Performance-related problems and exercise physiology

Chapter 16


Performance-related problems and exercise physiology





Chapter contents



INTRODUCTION


BIOCHEMISTRY OF EXERCISE AND FATIGUE



PRINCIPLES OF TRAINING



HEMATOLOGY OF HORSES IN TRAINING



ELECTROCARDIOGRAPHY



HEART RATE DURING EXERCISE



BLOOD LACTATE MEASUREMENT



EXERCISE TESTS



EFFECTS OF TRAINING ON BONE



SORE BACK IN THE PERFORMANCE HORSE




INTRODUCTION


There have been many developments in the science of training and fitness testing of horses over the last 40–50 years. Many of the research findings on blood lactate, heart rate and hematology in the exercising horse are of clinical relevance to veterinarians. The following sections provide an overview of the normal physiology and biochemistry of the equine athlete, both at rest and during exercise.


Veterinarians are frequently asked to examine horses that are either destined to be athletes or are not performing well. The main factors that contribute to good athletic performance include the structure and function of the respiratory, cardiovascular and musculoskeletal systems. It is not the purpose of this chapter to give details of all abnormalities of these or other body systems that affect performance, such as lameness and respiratory disease. The emphasis is rather on those techniques that are routinely used to assess performance, such as hematology and electrocardiography, and some new techniques based on the application of measurements made during or after standardized exercise tests. Some of these techniques are now routinely performed in large and successful racing stables. Unfortunately, veterinarians have been slow to apply many of the practical aspects of recent findings on exercise and performance in horses. This may be due to a lack of information on applied aspects of the many research findings in this area. One aim of the following sections is to review the research and present aspects of its practical application.


There are three principal aims of the application of science to training horses.



1. To develop techniques for testing fitness of horses for competition. Exercise testing may contribute to the financial success of training establishments in several ways. These include:



2. To design training programs that maximize fitness, reduce the incidence of injury and improve results in competition. Heart rate measurements during exercise and blood lactate concentrations after exercise can provide a guide to ideal training speed for development of stamina. This enables adjustments to the training routine in order to increase fitness without overtraining the horse.


3. To evaluate horses with poor performance.


Many of the techniques referred to in this chapter have been validated both in the laboratory and on the racetrack in Europe, USA and Australia. Moreover, similar techniques have been used by human athletes and sports science laboratories for decades and are now part of the daily routine of human athletic training. It has taken time to adapt and develop suitable protocols for horses. It is hoped that this chapter increases veterinarians’ understanding of the normal physiology and biochemistry of exercise in equine athletes and illustrates the limitations of some of the traditional methods of assessment of equine performance. This chapter also points the way ahead to the application of new techniques for monitoring fitness in horses.



BIOCHEMISTRY OF EXERCISE AND FATIGUE



ENERGY SOURCES


The processes of muscular contraction and relaxation require adenosine triphosphate (ATP) as an energy source. Hydrolysis of ATP to adenosine diphosphate (ADP) and a phosphate molecule liberates heat and energy. The energy is used in the process of actin–myosin crossbridge cycling in skeletal muscle cells, and muscular contraction and relaxation. The nature of the myosin ATPase enzyme in the muscle cell determines the twitch characteristics of a muscle fiber, and characterization of muscle fibers according to their actin–myosin ATPase activity forms the basis of muscle fiber typing.


The horse has very small stores of ATP within its muscles. These stores are quickly exhausted at the onset of exercise. In order for a horse to sustain exercise, it must be able to replenish ATP at a rate compatible with usage. The ability to utilize and regenerate ATP is a major factor limiting performance. There are several metabolic pathways that have the potential to contribute to ATP resynthesis. The relative importance of each pathway to overall ATP production is determined by the intensity and duration of exercise.


