Even though your body is still pumping out a stress hormone after nine sprints, your muscles stop breaking down sugar and making lactic acid during the tenth sprint — like your body’s alarm system is still on, but your muscles decided to ignore it.

Even though the person gets much weaker during their 10th sprint compared to the first, their muscles aren’t producing more lactic acid — which is weird because their stress hormone (epinephrine) is high, and you’d expect lactic acid to rise. So something is shutting down the energy system that usually makes lactic acid.