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...

When you push your muscles to their absolute limit with heavy lifting, your muscles quickly pull in more sugar and turn it into a form that helps them keep working hard — like revving up a battery to keep going.

When you lift super heavy weights until you can't do another rep, your muscles use up a lot of their stored sugar (glycogen) to keep going — this shows your body is breaking down that sugar fast to power your workout.

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.