The immediate sources of substrate for ATP resynthesis are high energy phosphate molecules stored within the cell. These substrates are most important at the onset of exercise, and during times of extreme energy demand, such as the sprint at the end of a race. Creatine phosphate (CP) represents the largest pool of high energy phosphate in the muscle. CP can donate a phosphate group directly to ADP. Thus, ATP regeneration is rapid, but CP stores are too small to support more than a few seconds of exercise. During times of extreme energy demands, a high energy phosphate group may be transferred from one molecule of ADP to another. One molecule of ATP is produced with one molecule of adenosine monophosphate (AMP), which is metabolized to inosine monophosphate and eventually ammonia.


Sustained exercise demands an energy supply beyond that available in stored phosphate bonds. This demand is met by the energy available in fuel stores derived originally from feeds. There are two primary energy sources: carbohydrate and fat. Carbohydrate can be metabolized aerobically (using oxygen) or anaerobically (without oxygen). Fats are metabolized aerobically. Proteins have a minor contribution as an energy source during exercise.



AEROBIC METABOLISM


When the demands for energy are low, aerobic metabolism is capable of meeting the requirements for ATP resynthesis. Aerobic metabolism is the primary pathway by which ATP is regenerated during endurance-type exercise. Aerobic metabolism also contributes greatly to the energy supply during high intensity exercise such as galloping. Anaerobic metabolism makes up the deficit in total energy resynthesis during high intensity exercise.


Aerobic metabolism is the process by which fats and carbohydrates are oxidized, culminating in the production of ATP, carbon dioxide and water. The oxidation of substrates produces hydrogen ions. Pairs of hydrogen ions are accepted by coenzymes nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), which are reduced to NADH and FADH. These coenzymes transport the hydrogen atoms to the enzymes of the respiratory chain, which are located within the mitochondria. These enzymes, the cytochromes, have electron transport functions and are capable of delivering electrons derived from oxidizable substrates to oxygen. In the process, derived energy is stored as ATP. Three moles of ATP are formed from each mole of NADH entering the respiratory chain, and two moles of ATP from each mole of FADH.


Fat is stored in depots around the body and as triglycerides within muscle cells. Mobilization of fat from the depots occurs under hormonal influence. Non-esterified fatty acids (NEFA) are transported to the muscle by the circulation, and move into cells along a concentration gradient.


Carbohydrates are stored in the muscle and liver in the form of glycogen. Glycogenolysis is activated by epinephrine (adrenaline). Glucose, transported to the cell by the bloodstream, can also be oxidized to produce energy.


Fatty acids enter the mitochondria and undergo a process known as beta-oxidation, which results in the production of hydrogen and acetyl coenzyme A (acetyl CoA). Acetyl CoA enters the tricarboxylic acid (TCA) cycle, where it is metabolized to produce four pairs of hydrogen atoms and CO2. Acetyl CoA is also the entry point of amino acids to the TCA cycle.


Glycogen and glucose are metabolized to pyruvate in the Embden–Meyerhof pathway by a process known as glycolysis that results in the production of hydrogen ions that can be accepted by NAD and FAD. Pyruvate may be metabolized to acetyl CoA, and enter the TCA cycle when sufficient oxygen is available. The oxidation of 1 mole of NEFA of average composition yields approximately 138 moles of ATP. One mole of glycogen is oxidized to produce 37 moles of ATP. The oxygen cost of oxidation of NEFA is approximately 10% higher than the cost of glycogen oxidation.


Glycogen and blood glucose are the substrates immediately available for aerobic metabolism. Mobilization of NEFA requires a considerable time, and their contribution as a substrate for aerobic metabolism increases as the duration of exercise increases.


The capacity of the horse to generate energy aerobically is primarily limited by the availability of oxygen in the working muscle. Potential limitations include the function of the upper airways, lungs and cardiovascular system, erythrocyte numbers, and capillarity and fiber diameter within the muscle. The concentration of enzymes in the muscles appears to be in excess of the levels required to fully metabolize the oxygen delivered by the blood.


Training enhances the capacity for oxygen delivery to the muscle, therefore increasing the capacity of the animal to generate energy aerobically. Mitochondrial density and enzyme concentrations also increase.



