what happens to your muscle cells causing them to become more tired and weak?

Answer 1

Muscle cells use oxygen when they work. When we excercise vigorously, the cells use oxygen faster than the blood can bring more oxygen to the cells and faster than the blood can carry away the CO2 created by the chemical reaction the uses the oxygen. When this happens, the cells create lactic acid. Too much lactic acid is bad for the cells. That is why weight lifters are careful not too exercise any single muscle too much too often. Excercise builds up your body's ability to deal with lactic acid and it makes the blood vessels bigger (when you get "buff", it is not because you are creating more muscle cells, it is because you are making the blood vessels larger and able to carry more oxygen to the muscle cells).

I hope this helps

Answer 2

Lack of oxygen -> lactic acid builds up causing fermentation

Answer 3

Lactic acid build-up from a lack of oxygen?