Experts used to believe that lactic acid build-up was what made your muscles get tired while working. But researchers recently proved that theory wrong: The only thing slowing you down is your brain. When you push harder than normal, the brain thinks you might damage your body. So it freaks out, reducing electrical output to your muscles until you quit.
The good news is that you can retrain your brain to chill by progressively pushing a little harder each time rather than making sudden changes. Add one extra set of an exercise a week and you’ll slowly but surely build up your stamina.