It comes from physics and how power is generated.
Power = (force x distance) / time
making specific to cycling we get...
Power = PedalForce*RPM
To generate equivalent power & speed at two different cadences.. say... 60 rpm vs. 90 rpm we have:
Power1 = Power2
PedalForce1*RPM1 = PedalForce2*RPM2
PedalForce1*60rpms = PedalForce2*90rpms
PedalForce1 = PedalForce2*1.333
Basically mashing your gears at 60rpms will require 33% more pedal-force than spinning at 90rpms for the same power & speed. This generates a lot more stress and strain on your knees at the lower RPMs. Especially when you try to go faster and start pushing harder and harder to keep up with the guys that are spinning. At 20mph average, mashing at 60rpms will feel like a lot of work, while spinning @ 20mph is a piece of cake. Forget it at 25mph..
An even bigger disparity occurs when you're going for maximum-speeds like in a sprint where you're pushing on the pedals at 100%. If you're trying to push a big gear at 100% pedal-effort at 60rpms, an equally-strong guy pushing on his pedals with EXACTLY the same force, but using a lower gear for 120rpms will generate TWICE the power (+100% more). He'll end up going 25% faster than you for the same strength, heh, heh...
Aside from fit issues, which should be optimized anyway, knee pains are often caused by low-RPM mashing. Even if you don't have any knee issues now, it's best to be preventative and spin to apply as low forces on the knees as possible to ward off future injuries. Plenty of people have posted about knee pains and going to higher-RPMs with easier gears alleviated that problem.
Last edited by DannoXYZ; 06-26-07 at 02:39 PM.