Electron Paramagnetic Spectroscopic Evidence of Exercise-induced Free Radical Accumulation in Human Skeletal Muscle

The present study determined if acute exercise increased free radical formation in human skeletal muscle. Vastus lateralis biopsies were obtained in a randomized balanced order from six males at rest and following single-leg knee extensor exercise performed for 2 min at 50% of maximal work rate (WR(MAX)) and 3 min at 100% WR(MAX). EPR spectroscopy revealed an exercise-induced increase in mitochondrial ubisemiquinone (UQ*-) [0.167 +/- 0.055 vs. rest: 0.106 +/- 0.047 arbitrary units (AU)/g total protein (TP), P < 0.05] and alpha-phenyl-tert-butylnitrone-adducts (112 +/- 41 vs. rest: 29 +/- 9 AU/mg tissue mass, P < 0.05). Intramuscular lipid hydroperoxides also increased (0.320 +/- 0.263 vs. rest: 0.148 +/- 0.071 nmol/mg TP, P < 0.05) despite an uptake of alpha-tocopherol, alpha-carotene and beta-carotene. There were no relationships between mitochondrial volume density and any biomarkers of oxidative stress. These findings provide the first direct evidence for intramuscular free radical accumulation and lipid peroxidation following acute exercise in humans.