, in doing so, partially obstructs airflow to the lung. Other problems that might be seen in the upper airway include dynamic collapse of the pharynx and paralysis of the right side of the larynx--again, these problems limit airflow to the lung. Exercise performance is impaired because of inadequate oxygen supply.
Measuring the amount of oxygen in the blood (blood gas analysis) is another way to assess respiratory function and the significance of any abnormality observed through the endoscope. This analysis is accomplished by inserting a small catheter into a blood vessel, thus allowing blood samples to be collected while a horse is running on the treadmill. If there is obstruction to airflow, it is reflected by a lowering of blood oxygen content (hypoxemia). This analysis also is useful in assessment of lower airway problems--severe inflammatory conditions of the lung (inflammatory airway disease and chronic obstructive pulmonary disease) can result in impaired oxygen transfer into the body.
To monitor heart rate and rhythm during exercise, a series of small electrodes are attached to the horse's body. These electrodes record the electrical activity of the heart and relay this information to a recording device. A printout of this information can be viewed after the test. Disturbances to heart rhythm are a reasonably common cause of poor performance, so these measurements are an important component of the overall performance evaluation.
Recent studies at the University of Pennsylvania showed that 55 of 348 horses evaluated for poor performance had clinically important irregular heart beats during exercise. Some of these horses also had evidence of an upper airway obstruction, and it has been suggested that hypoxemia predisposes the development of the irregular heart beat (Martin et al. 2000).
Work at the University of Pennsylvania also indicated that post-exercise ultrasound examination of the heart (echocardiography) is useful for detection of problems not evident during examinations performed at rest. This examination is normally performed within five minutes of the completion of the exercise test.
Other diagnostic procedures that are performed after an exercise test include endoscopy, bronchoalveolar lavage, and measurement of blood markers of muscle damage. Endoscopy normally is performed about one hour after exercise and is useful for assessment of exercise-induced pulmonary hemorrhage (EIPH). The endoscope is passed into the horse's trachea (windpipe) to check for evidence of blood and mucus.
For bronchoalveolar lavage (BAL), an endoscope or a specially designed flexible rubber tube is positioned in one of the lower airways (a bronchus). Then, a small volume of fluid is instilled and aspirated (taken in through suction) through the endoscope or tube. In essence, a portion of the lung is "washed." This sample is analyzed in a manner similar to that for a blood sample--the number of red and white blood cells is counted and the sample is examined under a microscope. This procedure is valuable for assessment of lower airway conditions that impair performance, and it helps in development of treatment recommendations. The BAL also is used to assess the extent of exercise-induced pulmonary hemorrhage.
To assess muscle damage incurred during exercise, blood samples for measurement of the enzyme creatine kinase (CK) are collected before and after exercise. This protein is contained within muscle cells and is released into the blood when cells are damaged. In the study performed at the University of Pennsylvania (Martin et al. 2000), 53 of 348 horses which undertook a standardized exercise test had marked post-exercise increases in blood CK. Although none of those horses showed any signs of tying-up after exercise, it is likely that ongoing low-grade episodes of muscle damage contributed to poor performance.
We have discussed only some of the diagnostic testing procedures that can be used in the evaluation of horses with poor performance. There is no set "recipe" of tests that is guaranteed to sort out these problems. Nonetheless, it has become clear that use of treadmill exercise testing in combination with a comprehensive clinical evaluation that focuses on the musculoskeletal, respiratory, and cardiovascular systems has greatly improved our ability to diagnose problems that can impair exercise performance. Although the prognosis for return to full function will depend on the specific problem, use of exercise testing often allows for earlier detection at a time when treatment still might be effective.
Couetil, L.L.; DeNicola, B.D. Blood gas, plasma lactate, and bronchoalveolar lavage cytology analyses in racehorses with respiratory disease. Equine Veterinary Journal Supplement 30: 77-82, 1999.
King, C.M.; Evans, D.L.; Rose, R.J. Acclimation to treadmill exercise. Equine Veterinary Journal Supplement 18: 453-456, 1995.
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.
Martin, B.B.; Beech, J.; Parente, E.J. Cytologic examination of specimens obtained by means of tracheal washes performed before and after high-speed treadmill exercise in horses with a history of poor performance. Journal of the American Veterinary Medical Association 214: 673-677, 1999.
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.
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.
Parente, E.J. Testing methods for exercise intolerance in horses. Veterinary Clinics of North America: Equine Practice 12: 421-433, 1996.
Seeherman, H.J.; Morris, E.A. Methodology and repeatability of a standardized treadmill exercise test for clinical evaluation of fitness in horses. Equine Veterinary Journal Supplement 9: 20-25, 1990.
Sloet Van Oldruitenborgh-Oosterbaan, M.M.; Clayton, H.M. Advantages and disadvantages of track vs. treadmill tests. Equine Veterinary Journal Supplement 30: 645-647, 1999.
About the Author
Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University