Answer
In short term light exercise (yoga, jogging) or when exercise stops, myoglobin in the skeletal muscles supplies oxygen for a limited period of aerobic respiration. But the supply of oxygen available in myoglobin is relatively small, and it is quickly consumed.
As a stop-gap measure the muscles remove Pi ions from ADP and from creatine phosphate (CP) and attach them to ADP to produce ATP ( ADP+Pi<>ATP). These two mechanisms constitute the phosphagen system. But this system is limited in the amount of ATP it can generate: soon it fails and the muscles turn to anerobic fermentation-- an inefficient method of ATP generation.
In anerobic fermentation the muscles first obtain glucose from the blood. but when this supply is spent, they break down stored glycogen by glycolysis to obtain more glucose . The pathway of anerobic fermentation produces 2 ATP per molecule of glucose and lactic acid as a waste product. This is a very inefficient way to generate ATP. It will supply ATP required to support about half a minute of maximum physical activity, for example, playing soccer or basketball.
Work Step by Step
For support of long term vigorous activity, the cardiopulmonary system must come to the rescue to supply enough oxygen to for aerobic respiration. This process can generate the number of ATPs necessary: it supplies 34-38 ATP per glucose molecule.
The rate of oxygen consumption by the myocytes is steady for about 10 minutes of exercise. At first, glucose and lactic are used about equally as fuel. But after about 30 minutes of activity, the rate of oxygen consumption declines, and lactic acid becomes the major contributor to the fuel supply.
