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.

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 creatine and oxygen to keep going.

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).

When strong athletes lift super heavy weights until they can't do another rep, the energy stores in their muscles—called ATP and creatine phosphate—get used up fast, dropping by a lot in just a few seconds.