When you flex a muscle in your arm, it’s not the muscle that’s shrinking, but the cells that comprise it. Each cell lining the muscle’s length has its own limited contractile ability, one driven by tiny motor and ratchet proteins that crumple up a cell and refuse to let it relax. Rather than being a single continuous muscular action, your bicep contracts as a result of millions of tiny, discreet contractions all working in concert. That being the case, it’s always been something of a misnomer to refer to robot muscles as muscles, per-se — despite being visually similar, they are really just servos, or hydraulic pumps, or whatever else. This week marks the first proof of concept for an attempt to change that, to make usefully strong artificial muscles that actually work like muscles — with all the advantages and disadvantages that come with it.