Which statement describes an early mitochondrial adaptation to endurance training?

• A. Increase in both subpopulations of mitochondria
• B. Decrease in mitochondria
• C. No change in mitochondria
• D. Only subsarcolemmal mitochondria increase

Answer: (A)

Endurance training rapidly stimulates mitochondrial biogenesis, increasing the number of mitochondria in skeletal muscle. In the early weeks of regular aerobic work, the muscle expands mitochondria across the fiber, not just in one region. Both subsarcolemmal mitochondria (just beneath the cell membrane) and intermyofibrillar mitochondria (between the myofibrils) increase, boosting the fiber’s capacity to produce ATP through oxidative phosphorylation. This coordinated rise is driven by signaling pathways that upregulate mitochondrial DNA transcription and the production of oxidative enzymes, so energy production improves quickly during sustained activity. If only one subpopulation were to increase or if mitochondria remained unchanged or decreased, the quick, comprehensive rise in oxidative capacity seen with training would not occur.