This is due to lactic acid accumulation in Emma's leg muscles.
Explanation:
In the absence of oxygen in the muscle cells, the cell switches from performing aerobic respiration to lactic acid fermentation. The lactic acid fermentation produces lactic acid as a product, which builds up in the muscle cells of the body causing a burning sensation and pain. This often happens during exercises.
This is the cause of Emma's pain in her lower leg muscles. Lactic acid has been formed and accumulated in the muscle cells of her leg during the exercise with her brother.
