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?
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It’s 2 fold. Cardiovascular workouts give you more stamina so you can lift things longer without getting as fatigued. You’re teaching your heart and lungs how to do the things. Resistance training breaks your muscles. When they repair they get stronger so they don’t tear next time.
Your nerves run out of the chemicals they use to signal the muscle to contract. When you exercise your body adapts by creating more nerve connections to the muscle and strengthening the ones already there.
Our body creates microtears all over the place and hat makes it harder to lift because we have less muscle strings helping to lift, then when we heal our body will heal the muscle tears and reheal layers over it. The same way your arm heals if you break the bone
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.
Maybe just check this out instead of the lengthy explanation I would write up; I think that’s quite a good breakdown:
What others mentioned isn’t actually wrong, however, it’s not a sufficient explanation for the scenario you described. Glycogen and other energy-storing molecules deficiency is more of a reason for general muscle fatigue you feel (the day) after a workout, not so much the reason why you can’t lift your dumbell for that sweet last rep you’re trying to crank out.
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