Beware, opposing running backs. The San Francisco 49ers are coming to hurt you -- not with overly aggressive handshakes, either.

Five Ring Circus gets credit/blame for this entry after setting off a discussion in the comments section of the Chat Wrap earlier. His boast about the 49ers' run defense included enough supporting evidence to spark a mini-investigation.

"Niners have ruined several fantasy seasons," Five Ring Circus mused, exhibiting the swagger a 5-1 record affords a fan. "Maybe Sando can keep a running total of RBs knocked out for significant period: Arian Foster, Felix Jones, LeGarrette Blount, Jahvid Best."

To be sure, San Francisco and Seattle run defenses have done the NFC West proud this season. The 49ers rank second in rushing yards allowed per game. The Seahawks rank first in yards per carry allowed.

Back to the claim about the 49ers knocking out running backs.

Foster tweaked a hamstring against the 49ers during preseason, and some fans insist a hit by the 49ers' Patrick Willis played a role. I'm not certain of that, but play-by-play records show Willis tackling Foster on what turned out to be his final play in the game.

The Cowboys' Jones suffered a separated shoulder when Willis and fellow linebacker NaVorro Bowman tackled him in Week 2. Willis made the primary tackle, with Bowman landing on Jones as the running back went to the ground.

Tampa Bay's Blount suffered a leg injury against the 49ers in Week 5. He came up limping after taking a big hit to the legs from free safety Dashon Goldson following a 29-yard reception midway through the third quarter. He did not touch the ball again during that game.

Best suffered a concussion against the 49ers last week. I'm not sure when.

For the regular season, the 49ers have injured three of their six opponents' starting backs. Next up: Peyton Hillis and the Cleveland Browns in Week 8. Hillis has missed one game already. He missed practice Thursday with a hamstring injury. If he plays in Week 7, he'll have to get through Seattle's run defense before facing the 49ers.

Update: Hey, great minds think alike. Nice job by Cam Inman writing about this earlier Thursday. Thanks for the heads up, Cam!

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2011 49ers Defense vs. Starting Running Backs