Fact: the Mets bullpen has fallen apart over the past week or so, hemorrhaging runs like a wound that just won't clot, regardless of how much dirt you rub in it or makeshift bandages you fashion out of paper bags and slug guts. A lot of theories have been floated that purport to explain the recent underwhelming play of the relief corps, from fatigue to poor supporting defense just regular crappiness. I just wanted to take a quick look at the state of the bullpen to see if they've really been overworked, or whether it's something else.

Dan Scotto made his case for improving the infield defense, particularly the right side, where Carlos Delgado has lead boots and Damion Easley doesn't quite move around like he used to. Anecdotally speaking, David Wright and Jose Reyes haven't been spectacular this year, but they've been at least decent and, well, they're not going anywhere. There's a little room for negotation at first and second, especially in the late innings, where maybe a run or two could be saved here and there. As Dan points out, the Mets' bullpen is chock full of groundball pitchers, which are an asset when your fielders are adept and less so when they're sluggardly oafs.

The idea that the Mets' starting rotation hasn't gone deep into games as an explanation for the bullpen's malaise has been bandied about quite a bit, but I don't know how much validity there is to it, really. We've all heard how Bob Gibson used to throw 93 complete games every year and how Cy Young would pitch nine days a week, but running the numbers shows that the Mets' bullpen hasn't been especially overworked relative to the rest of the league. Here are the average innings pitched per game start (IP/GS) for NL teams:

The Mets rank fifth in the National League in starting pitcher longevity, their rotation tossing more innings on average than all but four NL teams. That shouldn't be too surprising if you actually sit down and think about the construction of the Mets' starting staff. Sure, Pedro Martinez has been a crapshoot as has the fifth starter spot in general, but aside from that you've got four young-ish guys in John Maine, Oliver Perez, Johan Santana and Mike Pelfrey. Not too many years or innings on any of those arms, so it shouldn't be so much to ask them to buck up for six innings at a shot.

So the starters have been pulling their weight, what else? At first I thought that maybe the Mets had played a ton of long extra-inning games, so even if the rotation was going deep the bullpen could still be screwed if the Mets were constantly playing more than nine innings. Well, turns out that's not really the case, either:

The Mets are right about where they should be in terms of innings thrown by their relievers en masse. Their starters are fifth in most innings thrown and their bullpen is sixth in fewest innings per game. So, overall the Mets' relievers haven't been especially taxed.

What about individually? Well, Aaron Heilman has been abused. He has thrown 60 innings to this point, which actually only ranks him sixth in the NL. Still, that's an awful lot of innings through 111 games, so Heilman has probably been overworked a bit.

As for the rest of the bullpen, the next most-used Mets relievers are Billy Wagner and Duaner Sanchez at 47 innings apiece, tied for 40th in the NL. It may seem like a lot of innings for Wagner to throw as a closer, but fellow closers Salomon Torres, Kevin Gregg, Jose Valverde, Francisco Cordero and Brad Lidge have all thrown more. Sanchez, on the other hand, is a concern.

After having not thrown a pitch in a big league game since July 28, 2006, Sanchez has been one of the most oft-used relievers on the Mets this year. I'm not sure who's to blame for that, but I'm sure if you asked the Mets collectively at the beginning of the season whether they expected Sanchez to be so active by the first week of August I don't think many would have been on board with that plan. Even well-seasoned relievers wear down in the dog days of summer, so for someone who was basically out of baseball for 18 months it's probably too much to expect Sanchez to hold up under such circumstances.

Joe Smith could also be wearing down a bit, though he's young and has a couple of years of professional ball under his belt at this point. Pedro Feliciano, too, has shown signs of wear, and though he has appeared in just one fewer game than Heilman (58 to 57) he has thrown more than 20 fewer innings because he has been used far more selectively (i.e. often just against lefties and with runners on base). Feliciano has also been dreadful against righties this season, getting knocked around to the tune of .333/.416/.538 (!) in 90 plate appearances. That's like every righty who hits against him is Manny Ramirez. Feliciano has historically been at least decent against righties, but right now it's just an invitation to an ass-whooping to leave him out there against anything but lefties. The blame goes to the manager for not recognizing his utter ineffectiveness against the platoon advantage.

Aside from the relief appearances themselves, another thing to look at is the relative stress of those appearances. All appearances aren't created equal; I don't doubt that high pressure situations are more draining -- both physically and mentally -- than lower pressure spots. One easy way to compare relative duress is by using pLI, an indicator of mean leverage via the leverage index, an invention of Tom Tango. Thanks to the awesomeness of FanGraphs.com, we can easily compare the average pLI of all NL relievers. For the purpose of this exercise I've only included those pitchers who have compiled at least 40 innings pitched in relief.

The king of pLI has been the Giants' Brian Wilson, who has pitched in an average leverage situation of 2.35. Anything higher than 1.00 is considered high-leverage, though that scale is skewed somewhat for relievers since they tend to pitch in many more high-leverage situations than starters (late-game at-bats are typically more meaningful than early-game ones because of the reduced likelihood of coming back with so few innings remaining). Of the 62 pitchers who qualified for the above criteria, five are Mets. We can reasonably expect every team to have around four pitchers on the list (62 divided by 16 teams), so the Mets have one extra guy on there. Wagner is the highest at 14th overall (1.71 pLI). The next Met is Sanchez at 29th (1.34 pLI), then Heilman at 33rd (1.22 pLI), Joe Smith at 44th (1.05 pLI) and Scott Schoeneweis at 51st (0.88 pLI). I guess the good news is that the Mets' best reliever (Wagner) has been pitching in the most important situations. The bad news, at least in terms of figuring out what the problem is, is that the Mets' relievers don't seem especially taxed relative to the rest of the league. No clues here.

I guess fatigue seems like the most likely culprit at this point, though for different reasons for different pitchers. Wagner has actually been mostly terrific this season, though his last couple weeks have been particularly bad and that may be more due to injury than anything else. So, to summarize a bit:

Wagner: good, now hurt
Heilman: overused
Sanchez: too much, too soon
Smith: overexposed, maybe
Feliciano: something, ineffective against righties?

Wagner may be headed for the disabled list one way or another, and if the Mets can find an excuse to put Sanchez there to give him a breather it'd probably be for the best. Heilman could benefit from not pitching every day, and the Mets could consider calling up one (or both) of Jon Niese or Robert Parnell to get some work out of the bullpen if/when the aforementioned duo goes on the DL. Eddie Kunz will be a help, though he's not going to be used more than a couple of times a week, I wouldn't think.

So that's what I've got. Any other theories out there?