[greymule]

From the letter of the rules, we know that (1) a 1B or 3B coach cannot physically assist a runner, and (2) another runner can physically assist a runner.

So let's examine a case that falls into neither category. What if a runner trips coming home and a player from his dugout runs out, helps the runner up, and pushes him across home plate just before the ball arrives? What if the batboy assists the runner in such a way? The trainer?

Is there a rule that prohibits such action?

The usual reference books apparently have nothing on this (or assistance by a runner who has been put out), perhaps because if coaches can't assist a runner, it's obvious that other members of the offense can't, either.

Originally, the rule applied only to the 3B coach. It was later extended to include the 1B coach. I suspect that the rulesmakers simply never imagined anyone else assisting a runner.