During anaerobic exercise, lactate production is the most limiting factor of performance. Low oxidative muscle fibers do not contain a great concentration of lactate dehydrogenase, the enzyme that converts lactate to a usable form. Muscle is unable to continue working in the presence of a high level of lactic acid and, therefore, fatigue occurs. Lactate decreases the pH of the muscle (acidosis), resulting in a decreased ability to use carbohydrates for ATP production. It has been postulated that the decrease in muscle pH due to lactate accumulation also decreases the muscle’s ability to uptake calcium for contraction.

Heat is a by-product of the metabolism of fuels and is also a cause of fatigue in the equine athlete. As muscle temperature rises, certain enzymes that are used in the metabolism of fuels are unable to function correctly. High muscle temperature, along with decreased muscle pH, has also been related to the decreased ability of the muscle to uptake calcium for contraction.

Environmental conditions play an important role in the performance of a horse. It has a large effect on the ability of a horse to dissipate heat produced during exercise. Loss of heat by convection and radiation depend on a temperature difference between the skin and the air. When environmental temperature is low (10ºC), convection and radiation are able to dissipate a large amount of heat produced; however, when environmental temperature increases (36ºC), convection and radiation are ineffective, and the horse must rely on sweat as its primary method of heat loss. Humidity also limits the amount of heat that can be dissipated from the body. High humidity decreases the ability of a horse to sweat because the water vapor content in the air is high. In conditions of high heat and humidity, horses are severely limited in their ability to dissipate heat, and precautions must be taken to prevent dangerous elevations in body temperature.

Muscle glycogen depletion is not a factor in fatigue of horses working at high intensities, but it can affect horses working at prolonged, low intensities. During submaximal exercise, free fatty acids and glycogen are the major fuels for energy. When glycogen is depleted from the liver and working muscles, ATP production decreases and fatigue sets in. Although fat stores are not depleted, they cannot be metabolized without carbohydrates.