This comes up often and I have an interest in this subject. I did a simple test on me. The bike has been in use for at least 10,000 miles and is set up as well as I know how. I use the bike on trails, roads and hydraulic resistance trainer.
The test changed one variable only: Cadence.
Speed was held at 16.7 MPH.
Cadence of 78 RPM resulted in knee ache after 20 minutes and I stopped that after 30 minutes. HR 114 max. The word ("ache could be called pain")
Cadence of 88 RPM resulted in relief of the ache in both knee's after 5 minutes and no repeat of ache. HR 122 max. Every 15 minutes I got off the saddle and do one minute at 18 MPH. Duration of this test was one hour. Water consumption was one liter per 30 minutes. No food consumption.
I am pleasantly surprised that spinning at light load removed the discomfort.
The reason for this test:
I observed that some European bikers on the XC tour pedaled at low cadence. Lower cadence does result into lower HR. I thought that I should check this out.
Some of you know that I have Arthritis in both knee's but that has not prevented me from going XC or doing 6 hours on a hydraulic resistance trainer.
Perhaps this is of some interest for the 50+ bikers?