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.

High-intensity resistance exercise derives a substantial portion of its energy from the creatine phosphate system and aerobic fat oxidation, particularly during rest intervals.

When you push your muscles to their absolute limit with heavy lifting, a specific chemical in your muscles more than doubles — this might help your body keep making energy without running out of steam.

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.