Humidifier specs?

My neighbor thinks her knee pain is related to humidity. To test that theory, I looked into dehumidifiers. But I'm hopelessly confused. About the only specs I find are "pints/24 hours removed". Under what conditions? The "buyer's guides" talk about conditions ranging from wet to extremely humid.
I'm looking to take air at 68F from 65 to 40 percent RH or thereabouts. Looking at some of the engineering data suggests that the dew point is very low and it'll be a LOT harder to do that than to get from 100% to 80% on a 95F day. Outside temp is 45F or so this part of the year. The house is pretty "tight".
What are my options to construct an experiment? She has central air, so I could alternate between heat and cooling???? But that sounds like a lot more cooling than dehumidifying.
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