The top 3 running backs in the NFL after week 5 based on rushing yards are:
1. Arian Foster (Texans), 564 yds, 112 yds/game
2. Chris Johnson (Titans), 485 yds, 97 yds/game
3. Adrian Peterson (Vikings), 480 yds, 120 yds/game
Should be noted that Peterson was on a bye one week so his average is based on 4 games.
Now is you look at the average run defense ranking for the teams that each of their respective teams have played against:
Avg Run Def Rank (nfl.com) PFF Rating rank
Texans 16.7 19.2
Titans 15.6 10.4
Vikings 18.2 17.7
From these numbers I would say that the Texans and Vikings have played against slightly worse run defenses than the Titans. Now let's look at their own team's run blocking rank according to the PFF guys.
Run block team rank
Here we see a huge difference!! The Texans have one the best run blocking teams in the league compared to the Titans and Vikings who are in the bottom eight. Therefore Foster is being helped a great deal by his own team's blocking.