When athletes do their 10th all-out sprint, they still keep about 73% of their top speed even though their body isn’t burning sugar as much anymore—so they must be using other energy sources like...

When a person does their first super-hard 6-second sprint out of ten, their muscles get energy from two sources at the same rate: one that uses up a stored chemical called phosphocreatine (which drops by over half), and another that makes lactic acid (which jumps up to a high level).

Even when your muscles are tired from doing ten all-out sprints, they're still able to quickly recharge a special energy fuel called phosphocreatine during the short breaks—so you can keep pushing hard, even if you're not as powerful as before.

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.