Final answer:
The substrate preference for the working muscle during exercise is determined by the intensity of the activity. Low-intensity activities primarily use fatty acids for energy, while high-intensity activities increase carbohydrate usage to meet the quick ATP demand.
Step-by-step explanation:
Fuel Source Preference During Exercise
During low-intensity activities, muscles prefer to use fatty acids as the primary energy source through aerobic metabolism. This process is very efficient and allows for the production of larger amounts of ATP, necessary for prolonged activities. Fatty acids, stored mainly in adipose tissue, provide an almost unlimited energy reserve, making them ideal for low-intensity exercises where a smaller amount of glucose is also utilized. On the other hand, high-intensity activities require a rapid supply of ATP, leading muscles to rely on both anaerobic and aerobic metabolism, with an increased dependency on carbohydrates in the form of muscle glycogen and blood glucose. This shift occurs because high-intensity exercise surpasses the oxygen supply, prompting a greater need for glycolysis, an anaerobic process.
Based on this information, the correct answer to the question 'What determines substrate preference for the working muscle?' is A) intensity low intensity = more fat usage higher intensity = more carb usage.