Exercise-induced oxidative stress: Friend or foe?

•Muscular contractions stimulate reactive oxygen species production in active muscle fibers and skeletal muscles are a primary source of reactive oxygen species production during exercise.•Prolonged or high intensity exercise can result in both: (1) oxidative damage in skeletal muscle fibers and (2) accelerated muscle fatigue.•Exercise-induced increases in the production of reactive oxygen species in skeletal muscle plays a required role in skeletal muscle adaptation to endurance training.•Based on the available evidence, it appears unlikely that rigorous and prolonged exercise results in an oxidative stress level that is detrimental to human health.