an endurance runner begins experiencing muscle soreness towards the end of a long race. Explain, in detail, what is happening in her muscle cells. Your answer should include the molecules that are missing, how this is altering cell respiration, and what the cause of her soreness is

Her muscles are going through a stage of anaerobic respiration called Lactic Acid Fermentation and are producing lactic acid due to the fact that there aren't enough oxygen molecules present. This is causing there to be much less ATP produced but at a quicker rate. Her soreness is due to the building up of lactic acid in her muscles.

Explanation:

Cell respiration has two categories, anaerobic and aerobic. Anaerobic respiration happens when there is no or not enough oxygen present to go through Aerobic respiration. When you're doing high energy activities (such as running), your body doesn't have enough oxygen to go through the full aerobic respiration process and resorts to anaerobic respiration which, for animals, ends with Lactic Acid Fermentation. Doing this produces quick energy with the downside of also producing lactic acid which causes soreness in muscles.