On the Cello I'm nearing the end my of introduction to second position. My teacher told me last week that we'd be starting 4th position in a few weeks. (He said he likes to go from 2nd to 4th and then back to 3rd... I guess I'll discover why when we get there!) Extensions are still a struggle but I am definitely getting closer to the maneuverability I'd like. As a matter of fact this weekend was pretty productive in that regard.
As with piano, the muscles in the torso, the ribs, shoulders and neck, in the pelvis, etc. etc. are all involved and need to be worked on. It's not just the hands and fingers, ever!
Cello and piano practice continue to enhance one another. I consider myself somewhat fortunate as a cello student to have exposure to that instrument of torture we know as a piano. ;) The good kind of torture of course. Much of the time when I'm working on my left hand (and all that supports it) at the piano I feel it is work that benefits me at the cello just as much. And vice versa. I know my piano playing has improved since starting cello lessons.
I'm reading a couple of very good books at the moment. One is new--just published but the other I've had for, well... more than a few years but I never finished it. I've taken it up again.
The newly published book is Pedro de Alcantara's Integrated Practice. I'm not going to write anything about it until I've finished the book. Except to say that it's good. Really good.
The book that I've had on my shelf for the last 5 or so years is W. A. Mathieu's text Harmonic Experience. It's a tome and a textbook and pricey but ever so worth it. The first few chapters anyway (about as far as I've gotten) are a masterpiece. His is a fresh and exciting look at harmony. Now that I have a cello (capable of producing a drone) I can do several of the exercises from the book. Many of these exercises are singing one of the overtones against the drone (low C in most cases) to hear the exquisite silvery liquid beauty of harmony. (Something we don't hear too much of on modern well-tempered pianos!)
I'll have more to write about both these books as I progress. But do look them up. They are treasures.