Overtraining is a condition characterized by various functional disorders or pathological states caused by continuous fatigue, which occurs after a persisting imbalance between training-related load and physical function and recovery. Generally speaking, it's a state of imbalance between training and recovery, exercise and exercise performance, and stress and stress tolerance. Overtraining can cause various phenotypic changes or pathological remodeling, such as decreased skeletal muscle strength and exhaustive exercise endurance, skeletal muscle fatigue damage and dysfunction, skeletal muscle atrophy and loss, skeletal muscle glycogen depletion, skeletal muscle soreness and stiffness, skeletal muscle glucose intolerance, inattention, memory decline, anxiety, depression, abnormal emotions and behaviors, sleep disorders, cognitive function impairment, poor appetite, weight loss, liver/heart fat deposition, compensatory increase of liver/heart insulin signaling and glycogen storage, cardiac pathological hypertrophy, exercise-induced arrhythmias, myocardial fibrosis, ectopic and visceral fat deposition, and increased risk of injury. Unfortunately, its underlying mechanism is largely unclear. Recently, the enrichment of molecular and cellular signal pathway theory offers us a new explanatory paradigm for revealing its internal mechanisms. Based on the traditional explanation mechanisms and molecular and cellular signal pathway theory, we thoroughly analyzed the key mechanisms of health damage caused by overtraining from the perspective of oxidative stress, mitochondrial quality control disorder, inflammatory response, endoplasmic reticulum stress, cell apoptosis, and so forth. Specifically, overtraining-induced excessive reactive oxygen species (ROS) leads to serious oxidative stress damage in organisms at least via depressing Kelch like ECH associated protein 1(Keap1)/nuclear factor erythroid-2-related factor (Nrf2)/antioxidant response element (ARE) antioxidant pathway and activating p38 mitogen-activated protein kinase (p38 MAPK) signaling pathway. Overtraining induces mitochondrial quality control disorder and mitochondrial dysfunction, and thus triggers health impairment through inhibiting mitochondrial biogenesis and fusion, stimulating mitochondrial fission, and over-activating autophagy/mitophagy. Overtraining can also produce muscle, skeletal and joint trauma, then circulating monocytes are abundantly activated by injury-related cytokines, and in turn generate large quantities of proinflammatory IL-1 (3, IL-6, TNF-alpha, causing systemic inflammation and inflammatory health injury. Overtraining induces excessive pathological endoplasmic reticulum stress (ERS) and severe health damage via PERK-eIF2 alpha, alpha, IRE1 alpha-XBP1 alpha- XBP1 and ATF6 pathways which activated by proinflammatory signals. Overtraining also induces excessive apoptosis and harmful health consequences via Bax/Bcl2-Caspase 3-mediated mitoptosis which activated by oxidative stress and inflammation or even CHOP and Caspase 12-dependent ERS apoptosis. Nonetheless, it should be importantly emphasized that oxidative stress and inflammation are the central and preemptive mechanisms of overtraining and its health damage. Although the efficient strategies for preventing and controlling overtraining are scientifically and reasonably arranging and planning training intensity, training volume, and recovery period, as well as accurately assessing and monitoring physical function status in the early stage, yet various anti-inflammatory, anti-oxidant, anti-apoptotic, or anti-aging drugs such as curcumin, astaxanthin, oligomeric proanthocyanidins, silibinin, hibiscus sabdariffa, dasatinib, quercetin, hydroxytyrosol, complex probiotics, astragalus polysaccharides, semaglutide and fasudil also have an irreplaceable positive effect on preventing overtraining and its relevant health damage via depressing oxidative stress, mitochondrial quality control disorder, proinflammatory signals, endoplasmic reticulum stress, apoptosis and so on. We hope that this review can help us further grasp the features, mechanisms and regularity of overtraining, and provide an important reference for athletes and sports fan to conduct scientific training, improve training effectiveness, extend exercise lifespan, and promote physical and mental health.
