45 here, and hoping to get some fitness advice. Some history - started out long ago in the BMX crowd (obviously, single speed), and then moved into MTB single speed racing. Both times no physical problems. From there got into road riding and never could deal with all the gears, just not my style. So after a few years I converted my road bike into a three speed. I live in a very hilly area so the gearing was quite laxed being 42 x 21, 18, 16. I would use the 21 for climbing, the 18 for flat warmups, and the 16 for main flats. Then, wanting to get back into single speeding, I decided to finish the road convert and go all single speed. I figured I would go in the middle and use the 42x18 gear. This gear ables to get me up the climbs and spin rapidly on the flats. Thing is, I am now experiencing pain behind my left knee that was never there before. I know it's not a bike fit issue as the three speed never caused me any pain (same bike). Now what I don't know is if it is from climbing in the higher gear, or spinning in the lower gear... Anyone?