I can't do yoga because I have bad knees... it's rather the exact opposite of relaxing; excruciating.
But I can do splits (sitting or standing), backbends, back walkovers, handstands, flips... all sorts of gymnastics tricks. I just can't hold poses that require me to have most of my weight on my knees for extended periods of time (any kind of lunge, squat, etc.). Those poses (in all 3 kinds of yoga I've tried) account for about 2/3s of all the poses.
I can snowboard (because it's below freezing, and one's knees are never held in the same position for very long, and because the muscles are constantly shifting). I can ride horses. I can surf. I can swim and dive. But because I have loose tendons and ligaments in all my joints... yoga actually destroys my body. Every time I've attempted yoga classes I've ended up injured for several weeks. Which is neither relaxing, nor strengthening. The last time I royally screwed up my back. Yoga isn't for everyone, but most people enjoy it. ((Hint: Most gymnasts and dancers I know can't do yoga... because we sculpted our bodies in different ways and have different needs for our joints)). Runners... I've noticed... tend to really dig yoga.