Fitness for Your Horses

Whether your horse is competing at high levels or just being used for the occasional trail ride, it must have a certain level of fitness to perform well and endure the activity without injury. Asking the horse to do too much, too soon, can spell trouble. This is especially true for those pasture potatoes that have had little in the way of regular exercise, and are then suddenly expected to carry their equally unfit owner on a two-hour trail ride. Regardless of the discipline the horse is used for, they should be gradually adapted to greater workloads over time. But how do you know when your horse is fit enough?

Top-level athletes are put into rigorous training programs and are often in training year-round. However, the training program of most horses is usually interrupted. In Alberta, winter often means a substantial decrease in activity level because adverse weather may prohibit riding. Other horses may be given time off after the show season. Injuries may also require a period of lay-up while the horse recuperates. The question then becomes, how much fitness is lost and how fast does it disappear?

Training involves a combination of physical conditioning and task-specific schooling (i.e., schooling in the various tasks required of a specific event or competition). A discussion of schooling techniques for the various equine disciplines is beyond the scope of this paper. 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. This paper will also cover the principles of conditioning used to obtain and maintain fitness.

Basic Energetics of Exercise

In order to understand the adaptations that occur with physical conditioning, we must first understand the energy-generating processes involved in muscle contraction during exercise. An appreciation of these processes will also help you to design an appropriate training program for a particular event.

The immediate source of energy for muscle contraction is adenosine triphosphate (ATP). The energy released when a phosphate bond is cleaved from ATP is used directly by the contractile mechanism in the muscle. However, the concentration of ATP in skeletal muscle is very limited. If muscle contraction is to continue for more than a second or two, then ATP must be resynthesized. The replenishment of ATP is achieved by two distinct processes: 1) Anaerobic and 2) Aerobic mechanisms.

The generation of ATP by anaerobic processes occurs in the absence of oxygen. ATP is resynthesized anaerobically in the muscle from creatine phosphate or from carbohydrate, such as blood glucose or muscle glycogen. Breakdown of carbohydrate by anaerobic mechanisms is known as glycolysis, and results in the production of not only energy, but also lactic acid.

In contrast to anaerobic metabolism, ATP generated by aerobic mechanisms requires oxygen provided by blood circulation through the muscles. Carbohydrates and fats serve as the primary fuels for aerobic energy production. Carbohydrate sources include blood glucose and muscle glycogen. Sources of fat include fatty acids released from the adipose tissue, as well as triglyceride stores within the muscle. Another aerobic energy source is protein. However, the break down of proteins for energy is very inefficient and, therefore, does not contribute greatly to energy production during exercise.

Energy is generated more efficiently with aerobic metabolism. The net yield of ATP by aerobic metabolism is 36 ATP for each glucose molecule, whereas anaerobic metabolism of glucose produces only 2 ATP. Even more impressive, aerobic metabolism of a single fatty acid yields 138 ATP. Fats cannot be used as an energy source by anaerobic metabolism because the breakdown of fat requires oxygen.

The greatest advantage of anaerobic metabolism is that it is quite rapid, with glycolysis reaching peak energy production in about 30 seconds. By comparison, aerobic metabolism of substrates is a slower process because of the complexities of the reactions and the cardiovascular lag in supplying oxygen to the muscles. Nonetheless, aerobic processes are in full production within 60 seconds.

The relative contributions of aerobic and anaerobic pathways to the regeneration of ATP during exercise depend on both the intensity and duration of exercise. In general, as the intensity of the exercise increases, so does the contribution of anaerobic energy production. Conversely, as the duration of the exercise bout increases, the more muscles utilize aerobic energy. Trot and slow canter exercise on level terrain can be regarded as primarily aerobic. This means that ATP generated by aerobic metabolism can support almost all the energy demand of the exercise. There is very little contribution of either creatine phosphate or glycolysis, and the exercise may be continued for hours. At the other extreme, sprint exercise lasting less than 25 seconds, such as Quarter Horse racing and timed rodeo events, rely principally on anaerobic energy production.

It is important to remember that although one pathway may generate the majority of the energy at a given exercise intensity, both aerobic and anaerobic pathways are probably utilized in the muscle to some degree at all workloads. For example, most Thoroughbred and Standardbred races last between 100 and 200 seconds. Even though these are high-intensity events, anaerobic energy sources probably contribute less than 30% of the total energy output, leaving the majority of energy to be supplied aerobically. Submaximal events that involve intermittent bursts of activity, such as jumping and cutting, also involve significant energy production by both aerobic and anaerobic pathways.

Adaptations with Training

Five major systems are affected by an adequate period of physical conditioning:

1 Cardiovascular system - improved capacity to deliver oxygen to the working muscles.
2. Muscular system - improved capacity to utilize oxygen and more efficient 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 temperature.
5. Central nervous system - improved neuromuscular coordination, which means the horse is better able to complete the skills required for its particular discipline. All of these adaptations allow the fit horse to exercise more efficiently, as well as perform more work before fatiguing. Tired horses are more likely to take a misstep or overextend themselves; so proper conditioning may also prevent injury to muscle and supporting structures. The average amount of training needed to elicit these adaptations is presented in Table 1.