When you push your muscles super hard until you can't do another rep, your muscles build up a lot of lactic acid—this means your body is burning energy without oxygen and getting more acidic inside.

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 eight trained bodybuilders did arm curls with heavy weights, their muscles made more lactic acid the more sets they did — which means their bodies were working so hard they had to rely on a fast, oxygen-free energy system.

When you work out really hard and your muscles burn, the buildup of lactic acid might help your muscles refill energy faster—but after lifting weights, less lactic acid means slower recovery.

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.

When trained men do certain types of weightlifting workouts, their bodies produce more lactic acid compared to other workout styles, which means their muscles are working harder and under more stress.