When you exercise your muscles require oxygen to function properly. Your body deals with this need in three progressive ways. This is called your Aerobic system. The first part of this system is known as Glycolysis, the second The Lactic Acid System and the third is known as Oxidative Phosphorylation.
In the first 10 seconds of running, it seems effortless, very easy like 'running on clouds'. This is called Glycolysis. During this system you are running off a glucose reserve found in your muscles. This lasts approximately 10 seconds and takes around 2 minutes to regenerate, but can vary in different people.
After your muscles have run out of glucose, your body begins in The Lactic Acid System. Your legs start to feel heavy and it might become difficult to run much further. This is because your muscles are now producing acid. The only way you can stop your muscles from doing this is by slowing down or to stop moving altogether.
The third and final stage of your Aerobic system is known as Oxidative Phosphorylation. This is when your body is working hard to get oxygen to your muscles, in order to continue running. Your body does this by puffing and breathing very heavily. Your body has used more oxygen than you had avalible, so your body is now finding ways of getting that oxygen back. This is why after you engage in physical excersize you puff. It's your body's way of regaining oxygen.
Latic Acid
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