So here's something most of you probably never realized. Exercising your biceps will supposedly make your legs just as tired as working out your arms. And researchers now know why.
Researchers at Nottingham Trent University in England tested subjects on two different exercise setups: one group did two rounds of intense leg exercises while the other did arms followed by legs. Each group's second set of exercises showed about a 33% decline in performance — despite half of the subjects starting with fresh legs.
The hypothesis is, while fuel use is contained to the muscle that's being exercised, the buildup of metabolites in the blood affects the entire body. So the lactate, hydrogen and potassium your arms churn up while you're blasting curls will slow down your "fresh" legs almost as much as if you were doing squats.