I have made quite a few stills and my opinion, whether anyone agrees or not, is the PERP was wearing shin guards with a strap that wraps under the foot and I believe that may be what came loose. At some point, he may have fixed it. I did notice something flopping, as well, in multiple frames (not pixels). I believe the PERP was wearing short black boots of some sort, to the ankle maybe, not to the knee. I will post a picture of
similar shin guards, there are literally hundreds of types with the foot straps this is for example only.