Laboratory exercise testing

Laboratory exercise testing

Samantha Franklin and Kate Allen


The treadmill has been used for the scientific study of horses for over 100 years. Early studies were focused on energy metabolism in working horses using a rudimentary treadmill.1–4 However, during the war years, research declined and subsequently mechanization reduced the need for horses to be used in agriculture. It was not until the 1960s that there was resurgence in equine research, coinciding with increased use of horses for pleasure and sporting pursuits.

The first high-speed treadmill for equine use was developed in the 1960s in Stockholm, Sweden.5,6 The research group there, under the leadership of Sune Persson, was responsible for many of the early studies in equine exercise physiology and until the 1980s this was the only center to have these facilities. However, since that time, the use of high-speed treadmills has increased dramatically and is now used in many institutions throughout the world for exercise testing of equine athletes.7

Several types of exercise test have been described.8 Performance exercise tests are typically performed on the apparently healthy population to provide guidance for fitness improvement or as a basis for training athletes. They are also used in research to determine the effects of interventions on performance parameters. Clinical exercise tests are primarily used for individuals presenting with signs of poor performance or disease. The exercise test is generally used for diagnostic purposes.

Exercise testing can be performed in the laboratory or in the field and there are advantages and disadvantages to both types.

Field exercise tests can provide quantitative and objective measures of exercise performance within the performance or clinical disciplines and several authors have described exercise protocols for horses.9–12 Field exercise tests enable the horse to be examined in the environment in which it normally competes, a key advantage over treadmill exercise testing. Furthermore, field exercise tests enable the horse to be examined whilst ridden and with tack modifications that may influence clinical findings.13,14 However, exercise protocols are difficult to standardize and the types of tests that can be performed are more limited than those available in the laboratory. Field exercise testing is discussed in detail in Chapter 3.

Laboratory exercise tests are undertaken in a controlled environment with sophisticated instruments enabling greater precision and accuracy in the measurement of a larger number of physiological response variables. The benefits of treadmill exercise testing for both equine and human athletes are that exercise tests can be standardized and readily controlled, and a wide range of measurements can be made with no restriction on the size of the equipment. Because of this, laboratory exercise tests remain the preferred technique whenever feasible for human athletes.8 Treadmill exercise testing will be the focus of this chapter.

Indications for laboratory exercise testing in horses

Laboratory or treadmill exercise testing has been used extensively for research and for clinical exercise testing. Treadmill exercise testing has formed the basis of our knowledge of equine exercise physiology to date. Studies have been performed, covering a wide range of aspects relating to equine exercise. Many have been published in the proceedings of the International Conference on Equine Exercise Physiology that was first established in 1982 and is held on a quadrennial basis. These proceedings provide a valuable resource for exercise physiologists, veterinary surgeons and veterinary students alike.

Treadmill exercise testing for clinical diagnostic purposes in poorly performing equine athletes was first described in the early 1990s15,16 and is now routinely performed at many centers throughout the world. It is primarily used to assess the effect on performance of abnormalities found on the resting examination or to determine the reasons for reduced athletic capacity in horses in which no abnormality can be identified on resting examination. Abnormalities affecting the cardiovascular and respiratory systems may only be manifest during strenuous exercise due to the large reserves in the resting horse. It is well known that assessment of the upper respiratory tract (URT) at rest is frequently inaccurate at predicting the events that occur during exercise.1720 Also some cardiac arrhythmias and musculoskeletal disorders may only become evident during intense exercise.21,22 Furthermore, poor performance is often multifactorial and hence it is important to perform a thorough examination of all body systems before, during and after exercise.21,2325

In human athletics, exercise testing is used routinely to assess performance. Measurement of maximal oxygen uptake (VO2max) and anaerobic threshold are critical in the assessment of performance capacity (26). Similarly, in horses, information related to the functional capacities of the cardiorespiratory and musculoskeletal systems are considered to be the most valuable when attempting to evaluate performance potential. Several studies have shown that a high VO2max is associated with superior performance in racehorses.27–29 Despite this, the use of treadmill exercise testing for performance prediction, in horses, is not commonplace. More recently there has been a move towards investigating genetic markers of performance in both human30 and equine athletes.31,32 In particular, genes that affect muscular capacity have proven to be promising.3337

Preparation for exercise testing

Horse preparation

Ideally horses should be suitably fit for the assessment being undertaken. If unfit at the time of assessment, horses may not be able to exercise at a sufficiently high intensity that would be necessary to illicit abnormalities. Furthermore, it may not be possible to achieve exercise intensities that would be reached during competition. For racehorses, it is suggested that the horse should be in condition and prepared for the clinical exercise testing as if it were the date of a race.38 However, yearlings have been tested on high-speed treadmills without apparent problems.3942

