X-ray videos of bats in flight have revealed the movement of their skeletons and how they recycle energy for efficient flying, U.S. researchers say.

The videos revealed how the bats store and recycle energy in very stretchy tendons that connect their upper arm muscles to their bones, they said.

The study was designed to show exactly "how the skeleton moves with in an organism," lead researcher Nicolai Konow of Brown University said.

"Bats don't like being on the ground, so we just put them on the ground and filmed them as they took off," he told the BBC.

A change in the length of the bats' muscles and tendons revealed their stretchy, energy-storing properties, he said.

"Most small mammals have stiff, thick tendons so they cannot stretch or store energy in them like we do in our Achilles tendon when we run or walk," Knonow said.

But the fruit bats in the study stored energy as they stretched their bicep and tricep tendons during takeoff and climbing flight.

Just like a stretched rubber band will snap back, releasing this "elastic energy" gives the animal an extra power boost, Konow said.

"The fact that bats are able to pack such an incredible functional repertoire into such a tiny body utterly amazes me," he said.

The researchers presented their study at the Society for Experimental Biology's annual meeting in Valencia, Spain.