Mitochondria

Mitochondria are structures inside cells that produce over 90% of the body’s ATP.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 111.} The Krebs cycle, electron transport and the rest of the oxidative system run inside them and require oxygen.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 60.} Pyruvate from glycolysis and free fatty acids from fat enter mitochondria to be burned for energy.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 62.},^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 69.}

Muscle stores an oxygen reserve in myoglobin, which releases it to mitochondria when a client moves from rest to exercise and again during intense work.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 111.}

In practice

Aerobic training increases both the number and size of mitochondria, along with the oxidative enzymes inside them.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 111.} With intense, regular training, enzyme activity can double or triple.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 111.} Mitochondrial density rises more slowly than myoglobin concentration or capillary density.^{5International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 210.}

More mitochondria allow more ATP to be made aerobically, helping explain why a trained client’s lactate threshold occurs at a higher intensity.^{6Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 46.}

Mitochondrial density generally falls with resistance training.^{7Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 95.} The NSCA interprets this as muscle growth outpacing the supply of mitochondria, rather than a loss of mitochondria.^{7Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 95.}