Ballroom dancing is especially hard to capture, because the key dots on individuals get covered when people are very close together. These reference points are needed to capture movement accurately, posing a big challenge.

An interesting thing I've seen coming out is the idea of using large magnets. Essentially, you would have a large source magnet that blankets an area with a known magnetic field. Then, a node is moved around in 3D space which consists of 3 loops of wire in the X,Y,Z axes. This way, we can get a sense of the orientation and motion of the node based on the changing magnetic flux through the coil of wire.

Not sure how it compares to the optical method of motion capture, but pretty cool none the less :)