When bodybuilders lift heavy weights with their arms, their muscles get tired first because they run out of a quick energy source called phosphocreatine, but after the first set, the burn they feel comes mostly from acid building up in the muscles.

When eight trained bodybuilders did heavy arm curls, their muscle energy stores dropped a lot after just one set and still stayed low after three sets, showing their muscles were burning through quick energy fast.

When bodybuilders lift heavy weights and their muscles get more acidic, they tend to feel more tired in later sets — this might be why their performance drops.