QUESTION: I know a fielder cannot block a base without the ball, but can they straddle a base, home plate included without the ball? They are giving the runner a way to get to the base on a close play. Your expertise is appreciated. Thanks
ANSWER: Yes they can but they still face the risk of committing obstruction. Remember the runner gets their path to the base and is no required to slide.
Typically if you are in a situation where the defense is straddling the base you are looking at a close play and the runner will slide. I can't ever see calling obstruction on this play. If in the rare instance the runner comes in standing (they are never required to slide) and they adjust how they are approaching the base/plate based on the position of the defense, you could have an obstruction call.
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QUESTION: I'm not trying to be obtuse, but you make it sound theoretically possible to have obstruction on a steal of second with the fielder straddling the base if the runner adjusts their path because the fielder is in the way and doesn't yet have the ball.
If the baserunner has to adjust based on the position of the fielder, who is not in possession of the ball you have obstruction. I did say that you would rarely see this call.
From your reply "runner adjusts their path because the fielder is in the way and doesn't yet have the ball". This is the definition of obstruction. I couldn't have said it clearer myself.