Final answer:
The higher the level of physical training, the more energy comes from the oxidation of fatty acids during exercise under VO2max due to improved aerobic efficiency and myoglobin levels in muscles.
Step-by-step explanation:
The greater the level of physical training, the higher the proportion of energy derived from the oxidation of fatty acids during exercise below VO2max. During low-intensity activities which are typically below VO2max, the body prefers to use aerobic metabolism, which relies heavily on fatty acids as a primary energy source. This is due to the efficiency of aerobic respiration compared to anaerobic metabolism, producing around 36 ATPs per glucose molecule, and it also allows for a more extended exercise duration as the body has almost unlimited fat reserves. With increased aerobic training, the efficiency of the circulatory system improves, enabling a more sustained delivery of O2 to the muscles, promoting increased utilization of fatty acids over glucose, particularly as glycogen stores begin to deplete.
Moreover, aerobic training enhances myoglobin levels in muscles, which stores extra oxygen and supports more efficient muscle contractions with less fatigue. As a result, during aerobic exercises below VO2max, well-trained individuals are able to oxidize more fatty acids, sparing glycogen stores, and are capable of sustaining physical activity for longer periods.