From what I am reading from your post, this sounds like a simple run down. I don't see any protection being given to the runner in this instance. If, however, one of the fielders not in possession of the ball or about to recieve the ball do something that prevents the runner from either advancing or retreating safely, then we may have obstruction. From what you typed, though, I don't see anything that would require umpire involvement, other than watching to see if a tag is applied successfully.
Scott
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