aerobic and anaerobic exercise
The working ability of a muscle depends on sufficient blood flow and intake of oxygen and nutrition.
The body uses two different systems to supply the muscles with energy:
1) Aerobic system: Carbohydrates and fat react with oxygen to metabolize into carbon dioxide and water.
2) Anaerobic system: Carbohydrates are used for energy production without oxygen and metabolized into lactic acid or lactate.
At lower training intensities, the use of oxygen by your muscles will match oxygen intake. This type of activity can be sustained for long periods of time. At high intensities, however, your heart and lungs cannot supply enough oxygen to keep up with the demands of your muscles. As a result, your muscles begin to work anaerobically.
The problem with anaerobic energy production is that after a few seconds, waste products like lactate rapidly build up. Lactate blocks fat metabolism and forces the body to stop exercising. The metabolic turning point at which the body shifts from the aerobic to anaerobic energy system is called the anaerobic threshold (AT).
Your Anaerobic Threshold (AT) is the exercise intensity at which you begin to go anaerobic and build up lactate. If you want to improve your performance, it'ís useful to train for short periods (three to five minutes) at a heart rate just below your AT.
If you're out of shape, your AT might be at 70-80% or less of your maximum heart rate. However, if your training progresses intensively, your AT will increase. In very fit competitive athletes, AT can be at 90% of maximum heart rate.