Is there any evidence that low cadence is inherently dangerous, or is it just that, for a given power, if the cadence is halved the force at the pedals needs to be doubled. It seems likely that it is the high force that damages the knees rather than the cadence.
Yes, it's just that, the higher force
during rotation damaging the knee, since it's the equivalent of multiplying a persons weight while walking or them carrying far too great a load while walking.
During rotation of the knee joint, the bursa, which is a fluid filled shock absorbing bag between the faces of the bones, rolls with the movement. If the load is excessive, it can break down or squash aside and allow bone to bone contact causing abrasion. That's stage one of arthritis in the knee, a condition which never gets better and tends to be self aggravating with further exercise.
Your Quando isn't a problem in most conditions since it's power and moderate motor gearing means it can climb moderate hills without help, and quite steep hills with very little help. With the improved rolling characteristics of my Q bike version I drove it on a half discharged battery all the way up my 1 in 7 (14%) today without pedalling, showing that even in my area it's mostly not a problem.
The cadence problem arose for me for two reasons. First that I occasionally have an exceptional hill as you've remarked, but most because I tow a large often heavily loaded trailer, and 150 kilos uphill means it does need quite a bit of help.
The evidence of the knee damage and it's cause is abundant and has long been a matter for discussion in cycling circles. There was a lack of good gearing ranges in earlier days. The first cyclists had no gears and performed prodigious feats at very low cadences, then the introduction of the three speed hub gear still left a far from adequate range and could ease the situation a little, but those using them were often considered wimps. Early derailleurs were only 4 speed and it took a while for larger gear ranges to arrive.
I believe the true damage situation and it's cause was masked for years by the much shorter lifespans of yesteryear and the many other circumstances in those harder times which could also do physical harm. It was the growth of competitive cycling post World War II combined with medical advances which eventually revealed the nature and cause of the problem, this leading to realisation that cadences which were both higher and more consistent via better gear ranges were needed. Once it dawned on the cycling world that these also improved performance and records starting tumbling, the battle was won in the sporting area. The message has never spread to the general public, in part because the realisations mentioned coincided with the marked decline of cycling in favour of car use, leaving no-one to preach to.
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