Several of the Astros' most reliable relief pitchers have become more vulnerable lately, with Chris Sampson and Alberto Arias among those coming to mind. Since July 29, the bullpen--which had been so good--registered a 5+ ERA. We hear--usually after the fact--that some relief pitchers have been trying to pitch through pain or soreness. I'm not suggesting that the deterioration in reliever performance is solely related to workload. Sampson, Arias, Jeff Fulchino, and LaTroy Hawkins all had ERAs which were likely to see some regression to the mean.
How much of the pitching work load has been put on the bullpen so far this season? Apparently more than ever. Below you can see the percentage of total batters faced for seasons 2005 - current.
Relief Pitchers' Pecentage of Total Batters Versus Astros
2009 (current) 37.8%
Is it any coincidence that the very low reliance on the bullpen in 2005 coincided with a World Series appearance by the Astros? The Astros' reliance on the bullpen has been creeping upward since then. At this point, I think it's fair to say that the bullpen performance is likely to decline further at the current rate of usage, unless something changes. One possible solution is to add fresh arms to the bullpen as the current arms decline. Another possible answer is to get better performances out of the starting pitchers, so that they can pitch deeper in games. I'll leave it to you to guess how likely those scenarios might be.