Many years ago when I was ~15 I rode road bikes and pushed myself pretty hard. I developed knee problems and was told by sports doctors that I should be using a shorter gear and turning ~100RPM cadence. Those knee problems eventually forced me off the bike. Now 20 years later I've been slowly getting back into it over the last couple years - not really a whole hearted comeback since free time is nearly nonexistent with a young family.

With the riding I've done one knee feels perfect the other is maybe 90%. I can feel something is not exactly right but I definitely wouldn't say it's painful. The effects don't last, as in my knee doesn't hurt after the ride or the next day, just the lingering feeling that things maybe aren't 100%. I've been keeping my cadence around 95RPM but I almost feel that in itself is putting some stress on my knee. The last while with the bike on the trainer I've tried slowing down to about 75RPM and I don't feel any more stress on my knee compared to 95RPM, maybe even a little less. But the 75RPM makes for a much more enjoyable workout. I feel like I can put some power down rather than spinning like crazy like running full speed down a steep hill if that makes any sense.

So I guess the question is: is 75RPM considered too slow of a cadence? or should I be sticking with the higher cadence given my possible or pending knee problems?

Its called getting old. Actually, for someone just getting back on a bike, 75 is a good baseline to train your knees until there strong enough for more strenuous workouts. It took me 500 miles of low 70s cadence riding until my knees felt strong enough to pedal 90-100rpm without much soreness or tightness in the muscles and joints. And as always, drink plenty of water during and after your ride.

You'll have to work it out for yourself, but it's not just the cadence, it's the cadence in relation to the speed for a given terrain. Your trainer experience is not really relevant unless you're going the same speed at 75 vs 90+ and on a high quality trainer.

I've had knee problems due to hockey and "jackass" type injuries. I find that I have to maintain higher cadences in cold weather especially. But I've trained so that I am comfortable with cadence in the 90-100 range in any temperature, so occasionally mashing at 60 on big hills is not a problem since it's not sop.

A low cadence in it self isn't bad for the knees. Pushing a high gear at a lower cadence vs spinning at a lower gear is what is bad on the knees. If you all you can pedal is 75 rpm thats fine. If you can spin at 90 rpm in the big crank, but instead do 60 rpm in the big crank then that is when you are likely to strain your knees.

Are you sure the bike is fitted correctly and the saddle is at the correct height?
FYI: After my knee surgery a few years ago my sports med doc prescribed riding the bike as my physical therapy.

I'm with RonH. It sounds like a fitting problem, not a cadence problem.

In addition to saddle height and setback, I'd consider cleat location and pronation matters.

I used to have issues with my right knee, similar to your description. At a fitting it was discovered that my right foot tips in towards the crank. I use SPDs and Specialized makes wedges that fit inside the shoe to correct this. This eliminated issues in my right knee. (Last I heard, Trek was also coming out with a set of wedges for SPD shoes, but I've been unable to verify this.) There are other wedges that fit between road cleats and the shoe.

For some unknown reason, I tend to sit slightly crooked on the saddle. I've been unable to correct this on my own. Moving my cleats (one forward, one aft) has largely corrected the matter. It wasn't bothering my knee before, but it did for a few days after the change until I got used to it. While it wasn't bothering my knee to begin with, perhaps you have a similar issue that's causing you problems--leg or femur length differences for instance.

Finally, have someone watch you from the front and back while you're pedaling. Have them look to be sure the trouble-prone knee is moving straight up and down, not moving in an oval. I see that a lot on other cyclists as I'm riding around.

I concur with the others. Make sure your position on the bike is correct before worrying about cadence.

Thanks for all the input.

I'll play with the positioning/mechanics to see if things can improve. I don't think that the bike is causing the issues with my knee since even when I hadn't been on it in a few months I could still tell there was something up with my knee... but still no harm in trying. I just want to do everything I can to make sure it doesn't get worse. At any rate I'll stick with the smaller gears & higher cadence.

As a side note I do think the frame is a little too small. It's a Giant FCR2 size:large and I feel a little cramped - more so with my arms & back. The saddle position feels OK. I recently bought a Giant XTC0 size:XL and it feels great though by the #'s it should be too big for me since AFAIK I'm 6'0". Maybe I'll see if I can find a XL frame for the FCR2. But again I think that is for back/arm comfort, not sure if it would affect my knees any.