ANAEROBIC METABOLISM


At the onset of exercise, the delivery of oxygen to the muscles does not instantaneously reach the level required to support aerobic metabolism. Approximately 30s of exercise is required before maximal aerobic activity is achieved. During this time, there is an increase in heart rate and ventilation, an increase in the oxygen carrying capacity of the blood as the splenic erythrocyte reserve is mobilized, and redirection of blood flow to the skeletal muscles. Increases in body temperature may enhance enzyme activity. During this period, the deficit in energy production is met by anaerobic metabolism. During intense exercise, the maximal accumulated oxygen deficit can be measured, and this measure reflects the anaerobic capacity.


When insufficient oxygen is available to support the activity of the TCA cycle and respiratory chain, or when the rate of pyruvate production is very high, pyruvate may act as a recipient of hydrogen atoms from NADH, resulting in the production of ATP and lactate. No oxygen is required in this reaction.


One mole of glycogen yields three moles of ATP when metabolized anaerobically. When glucose is the substrate, two moles of ATP are produced, as one mole of ATP is used to phosphorylate the glucose on its entry to the cell. In contrast to the reactions of aerobic metabolism, which occur in the mitochondria, the processes of anaerobic metabolism occur in the cytoplasm.


Lactate diffuses from muscle cells to the bloodstream and is transported to the liver where it is oxidized to pyruvate by NAD in the cytoplasm. The pyruvate is converted to glycogen and stored in the liver in an energetically expensive series of reactions. Fatty acid oxidation may provide the energy used in these reactions. During exercise, mobilization of liver glycogen stores helps to maintain blood glucose concentrations. Lactate ions are also metabolized aerobically in slow twitch muscle fibers during and after exercise.


Anaerobic metabolism reaches its peak capacity within 20–30s of the onset of exercise. It is best viewed as a supplement to aerobic metabolism that enables the total demands for energy to be met. The balance between aerobic and anaerobic metabolism depends on the rate at which NADH can be oxidized. For carbohydrate metabolism to continue, a supply of NAD is required. Resynthesis of NAD can be viewed therefore as a key step in ATP generation. As the duration of exercise increases, or as speed falls, the contribution of anaerobic metabolism to the overall energy equation becomes smaller.


In a horse galloping over 1200 m, aerobic metabolism accounts for appro-ximately 65% of the ATP generated. Demands for ATP resynthesis during exercise at a velocity slower than 18s/200 m (11 m/s, or 650 m/min) can be almost fully met by aerobic metabolism in most horses. Post-competition measurements of blood lactate concentrations in showjumpers have shown that anaerobic metabolism is significant in that form of exercise.



FATIGUE


Fatigue can be defined as the failure to maintain a desired power output during exercise. Many factors contribute to the development of fatigue, and the importance of individual contributors depends on the intensity and duration of the exercise. At a biochemical level, the most important factors are depletion of high energy phosphate reserves, changes in metabolite concentrations, and the depletion of glycogen stores. Depletion of intracellular ATP will also inhibit the cell membrane sodium–potassium pump, so there is accumulation of potassium in the extracellular fluid. Cycling of calcium between the cytosol and the sarcoplasmic reticulum will also be compromised, and fatigue is associated with increased calcium ion concentration in the cytoplasm.


During endurance exercise, the biochemical cause of fatigue is depletion of glycogen reserves in the muscle. Muscle biopsy studies have shown that muscle fibers are recruited progressively as glycogen stores in other fibers are exhausted and their power output decreases. When all fibers have depleted their glycogen reserves, exercise must stop. The oxidation of fatty acids alone is not sufficient to maintain the required power output, although the energy held in fat stores would support many days of continuous slow exercise. However, increased use of fatty acids as a substrate for aerobic metabolism has the effect of sparing glycogen reserves and increasing the potential duration of exercise. Aerobic training enhances fat utilization by increasing the concentrations of enzymes involved in fat metabolism and increasing mitochondrial volume.


Appropriate training will increase the capacity of the horse to use fatty acids as a substrate. The value of glycogen loading, of proven benefit to human endurance athletes, remains unclear in the horse. It is possible that the very large glycogen stores that are found in equine muscle may approach the limit of storage capacity. Hyperthermia, dehydration and electrolyte imbalances (q.v.) also contribute to fatigue and the “exhausted horse” syndrome during prolonged exercise.


