Hehe, DON'T DO IT!
Just wondering, roller or ice? I tend to bruise up my knees a little more outdoors since concrete can be less forgiving than the ice. How tall are you by the way?
Goalies who do the butterfly tend to develop more knee and hip problems. Most goalies who start out instinctively do a non-butterfly technique where the pad face ends up on the ice surface as opposed to being at the shooter. You cover less net, but your knees take less of a pounding. A guy like Brodeur does a half-butterfly, I've also seen it referred to as the 'right knee down' technique. Brodeur says he's quicker to recover than other butterfly goalies.
Example:
Butterfly:
http://www.kazlaaz.com/goalies/tips/butterfly01.jpg
Half-butterfly:
http://www.hockeyrulz.com/photos/brodeur03.jpg
The difference being, in the Brodeur pic, his right pad is lying on the ice as opposed to being perpendicular to the ice and facing the shooter. A lot of beginning goalies will have both pad faces on the ice. Like I said, it's a lot easier on the knees since you're using your pads as cushioning......as opposed to maximizing the amount of net they're blocking.
Depending on the level you're playing at, just remember that 70-75% of goals are along the ice.
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What is your price range on leg pads? Each company (aside from Brian's) seemingly makes pads in three different price ranges: $200-300 for roller hockey/beginning ice, $400-$500 for ice, $900-$1000 for those who obsess about hockey.
If you aren't certain you're going to be a goalie for more than a couple seasons, I'd go with the cheaper pads. But if you think this might be a more permanent thing, I'd at least spring for the $400-500 range.
The lower level pads offer zilch in terms of knee cushioning. I bought $99 CCM roller pads for Xmas, and my knees were just taking a beating to the point where I had to tape volleyball knee pads to the pads.
The $400-$500 pads will have more knee padding, as well as things like thigh boards. The $1000 pads will have some more bells and whistles that probably aren't needed if you're a beginner.
Here's a pic of pads with built in knee pads:
http://lib1.store.vip.sc5.yahoo.com/lib/hockeygiant/beastback.jpg
Here are my recommendations:
$99 - 2004 CCM Roller pads:
http://www.discounthockey.com/ccmrgsegonew.html - I have these. I don't like them very much (lots of rebounds), but you can't beat that price.
$199 - 2005 CCM Roller pads:
http://www.discounthockey.com/ccmpfs30sego.html - This year's model, pretty nice in terms of knee protection for the price. The pad itself is kinda ugly and seemingly made of some synthetic material.
$199 - 2004 Mission Motion Lite:
http://www.discounthockey.com/mimoligole.html - Had two buddies buy these in the last week. The pad itself is nicer than the CCMs, but the knee protection isn't as good. Not sure which one I'd recommend between these and the $199 CCMs.
$299 - 2005 Reebok Seniors:
http://www.peranis.net/prodHome.ihtml?sid=2&cid=27&pid=3987&dept=3 - These just came out. Just from looking at them, it seems to have the same construction as the Mission with the knee padding of the CCM.
$409 - 2004 Koho 490's:
http://www.goaliemonkey.com/koho490-pads-sr.html - I don't recall any extra knee padding on these. I'd personally spring the extra $50 on either of the next two.
$460 - 2004 Heaton Z's:
http://www.goaliemonkey.com/gpheatonhelitez-sr.html - I like these pads a lot, but couldn't get myself to buy them. They are nearly identical to the $1,000 versions. The knee and calf padding is there.
$460 - 2005 Itech 7.8:
http://www.goaliemonkey.com/itech-gl78vampelitesr.html - Just came out, these look very simliar to the Heaton Z's (no calf padding).
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Anyways, feel free to ask more questions. Buying goalie equipment is a big investment, so it's best to take an afternoon and try everything on since every goalie is gonna like his pads a certain way. The hockey shop should be more than understanding.