Which of the following factors increases during intensive training?

Oxygen uptake refers to the amount of oxygen your body can utilize during exercise, and it significantly increases during intensive training. This improvement is largely due to enhanced cardiovascular efficiency and an increase in the number and size of mitochondria in the muscle cells, which are responsible for energy production. As training intensity rises, the demand for oxygen during physical activity increases, leading to adaptations that improve the body's ability to transport and utilize oxygen more effectively. This physiological adaptation is crucial for endurance athletes, as higher oxygen uptake enables sustained performance at increased intensity.

In contrast, body fat percentage typically does not increase during intensive training; effective training usually promotes fat loss as the body burns more calories. Similarly, while caloric intake may rise for some individuals who are trying to fuel their bodies adequately for high levels of activity, it's important to note that this is not a direct result of training intensity but rather a behavioral response to increased energy expenditure. Resting heart rate generally decreases with improved cardiovascular fitness due to more efficient heart function, meaning that it does not increase. This highlights the significant role of intensive training in enhancing one's aerobic capacity through increased oxygen uptake.