Before commencing the exercise test, a thorough clinical examination of all body systems should be conducted in order to rule out obvious disease. It is important that horses are evaluated for lameness prior to exercise testing for two reasons. Firstly, lameness may be the reason for poor athletic performance and secondly, there is the potential to exacerbate any pre-existing lameness during high-speed exercise. Mild lameness is a common finding in racehorses referred for poor performance investigation21,23 and it is often difficult to convince trainers or owners that the lameness might be the cause of the poor performance. Furthermore, as described previously, poor athletic performance is frequently multifactorial and hence it may be important to determine whether or not other conditions are contributing to the poor performance. In such cases it is important to carefully weigh up the potential advantages of performing a complete examination with the potential disadvantage of exacerbating the pre-existing lameness.

When performing treadmill exercise, it is recommended that horses be shod in order to prevent foot soreness. Friction associated with the movement of the foot against the treadmill belt results in heat build up and may lead to secondary foot problems in horses undergoing repeated exercise sessions.43 It is not uncommon for loose shoes to come off during treadmill exercise resulting in a potential risk of injury to the horse or to any persons who may be positioned behind the treadmill. It is therefore important that shoes are checked before commencing exercise and any loose shoes removed and refitted. Studs should also be removed in order to reduce wear on the treadmill belt.

Treadmill instrumentation/equipment

A number of companies now manufacture high-speed equine treadmills throughout the world. The main requirements are the ability to incline to a slope of 10% (6°) and to reach speeds of 14–15 m/s, with rapid acceleration. Treadmills may be mounted in a pit in the ground or above ground and at least 4–5 m of space in front of, behind and on either side of the treadmill is needed to allow for equipment, personnel and to enable the horse to move on and off the treadmill.43 It is thought that treadmills situated in the ground are safer.43

The room in which the treadmill is housed should ideally have full climatic control with large fans placed in front and overhead to prevent excessive hyperthermia during exercise (Fig. 2.1). It has been shown that horses undertaking treadmill exercise reach higher temperatures than horses exercising overground44 and this is likely to reflect the reduced capacity for convective heat loss during treadmill exercise. Both the velocity and volume of air moved by the fans need to be taken into account. Air velocity should match that at which the horse is exercising and fans of at least 50 cm diameter are normally required to generate sufficient volume.45

The use of a safety harness, whilst not essential, is strongly recommended during exercise. This will ensure that horses do not come off the back of the treadmill should they stop suddenly. An emergency stop should also be available in order to stop the treadmill belt if the horse gets into difficulties. Finally, it is recommended that horses be fitted with boots in order to minimize the risk of injury to the limbs, particularly during training when they are not yet fully accustomed to treadmill exercise (Fig. 2.2). Overreach wounds and grazes to the limbs are the most common form of injury incurred during treadmill exercise.46

Acclimation to treadmill exercise

Although treadmill exercise may on first appearance seem alien, horses readily adapt to treadmill exercise, with less than 1% of tests abandoned because of behavioral issues.43,46 It is recommended that horses undergo a period of training or acclimation before commencing exercise testing. The aim is to introduce the horse to exercising in a novel environment, improve coordination and hence reduce risk of injury and to reduce anxiety and excitement that might otherwise influence any physiological parameters being measured.

The most important part of the familiarization process is to have a competent horse handling team. When introducing a horse to the treadmill for the first time it is useful to have at least four people involved, including two handlers (one on each side). In addition, one person will be required to control the treadmill and an additional person may be required to encourage the horse to move forward (Fig. 2.3). During subsequent sessions one handler is likely to be sufficient. The protocol that we have used successfully at our center is as follows:

When naïve horses are first introduced to treadmill exercise, they adapt a wide-based stance and typically lower the hindquarters and withers.47 However, during the first two minutes a rapid adaptation occurs and by five minutes horses are able to stabilize their gaits.47 During subsequent sessions, horses stabilize more quickly although there may be considerable variation between individuals.

The number of training sessions varies between clinics. Some perform acclimation runs over several days, whilst others do not perform extensive training prior to clinical exercise testing.46 The number of acclimation runs performed will depend on the individual horse’s ability to adapt to treadmill exercise, whether or not it has previously been exposed to treadmill exercise during routine training and the type of tests being performed.

