Before we get into the scientific nitty-gritty, let's do a little experiment. Stand up right now and do 10 rapid Hindu squats. (For those who don't know, this is a deep knee bend that is favored in the grappling arts.) Make sure you do 10 of them hard and fast. Now it's time for a quick self-assessment. How do your legs feel?
There is a possibility that your lungs puffed a wee bit and that your heart rate rose, but I'll wager that the low number of repetitions did not make any fatiguing demands on your legs. They're probably good to go for another 10.
Since what you're feeling is not leg fatigue but more likely related to elevated heart and respiration rates, do 10 more — but this time, I want you to time them a little differently.