Most will agree that thorough preparation of a horse is key to success in athletic competition. With spring hopefully just around the corner, this is an opportune time to discuss conditioning. Of course, the nature of a conditioning program will vary greatly, depending on the goal in mind and the length of lay-up prior to the start of training. Top-level athletes often are in training year-round and, therefore, maintain a high degree of fitness. Similarly, for a horse owner in warm climates (Southern United States), a regular program of training and competition might be maintained year-round. However, for horses located in cooler climates, winter often means a substantial decrease in activity level and an inevitable loss of fitness. So, as spring rolls around, you need to develop a plan for bringing the horse back to fitness.

Even for horses used strictly for pleasure trail rides and the like, the transition from a sedentary lifestyle to one of moderate activity can pose problems. Asking the horse to do too much, too soon, can spell trouble. In particular, horses which have had little in the way of regular exercise might be at greater risk for injury compared to their trained counterparts.

Often, the late winter/early spring period also is the time when young horses are first introduced to formal exercise training. Again, the major concern is that this initial period of formal conditioning should not place the horse at increased risk for musculoskeletal injury.

Training involves a combination of physical conditioning and task-specific schooling (i.e., schooling in the various tasks required of a specific event or discipline). The process of physical conditioning (or exercise training) refers to the physiological adaptations associated with a period of regular exercise. That is, a horse is taken from an "unfit" to a "fit" state.

A discussion of schooling techniques for the various equine disciplines is beyond the scope of this article. Rather, we will focus on how the horse's body adapts to the rigors of regular exercise, with particular attention to the time course of these adaptations. The next article (in the March issue) will consider 1) how different exercise protocols (e.g., slow vs. fast work) alter the magnitude of these training responses, and 2) assessment of fitness and application of fitness measurements to horses in training.

Training Principles

There are several important principles of physical conditioning. The first is the progressive loading principle. For a conditioning or training response to occur, the horse must be subjected to an exercise load. Exercising at a level higher than that normally performed will induce a number of specific adaptations that, in combination, enable the horse to function more efficiently. A given amount of exercise (e.g., 15 minutes of trotting per day) will result in a certain level of fitness. However, without a further increase in training load (an increase in training duration, intensity, or both), there will be no further increase in fitness. Therefore, training requires a progressive loading--each new level of training is maintained until the body has adapted to the added stress, after which a further increase in training load can be applied. Other horses can continue to gain fitness at a given exercise level for quite some time. In other words, a horse could begin to gallop one mile per day and gain fitness daily for at least 30 days, or even longer. At the end of the 30 days, the trainer has to increase the work because the horse feels so good. On the other hand, excessive overload or rapid increases in training load over a short period of time inevitably will result in failure of one or more body systems. The worst-case scenario involves excessive overloading of the supporting structures of the limbs (bones, cartilage, ligaments, and tendons). Overloading of these structures will manifest as injuries such as fractures and strains of ligaments and tendons.

The second principle is that of specificity--for any equine discipline, performance is most effectively improved by training the specific muscles and systems involved in that discipline. In other words, training must be focused on the specific demands of the competition exercise. Sprint training that involves a series of short, intense work efforts will not provide an appropriate training base for horses competing in endurance rides. Similarly, long duration, low intensity exercise, an important component of endurance horse training, does not adequately prepare a racehorse for an all-out effort over distances of six furlongs to one mile.

Another important principle is that of individual differences. For a given training program, there will be wide variations in the training response of an individual horse. Some horses will respond more quickly than others and will better tolerate faster increases in training load. The magnitude of the overall training response also will vary among horses.

Genetic factors play a major role in this variation in training response, but another consideration is the state of fitness at the beginning of a training program. A horse which has been inactive for a long time (12 months or more) will require a longer period of training to reach a certain level of fitness compared to a horse which has had a six- or eight-week layoff after a season of training and competition. Training programs must be individualized in order to attain maximum benefit while minimizing the risk of injury.

