This amazing video was taken by a research team headed by Nicolai Konow of Brown University, in order to study how bats became the only mammal that can fly.
The team specifically looked at a species called Seba's short-tailed bat, and used technology called XROMM (X-ray Reconstruction of Moving Morphology) that integrates three-dimensional renderings of bone structure into X-ray video, allowing for detailed analysis of muscle mechanics and anatomy during an animal's movement. Their analysis—which they presented today at a meeting of the Society for Experimental Biology—showed that the bats first stretch out the tendons that anchor their biceps and triceps muscles to their bones, then compress the tendons to release energy and power their flight upward.
Read more about the research at Smithsonian's Surprising Science blog. Link