Mitochondria form a reticulum in skeletal muscle. Exercise training stimulates mitochondrial biogenesis, yet an emerging hypothesis is that training also induces qualitative regulatory changes. Substrate oxidation, oxygen affinity and biochemical coupling efficiency may be differentially regulated with training and exposure to extreme environments. Threshold training doses inducing mitochondrial up-regulation remain to be elucidated considering fitness level.

SUMMARY: Muscle mitochondrial are responsive to training and environment, yet thresholds for volume vs. regulatory changes and their physiological significance remain to be explored.