Many people say that lactic acid causes muscle fatigue. It's not that simple! As this article discusses, the lactate ion is used as a source of fuel in the mitochondria.

Here are some comments from a retired exercise physiologist in response to the NY times article:

There is a difference between muscle soreness and muscle fatigue.

Muscle soreness is what one experiences 24-48 hours after strenuous exercise or physical activity and is usually caused by muscle trauma ("pulled muscle fibers") and/or inflammation (where interstitial fluid accumulates in the injured muscle and spaces) all resulting in the stimulation of pain receptors in the area.

Muscle fatigue has many causes, one is the lack of sufficient metabolic fuels for the muscles (glycogen or glucose). Short-term high intensity exercise (in the "anaerobic zone") does produce lactic acid and leads to the fatigue in untrained (out-of-shape) muscles. The lactic acid will dissociate in the muscle tissue (into free Hydrogen ions [H+] and lactate ions). The lactate ions ARE definitely still a fuel source. They enter pathways to yield further ATP production. But the free H+ ions form an acidic environment around the muscles which DOES lead to a decrease in muscle tension and fatigue.

One hypothesis is the accumulation of free H+ ions around the muscle sarcomeres interferes with the formation of cross-bridges between actin and myosin fibers. It blocks the troponin-tropomyosin action and thus fewer cross-bridges form or are detached and that leads to the inability of the muscle to maintain tension, thus fatiguing. The excess H+ ions are undoubtedly responsible for the "burn" in the anaerobic exercise.

As one improves his/her fitness level through training, a number of adaptations do occur within the muscles. One is enhanced capillarization in the trained muscles (bringing more nutrients and removing metabolic wastes). Another is an increase in the size and number of mitochondria within the working muscles (more and larger mitochondria enable faster, more efficient metabolic pathways). Another adaptation is the production of buffers within the blood and interstitial fluids thus 'neutralizing' the excess H+ ions that may accumulate. Elite athletes thus are able to perform longer and more efficiently because of these adaptations.

The lactate ions are metabolized both in the muscles and the liver (by the mitochondria) as a further source of producing ATP. Actually the cardiac muscle "feeds" most often on lactate ions.