Although the passage of electrons down the electron transport chin performs an essential role in the process of aerobic ATP production, this pathway also produces free radicals (30). Free radicals are molecules that have an unpaired electron in their outer orbital, which makes them highly reactive. That is, free radicals quickly react with other molecules in
the cell, and this combination results in damage to the molecule combining with the radical.
Historically, it and was believed that increased aerobic metabolism during exercise promotes increased production of free radicals in the mitochondria of the working muscles (30). However, new research indicates that this is not the case. Indeed, although
exercise results in increased production of free radicals in the active skeletal muscles, this increase in muscle free radical production is not due to oxidative phosphorylation in the mitochondrion (22). For additional information on the sources of free radical production during exercise, see Sakellariou et al. (2013) in the Suggested Readings.