Energy and the Performance HorseBy Dr. Joe Pagan · December 10, 2012
Energy is a measure of a feed’s potential to fuel body functions and exercise. Various pathways and substrates are used by the horse to produce a chemical intermediate that fuels muscle contraction during exercise and depends on the intensity and duration of the exercise.
The main productive function in horses—racehorses, draft horses, trail horses—is work. The basic driving force behind the various types of equine performance is the conversion of chemically bound energy from feed into mechanical energy for muscular movement.
Because horses do not eat continuously while they exercise, feed energy must be stored in the horse’s body for later release. The horse can utilize a number of different storage forms including intramuscular glycogen and triglycerides as well as extramuscular stores such as adipose tissue and liver glycogen. Many factors determine the proportion of energy derived from each storage form including speed and duration of work, feed, fitness, muscle fiber composition, and age of the horse.
Work capacity depends on the rate at which energy is supplied to and used by muscles for contraction. A molecule called adenosine triphosphate (ATP) is used to produce muscular activity. The most direct way to form ATP is by the cleavage of another compound, creatine phosphate (CP). However, since muscle contains only small amounts of CP and ATP, the supplies are exhausted after a short duration of exercise. Prolonged exercise would not be possible without a way for ATP to be resynthesized at the same rate at which it is used. Two fundamental reactions resynthesize ATP:
(1) Oxidative phosphorylation breaks down carbohydrates, fats, and protein into energy (ATP) with the involvement of oxygen. The use of oxygen qualifies this as an aerobic reaction.
(2) Glycolysis breaks down glucose or glycogen into lactic acid. This reaction doesn’t use oxygen and is considered anaerobic.
Large quantities of energy can be derived from the utilization of intramuscular (triglyceride and glycogen) and extracellular (free fatty acids from adipose and glucose from the liver) fuels.
The horse has three basic types of muscle fiber: I, IIA, and IIB. These fiber types have different contractile and metabolic characteristics. Type I fibers are slow-contracting fibers, while types IIA and IIB are fast-contracting. The type I and IIA fibers have a high oxidative capacity and can thus utilize fuels aerobically, while type IIB fibers have a low aerobic capacity and tend to depend on anaerobic glycolysis for energy generation. All three fiber types are very high in glycogen while only type I and IIA have triglyceride storage.
The amount of ATP used by a muscle depends directly on how fast it is contracting. While walking, the muscles contract very slowly and expend relatively small amounts of ATP. During this type of exercise, type I fibers are primarily recruited and energy generation is entirely aerobic. At this speed, the muscle burns predominantly fat. Fat stores are plentiful and they can be mobilized and metabolized fast enough to regenerate what ATP is used at a walk.
As speed increases from a walk to a trot to a canter, type I fibers alone are no longer capable of contracting rapidly enough to propel the horse. At this point, type IIA fibers are also recruited. These fibers are also aerobic, but they use a combination of glycogen and fat for energy generation. Glycogen (or glucose) can be metabolized aerobically twice as fast as fat for ATP generation and as speed increases, fat becomes simply too slow a fuel for energy generation. As the horse increases speed to a fast gallop, type IIB fibers are recruited and energy generation no longer remains purely aerobic. Anaerobic glycolysis is the fastest metabolic pathway available to generate ATP and the horse must depend heavily on this to maintain high rates of speed. Anaerobic glycolysis results, however, in lactic acid accumulation and fatigue soon develops as the pH in the muscle begins to fall.
The endurance horse typically travels at speeds that can be maintained almost entirely through aerobic energy generation. Only during hill climbing and for short intervals is the horse’s ATP demand too great for aerobic regeneration. Fatigue in endurance horses is much more likely to result from glycogen depletion than from lactic acid production.
Racehorses, eventers, and many of the western performance horses perform at much higher intensities of exercise. These horses depend heavily on anaerobic glycolysis for energy generation, and fatigue is most likely to result from lactic acid accumulation rather than glycogen depletion.
Substrate utilization in the horse can be investigated by using biopsy techniques of both the muscle and the liver. These biopsies are safe and can be taken repeatedly to determine how much muscle glycogen is used at different intensities of work. In addition, substances in the blood and respiratory gases can be used to paint a metabolic picture of substrate utilization during various intensities of exercise.
The middle gluteal muscle is the most convenient muscle to biopsy when studying intramuscular substrate utilization. This muscle typically contains between 500 and 700 millimoles (mmol) of glycogen per kilogram (kg) of dry weight. During endurance exercise (7.5 to 11.5 mph), horses will typically use muscle glycogen at a rate from 0.5 to 1.5 mmol/kg/min. The remainder of the energy generated at this rate of speed comes from fat oxidation. As speed increases, muscle glycogen utilization increases. At a speed of around 650 meters/min (a 2:25 mile), ATP production can no longer be completely satisfied by aerobic pathways. At this point, anaerobic pathways become an important source of energy.
As the horse crosses the anaerobic threshold and shifts to this type of energy production, the use of glycogen and the accumulation of lactic acid increase exponentially. The reason for this increase is that anaerobic glycogen utilization is 12 times less efficient than aerobic glycogen utilization. When glycogen is metabolized aerobically, 36 ATPs are produced, but when glycogen is metabolized anaerobically, only three ATPs are produced and two molecules of lactate are also produced. These factors explain why horses can perform at a slow, steady pace for much longer than they can sustain maximal exercise.