Monday, January 26, 2009

Back to Backs

Doogie2K did a recent article on the physiology of goaltending, focusing on the effects on the body of playing goal and why it is difficult to recover in time for optimal performance in back-to-back games. I found the article to be very interesting, and wanted to do a bit of additional research to try to quantify the back-to-back effect.

I don't have a quick way of figuring that out, so I decided to use the brute force method of scrolling through game logs. I took a sample of the top 10 goalies in games played since the lockout, and compared how they did in the first game vs. the second game of all of their back-to-backs over the last 3 seasons. What I found, somewhat surprisingly, was that the numbers were pretty much identical in both halves of the back-to-backs:

First Game: 131-80-13 (.614), 2.40, .918, 29.2 SA/60
Second Game: 124-72-27 (.617), 2.43, .918, 29.8 SA/60

However, we need to account for two major lurking variables: travel and strength of opposition. If teams are aware of the potential factors influencing back-to-back performance, they are more likely to avoid starting their goalies in back-to-back games. Therefore it is likely that the goalie will face only weaker opponents in the back-to-back games he does start. We would also expect that goalies would recover better if they did not have extensive travel between back-to-back games.

Here is the breakdown by Conference:

East: 2.40, .919, .628 in 1st game; 2.36, .922, .598 in 2nd game
West: 2.42, .914, .585 in 1st game; 2.53, .912, .651 in 2nd game

I expected the numbers from the Western Conference goalies to be especially influenced by weaker opposition, because of the more extensive travel teams have to endure. That seems to be supported by the increase in winning percentage in the second game, despite a drop in goaltending statistics.

The Eastern numbers suggest that the goalies have similar performance in back-to-backs, but the rest of the team of the team plays a bit worse (scoring rate drops slightly and shots against go up by 0.7 shots per game, resulting in a worse record).

The weaker opposition effect is less likely to be present for goalies who play in nearly every game. I broke it down by only looking at back-to-back results from seasons where a goalie played 70 games or more.

First Game: .641 win %, 2.21, .922, 28.4 SA/60
Second Game: .609 win %, 2.41, .916, 28.8 SA/60

Compare that to goalies who didn't play 70 games or more that season:

First Game: .563 win %, 2.71, .910, 30.2 SA/60
Second Game: .634 win %, 2.45, .922, 31.4 SA/60

These numbers suggest that schedule strength is a factor, and that teams do avoid starting goalies in back-to-backs against quality opponents. As always with goalie stats, these numbers are dependent on the rest of the team - we don't know what the impact is on the skaters, so maybe the numbers for the workhorse goalies have a lot to do with the type of shots being allowed. However, I think it is likely that goalies can be expected to perform somewhat worse in back-to-back situations, and the evidence suggests the effect is probably somewhere around .005-.010 in save percentage.

The sample sizes are fairly small for each goalie individually. However, for what it is worth Turco, DiPietro and Vokoun did better in the second half of back-to-backs, while Luongo, Kiprusoff, Nabokov, Brodeur and Giguere did worse, and Miller and Lundqvist were about the same. A lot of that probably had to do with strength of opposition.


Doogie2K said...

It makes sense intuitively that goalies would mostly take both halves of B2Bs when the opponent is weaker or when they play almost every game, anyway. The problem is teasing out the team effects, since everyone's going to be tired after playing the previous night. Interesting that we again see the shot-to-save-percentage inverse relationship throughout the data; I wouldn't have expected that to hold in a fatigued goaltender, but then maybe just one B2B in and of itself isn't enough to completely blow a goaltender with modern conditioning away, in terms of overall performance.

I suspect that the larger effect of B2B play, however, would be increased injury risk, which I touched on a little bit in the latter half, and arguably should have found more to say about. Of the guys you mentioned, more than half have suffered some sort of serious (10+-game) injury at some point since the lockout. With some help from TSN, we have Kiprusoff's knee, Luongo's groin, Brodeur's biceps tendon (which strikes me as a strong candidate for being an RSI, though I'd have to double-check; I'm only a student), Giguere's groin, and DiPietro's...well, entire lower body, plus we can toss in Hasek's legendary upper legs, which seemed to have problems at every turn (hips, groin, and hamstrings at different points -- if he'd injured his quads, too, he'd have hit for the cycle). If it were ever possible, I'd be very interested in seeing some scans of a goalie's muscles from before and after a B2B, and perhaps also compare the end-of-season tissue damage between a 70-game goalie (who will inevitably play a bunch of B2Bs) and a 50-game goalie (who likely seldom would).

Scott said...

Great article CG. This is one of the cases where there are just so many factors to keep track of. How big did your sample end up being for 70gm/season goalies?

The Contrarian Goaltender said...

How big did your sample end up being for 70gm/season goalies?

I had 138 back-to-backs for the 70+ game guys, so 276 games in total over the 3 seasons.

The Contrarian Goaltender said...

Doogie2K: Interesting comments on injury risk. That would be an interesting topic of study. First of all, I'd like to know how injuries correlate to games played, so we'd need find the injury rate of backup goalies to compare to starters. I would imagine injuries are dependent on style as well (I would expect someone who plays like Hasek to get injured more often than someone with a more traditional style, for example).

One thing I would like to check (although it would be a very involved study to undertake) is whether career workload influences results, i.e. do goalies decline depending on the number of shots they have faced, or games they have played, or simply because of age effects alone? From the work you have done, perhaps the number of back-to-back games or high-workload seasons or long playoff runs would also have an impact.