Energy production during strenuous exercise can be divided into two stages. 

Let’s look at what is happening in Anna’s muscle cells. 

Stage 1:

For the first 2-3 minutes, Anna’s body uses up the blood glucose to produce energy through glycolysis. After about 3 minutes the body uses up all the blood glucose and Anna’s muscle cells start breaking down her muscle glycogen to glucose to produce energy through anaerobic glycolysis. Remember, anaerobic glycolysis is a faster way to make ATP during strenuous exercise. This leads to an increase in lactate production and a drop in pH.  

Stage 2:

After about 8-10 minutes, energy production shifts to oxidative phosphorylation. The muscles start using pyruvate from glycolysis AND free fatty acids, glucose, and amino acids from the bloodstream to produce more ATP to allow Anna to keep running.

Compared to Anna, what is happening in Beth's muscle cells that lead to her symptoms?