Your muscles store sugar (glycogen) to use as quick energy when you lift weights.

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 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.

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.

Even after a really tough weightlifting session, your muscles don’t run out of fuel fast enough to slow you down, and they can refill just fine even if you don’t eat carbs for a while.