As we grow older, our mitochondria collect more and more mutations. Eventually, mutations can reduce the energy available to the cell. If the energy drops low enough, the cell can malfunction. If the energy drops even further, the cell will stop working. If muscle cells can't get enough energy through cellular respiration, they have to get energy without oxygen.
Lactic acid can cause sore muscles.
For example, the soreness people feel after running a marathon is caused by a buildup of lactic acid. Some people with MT have very high levels of lactic acid in their blood. This rare condition is called lactic acidosis. There is a blood test for lactic acid levels, but experts disagree on how to interpret the results. Nausea, vomiting, fatigue and recent weight loss may be signs of lactic acidosis.