Yeah, the arthritic folks I've ridden with - like zero knee cartilage - ride seated, take ibuprofen, and spin 90 on the climbs. Itty-bitty gears unless you're aerobic capacity is very high. But it depends on what one means by "arthritic." I went XC skiing today with a woman with diagnosed arthritic knees who has found that exercise is very, very helpful, to the tune of 10 miles, 4 hours, and 1100' today before lunch. it is really sweet exercise, zero impact, lots of small muscle recruitment as well as the major movers. One doesn't get that combo on the bike.
She'll ride tandem with her husband and us on Sunday, 40 miles and 2100', hard as we can go. Good medicine. We don't stand much, just butt breaks or at a hill crest for momentum.
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