The beneficial effects of exercise training are reversible. Detraining refers to the reversal of training adaptations when a program of regular exercise is discontinued. In some cases, this period of detraining is planned, such as when the horse enters a lay-up period at the end of the competition season. In other circumstances, training programs are interrupted by injury or illness.

Training Adaptations

All training programs have three general goals in mind:

1) Improvements in energy transfer--greater efficiency in the processes involved in the conversion of stored chemical energy to mechanical energy (refer to the Sports Medicine article in the January 2000 issue);

2) Structural adaptation--for example, strengthening of the horse's supporting structures (bone, tendons, and ligaments) so that there is lower risk of injury under the stresses and strains of exercise;

3) Improvements in coordination and skill level.

Specific Adaptations

Let's now consider the specific adaptations that occur during an exercise training program. In general terms, adaptations of five major systems can be expected:

1) Cardiovascular--improved capacity to deliver oxygen to the working tissues;

2) Metabolic machinery in skeletal musclE--improved capacity to utilize oxygen and greater efficiency of fuel utilization;

3) Supporting structures (bone, tendon, ligaments, muscle)--an increase in the size and/or strength of these structures;

4) Temperature regulating system--greater ability to lose body heat during exercise, thus avoiding excessive increases in body

5) Central nervous system--improvements in neuromuscular coordination, which means the horse is better able to complete the skills required for its particular discipline. From a psychological viewpoint, the horse also becomes adapted to the routines associated with training and competitions.

Aerobic Capacity

A basic level of aerobic fitness is a vital foundation for all forms of exercise. Aerobic fitness or capacity refers to the maximum rate of oxygen consumption (the abbreviation for the rate of oxygen consumption is VO2max, while maximum aerobic capacity is termed VO2max). The VO2max is a measure of the function of the respiratory and cardiovascular systems and involves a series of steps that culminate in the utilization of oxygen in the tissues. These steps include the diffusion of oxygen across the lung surface, loading of oxygen onto hemoglobin in red blood cells, delivery of oxygen to the peripheral tissues via the cardiovascular system, extraction of oxygen from the blood, and use of oxygen by the metabolic machinery within working tissues (see figure on page 68).

A number of these oxygen transfer steps undergo adaptation during training. Together, these adaptations result in a substantial increase in VO2max. Within two to three weeks of the start of a regular program of exercise, there is an increase in blood volume (due to increases in the number of red blood cells and the volume of plasma, the non-cellular component of blood). These increments in red cell number and total blood volume result in an increase in oxygen-carrying capacity; i.e., more oxygen can be transported in the blood to the working tissues. Over a longer period of training (three to six months), there is an increase in the number of small blood vessels within skeletal muscle--this improves the efficiency of oxygen extraction from blood, thus providing more oxygen for the tissues.

Relatively little is known about changes in heart size during athletic training in horses. However, a recent study in Britain demonstrated a significant enlargement of the left ventricle of 2-year-old Thoroughbreds after four months of conventional race training. Similar increments in heart size occur in other species and, together with an increase in blood volume, contribute to an increase in maximum cardiac output during exercise. These adaptations also contribute to the overall increase in VO2max with training.

A further cardiovascular adaptation to training will be apparent during submaximal exercise (trotting and cantering) and during recovery from exercise. Most studies have shown a reduction in heart rate (up to 10 beats per minute) during submaximal exercise after training. Recovery of heart rate following exercise also is faster in well-trained horses, particularly endurance athletes. Thus, monitoring of heart rate during and after exercise is an important tool in the assessment of fitness. (Next month's Sports Medicine column will further explore this issue.)

A big component of the increase in VO2max associated with training is an increase in the oxidative capacity of muscle--per unit time, the working tissues of the body are able to utilize more oxygen. Aerobic metabolism of fuel stores (glycogen and fat) occurs in small structures called mitochondria that are located within muscle fibers. Exercise training results in increases in the size and number of mitochondria within working skeletal muscle. Concurrently, there is an increase in the quantity of enzymes involved in the chemical reactions that drive the aerobic metabolism of body fuels.

