Which statement about free radicals production during exercise is true?

• A. They are harmful and never beneficial
• B. They are important signals for muscle adaption to exercise training
• C. They have no signaling role
• D. They only appear during fatigue

Answer: (B)

Free radical production during exercise acts as a signaling mechanism that promotes muscle adaptation. As you exercise, higher oxygen use and metabolic flux raise reactive oxygen species from sources such as mitochondria and enzymes like NADPH oxidase and xanthine oxidase. At moderate, controlled levels these reactive species trigger signaling pathways that lead to beneficial adaptations, including increased antioxidant defenses (like SOD and glutathione peroxidase), greater mitochondrial content through pathways involving PGC-1α, and improved stress resilience. The idea isn’t that free radicals are purely harmful; they are essential messengers that help the muscle acclimate to training. Of course, excessive ROS can cause damage if antioxidant systems are overwhelmed, but with regular training, the redox system adjusts to support adaptation. They don’t only appear with fatigue, and they aren’t exclusively harmful, which is why the statement about them being important signals for muscle adaptation is the best answer.