HIIT improves both energy systems for endurance:
Anaerobic Energy System
Anaerobic literally means "Without oxygen." The anaerobic energy system is what provides energy in all out efforts of up to 1 minute. For the first 10-15 seconds, the phosphate pool is used up and after that, glycolysis and lactic acid are involved in the effort.
During 10-15 second bursts, there is a very small amount of lactic acid produced. Rest periods of 30 seconds to a minute will provide complete recovery of the Adenosine Triphosphate-Creatine Phosphate (ATP-CP) system. During efforts of more than 10-15 seconds, a large amount of lactic acid is produced and such efforts are extremely taxing on both the athlete's muscles and their Central Nervous System (CNS).
Aerobic Energy System
Aerobic literally means "with oxygen." This energy system is utilized during prolonged exercise over a period of at least 3-4 minutes. As long as there is enough oxygen to provide energy, the fatigue that you experience will remain at a low level.
This is the reason why many track and field athletes train at higher altitudes where there is less oxygen. By training at high altitudes, they can increase the number of red blood cells which will help them to perform for a longer period of time with little to no fatigue throughout.
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