How does increased upper-body strength affect riding, and which muscles make the biggest difference? I am a college student, in Air Force ROTC, and I bike for 1.5-3.5 hours a day.  My cardio and core strength are great, but I haven't found the motivation to build my upper body.  I'm a skinny build, so it will take some work.

My question is: how will increased upper-body strength affect cycling, primarily all mountain and freeride type riding?

On top of HOW more upper body strength affects riding, which upper-body muscles will make the biggest difference?
For AM and FR (All Mountain and FreeRide) style riding, a strong upper torso is a huge improvement.  Like your lower body, you want a lot of it to be endurance as well as strength.  Keeping the front end straight and using your arms like shock absorbers as much as your legs will eventually take it's toll.  I have been on many a mountain bike ride in the spring after hours of snow riding in the winter (much closer to road riding) and had arms that were totally shot after only an hour or so while my legs were still quite fresh.  My personal feeling is that my shoulders and arms (uppers and forearms) seemed to be suffering the worst.

One of the main overlooked advantages of a strong upper body is being able to pump (like a pump track) while mountain riding to maintain speed.  I've seen strong BMX kids roll multiple laps around a pump track without pedaling at all, while a competitive road guy barely made a lap before his arms gave out.  Much of that can be translated to trail riding where you can use your upper body to extend your legs.