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...

Lifting weights once can use up a lot of the sugar stored in your thigh muscles, which your body needs for energy during hard exercise.

After a short, super-hard workout, your muscles refill their energy stores way faster than after a long, slow one—even if you eat the same amount of carbs.

When muscle glycogen levels are much lower than normal, the biological signals that trigger muscle growth after weight training are less active.

When muscle glycogen levels drop below one-third of their normal storage capacity, the molecular signals that promote muscle growth after weight training are reduced.

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.