Best wishes to all of you -- knee problems can be a witch with a b. I have been fortunate -- two lateral patellar dislocations, the first in high school, the second at age 45, good recovery.
There is no one-size-fits all solution, but I think the best way to get aerobic exercise for anyone with knee problems is cycling in a low gear (low gear ratios have even become fashionable in the Tour de France
), making sure seat height and foot rotation (toe-in/toe-out) are correct for you.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069