Muscular fatigue during exercise results in a decrease in the ability of muscle to produce force. Fatigue can occur during both submaximal and maximal intensities of exercise. The mechanisms of fatigue in events of different intensity and duration have been subject to much research, but no single cause of fatigue has been identified; fatigue is therefore usually referred to as multifactorial. Fatigue during exercise is a common presenting complaint in veterinary clinical practice. It can be caused by many diseases and dysfunction of many body systems, and these problems are discussed in other chapters. This chapter focuses on fatigue during exercise in healthy animals.

Fatigue is a normal consequence of exercise that is continued at high intensity or for prolonged periods of time. The decrease in force production can be regarded as a safety mechanism. If fatigue did not occur, or were delayed greatly, structural damage to muscle cells and supportive tissues could occur during intense exercise. Most knowledge concerning fatigue in animals has been described in horses because of the ease of studies in laboratories with high-speed treadmills that enable investigation of the respiratory, cardiovascular, and metabolic responses during exercise, and of collection of muscle biopsies before and after exercise. Fatigue in these studies is usually defined as inability or unwillingness of the horse to maintain the same velocity as the treadmill during tests in which velocity is increased every 1–2 min. Fatigue during intense treadmill exercise is associated with changes in gait, including increases in stride length and decreases in stride frequency.

Biomechanically, fatigue is associated with a reduction in (neuro)muscular control of joints, especially high motion distal limb joints such as the fetlock joint. Electromyography studies of fatigued muscles have shown both a reduced latency for muscular contraction and reduced force generated, leaving high load joints vulnerable to injury. Because gait and biomechanical changes may be important predisposing factors for injury to the locomotor system of horses, training of horses should avoid exercise intensities and durations that result in locomotor changes due to fatigue.

Fatigue has been classified into 2 types—peripheral and central. Peripheral fatigue has been described as fatigue due to altered muscle function. Studies of muscle metabolism after exercise have relied mainly on muscle biopsies and measurement of the concentrations of muscle glycogen and creatine phosphate, ATP, ADP, inosine monophosphate, inorganic phosphate, glycolytic intermediary products, protons, and other metabolites. The fundamental failure in muscular fatigue is failure of ATP resynthesis and accumulation of ADP and inorganic phosphate ions. Central fatigue is attributed to signals arising from the CNS, directing a decrease in performance via a change in the frequency of action potentials in motor neurons. It may occur secondary to pain, dyspnea, perceptions of exertion, hypoglycemia, hyperthermia, and altered metabolic (ammonia accumulation, altered amino acid metabolism) and/or ionic extracellular environments. However, central fatigue in response to these stimuli is probably highly variable. For example, some horses can continue endurance exercise despite severe hyperthermia, dehydration, and plasma electrolyte disturbances.

Last full review/revision July 2011 by Catherine McGowan, BVSc, MACVSc, DEIM, DECEIM, PhD, FHEA, MRCVS