Biologically this process is called damage because it changes a natural process by speeding it up, but there is no evidence that this is harmful. We don't know. If free radical generation were a problem, one might expect to see a greater incidence of disease related to free radicals in athletes. Although there is uncertainty, some researchers remain concerned that excessive exercise may lead to health risks. Kenneth Cooper, M.D. founder of the Cooper Institute for Aerobics Research and author of the landmark book Aerobics in 1968, discusses possible harm in his new book Antioxidant Revolution.
Regular training improves endurance, strength, and flexibility.
The better your training, the more your performance improves. But everyone reaches a point at which further increase without adequate rest becomes stressful. Overuse of skeletal tissue produces injuries. Although moderate exercise improves your immunity, continuous heavy training, such as that needed to prepare for marathons and triathlons, weakens your immune system. This increases problems such as upper respiratory infections.