Originally Posted by Carbonfiberboy
Here's a total oddity that might contribute to the discussion. I mostly climb in the saddle. I'm not a hollow-boned waif. From time to time, I get up to rest or power over a little bump. Every time I do that, my HR will pop up about 5 beats, even though I hold the speed the same and shift up to a more appropriate resistance. However, riding a spin bike with that heavy flywheel at a climbing resistance and low cadence, if I get out of the saddle at the same resistance and cadence, my HR drops about 2 beats. That has to be something to do with the flywheel effect on what is effectively a fixie. So is that why some people enjoy climbing with their fixies?
your legs can produce more torque when you're standing on your pedals, because you're now using gravity to assist in your downstroke; but your body also has to do more work on the upstroke (both to lift your leg against gravity and retain balance). So you can make yourself climb faster when out of the saddle, but you're forcing your body to do more work, and that work increases in relation to your weight.
On a fixed wheel, the momentum from the wheel is also assisting on your upstroke, while gravity is still providing the same boost on the downstroke as on a freewheel bike, which is why your body would do less work and your HR drops.