A week ago, commenter MGM2, a proprioceptive researcher by trade, shared his observations of Manning's play, in which he detected a serious leg injury.
MGM2 has graciously permitted us to repost his comments here; we've rearranged them a bit, but you can see his original words here.
Unfortunately for the Broncos and Manning, MGM2 believes Peyton has a repetitive stress injury, one he is unlikely to bounce back from.
While we're of course disappointed by MGM2's prediction, we are extremely grateful for his input.
As an expert in body mechanics, I'm very sad to say that Manning is certainly dealing with an overuse injury. He reconfigured his throwing motion post-surgery to generate power from his base, and specifically rotation from his inner thighs. This is why he looked so dynamic and mobile at the beginning of last season.
The problem is that this is a fundamentally unsound mechanic, and eventually leads to repetitive stress injuries of the overworked muscles (in this case his thighs). We saw the result last year as he lost strength and mobility as the year progressed.
This year, the tipping point has been reached, and it seems that his thigh problem is getting worse every week no matter how much rest he gets. I'm sure we've all noticed how much less strength is in his throws recently, especially those quick outs to the sideline (when you need full rotational force). Also, look at his footwork- he is unable to push off his inner thighs and is much weaker stepping into the pocket.
This hasn't been terribly noticeable because it's a slow onset type of injury. But if you need proof, fire up any game from early last season and compare it to the past few weeks- it's rather obvious. This isn't 'age' or 'weather' but simply his muscles weakening from overuse.
The throws that have looked bad have mostly been to the sideline, where rotational force of the torso is critical. Overall, he has been compensating really well, but he is clearly losing strength and accuracy.
If I were going to gameplan for Manning's condition, I would come out throwing and then try to run later. With an RSI, strength and accuracy get worse as the game goes on, and the muscles tighten and weaken.
It makes me very sad to say I think we've seen the last of Manning's greatness. It's possible he can rework his throwing motion again this offseason, but very unlikely. It was a tremendous project to rework his motion post-surgery, and this would require similar relearning.
All I can say is that I really hope I'm wrong.
I'm a proprioceptive researcher, soon to publish works on the function of mammalian proprioception in muscle development and body mechanics (look up proprioception if you don't already know it- it's worth knowing). But it means I study and analyze human/animal movements all day, everyday (including sports, yay!)