For billions of years, the cells used one of the two paths to make energy (ATP - the energy coins). It was a time when Earth's atmosphere was lacking oxygen. This was inefficient .. you could only generate 2 ATP energy coins with the old methods. Later Oxygen become abundant in the atmosphere and the cells evolved to make use of this new change. They came up with a new process something we today call 'cellular respiration' which takes in Oxygen as one of the inputs. This generates 36 ATP coins.

Great engineering design by these cells. When you have a good supply of oxygen in your body, the cells can use the Mitochondrion to generate 18 times more energy compared to when the cell has to generate energy by the old ineffcient method. Pranayams are a set of breathing exercises which helps you pump enough oxygen into your body. And so you should do Pranayams ( breathing exercise in Yoga) to make more of ATPs when you feel tired. What about the old process? What is it used for now in our body? When we exercise a part of our muscle a lot, the oxygen supply is consumed heavily and for more energy the system turns on Glycolysis, the old engine. Glycolysis, the old process, is the reason that Lactates are generated and they cause the formation of Lactic acid which results in the pain sensation. But more than as a reserve energy source, old process that produces Lactic acid in humans basically works as a safety mechanism today, a feedback to prevent us from overworking our muscles so that we don't damage them. Lactic acid makes the environment acidic and the whole process of Glycolysis even less efficient. So, if the goal of evolution was to have it as a alternate source of energy this formation of acid medium makes less sense. That typical pain/ fatigue stops us from overworking the muscle further. The alternate energy source works as a motivation for triggering the safety mechanism basically.