The accumulation of lactate in the cytoplasm is a characteristic of cells generating energy anaerobically. Traditionally, the development of intracellular acidosis resulting from the production of lactic acid by anaerobic metabolism has been believed to be a major factor contributing to fatigue. However, at physiologic temperatures, reduced intracellular pH has little direct effect on muscle contraction and fatigue. Glycolysis may provide a high and sustainable supply of ATP without causing muscle fatigue.


The development of fatigue during high intensity exercise is related to oxygen delivery to the muscles. Oxygen modulates intracellular metabolism to influence the rate of change of metabolites—mainly inorganic phosphate (Pi) —and may affect fatigue by influencing the rate of Pi accumulation. In muscle cells, an increase in Pi may reduce crossbridge force production and myofibrillar Ca2 sensitivity, and inhibit Ca2 release and/or reuptake. When oxygen delivery is limited, there is greater reliance on substrate-level phosphorylation for ATP resynthesis, resulting in disruption of metabolic homeostasis and a decline in developed tension. Studies in humans have found that hypoxic conditions that decreased aerobic capacity had no effect on performance for sprints of up to 60s, and had very little effect on sprints of up to 120s, indicating that maximal human running speeds for short- and intermediate-length sprints are relatively unaffected by large reductions in aerobic power. The ability to maintain power output at a time when aerobic capacity was decreased suggests that additional metabolic energy must have been derived from anaerobic sources during exercise in hypoxic conditions, that maximal metabolic power outputs during sprinting under normoxic conditions are not limited by rates of anaerobic metabolism, and that human running speed is largely independent of aerobic power during all-out sprints lasting less than one minute.



PRINCIPLES OF TRAINING



INTRODUCTION


Successful performance of a horse in an athletic event will depend on its adaptation to the physical demands of the event. The degree of adaptation will depend on inherited characteristics of heart size (q.v.), muscle fiber types and other factors. It will also depend on adaptations that occur in response to the stimulus of regular exercise, or training. A key aim of training the horse is increasing fitness in order to delay the onset of fatigue during exercise. In some events, such as dressage and showjumping, skill is also likely to be a very significant factor influencing performance.


The development of fatigue during exercise is likely to be multifactorial. The body of research in exercise physiology encompasses five general models to explain the processes affecting performance:



1. The cardiovascular/aerobic model of fatigue is based on the idea that increasing the ability of the muscles to utilize oxygen, thus delaying the onset of lactate accumulation in the blood, will delay the onset of fatigue.


2. The energy supply/energy depletion model is based on the idea that fatigue during high intensity exercise may be due to the inability to supply ATP at a sufficient rate to support exercise.


3. The muscle recruitment (central fatigue)/muscle power model holds that brain concentrations of serotonin (and perhaps dopamine, acetylcholine and other neurotransmitters), alter the neural drive to exercising muscles, or that reflexes generated in the exercising muscles reduce recruitment of alpha motor neurons at the level of the spinal cord.


4. The biomechanical model states that biomechanical characteristics of the muscle (such as the capacity to act as a spring), affect the demands for force production, and therefore limit metabolite accumulation and heat production.


5. According to the psychological model, the ability to sustain exercise results from a conscious effort.


Physical fitness is associated with the capacity of the horse to do work. Therefore, in order to achieve fitness, it is necessary to enhance the function of all systems that may limit work capacity. Physical training that increases the work capacity of the horse must impact on one or more of the factors that contribute to fatigue. As the ability of the horse to use and regenerate ATP is a vital factor limiting performance, achieving an increase in the capacity of the horse to generate and utilize ATP is a key aim of training. In addition to enhancing energy generation capacity, training should increase the strength of the muscles, tendons and skeleton, enabling the horse to generate higher work outputs without succumbing to injury.


