the beginning of an activity, and some muscle GSDs can achieve second wind within about 10 minutes from the beginning of the aerobic activity, such as walking.

In experienced athletes, "hitting the wall" is conventionally believed to be due to the body's glycogen stores being depleted, with "second wind" occurring when fatty acids become the predominant source of energy. The delay between "hitting the wall" and "second wind" occurring, has to do with the slow speed at which fatty acids sufficiently