Changes in behavioral responses and physiological parameters such as heart rate and plasma lactate concentrations occur following a period of acclimation and this is important to consider if you intend to make physiological measurements during the exercise test. King et al.48 found that, in general, one acclimating run resulted in reasonably consistent submaximal heart rate and plasma lactate concentrations although there was significant inter-horse variation. Subjectively it was believed that most horses benefited from a second training run. Rose and Hodgson49 reported that for reproducible results, at least four training runs were necessary. However, in another report, the same authors suggest that repeated treadmill exposure may have an adverse effect on acclimation, due to increased apprehension and excitement in anticipation of exercise.48 We therefore recommend that horses receive two or three acclimation runs prior to exercise testing but that final decisions should be based on individual animal’s ability to adapt to treadmill exercise. For horses that will be required to wear a respiratory mask during clinical exercise testing, a training session with the mask in place is recommended.

Treadmill safety

During any exercise at high-speed there is a potential risk of injury to the horse. A recent multicenter retrospective study investigated the risk of injury in 2305 horses undergoing treadmill exercise testing.46 The risk of major injury (such as a fracture, tendon injury or severe myopathy) was 0.6%, whilst the risk of minor injury was 4.7%. Similarly, another report suggested that substantial injuries (e.g. tendon injuries) occurred in less than 1.5% horses and other injuries occur in less than 5% of horses.43 These injury rates are, in fact, not dissimilar to those sustained during high-speed exercise during racing50,51 and eventing.52 Furthermore, in an earlier study in which comparisons were made between racehorses that were trained predominately on the track or on the treadmill, the treadmill group were perceived to have lower injury rates.53

Anecdotally, it has been suggested that horses may be at increased risk of tendon injury when exercising on a treadmill. However, in the study performed by Franklin et al.46 there were only two cases of tendon injury following treadmill exercise. A previous study by Takahashi et al.54 found that the peak forces in the superficial digital flexor tendon (SDFT) were lower during inclined running compared to the flat and hence uphill exercise (as is often performed on a treadmill) may reduce the risk of SDFT injury. Similarly, stress fractures were identified in only three horses. It has been found that bone strain is 75% less during gallop on a high-speed treadmill than that measured on a woodchip track.55 Also, in human athletes, significantly higher tension and compression strains and strain rates have been measured in the tibia during overground running compared with treadmill running, suggesting that treadmill runners are in fact at a reduced risk of tibial stress fracture than overground runners.56

The most common injuries incurred by horses during treadmill exercise are due to abrasions and overreach injuries.46 The risk of injury can be minimized by using an experienced handling team, providing adequate training before exercise testing, ensuring shoes are firmly fitted and that appropriate protective clothing, such as boots are fitted and making use of a safety harness.

Exercise test protocols

Whatever the reason for exercise testing, it is important that standardized procedures are followed so that data can be compared against subsequent tests for the same horse or with measurements from other horses of similar age and fitness level.

In human exercise testing, three factors are found to contribute to a good performance test57 and these can also be applied to equine exercise testing:

Validity of treadmill exercise testing

Clearly treadmill exercise testing will always struggle in terms of validity due to the lack of a rider and differences between treadmill and track surfaces. In fact, Evans58 went as far as to state that attempts to design treadmill exercise tests to replicate competition conditions may be a fruitless endeavor. Several studies have shown that there are significant differences in locomotion and physiological responses between treadmill and overground exercise.59 There are differences in stride and stance duration during treadmill locomotion,60,61 likely associated with changes in belt velocity during hoof impact.62 Furthermore, the energetic cost of locomotion is lower on the treadmill. Hence it is not surprising that many studies have found that both heart rates and blood lactate levels are lower in horses exercising on a level treadmill compared with exercise at the same speeds on the track.44,6365 These differences may be compensated for by increasing the treadmill speed by approximately 10%64 or by increasing the incline by 2.4–3.7%.63,66,67 In humans, it has been found that running on a 1% incline is more representative of the energy cost of running outdoors.68 The increased cost of locomotion during outdoor running has been attributed to the need to overcome air resistance.69 Other factors such as the stiffness of the treadmill itself may also influence the economic cost of locomotion.70 For horses, it is necessary to increase the incline further to account for the lack of a rider or harness during treadmill exercise. Metabolic workload can be increased by increasing the load carried.71 However, inclined running results in a much greater increase in metabolic rate than increasing the load.71 The oxygen cost of running on a 10% incline has been calculated to be 2.5 times greater at any speed than running on the level.72,73 During clinical exercise testing, inclines are commonly increased up to 10% making these tests more strenuous than routine field exercise tests.7476