This increase in muscle oxidative capacity has two very important implications. First, there is a more efficient utilization of fuel substrates. During submaximal work, there is an increase in the amount of fat utilized with a corresponding decrease in the utilization of blood glucose and muscle glycogen. In this way, the body's limited carbohydrate reserves are spared, allowing the horse to exercise for a longer period of time before development of fatigue. Second, the increase in oxidative capacity allows the horse to sustain a higher workrate during prolonged exercise without buildup of lactate in muscle.

How quickly does VO2max increase during training, and what is the magnitude of the increase? Surprisingly, there is a substantial increase in VO2max within the first few weeks of training. In one recent study in Thoroughbred horses, there was a 9% increase in VO2max after 10 days of training. Even with longer duration training, the most substantial increase in VO2max occurs within the first six to eight weeks of training (see chart on page 74). The early changes in aerobic capacity are related to increases in oxygen-carrying capacity of the cardiovascular system, whereas the later alterations are more due to an increase in muscle oxidative capacity, in particular the increase in mitochondrial volume.

With a progressive increase in training load, increases in VO2max of up to 30% have been measured. Such increases in VO2max will confer a marked increase in overall work capacity--the horse is able to sustain a higher running speed for a longer period of time.

Less well known is the timecourse of the decrease in VO2max during detraining. Some studies have reported that two to four weeks of detraining reduces VO2max to values near those before training. However, the speed of this decrease during detraining might be influenced by the duration of training. Following an eight-month period of intensive exercise training in Standardbreds, VO2max decreased very slowly during detraining; after 12 weeks of inactivity, values still were 15% above those measured before training. These observations have implications for management of horses which have their training interrupted by illness or injury. For horses which have been in training for long periods, aerobic capacity might be maintained during an enforced lay-up (four to six weeks). Maintenance of aerobic capacity might allow those horses to attain their former level of fitness more quickly after the resumption of training. On the other hand, an enforced lay-up might weaken the supporting structures of the limbs, dictating a more gradual increase in training level.

Horses which have been trained up to a level of competition and then been given even an extensive layoff, return to fitness much faster than horse which never have reached that competitive level of fitness. This is important because most competing horses which are given an extensive layoff have had soundness problems, and their careers might be lengthened if they are not overtrained while returning to fitness.

Adaptations of Bone and Tendon

Compared to the cardiovascular and muscular systems, the supporting structures (bones, cartilage, ligaments, tendons) appear to adapt more slowly during training. If the training workload is greater than the capacity of these structures to adapt, injuries will occur. Practically speaking, the speed of adaptation of bone and other supporting structures can be regarded as the rate-limiting step in the preparation of a horse for competition. For completely untrained horses, the cardiovascular and muscular systems are well adapted to exercise within a 10-12 week period, whereas up to six months might be needed for adaptation of the supporting tissues (see chart on page 74).

Relatively few studies have examined how the bones, tendons, and ligaments of the horse's limbs adapt during training. However, several laboratories around the world are working in this area. Some studies have demonstrated increases in the density of the metacarpal (cannon bone) and third carpal bones of horses during training. However, the intensity of training has an important bearing on this response; submaximal training (trotting) results in minimal change in metacarpal mass or density, whereas training at higher speeds (galloping exercise) does result in an increase in bone density.

Bone density is an important determinant of strength. Thus, the increase in bone density is an important adaptation to the added stress of exercise training. Although the exact timecourse of these changes is unclear, recent studies have detected increases in bone density after four to five months of training.

Exceptions to bone density equated with strength can be found where bones have become sclerotic, or more dense, as a result of exercise, and the sclerotic bone is not efficient in dispersing energy. Some examples include sclerotic third carpal bones and metacarpal condyles in racehorses and sclerotic navicular bones in all horses.

