How Do Your Muscles Continue To Get Energy During High Levels Of Activity When There Is Not Enough Oxygen?

Your muscles use Anaerobic Respiration or more commonly fermentation of Glucose.

Glucose ---> Lactic Acid + Energy (ATP)

C6H12O6 ---> 2C3H6O3 + 2 ATP

The energy released is about 120kJ per mole Glucose.