Other factors that limit performance, in particular skills and psychology, are beyond the scope of this chapter. Skills training may result in improvements in gait that result in increased efficiency of movement and a decrease in energy demands. In addition, training of this type may improve the psychological condition of a horse, making it more willing to work. The benefits of physical training can easily be undone by inadequate provision of rest periods before competition, so that the horse becomes tired and disinterested in competing.



AEROBIC TRAINING


In order to design a training program aimed at improving energy economy, it is necessary to know the contribution of the various metabolic pathways to ATP resynthesis in the athletic endeavor for which the horse is being trained.


Aerobic metabolism makes a significant contribution to ATP resynthesis during exercise of any intensity. All horses being trained for any purpose require aerobic training as part of their program. In events that require relatively low rates of energy turnover, such as dressage or endurance riding, aerobic metabolism can satisfy the demands for ATP resynthesis. As exercise intensity increases, the contribution of anaerobic metabolism becomes relatively more important, supplementing the energy produced aerobically to meet the total energy requirements for exercise.


When anaerobic metabolism makes a significant contribution to energy generation, a horse will be unable to sustain exercise as metabolic changes in the muscle will result in the onset of fatigue. If the capacity of a horse to generate energy aerobically is increased, its reliance on anaerobic metabolism to generate energy during exercise at a given intensity will be decreased. Therefore, the onset of fatigue will be delayed.


The maximum work capacity is determined by the sum of the aerobic and anaerobic capacities. Therefore, training that enhances anaerobic capacity will enhance performance in horses working at maximal intensities.


There are several basic principles that apply to training. Training will produce changes in the function of all parts of the body, but most importantly in the musculoskeletal and cardiovascular systems. In order to produce changes in a specific system, that system must be stressed in the training process. This has been described as the overload principle. However, it is important to avoid fatiguing the animal during training. It is therefore beneficial to have a method of measuring training intensity.


The changes produced by training are influenced by how often the horse is trained, the duration of training and the training intensity. Training-induced changes will be lost if a horse ceases regular exercise, although maintenance training programs will preserve the adaptations.


Aerobic training results in increases in circulating blood volume and stroke volume, which produce an increase in the amount of oxygen transported to the muscle. Within the muscle, there is an increase in the capillary distribution to individual muscle fibers, and fiber cross-sectional area. Mitochondrial volume, enzyme concentrations and myoglobin concentration increase within the cell. Training also appears to increase intracellular glycogen stores.


The metabolic cost of exercise is largely unaffected by training. A horse working at a given submaximal speed has a set requirement for oxygen delivery. As stroke volume increases during training, heart rate will fall. Heart rate provides the best indicator of the intensity of aerobic training, and heart rate changes can be used to monitor aerobic fitness.


The aerobic capacity of a horse can be increased by training at a relatively low intensity. Exercise at a heart rate approximately 50% of maximum appears to be sufficiently intense to maximize gains in aerobic power (maximum heart rate in the horse is in the range 195–240bpm). Regular monitoring of the heart rate allows the training intensity to be increased as the horse becomes fitter. Exercise that results in heart rates in the range of approximately 70–90% of maximum is likely to be associated with metabolic changes in the muscle. These changes promote development of buffering capacity in muscle cells.


Training sessions aimed at enhancing aerobic power should last for at least 30 min. Where horses are being trained for endurance riding, at least one session per week should be in excess of 1 h in duration, in order to ensure that mobilization of NEFA has occurred in the fat depots of the body.


Skeletal remodeling is an important factor contributing to the maintenance of soundness through training and competition, and should be considered an important goal in this stage of training. Because the intensity of work is low during aerobic training, the skeleton is not exposed to extreme stresses. Only a minimal increase in the exercise intensity is required to promote bone remodeling. Although increases in aerobic capacity are rapid, morphologic changes in the skeleton and muscle occur more slowly. This must be considered when planning the duration of the aerobic component of the training program.


Once a horse has achieved a high aerobic capacity, specific training is not required to maintain it. The higher intensity work given to train the anaerobic system will maintain aerobic power. If a horse is not undertaking high intensity work, relatively low intensity exercise such as trotting will enable it to maintain considerable aerobic capacity.



