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You're lifting weights. The first time it seems easy, but each lift requires more and more effort until you can't continue. Inside your arms, the muscles responsible for lifting have become unable to contract. What's going on? Christian Moro explains how exactly our muscles work and what causes them to tire. Lesson by Christian Moro, animation by Nichola Latzgo. Watch the full lesson at: https://ed.ted.com/lessons/the-surprise...