Exercise requires an integration of many systems, each containing varied elements, and any factor that upsets this integration could cause fatigue. Horse owners often have trouble identifying fatigue because its multidimensional nature varies with activity, training and physiological status of the individual, and environmental conditions.
The onset of fatigue is most often associated with either the accumulation of metabolic by-products or a decline in muscle glycogen concentration.
Key factors causing fatigue in humans parallel those factors causing fatigue in horses. In both human and horse endurance events, heat stress and substrate availability have been indicated as key factors in fatigue. In the horse, substantial depletion of muscle glycogen has been reported during endurance rides, supporting the idea that substrate availability may play a key role in fatigue.
Kathleen Crandell, Ph.D., an equine nutritionist on the staff of Kentucky Equine Research, explained that changes in muscle lactate and pH are also unlikely to be the cause of fatigue in endurance events. Even though some metabolic acidosis occurs, the changes are unlikely to be of sufficient magnitude to induce fatigue. Heat stress, however, has been shown to play a role in reduced performance in such events.
In sprinting events such as Thoroughbred racing, the onset of fatigue is most likely related to changes in the chemistry of the muscle cell. Lactate concentrations rise following intense exercise, and the resultant fall in cell pH affects both energy production and excitation contraction in the muscle.
All heavily exercised horses can become dehydrated as they lose significant amounts of water and electrolytes through sweat. Electrolytes are responsible for maintenance of acid-base balance and osmotic regulation of body fluids. Without electrolytes, the body is not capable of maintaining the right amount of fluid in and around cells. Electrolytes also play key roles in transmission of nerve impulses and muscle contraction. Horses with insufficient electrolytes risk loss of stamina and early onset of fatigue. To combat this, Kentucky Equine Research recommends the use of Restore SR.