Why does lactic acid build up in muscles when feeling fatigued?

Answer 1

The question puts feeling fatigue as preceding the lactic acidosis. But it's not quite that way. Let me explain through examples.
Example One: A 40-year old man who injured his spinal cord at age 25 developed muscle tremors--both large and small tremors. Even though he spends 90% of his day in bed because of pain and inability to walk very far, the nerves in his muscles continually "fire" causing small to large movements even when he's asleep. After a week of more severe pain, he awakens one morning with a very dry mouth, dry eyes, a nagging headache, pain in the stomach (high) as if hungry but he's too nauseated to eat or even drink... and he's "so tired" he can barely stay awake. He eats and drinks very little, sleeping off and on to ignore the stomach pain. These symptoms worsen over the next day until he is doubled over from the stomach pain. He's taken to the ER, worried about his appendix. But blood labs show a drastic shift in electrolytes and high lactic acid. The doctor starts 1,000 cc IV bag open wide to infuse quickly; a second bag follows. Within one hour, his headache, stomach pain, and nausea is gone. He feels "much better", is alert, and eager to get back home. The doctor explains that the constant spasms and tremors in the man's muscles lead to lactic acid build up, triggering an episode of dehydration. The symptoms caused the man to reduce food and fluids, thereby worsening the dehydration, which then caused more severe lactic acid levels. The continued muscle activity also pushed lactic acid higher. The man's tiredness and fatigue were a symptom of lactic acidosis secondary to dehydration initiated from the tremors.

Example Two: A 22-year old woman is a seasoned marathon runner who's entered her 40th race. Air temps near the 90s with high humidity. She's running with excellent times, but ignores offers of bottled water along the first section of the route. She had just recovered from the flu the week before. Though she didn't realize it and felt "energized" and in perfect health, she was already slightly dehydrated. Taught to ignore minor symptoms, she pushed on despite having some stomach cramps--she thought it was just a result from having the flu earlier. Halfway through the marathon, her large muscles (legs) began to cramp as if a "Charley Horse" in her calves. She kept running, but did start to drink some water. Within the next half hour, she begins to feel heart palpitations---something she only experienced once before when she was fevered and quite ill. On the last stretch of the course, a balmy wind kicks up; she notices her eyes feel drier but blames it on the wind. Within the next 500 yards, she collapses. Medics start an IV, per regular protocol for any patient. With 500cc into her system, her palpitations stop and she feels less weak. By 1,000 ccs, the cramping has stopped. But now in the ER, her bloodwork shows lactic acidosis. Drs push 2 more bags of 1,000ccs by IV. After 5 hours in the ER and routine tests (EKG, bloodwork, urinalysis), she is sent home feeling much improved.

Lactic acidosis causes fatigue... fatigue does not cause lactic acidosis. Lactic Acid builds up when the muscles are not able to clear the acid. Muscle movements (any) releases lactic acid, but under normal conditions, the body should be able to handle this task efficiently. But in some conditions (movement disorders) or diseases (kidney disorders), the body doesn't clear lactic acid as well. Dehydration makes the problem worse, and reduces the body's ability to clear lactic acid from the muscles. Because the heart is muscle, if untreated, lactic acidosis can affect how well the heart contracts and cause arrhythmias (irregular contractions). Fluid replacement is one critical treatment initially, and most people recover quickly from IV fluids. However, the underlying problem that caused the lactic acidosis elevation may still exist--for example, the guy with the muscle tremors will be more prone to LA and dehydration. The female runner, on the other hand, was put at risk from having the flu right before a marathon (because flu can cause dehydration). She will just need to be sure to hydrate well before, during, and after running.

Again, fatigue occurs because of the build up of lactic acidosis and likely dehydration.