The legs are those of the American Ballet Theater ballerina Christine Dunham. The same dancer can be seen in 1X.Com published images here and here and here.

This shot is more than it seems. The dancer is standing on a small platform covered in black velvet. The platform is on roller bearings on a motorized 12-foot track set up to move the dancer from upper right to lower left. The exposure is a combination of continuous spotlight and occasional electronic flash bursts. During the time exposure, she continuously moved her legs (a bourrée, for those dance aficionados out there), to keep the image from appearing too static.