I've had that back pain - and I've reached for my back, and cried out in pain. When you had that back pain - did you just shake your head and wobble around? I've never had or seen anyone who has intense back pain just stumble like a drunk person and shake their head and not at all reach for their back or cry out in pain.
There comes a point where "proof" can be replaced by common sense. If you watch the tape and think that him shaking his head, wobbling and stumbling like someone who was knocked senseless is from back pain then I don't know what to tell you. It simply is not plausible. Common sense,