Muscular Fatigue and Exercise in Horses

The decline in a muscle’s ability to generate contractile force, preventing the horse from maintaining exercise intensity. 

• Physiological Role: It acts as an intrinsic safety mechanism.

• Protection: Without fatigue, structural damage could occur to the myocytes (muscle cells) and supportive tissues.

Influencing Factors

• Environmental: Ambient temperature (high heat increases risk).

• Physiological:

  • Hydration Status: Dehydration accelerates fatigue.

  • Electrolytes: Imbalances in serum electrolyte concentrations.

  • Metabolic Byproducts: Accumulation of lactate, ADP, and inorganic phosphate.

  • Substrate Depletion: Depletion of muscle glycogen and failure of ATP to resynthesize.

• Psychological/External:

  • The horse's desire to work and external motivators.

  • Pain and perception of exertion.

Types of Fatigue

• Peripheral Fatigue (Muscle-Level):

  • Cause: Secondary to altered muscle function and the failure of ATP to resynthesize.

  • Clinical Biomarkers: Lactate, ammonia, hypoxanthine, and inflammatory mediators (IL-1, TNF-alpha).

  • Cellular Changes: Intracellular acidosis and oxidative damage.

• Central Fatigue (CNS-Level):

  • Cause: Alteration in signals from the Central Nervous System (CNS) that modify action potential frequency in motor neurons.

  • Triggers: Hypoglycemia, hyperthermia, ammonia accumulation, and changes in amino acid metabolism or serotonin levels.

  • Clinical Signs: Decreased motivation, lethargy, and loss of muscle coordination.

Clinical Markers & Diagnosis:

• Direct Measures: Muscle biopsies to check glycogen, ATP, ADP, and pH levels.

• Indirect Markers: Serum levels of lactate, lymphocytes, and markers of oxidative damage like glutathione peroxidase.

• Metabolomics: Identifying distinct differences in plasma metabolites between horses that successfully complete endurance races and those that do not.

Management & Treatment

• Immediate Action: Rest and cooling strategies.

• Fluid Therapy: Rehydration and the restoration of normal serum electrolyte concentrations.

• Conditioning: Monitoring mRNA expression in muscle tissue to track adaptations in glucose and fatty acid utilization.