Mitochondrial Adaptations to Exercise Training in Equine Skeletal Muscle: A Narrative Review

The horse represents one of the most physiologically specialized athletic mammals, capable of sustaining both high-intensity and prolonged exercise. Central to this remarkable performance capacity is the metabolic adaptability of skeletal muscle and its mitochondrial network. This narrative review synthesizes current evidence from equine, human, and rodent studies on exercise-induced mitochondrial remodeling in equine skeletal muscle. A comprehensive literature search was conducted across PubMed, Web of Science, and Scopus using terms related to equine exercise physiology, mitochondrial biology, and skeletal muscle metabolism. Preference was given to peer-reviewed original research and review articles. Mitochondria regulate oxidative phosphorylation, substrate oxidation, redox signaling, and cellular responses to metabolic stress induced by exercise. Training induces extensive mitochondrial adaptations, including mitochondrial biogenesis, remodeling of the respiratory chain, enhanced oxidative phosphorylation efficiency, and increased metabolic flexibility. These adaptations are believed to contribute to improvements in aerobic capacity, delayed fatigue onset, and enhanced recovery following exercise, although direct mechanistic evidence in horses remains limited. In equine skeletal muscle, mitochondrial plasticity is closely linked to muscle fiber composition and the distribution of oxidative and glycolytic fibers. Exercise-induced signaling pathways involving AMP-activated protein kinase (AMPK), Ca²⁺-dependent kinases, and the transcriptional coactivator PGC-1α regulate mitochondrial biogenesis and metabolic remodeling. In addition, mitochondrial dynamics, including fusion, fission, and mitophagy, maintain mitochondrial quality and functional efficiency during repeated training stimuli. Experimental studies in Thoroughbred and Standardbred horses demonstrate that training has been associated with increases in mitochondrial density and respiratory capacity in equine skeletal muscle, contributing directly to improved aerobic performance and metabolic efficiency. However, mitochondrial adaptations must be interpreted within the broader context of musculoskeletal adaptation, as metabolic improvements may occur faster than structural adaptation of tendons and ligaments. This review synthesizes current knowledge on exercise-induced mitochondrial remodeling in equine skeletal muscle, while highlighting the limited mechanistic evidence available in horses and the need for more standardized longitudinal studies.