After a certain number of repetitions usually the muscle being stressed starts aching, the pain grows until it becomes unbearable then one can still continue pushing the muscle to higher extremes but after a while it does not matter how hard one pushes and how good one became at ignoring the pain, because the muscle will simply stop moving.
After a few seconds of rest, usually even 5 or 10 seconds are enough, the muscle is ready for more repetitions.
And this repeats until one is bored to death or can no longer tollerate the pain.
What mechanism makes it so that your muscle stop working and ignore your signals to to keep pushing but they work again after a few seconds?
Is it a mechanical block in the structure of muscle fibers or is it the brain trying to stop you from working out?