ANAEROBIC TRAINING


Anaerobic training is often the most neglected aspect of training. During this phase, the aim is to increase the capacity of the anaerobic pathways to generate ATP, and to increase the capacity of the muscle to cope with changes in metabolite concentrations and intracellular acidosis.


The point at which anaerobic work should be introduced to the training program is debatable. At least 4–6 wk of aerobic work appears to be necessary to provide an appropriate level of background fitness. However, many human athletes include an anaerobic component from the onset of training. As they become fitter, the anaerobic proportion of the total training load is increased. Some horse trainers believe that excessive low intensity training of racehorses results in a loss of speed.


An appropriate training load to stress the anaerobic system is one that results in post-exercise blood lactate concentrations in the range 4–10 mmol/L. Heart rates in the range of 85–95% of maximal are also appropriate. At this exercise intensity, anaerobic metabolism is making a significant contribution to energy generation. The resulting metabolic changes cause an increase in sprint capacity and an increase in intramuscular buffering capacity, which enables the horse to sustain intense exercise for longer periods.


It has been suggested that exercise eliciting a heart rate of 200bpm should be sufficient to generate lactate concentrations in the range 4–10 mmol/L. However, some horses working at this speed will have lactates either higher or lower than this range. In some horses, the target heart rate will be appropriate and may be used as an approximate guide when lactate measurement facilities are not available. New technology that combines simultaneous measurement of heart rate and velocity with a global positioning system (GPS) has great potential for improving the monitoring of exercise intensity during horse training.


During anaerobic training, the horse can quickly become fatigued. Measuring blood lactate concentrations to monitor workload enables the trainer to avoid fatiguing the horse. Training-related anorexia is generally seen in horses working frequently at an intensity that generates blood lactate concentrations greater than 10–15 mmol/L. In addition, an excessive anaerobic training load may result in glycogen depletion in the muscles, and a “tired” horse. Regular light training days should be provided to enable the horse to replenish its glycogen stores.


The optimal amount of anaerobic exercise in each training session and the optimal frequency of training sessions have not been determined. When the exercise intensity is closely controlled by regular blood lactate analysis, horses can cope with an exercise volume far in excess of the amount that has traditionally been considered the limit. However, in general, it would be unwise to regularly use very high intensity exercise training on more than two days per week.


Speed is an important factor contributing to the occurrence of injuries. Working horses uphill increases the intensity of exercise at a given speed, enabling an appropriate training stimulus to be generated at lower speeds, and minimizing the risk of injury. However, there has been a report that uphill treadmill exercise also increases the severity of exercise-induced pulmonary hemorrhage after intense exercise. Excessive use of training at speeds >900 m/min in Thoroughbred horses increases the risk of injury.


For horses involved in events where maximal exercise intensities are reached, speed training is the final component of the training program. The aim of this phase is to increase maximum speed and strength, and to increase stores of high energy phosphates. In order to achieve this, high speeds, generally higher than mean race speed, are required. Traditionally, sprint training of racehorses has been limited to a few maximal sprints at the end of training gallops. In human athletics, interval training techniques with up to six repetitions in each session have been used.


During the sprints, the horse should not reach the point of fatigue. A fatigued horse is unable to work at maximum speed, and therefore does not receive the appropriate training stimulus. In addition, the risk of injury is increased in a fatigued horse because gait becomes abnormal. The amount of sprint training required to achieve the maximum performance benefit in the horse is unknown. The distance over which the horse is sprinted should equal, or slightly exceed, the distance over which it will sprint during competition.



TRAINING FOR SPECIFIC PURPOSES



Racehorses


The discussion of training above provides general guidelines to racehorse training. The duration of each phase of training must be balanced between the time required to optimize performance and the costs of keeping a horse in training. As a general rule, a program consisting of 4 wk slow aerobic training, 4 wk medium pace and 2 wk sprint training can be considered as a bare minimum. The sprint training component includes skills work such as practice starts from stalls. Approximately 4 days before the race, the workload should be decreased to enable the horse to replenish intramuscular glycogen stores.


