Sometimes referred to as oxygen debt (now an outdated term), EPOC refers to the amount of oxygen needed to recover after exercise. EPOC enables lactic acid to be converted to glucose, carbon dioxide and water (using oxygen). It explains why we continue to breathe deeply and quickly after exercise.

The pain felt in the muscles the day after exercise.

With oxygen. When exercise is not too fast and is steady, the heart can supply all the oxygen that the working muscles need.
Summarised as: glucose + oxygen ? energy + carbon dioxide + water.

Without oxygen. When exercise duration is short and at high intensity, the heart and lungs cannot supply blood and oxygen to muscles as fast as the respiring cells need them.
Summarised as: glucose ? energy + lactic acid.

The aerobic training zone allows the aerobic system to be trained. To define aerobic training zone: 1). Calculate maximum heart rate (220 bpm) minus age: 220-age 2). Work at 60-80% of maximum heart rate