What in the world ?!?!?
We follow some pretty smart people on Twitter, probably like you do. In recent marathons this strange phenomenon was noted. There as been some great thought and productive chatter on what this might be.
One postulation was this:
Myokymia, a form of involuntary muscular movement, usually can be visualized on the skin as vermicular or continuous rippling movements
Another reasonable one was that with a fast shutter speed it could just be a fascial.fatty, skin/ muscle interface rippling effect after a loading response. Yes, even though the appearance is on the swing leg.
It certainly is strange and we are doing our homework into possibilities like the other wisemen/women.
Certainly, you will see similar horizontal striations in bodybuilders with low body fat, but we wonder about that hypothesis because those are more individualized to the different muscle bellies, this appearance here seems to span the entire quad and the rectus femoris just isn't that hearty we believe.

Have you seen it ? Thoughts ? Brainstorm . . . .

Thanks Ross Tucker, Adam Meakins and the others for a fun topic to ponder.