Currently, an incremental training program that gradually increases the length, speed, and repetition of galloping is recommended for enhancement of bone strength. Consistent with the principle of progressive loading, initial gallops should be at low speeds (canter), with an increase in speed after periods of three to four weeks. The number of repetitions can be increased in a similar manner. For young racehorses, some degree of shin soreness ("bucked shins") is likely to occur and close monitoring is required, particularly as training gallops approach maximum speed.

Injuries to the tendons of the lower limbs (bowed tendons) are very common and an important reason for lay-up and, in many cases, retirement from athletic competition. Unfortunately, even less is known about adaptations of tendon tissue during exercise training. However, a recent series of investigations have found that the tendons of mature horses have a limited ability to respond to training and over time, repeated trauma to the tendon likely predisposes to injury. In contrast, the tendons of young horses (less than two years) strengthen in response to training. Thus, contrary to the common belief that exercise training of immature horses is detrimental, the results of these recent studies raise the possibility that early training might enhance development of the supporting structures of the limbs and perhaps reduce the incidence of injury during training and competition.

Further research is required to clarify this issue. In the meantime, injury to the structures of the lower limbs--particularly tendons and ligaments--remains a major concern for all horses in athletic training. These structures should be monitored closely during training; on a daily basis each limb should be palpated for signs of swelling, heat, and pain. While the limb is bearing weight, the suspensory ligament and deep and superficial flexor tendons should be palpated for heat and swelling. This examination can be repeated with the leg raised. If abnormalities are detected, ask your veterinarian to investigate further.

Although more studies are needed to determine the best method for restoration of glycogen reserves, it currently is recommended that the horse be fed a grain meal in the one- to two-hour period following hard exercise.

Increase Aerobic Capacity

One of the important adaptations associated with exercise training is an increase in aerobic capacity, that is, the maximum rate at which the body can consume oxygen (V.O2max). There are two main reasons for this increase in aerobic capacity. First, the capacity of the cardiovascular system to transport oxygen to working muscles is enhanced. In blood, oxygen is transported within red blood cells (bound to hemoglobin). With training, there is an increase in the number of red blood cells and thus an increase in the oxygen-carrying capacity of blood. In addition, improvements in heart function enhance delivery of oxygen to the tissues. The second reason for the improvement in V.O2max is an increase in the capacity of the muscles to consume oxygen. Within muscle, there are increases in both the number of mitochondria (where oxygen is consumed) and the activities of enzymes that drive energy-generating biochemical reactions. This improvement in aerobic power lessens the reliance on anaerobic metabolism for energy and improves the efficiency of fuel use during exercise.

Overtraining: Can Your Horse Get Too Much of A Good Thing

There can be a delicate balance between the level of training required to reach and maintain peak fitness and "pushing the envelope" too far, with a resultant decline in performance. Human athletes in heavy training, particularly those involved in endurance events, can experience a syndrome of overtraining or "staleness." Symptoms of this overtraining syndrome include mood changes (depression, irritability, general malaise), chronic fatigue, loss of interest in training, persistent poor performance, and increased susceptibility to infections. The precise cause of this syndrome is unknown, but it occurs more commonly in athletes engaged in very high volume training. The symptoms of overtraining persist unless the affected athlete rests, and recovery might require weeks or months.

A similar syndrome has been reported in horses. Researchers in Sweden long have recognized overtraining syndrome as a major cause of poor performance in Standardbred trotters. Signs of overtraining in those horses include poor race performance, increased heart rate and blood lactate concentrations during standardized exercise tests, unwillingness to train, weight loss, and poor appetite. In addition, heart rate might be slower to return to resting levels following a bout of exercise. As in humans, the cause of this syndrome is not known, but its onset often follows a sharp increase in the amount of fast work training.