Treadmill reliability and sensitivity

Research in human athletes has shown that time-to-exhaustion exercise tests have a coefficient of variation (CV) of >10%, whereas time trials have been shown to have a CV of <5%. A sensitive protocol is one that is able to detect small changes in performance. The difference between finishing first and second in human sporting events is suggested to be <1%.57 Therefore, it is important to be able to detect small changes with performance protocols. The repeatability/reliability of equine treadmill exercise tests is not well investigated. However, treadmill exercise testing has the advantage over field exercise testing in that it is much easier to standardize the exercise test. Factors such as treadmill speed, environmental conditions and the running surface can be replicated on multiple occasions. The reliability and sensitivity of treadmill exercise tests will depend to a greater extent on the reliability and sensitivity of the equipment being used to measure physiological parameters rather than the treadmill exercise protocol per se. Variability in measurements may also arise due to factors such as operator differences in the timing of measurements and the time taken to reach steady state values. Variables that depend on regression coefficients from the results of measurements made during submaximal exercise are likely to show increased variability compared to maximal values. Factors such as excitement and anticipation of exercise that cause release of adrenalin, may result in artificial increases in physiological parameters such as heart rate, blood lactate and PCV during submaximal exercise and hence adequate time for acclimation to treadmill exercise is important.

Several studies have been performed to assess the repeatability of measurements made during treadmill exercise tests. However, there are difficulties when making comparisons between papers as different measurements of variability are used. Measurements of HRmax have been found to have coefficients of variation between 0–7%.15,77,78 A number of studies have examined the repeatability of VO2max and values between 1 and 11% have been reported.15,7779 Studies examining the repeatability of blood lactate measurements have had conflicting results, with one study reporting a wide variability (21.4%) in peak blood lactate concentrations15 whilst another study found only a 2–11% variability.80

Main types of exercise test protocols

Two main types of exercise testing protocols have been described in horses: The incremental standardized exercise test and the high-speed single step exercise test. Both are performed after an initial warm-up phase. The incremental exercise test was first described by Seeherman and Morris15 and since that time this protocol has been adopted for clinical exercise testing. It involves the treadmill speed being increased typically every 60 sec (Table 2.1). Physiological measurements can be made at the end of each speed step in order to examine the relationship with speed. Depending on the requirements for the test, this protocol may be terminated at submaximal intensities or may be continued until the horse fatigues. The latter is important when determining maximal values of oxygen uptake and heart rate as well as for ruling out possible cardiorespiratory causes of poor performance.

During experimental trials and clinical exercise testing of Thoroughbred racehorses, tests are frequently performed on an inclined treadmill (up to 10%). This allows for exercise intensity to be increased without the need to attain excessively fast speeds.15,84 A 10% slope provides a compromise between a steep slope that would be unrepresentative of track exercise and faster speeds that may compromise horse safety.49 Inclined running does not appear to affect stride length or duration.60,85,86 However, there are changes in hindlimb kinematics and muscles are recruited differently when exercising on a slope, because the hindlimbs have to carry more weight and provide greater propulsion.8688 Also, it has been shown that exercising on a 10% gradient results in significantly greater maximal values for cardiac output89 and oxygen consumption,90 and this should be taken into account when comparing results from different exercise trials.

It is acknowledged that the incremental exercise test does not reproduce racing conditions. Nevertheless this type of testing has been used extensively for clinical evaluation of racehorses. The number of steps completed and the total run time provide an index of performance capacity and degree of fitness. It has been argued that the incremental test may not be an accurate assessment of a racehorse exercising over sprint distances of a mile or less. Data analyzed at our center has indicated that National Hunt racehorses reached higher speed steps than flat racehorses during incremental step tests91 confirming that this type of exercise test has a substantial stamina component and is appropriate for testing of horses that run over longer distances (>1 mile). However, since flat racehorses reach faster speeds during racing, a more appropriate test for sprinters would be to start the exercise test at close to maximal speeds and maintain this until the horse fatigues.81 Several centers therefore use a high-speed single step exercise test for clinical exercise testing of sprint racehorses. Again, this may be performed on a level or inclined treadmill (Table 2.1). Speeds and distances for Thoroughbred and Standardbred racing are included in Table 2.2.

For non-Thoroughbred racehorses, exercise tests tend to be performed at slower speeds and on a lower incline. The exact testing protocols vary. For Standardbreds or trotters, the incline is generally set lower (0–4%) than for Thoroughbred racehorses. This is because at high speeds, the energy cost of trotting is greater than that associated with galloping96

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Jun 18, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Laboratory exercise testing

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