Muscles indeed will fatigue during extreme exercise but there are other less obvious ways that muscle can fatigue….. let’s look closer. Muscle fatigue is defined as an exercise-induced reduction in the maximal force capacity of the muscle. This reduction of muscular capacity is largely due to lactic acid and metabolites because of anaerobic conditions, that is, the muscle is not getting enough oxygen. Fatigue can occur because of an accumulation and/or depletion of these metabolites….build up of lactate and therefore hydrogen ions or depletion of energy in the muscle cells like ATP and glycogen. Undoubtedly in your science classes you have learned how the body creates energy via the Krebs Cycle. However, our body during exercise often cannot get enough oxygen to keep our muscles working at 100% efficiency and the muscle begins to use a mechanism of anaerobic respiration to gain a sort-of supplemental energy (this is much less efficient but prevents the muscle from suddenly not being able to function). Chemically, this is glucose being cut in half to form two lactic acid molecules and energy. The harder the muscle works, the more lactic acid build-up. The lactic acid interferes with muscle function as it increases in amount therefore worsening the fatigue. There can also be a neuronal component to muscle fatigue where the muscle has the capacity to exert a force but cannot by voluntary effort. This can be measured as the difference between force exerted voluntarily and force exerted when say an electrical stimulus is applied to the muscle causing an involuntary twitch. Muscle fatigue is not necessarily caused by intense exercise per se. Muscle fatigue from sitting in a particular position for a long period of time can result in fatigue because you are causing that muscle to work in a particular way for an extended period of time (and may be cutting off circulation which supplies oxygen). If your muscles are not accustomed to a particular motion, even a short duration, fairly low intensity exercise can result in the fatigue of the muscle(s) involved (which could be caused by something as simple as carrying too many books home from school). I hope this has answered your question. Keith Anderson Brigham & Women's Hospital, Boston, MA
Try the links in the MadSci Library for more information on General Biology.