"It's the first time engineered muscle has been created that contracts as strongly as native neonatal [newborn] skeletal muscle," Duke researcher Nenad Bursac told the BBC.
To make the muscles as responsive and strong as the real thing, they created little gaps in the fibers where muscle stem cells could grow. This also allowed them to heal themselves, as Alan Boyle explained at NBC News:
When a natural-born muscle is injured, the satellite cells are activated to begin the regeneration process. The researchers found that their lab-grown muscles did likewise when they were damaged with a toxin found in snake venom.