Purpose: There is a paucity of research investigating the expression of mitogen-activated protein kinases (MAPK) in chronically trained (CT) athletes. Thus, it is unclear how MAPK may contribute to performance and muscle adaptation in CT subjects. The purpose of this study was to determine MAPK total protein, and phosphorylated expression of extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun N-terminal kinase (JNK), and p38-MAPK (p38) between untrained, and chronically trained runners. Methods: Tissue samples were analysed from sedentary (SED; n = 5) controls and chronically trained runners (CT; n = 5). Resting muscle biopsy samples were analysed for total-MAPK - and ratio of phosphorylated/total (p-MAPK) - ERK1/2, JNK, and p38-MAPK via western blotting. Mann-Whitney U tests and effect sizes were utilized to determine differences in total MAPK protein content and phosphorylation status between SED and CT subjects. Results: There was no difference in total-MAPK expression between SED and CT (p > .05). p-p38-MAPK tended to be greater for CT compared to SED (p = .07). There were moderate effect sizes of decreased pERK/total-ERK (d = -0.69) and increased pJNK/total-JNK (d = 0.54) in CT compared to SED. There was a positive correlation between p-p38-MAPK/total-MAPK and the percentage of type I fibres (r = 0.73, p = .016). Conclusion: Contrary to previous studies, chronic endurance training does not greatly influence total MAPK protein expression in chronically trained runners. However, resting phosphorylation of p38-MAPK may contribute to enhanced oxidative metabolism at chronically trained levels. These alterations are likely involved in the different physiological adaptations that occur following long-term training or at highly competitive levels.
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