ADVANCED FITNESS PERFORMANCE
Physiology
What limits my ability to swim or run faster and longer?Exercise is forceful movement of your muscles. Your muscles metabolize glucose or fats and make chemical energy packets called ATP. Your muscles then turn the ATP into mechanical energy and produce the forceful movement you ask of them. These ATP packets can be produced from either aerobic metabolism ( using oxygen ) or anaerobic metabolism ( not using oxygen ).
Since aerobic metabolism is much more efficient at producing ATP and muscle force ( one glucose molecule produces 37 ATP ) your muscle will always metabolize glucose or fats by using oxygen. When a muscle cell is no longer delivered the amount of oxygen it needs for the task at hand it changes to anaerobic metabolism. Although the muscle cell continues to produce ATP and forceful movement, it does so at a great cost. Anaerobic metabolism is not only terribly inefficient ( only 3 ATP per glucose molecule ) but it also produces lactic acid. Although the large majority of muscle cells in a muscle are working aerobically, at peak exercise more and more cells switch to anaerobic metabolism. Lactic acid builds up in the blood and in the muscles causing cramping, fatigue and shortness of breath.
Then the answer to the above question is that you are limited by the amount of oxygen you are able to deliver to your muscle cells. The muscle cells that cannot get the oxygen they need start to produce lactic acid. The lactic acid builds up and eventually makes all your muscles stop because of weakness and fatigue.
You are sitting at your desk reading. Your muscles are relaxed. They are metabolizing glucose and fats with oxygen (aerobic metabolism). If you got up from your chair and started to climb steps at a faster and faster rate, you would reach a point at which some of your muscle cells would not be getting enough oxygen delivered to them by your blood vessels and heart. These muscle cells will start to produce lactic acid. As you go faster and faster up and down the stairs more and more cells are working anaerobically (without oxygen) , producing and accumulating lactic acid. You then need to stop.
So how are you different than Lance Armstrong?Lance can supply more oxygen to his muscle cells. He has a tremendous aerobic capacity which is the ability to deliver oxygen to his muscles. Yes, eventually some of his muscles cells start to metabolize glucose anaerobically and produce lactic acid. His muscles will cramp up become fatigued and he will become short of breath. The difference is that he is able to exercise longer and harder because of his aerobic capacity.
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