Horses that have achieved race fitness do not require intensive training in order to maintain fitness. Low intensity exercise with weekly gallops appears to be sufficient. In Standardbreds, which may race at intervals of <7 days, racing may be the only high intensity exercise required.



Endurance horses


As a general rule, training volume is the key to success in preparing endurance horses. The more training exercise given to a horse, the greater the improvements in performance. Some training sessions must be long enough to ensure that mobilization of fatty acids occurs (at least 1 h). Initially, low intensity exercise should be given to increase aerobic capacity and stimulate bone remodeling. Once preliminary aerobic training is completed, an appropriate training speed is 80–100% of speeds in endurance races.


A characteristic of elite endurance athletes is that lactate does not begin to accumulate in the blood until the intensity approaches that generating maximal heart rates. Training intensities that generate lactate can shift the lactate accumulation point closer to the maximum heart rate. All endurance horses should receive some anaerobic training. Many endurance riders are reluctant to allow their horses to canter or gallop, as they believe this may teach them bad habits for the race. If the trainer is reluctant to increase the speed of exercise, beneficial results may be obtained by working the horses at lower speeds up hills or on inclined treadmills. Treadmill training is a regular feature of many top class human athletes.


One to two weeks before an event, the horse should be given one training session where at least half the distance of each race leg is covered. The greater the distance covered in this session, the greater the benefits to performance. During the week prior to the event, training should be tapered to ensure that intramuscular glycogen stores are maximized.






INTERVAL TRAINING


Interval training has achieved widespread acceptance with Standardbred trainers, and is commonly used by human athletes. The aim of interval training is to increase the volume of work done in an individual training session. In order to achieve this, exercise is provided in bouts, with a recovery period between each bout.


There are five factors to be considered in designing an interval training program. These are:



The final program depends on the goal of training. Where the anaerobic system is the target, intervals are generally conducted at 10% below race speed. Recovery between bouts does not need to be complete. Heart rate monitors are commonly used to measure recovery, with a heart rate of approximately 120bpm the goal. The distance of each interval does not appear to be particularly important. Where enhancement of sprint capacity is the aim of training, the horse must be fully recovered before a heat is commenced. Trotting a horse between exercise bouts will result in a more rapid fall in blood lactate concentration than walking or cantering.


Flexible interval training programs tailored to meet individual requirements can produce good results. However, repeated high intensity interval training can also cause overtraining in racehorses, with weight loss and loss of performance.



OVERTRAINING


Overtraining can be defined as an imbalance between training and rec-overy. Overtrained animals appear fatigued and stale. Their performance deteriorates and they may lose weight. Short-term overtraining can be corrected by rest for a period of days to weeks. Where the severity of the overtraining is greater, a recovery period of several months is required.


Overtraining appears to be associated with dysfunction in the neuroendocrine system. The blood cortisol response to intense exercise is reduced in overtrained horses. The syndrome is not associated with exhaustion of the adrenal glands (q.v.). Rather, there is a downregulation of the hypothalamic response (q.v.) to the exercise stimulus. The syndrome cannot be diagnosed by routine hematology or biochemistry in resting horses.



HEMATOLOGY OF HORSES IN TRAINING



INTRODUCTION


Routine blood testing is used by trainers in a variety of ways. Many of the claims of the usefulness of blood tests have been overrated. Blood testing can be used routinely to check the health status of the animals in a stable. However, this becomes expensive where large numbers of horses are involved, and the costs of such a program may well exceed any benefits that are gained.


Blood tests should be considered as a diagnostic aid in evaluating loss of performance, as they can be used to identify horses suffering from subacute diseases that may impair performance. In addition to the direct loss of performance, the stresses associated with racing may exacerbate a pre-existing condition, with the result that far more severe problems develop. This is particularly true of horses with respiratory disease.


