system
Given the following post, respond as if you're commenting on Reddit:
human
Do you burn more calories as an exercise goes on?

For example say do I burn more calories on the last squat of the set versus the first squat of the set.
I'm curious because you are struggling more as the exercise goes on so it would take more energy to continue?
gpt
What you're noticing is likely not an increase calories being burned, but simply muscular fatigue. 

Your muscles use a molecule called atp to provide the energy to contact fibers. Atp is synthesized in part from creatine phosphate, another molecule stored in your muscles. When you use your muscle enough to deplete your creatine store, your body switches to glucose to provide energy, which is stored in your muscles as glycogen. As your exercise goes on, the amount of glycogen and creatine in your muscles goes down, making it harder to produce the same force since you have less energy sources in your muscles. 

In addition, the byproducts of the chemical reactions that allow your muscle fibers to contact can interfere with ion levels in your muscles. Various Ion levels are very important to allow your muscles to react properly to signals to contact from your nerves, and high levels of these byproducts can reduce your your sensitivity to nervous signals, making you weaker.