The runner was impeded so by rule that is obstruction. Doesn't matter if it was intentional or not.
So feet up or not he was doomed from the start because he dove for the ball......
But yeah he did trip over Middlebrook's waist. The feet had nothing to do with him tripping.
I repeat, a ridiculous ending to the game. Calling interference when Middlebrooks was on the ground due to throw. There was no way for Middlebrooks to get out of the way.