Whether overtraining syndrome occurs in other horse populations is less certain. However, racehorse trainers all over the world also have described behavioral ("staleness") and performance alterations in some horses which have been ascribed to overtraining. Recently, researchers at the University of Sydney reported on a study in which a state of overtraining was induced in a group of Standardbred horses. Thirteen Standardbred horses were trained over a 34-week period, comprising three phases: 1) endurance training (seven weeks); 2) high-intensity training (nine weeks); and 3) overload training (18 weeks).

Before the start of Phase 3 (overload training), the horses were divided into two groups: overload training and control. The overload training group was exercised at greater intensity, frequency, and duration compared to the control group. For example, three days per week (the horses were trained six days per week), the overload trained horses completed 10 to 15 half-mile interval workouts at a speed greater than each horse's maximum aerobic capacity (V.O2max), for a total distance of about six miles. On the other training days, the horses galloped at lower intensity (approximately 85% of V.O2max) for 3 to 4 miles. In contrast, the control group horses performed a fewer number of intervals (2 to 3) at a reduced intensity (100% V.O2max) and the duration and intensity of exercise were also lower on days of moderate intensity training.

Signs of overtraining syndrome developed after 31 weeks of training (or 15 weeks after the start of overload training). These included a reduction in body weight (4%-5%) without a decrease in feed intake, a 10% to 20% decrease in run time during an incremental treadmill exercise test, and a variety of behavioral disturbances, particularly reduced willingness to train. The signs of overtraining persisted after two weeks of a greatly reduced training load, indicating that these manifestations were due to overtraining rather than "overreaching" (poor performance due to fatigue and insufficient rest periods between workouts). Unfortunately, there were no consistent blood markers of overtraining in this group of horses.

Although the training regimen required to induce a state of overtraining was quite severe, the results of this study confirm that overtraining can occur in horses. Importantly, there is likely to be a great deal of individual variability in the susceptibility to overtraining; one horse might be able to maintain or improve performance under rigorous training, whereas another horse might develop overtraining in response to a similar or even lesser amount of work.

A very important aspect of overtraining is the cumulative effects of training on soundness. We now know that most injuries are the result of cumulative stresses rather than a single incident. Racehorses, sport horses, endurance horses, and performance horses of all kinds suffer overuse injuries. These must be detected early to prevent career-threatening conditions.

Training programs must be tailored to the individual horse and adjusted if problem signs develop. Once a horse reaches a certain level of fitness, the frequency and duration of exercise required to maintain that level of fitness are much less than that required for its improvement. A common sense approach is vitally important; horses must be given adequate rest (either none or only light exercise) between hard training and competition exercise bouts. The horse's mental attitude, both at rest and associated with exercise, should be closely observed. Ideally, the horse's feed intake and body weight should be monitored throughout training. This approach will avoid overtraining and lessen the risk of injury.


Conditioning Sport Horses, by Hilary M. Clayton. Sport Horse Publications, Saskatoon, Saskatchewan, Canada, 1991.

The Athletic Horse: Principles and Practice of Equine Sports Medicine, by David R. Hodgson and Reuben J. Rose. W.B. Saunders Company, Philadelphia, PA, 1994.

R.K. Smith, H. Birch, J. Patterson-Kane, et al. Should equine athletes commence training during skeletal development?: changes in tendon matrix associated with development, aging, function, and exercise. Equine Veterinary Journal Supplement 30 (1999): 201-209.

C. M. Tyler, L. C. Golland, D. L. Evans, et al. Changes in maximum oxygen uptake during prolonged training, overtraining, and detraining in horses. Journal of Applied Physiology 81 (1996): 2244-2249.

C.M. Tyler-McGowan, L.C. Golland, D.L. Evans, et al. Haematological and biochemical responses to training and overtraining. Equine Veterinary Journal Supplement 30 (1999): 621-625.

R.J. Geor, L.J. McCutcheon, H. Shen. Muscular and metabolic responses to short-term moderate intensity training in Thoroughbred horses. Equine Veterinary Journal Supplement 30 (1999): 311-317.

About the Author

Ray Geor, BVSc, PhD, Dipl. ACVIM

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

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