[Kevin]

[B]
Berg seems limp just before the beheading. It is not clear if he was moving after the time skip in the tape. While on the ground, Berg's body didn't seem move except in response to the captors movements

I didn't watch the video, however, if he was beheaded from behind (which is kind of what I'd imagine happened), one of the first things that would have been severed would have been the spinal cord. That would explain the lack of struggle -- instant paraplegic.