85kg rider & bike, 6% grade, 80rpm cadence, 170mm cranks, 15.7mph = 447W
Force required to simply push down the pedal (70 degree effective pedal stroke) = 181 lbs.
Downward pedal force assisted by an upward pull over the last 45 degrees of the upstroke (115 degree effective pedal stroke) = 110 lbs.
When I'm climbing hard out of the saddle in a big gear, I know there is significant pull over the top from my back leg, so that's why I included the second number.
Of course, instead of crunching that big gear, you could spin at 95 rpm and the first number goes down to 152 lbs. The second number drops to 93 lbs at 95rpm.
http://www.analyticcycling.com/ForcesPower_Page.html
I can climb at 450+W in the saddle and I don't do any gym work, so none of those forces are beyond the bounds of human capability.