system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what happens in our muscles when after a few times, it becomes physically impossible to lift a heavy item another time, at least until we had some rest? how does exercise change this?
gpt
The short answer is your muscle cells have used all of their energy reserves and need time to replenish and make more. 

Okay so your cells use a chemical called ATP as a type of chemical energy storage. Your cells break ATP apart and use that energy to do things like contract muscle cells. There are several ways that your cells can make ATP, some are fast but not very efficient/don't produce much, while  other methods or slow but are highly efficient. However, there is only so  much ATP your cells can store at one time. So, as your lifting the weights, your muscle cells are burning through their ATP reserves and desperately trying to keep up. Eventually you reach a point where only the fastest method of making ATP, called glycolysis, can keep the cell alive, but this isn't enough ATP to power the contraction of your muscles. Once you take a break for a few minutes your muscle cells have had time to build up their ATP reserves again by having some of the slower metabolic pathways active.