• Phase 1: Endurance training at speeds less than 600 meters per minute (m/min)

• Phase 2: Combined aerobic and anaerobic training at 70% to 80% of maximal speed (750 to 850 m/min)

• Phase 3: Anaerobic training for development of speed and acceleration

Phase 1: Endurance training

Endurance exercise at the start of any Thoroughbred training program is vital for the racing future of the horse. It involves exercise at slow speeds (trot and canter at speeds up to 600 m/min) over long distances. The distances used vary greatly among trainers.

Such exercise rapidly increases maximal oxygen consumption (O_2max). Within 2 to 6 weeks, 10% to 23% increases in O_2max occur. However, it is likely that prolonged endurance training over many months will result in gradual improvement in the maximal aerobic capacity of the horse, as in humans.

The other important role of the initial weeks of endurance training is the development of strength in bone and soft tissue in the limbs, that is, the response to Wolf’s law. This will result in remodeling of all of the major bones, ligaments, and tendons used in locomotion and support of the animal during racing. However, little is known about the durations or intensities of exercise that promote optimal adaptive responses in bone or soft tissue (see Chapter 13). A 14-week period of treadmill training has demonstrated the effects of training on third metacarpus bone quality in 13- to 14-month-old Thoroughbred horses. The training did not produce any clinical signs of bucked shins but did result in several important adaptive responses in the cortices of the metacarpal bones of the young horses compared with unexercised controls. These included increased subcortical bone, bone mineral content, and bone stiffness.

It is likely that the greatest adaptation will occur if the “overload” principle is followed. This principle is relevant to the adaptation of many body systems in horses in training. It refers to the necessity for a gradual increase in the training stimulus every 2 to 3 weeks. This period gives time for adaptation to the current training demands before increasing the stress of the training to induce further adaptations. Many of the problems found in Thoroughbred training, such as bucked shins and periodic inappetence, are probably related to rapid increases in the intensity of training such that time for adaptation has been insufficient.

Endurance exercise training of Thoroughbreds can be conducted in many ways besides the traditional use of a jockey. Treadmill exercise at the trot and canter, jogging horses behind trucks or beside horses in carts, and swimming have been used successfully. It is important not to rely exclusively on exercise without using a jockey, especially in young horses, since this period of training is as important for development of the horse’s behavior as it is for the physical adaptations to training.

Swimming should be used sparingly in endurance training of Thoroughbreds. It does promote cardiovascular fitness but probably does not develop limb strength or gait coordination. Frequent use of swimming also breaks one of the major rules of training, that is, specificity. Specificity refers to the need for training to mimic the gait that is employed in competition so that structural changes in the limb are appropriate to the stresses of competitive events. Although a disadvantage in training, the reduced limb load bearing in swimming is an advantage for rehabilitation programs.

Typical slow-speed training in Australia involves only 4 to 5 weeks of training at the trot and canter over 3 to 5 km (2 to 3 miles) per day before moving on to faster exercise at greater than 20 seconds per furlong. Thoroughbred trainers in England tend to employ greater durations of slow exercise over a period of 3 months or more, especially in 2-year-old horses. Subjective assessments indicate that English 2-year-olds have a lower incidence of bucked shins compared with their Australian or North American counterparts. Rapid preparation of 2-year-olds in Australia has been identified as an important contributing factor in the incidence of bucked shins and other joint ailments.

It is not possible to make specific recommendations about the duration of the trotting and cantering endurance training of Thoroughbreds. Generally, most Thoroughbreds can canter about 10,000 meters at about 500 m/min in one bout 6 days per week after a suitable 3- to 5-month prior training and adaptation period. More prolonged endurance training also has been recommended. An obvious balance has to be struck between the likely adaptive advantages and the financial cost of prolonged training periods. If conditions are hot and humid, it is appropriate to break up the endurance training into 10- to 15-minute sessions if relatively high-speed cantering is being used. This will obviate the risk of exhaustion caused by hyperthermia or heat stroke.

Slow-speed training has been subdivided into a slow, long-distance training phase and a phase at slightly faster speeds termed cardiovascular fitness work to improve the oxygen transport system. This division is entirely arbitrary and varies according to trainer, horse, track, and climate. This seems not be of great consequence, as no differences have been demonstrated in the degree or rate of change in cardiovascular fitness during training at different speeds of submaximal exercise. However, the recommendation that a prolonged period of exercise at speeds up to about half pace (800 m/min) with gradual increases in the stress of training every few weeks is likely to be physiologically appropriate.