Before I broke my leg this summer, I could squat more than my body weight, bench within 10% of it, do maybe 10 pullups, and had somewhere around 21% body fat; so I considered myself in at least passable shape.  But I always felt like my riding would be better if I were a lot stronger, and/or if I had less dead body weight (I'm not that great a rider, although I'd only been at it for about 2 1/2 years).

So... do all you good riders have really high strength-to-weight ratios, or what?  Because I know how much strength it took to do the lame tricks I was doing, and then I see other guys killing it and think "Jeez, they must be all muscle or something."   

Obviously I just need to put more hours in on the bike once I get my leg back.  But it seems like the strength/weight thing would help too.  And my bike's not too much of a supertank, it's maybe 26-27 lbs with front and back brakes.

Thoughts? 

sounds like your problem might just be endurance, rather than strength.

are you saying your arm muscles hurt after riding for an hour? 2?

my legs might be sore the day after a long session, but i've never left
my spot with muscle pain.
another thought is it shouldn't be that hard to hold a trick once you're
in the sweet spot.

idk

I'm by no means a good rider, but I have been described as freakishly strong by many.

During Festivus, I usually win. (I think that reference might be lost on a lot of the younger dudes on here...  )

I'm not much for the for feats of strength....but I'm a champ for the airing of grievances! 

I usually get up in the morning and rip a phonebook in half. I don't have a front door, I just pay some masons to brick up my entryway every night so I can smash through it like the kool-aid man once a day. I bench cars. When I do squats Atlas goes "whoa".

TJ-

"HULK SMASH"

I usually get up in the morning and rip a phonebook in half.

no lies: a guy i ride with can do just that.

I am far from pro or even top-am level, but the tricks I can do take virtually no strength once you know what you're doing.

I may have posted something like this before, I'm kinda into exercise physiology... And Havok is very right. Endurance is probably your problem. The other thing to remember is kind of what Wolverine said... But to make it a little more tech lets call it by its real scientific name,  the SAID principle... this stands for:
Specific
Adaptation to
Imposed
Demands
 
Basically, this principle of exercise means that whatever you practice or work at you will get good at. Yes, seems simple, but you will fine many people trying to "cross train" and wasting time with exercises that will not help them in their sport.
Here is a link to an article that I really like by an old BMX racer/ medical doctor and Super Slow trainer. He hits on some really good points.
http://www.ultimate-exercise.com/bmx4.html

Example: I'm 37... yes, I'm old, hahahaha.
I started riding flat again about 1.5 years ago after about 3-4 years off a flat bike. I can ride a road bike full speed, 25-27MPH for about 20-30min. without a problem and without being winded. But I can barley get a 30-40 second link done without coughing up a lung and without my knees about to give out. Riding a road bike gives me "generic endurance", it is not specific to flatland and the demands that my body needs to preform the actions that the link needs.
If you want to become good at flatland, practice flatland. If you want to build strength without the stress and danger of hurting yourself you need to look for exercises that mimic the SAME actions that you would do in that trick in order for it to truely help you. As for endurance... Well any cardio exercise will help some, but again the demands are different, so you really need to practice what you need the endurance for. Both strength and enduance training will help anyones game, that's a given, but it will help more if it is done right.
Hope this helps some of yall a little.
PS
jm, aren't you a yogi? You should do a yoga/flatland bike strength class or something. I'd come to TX to take it! But instead of a yoga mat, you use your bike, that would be a cool class. Or at least I think it would be.
Rob