Many trainers use routine blood testing in the belief that it will enable them to assess the fitness of their horses and estimate their performance in competitive events. Although certain hematologic changes do occur as a result of training, hematologic evaluation cannot be used as a guide to fitness, or to identify horses that will win races. In a comprehensive study of Thoroughbred horses conducted in Australia in the late 1970s, attempts were made to correlate the pre-race hemogram with subsequent performance. From the results it was apparent that no relationship existed between the hemogram and subsequent racing performance. Similar results have been found in a subsequent study of Standardbreds.


The usefulness of hematologic evaluation as an indicator of fitness is limited by the variability in the results obtained. The sources of variability are: the state of the horse; the handling of the sample; variation in the normal values recorded in different laboratories; and individual variations between horses. Erythrocytes, as a step in the oxygen transport chain, represent a potential limitation to performance. However, many other factors affect performance in a healthy horse, and the blood cannot be considered the key limiting factor. Transport of horses can have a significant effect on their blood indices.



RED BLOOD CELLS


The horse holds a large store of erythrocytes in its spleen. Under stimulation of the sympathetic nervous system, the spleen contracts, injecting the stored cells into the circulation and resulting in an increase in the hematocrit. Splenic contraction occurs in anticipation of or during exercise. However, changes in the hematocrit (q.v.) are minimal following endurance exercise unless dehydration has occurred. Hematocrit values >0.55L/L, and plasma protein concentrations >90g/L in endurance horses are signs of potentially dangerous fluid loss.


The act of blood sampling may be sufficient to cause mobilization of the splenic erythrocyte pool in some nervous horses. This can result in large increases in the hematocrit. Because the temperament of the animal is a variable that cannot be controlled, comparison between horses becomes meaningless.


In order to minimize the variation in results obtained in multiple tests conducted on the one animal, conditions of sample collection should be standardized. Minimal restraint should be used, and the same person should collect the sample on each occasion. The sample should be collected before the horse is fed or exercised. Feeding is associated with an increase in hematocrit and plasma protein concentration, and a fall in serum potassium concentrations.


Stallions tend to have a higher hematocrit than geldings or mares. The hematocrit in Thoroughbreds is usually higher than in other breeds. Red cell numbers increase as a horse becomes fitter, and as it ages. In general, training results in an increase in erythrocyte numbers and total blood volume. However, changes in horses undergoing endurance training are minimal. Species and state of training must be considered when analyzing erythrocyte values.


Poor performance has been reported in racehorses with a resting hematocrit <0.36L/L. If abnormally low red cell indices or hemoglobin concentration are suspected, collection of a blood sample within 5 min of a fast gallop should be used to confirm the condition. Normal hematocrit after a fast gallop is in the range 55–70L/L.


In horses used for endurance exercise, the lower limit of normality may be considered to be 0.30L/L. Resting hemograms are a useful aid in the detection of anemic horses. In such animals, the hematocrit is lower than the lower limit of normality at the laboratory conducting the analysis. In addition to poor performance, signs include pale mucous membranes and a rough coat.


Two common causes of anemia (q.v.) in horses are chronic inflammation, which causes bone marrow suppression, and a heavy parasitic burden (q.v.), which causes increased blood loss. Subclinical infections, which have the potential to impair performance, may cause bone marrow suppression. Because reticulocytes do not appear in the bloodstream, a bone marrow biopsy (q.v.) is required to determine whether anemia is regenerative or non-regenerative. Anemia is rarely due to a primary iron deficiency, and iron supplements should not be considered a treatment for the condition. The animal must be examined to determine the underlying cause. Iron supplementation has been associated with severe phlebitis when injections are incorrectly administered. Fatalities have also been reported to occur following iron injections. Erythrocytes require 5–7 days to mature. It is therefore very optimistic to expect that any treatment for anemia is going to help the performance of the horse in a race two days later.


It has long been argued that polycythemia (q.v.) is the mechanism or hallmark of overtraining syndrome in Swedish Standardbred trotters. However, a longitudinal study of overtraining that included a control group failed to confirm that overtraining causes red cell hypervolemia.


If the resting hematocrit is in the normal range, no further interpretation can be made from the results. There is no relationship between resting hematocrit and the post-exercise hematocrit, which can be 55–70L/L.

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Performance-related problems and exercise physiology

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