Final answer:
The creation of new mitochondria in cells is termed mitochondrial biogenesis, a physiological response to endurance exercise training that enhances the muscle fibers' capacity for aerobic metabolism.
Step-by-step explanation:
The process of synthesizing mitochondria within cells, such as what occurs with increased volume in skeletal muscle fibers due to endurance exercise training, is called mitochondrial biogenesis. Endurance training specifically targets slow-twitch or type I fibers, increasing their efficiency by adding more mitochondria to support aerobic metabolism and ATP production. Additionally, endurance exercise elevates the amount of myoglobin found in the sarcoplasm and stimulates angiogenesis, the formation of capillary networks, to better supply oxygen and remove metabolic waste. In contrast, resistance training predominantly leads to hypertrophy, the enlargement of muscles due to the addition of structural proteins, without significantly increasing mitochondria or capillary density.