Compared to the large number of preclinical studies investigating the impact of endurance training on the activity of antioxidant enzymes in skeletal muscles, human studies on this topic are limited. The obvious explanation for the small number of human studies is the invasive nature of obtaining skeletal muscle biopsies. Nonetheless, several clinical studies exist and collectively, these studies confirm that endurance exercise training increases the activities of key antioxidant enzymes in human skeletal muscles. Although an early study concluded that short-term aerobic exercise training does not increase antioxidant enzyme activities in human skeletal muscles [55], subsequent studies have consistently demonstrated that weeks-to-months of endurance exercise increases antioxidant enzyme activity in the trained muscles. For example, six months of endurance exercise training increases the activities of total SOD, CAT, and total GPX in skeletal muscle by 31%, 57%, and 51%, respectively [56]. Similarly, another study concluded that moderate-intensity endurance exercise training increases the expression of SOD2, GPX1, and PRDX5 in skeletal muscles by 66%, 62%, and 37%, respectively [57]. Endurance exercise has also been reported to increase both SOD and CAT in skeletal muscles of obese sedentary men [58]. Finally, a retrospective study concluded that older adults engaged in regular bouts of moderate-to-vigorous physical activity have higher levels of total SOD, total GPX, and CAT activity in locomotor skeletal muscles compared to inactive individuals [59]. Similarly, another study investigating the impact of life-long exercise on muscle antioxidants in humans concluded that endurance exercise increases the abundance of both SOD2 and PRDX5 protein in the trained locomotor muscles [60]. Collectively, these clinical studies are generally consistent with the preclinical studies indicating that endurance exercise training increases the activities of key antioxidant enzymes in trained skeletal muscles of humans.