Final answer:
The use of fatty acids as an energy source increases during sustained muscular work because they become the primary fuel for endurance activities after glycogen reserves start to be depleted.
Step-by-step explanation:
During sustained muscular work in humans, the use of fatty acids as an energy source increases, as they are the primary fuel for endurance activities. During low to moderate intensity exercise that uses aerobic metabolism, fatty acids become the predominant fuel source for the exercising muscles. This happens after the body's glycogen reserves start to decrease. As exercise intensity and duration increase, the body transitions from primarily using stored glycogen to utilizing fatty acids as the stores of glycogen are depleted. With fat reserves in the body being relatively high, they allow for prolonged energy sustenance for endurance activities.
Fatty acids are stored in the muscles but also largely in adipose tissue. During extended periods of exercise, fatty acids are mobilized and oxidized to provide a steady supply of energy. The process of converting fatty acids to energy through beta-oxidation makes them a key component of sustained energy release. Fatty acids offer a higher energy per unit mass compared to glucose, explaining their key role as an energy supplier during sustained muscular efforts.