Final answer:
Amino acids can serve as a primary fuel for muscles, especially during energy shortage from typical sources like carbohydrates and fats. They are used as an alternative energy source, contributing to ATP production through the Krebs cycle.
Step-by-step explanation:
Amino acids can indeed be used as a primary fuel for muscles, especially under conditions such as fasting or starvation when the usual sources of energy (carbohydrates and fats) are scarce. Although typically only about 10% of our energy comes from amino acids, they become a significant energy source during these times. Amino acids are converted into metabolic intermediates that enter the Krebs cycle, aiding in ATP production, which is essential for energy.