This section discusses the effects of endurance exercise training (i.e., continuous exercise at submaximal intensity) at varying exercise intensities (e.g., 50–75% VO 2 max) on skeletal muscle antioxidant enzymes in both preclinical and human studies. We begin with a discussion of the data from preclinical studies investigating the impact of endurance exercise on antioxidant enzyme activity in skeletal muscles.
4.1. Preclinical Studies Reveal That Endurance Training Increases the Activity of Key Antioxidant Enzymes in Skeletal Muscles
39,40,42,43,44,45,46,47,48,49,50,51, The first study demonstrating that endurance exercise training increases muscle antioxidant enzyme activity appeared in 1983 [ 3 ]. In the decades following this groundbreaking report, numerous studies have provided details about the impact of endurance training on the activity of antioxidant enzymes in skeletal muscle. We begin with a summary of investigations into the effect of endurance exercise training on the activities of SOD1 and SOD2 in locomotor muscles. Although some studies report that endurance training does not increase SOD1 or SOD2 in trained limb muscles [ 38 41 ], many studies agree that endurance exercise training increases the activities of both SOD1 and SOD2 in skeletal muscles [ 3 52 ]. Interestingly, the magnitude of endurance training-induced increases in muscle SOD1 and SOD2 activity across studies ranges between 20–112%. The explanation for the variance across research findings is not clear but methodological diversity in the assay of SOD activities together with differences in exercise training protocols are possible contributors to this lack of accord. For example, 10-fold variances exist in the sensitivity between methods used to assay SOD activity [ 53 ].
2 max) and/or longer durations (>30 min/day) typically report significant increases in the abundance/activity of both SOD1 and SOD2 [ Hence, assays of SOD activity with low sensitivity would not detect small-to-moderate differences in SOD activity between tissues. It is also possible that the failure of some studies to report exercise-induced increases in SOD activity in muscles is due to a low intensity and/or short duration of the exercise training sessions [ 51 ]. Indeed, studies exercising animals at higher relative intensities (e.g., >50% VOmax) and/or longer durations (>30 min/day) typically report significant increases in the abundance/activity of both SOD1 and SOD2 [ 4 ]. Figure 3 illustrates this point and shows that total SOD activity (SOD1 + SOD2) increases in the rat gastrocnemius muscle as a function of both exercise intensity and duration [ 51 ].
Although the effect that endurance exercise training has on total SOD protein/activity in skeletal muscle has received significant research attention, investigations into the impact that endurance exercise has on specific SOD isoforms in muscle has been studied sparsely. While one study concludes that endurance exercise training increases SOD2 protein content/activity in skeletal muscles without increasing SOD1 abundance [ 42 ], other studies conclude that endurance exercise training increases the protein expression of both SOD1 and SOD2 in trained rat skeletal muscles [ 49 54 ]. The explanation for these divergent findings is not clear but could be due to differences in the muscles selected for study (i.e., different exercise recruitment patterns between the limb muscles investigated).
Similar to SOD, variations exist in the literature about the impact of endurance exercise training on total GPX activity in skeletal muscles. Nonetheless, most preclinical studies conclude that endurance exercise training results in significant increases (e.g., +20–177%) in total GPX activity in skeletal muscles (reviewed in [ 6 ]). Similar to the exercise-induced increases in SOD, an exercise dose–response relationship exists between the exercise intensity/daily training duration and the magnitude of increases in total GPX activity in skeletal muscles ( Figure 4 ) [ 51 ].
Currently, most published studies conclude that endurance exercise training increases total GPX activity in the trained muscles. However, limited information exists about the impact of endurance exercise on the expression of the specific GPX isoforms within skeletal muscle fibers.
Numerous studies have also investigated the impact of endurance exercise training on the expression/activity of catalase in rodent skeletal muscles. Unfortunately, no clear consensus exists as to whether exercise training increases catalase activity in skeletal muscles. Indeed, studies have reported that endurance exercise training increases, decreases, or does not change catalase activity in skeletal muscles [ 4 ]. The explanation for these divergent findings remains unclear but may be the result of challenges in the assay of catalase activity; see reference [ 4 ] for a detailed discussion of these problems.
Finally, while it is established that both PRDX and TRX are key antioxidants in cells, the impact of endurance exercise training on the expression/activity of these proteins in skeletal muscles of animals remains unknown. Clearly, this is an important topic for future research.