Oxidative system

The oxidative system is the aerobic energy system that uses oxygen to make ATP in the mitochondria.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 60.} Its pathway includes the Krebs cycle and electron transport, with oxygen as the final electron acceptor.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 60.} It supplies most ATP at rest and during aerobic activity, mainly from carbohydrate and fat, including during treadmill walking, water aerobics and yoga.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 46.}

Of the energy systems, it delivers ATP slowest but has the largest capacity.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 69.} Oxidizing the fat in one triglyceride made of three 16-carbon fatty acids yields over 300 ATP.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 69.} Protein contributes little during short exercise and an estimated 3 to 18% of energy during prolonged activity.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 69.}

At rest, about 70% of ATP comes from fat and 30% from carbohydrate.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 46.} As intensity rises, fuel use shifts toward carbohydrate; during high-intensity aerobic exercise, it supplies almost 100% of energy if enough is available.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 46.}

In practice

• Fat oxidation predominates from rest through light to moderate exercise.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 51.} Glycogen becomes the preferred fuel above roughly 60 to 75% of maximal oxygen uptake.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 51.}
• Aerobic training increases the number and size of mitochondria, and intense, regular training can double or triple oxidative enzyme activity.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 111.}
• Resistance training relies mainly on anaerobic metabolism and doesn’t appear to improve oxidative enzyme activity, so clients seeking aerobic fitness need aerobic-specific work.^{6Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 95.}