What is muscle fatigue? By definition it is know as muscle weakness. In general, when we refer to muscle fatigue, we are usually talking about muscles that cannot exert any more force. It usually occurs after performing lots of reps or lifting heavy weights. You know that feeling that comes after a long set or two where you just find yourself having to rest at the end of the bench? That is a great example of muscle fatigue. But what really causes muscle fatigue? Ironically the pathway that provides your muscles the energy to contract is also the reason why muscles become tired.
You see, in order to do work, the muscles need ATP or adenosine triphosphate. The harder the activity, the more the ATP that is needed to combat the fatigue that comes with the increased activity. The problem is that the body does not have a very efficient way to produce large amounts of ATP rapidly. The series of chemical reactions that makes ATP is called the anaerobic pathway, and unfortunately this pathway has some very nasty by-products, one of them being lactic acid. When lactic acid builds up in your muscles it begins to burn. This is the limiting factor in doing anything that requires ATP to be produced through this pathway.
The amount of lactic acid that can build up in your muscles before they fail you increases as your muscle size increases. It makes perfect sense because the stronger you are, the bigger your muscles are. The bigger your muscles are, the more space they have to allow for lactic acid build up. So the next time your muscles get fatigued, don’t get angry, get happy, because you just worked your way to failure! And in this case, failure is not a bad thing at all.