Thanks 'p'. yoga sounds like a great idea. might give it a go. I dont think the missus would be up 2 it though. unless i could convince her that its good for arthritis. she suffers it real bad.
That said, not all yoga classes are created equal. I would say a more gentle practice may suit someone with arthritis better. Particularly Iyengar yoga has a philosophy of adapting yoga to help people with ailments. Yoga teachers aren't (usually!) doctors though, so I would encourage your girlfriend to talk to her physician about yoga. Or possibly Tai Chi. A good Tai Chi class will definitely help you develop a certain amount of strength (particularly in the legs).
As some added motivation, here are a few "ripped" Yogis. These guys aren't tanks, but they are good yoga practioners who have some muscle:
Andre Sidersky
Glenn Ceresoli:
Some guy from New Zeland called Shane lol! (he's all over this site):
Now I'm not saying that these guys ONLY do yoga, but I'd say a lot of their strength comes from their yoga. I say this because to get to the level they are at requires a lot of practice (although I think Shane from yoga.nz does mention some strength training too).
Good luck!
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"Let your food be your medicine, and your medicine be your food"
Hippocrates.