The ache you feel in your muscles, and the fatigue you feel after a good work out, is due to an event called "excess post-exercise oxygen consumption", EPOC for short, or more commonly referred to as "afterburn". The feeling you get after a hard workout, or even a mild one, might not be very comfortable, but it's an important process when it comes to getting in shape.
This fatigue and slight ache are caused by the rapid burn and decline of oxygen in your body, which happens during and a while after your workout. EPOC refers to the body's process of restoring the body's oxygen and stored fuel, as well as the oxygen deficit. This deficit was known as the body's “oxygen debt” in past decades, and it's still often called that today.
The body needs oxygen to restore the lactic acid in the muscles, as well as repair cells and bring your pulse and breathing rate back to normal. Your body uses the things it needs – water, food and oxygen – to create energy stores in the muscles. It takes an increased amount of oxygen to replenish these stores. This is also why you're advised to wait a day or two between work outs, to allow your body to fully recover.
How Long Does EPOC Last?
It makes sense that EPOC is highest right after the workout, but it can still be detected even after 24 hours of working out. A few specific studies to find out exactly how long the effects of EPOC can take place after a work out found out that it only decayed to 13% three hours after working out. Sixteen hours later, it had dropped down to 4%. One study detected EPOC even after 38 hours after exercise.
There are two types of exercise, and both are equally important to staying fit and healthy. Aerobic exercise burns more calories and works to strengthen your heart and lungs, as well as your muscles and bones. It consists of using large muscle groups for an extended amount of time to deliver oxygenated blood throughout the body. Examples of aerobic or cardio workouts, are cycling, swimming, jogging or aerobic dancing.
Anaerobic exercise is strength straining, and unlike aerobic workouts, you can only do it for a short amount of time because your oxygen levels quickly drop (EPOC). It's more taxing on the body than aerobic workouts. You might be advised to do aerobic and anaerobic exercises together, since aerobic exercise can help bring oxygen back to the muscles. For example, you could alternate between doing cardio and strength training.
Aerobic exercise burns more calories during the workout, but doesn't cause EPOC. You can tone your muscles as well as burn more calories even after the workout by doing anaerobic exercise, as well. Since EPOC is still in effect, it's requiring more energy to get your body back to normal, which means more oxygen intake as well as more energy (calories) being burned.