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So, many of us have issues with that thumb. Don't let it bend back, or risk injury. Keep it behind the second finger. Don't use it to grasp the cello, you need to slide it freely. There are other issues, so please mention those, too.
I want to share a tip about grasping the cello with a thumb hold. We are not wrestling with the cello, so we do not need a "thumb cello hold". We should be letting the cello rest and balance against our bodies. That is why seeing a cellist in a proper position is such a beautiful thing. They are one. When done right, to me, it looks like one object, the body and the cello combined.
I was told, have read, and have seen videos stating this, that to get out of the habit of a "thumb cello hold", do not use you thumb. You just let your cello rest against you as normal, but do not put the thumb behind the cello. From the videos I saw the thumb is pointing up, parallel with the cello fingerboard, but not touching it.
Play your cello this way for a while, not just talking hours here. Then, when you feel you are stable with the cello, just let the thumb curve gently behind the fingerboard behind the second finger. You don't need the "thumb cello hold". Your thumb should just be noticed when you are pressing your fingers on the fingerboard. It gives the fingers a little more strength, that is the best way I can explain it. Maybe stabilize is better.
If you feel yourself doing that "thumb cello hold", repeat this. Sometimes we need little reminders.
You are basically stroking the back of the cello fingerboard as you shift or do string changes, or extended notes. You cannot do this if you are wrestling with your cello with a "thumb cello hold". With a "thumb cello hold", shifting will be difficult, if possible at all. Stretches will be difficult, if possible at all. You could injure your thumb.
This is what I have been told, read and saw in videos. I am not an instructor, and definitely not an real experienced cellist, or an issue free cellist. I am now taking viola lessons, but am still working on cello on my own. I love the beautiful graceful cello. I hope this works for or helps you.
I used to have just that ”thumb cello hold”, but it’s much better now. I practiced without using the thumb and tried to feel the weight of my hand pressing the strings. It was great method to loosen the thumb, but it worked only little time at first. Then I found myself squeezing it again. My only solution to this problem has been time and practicing more. One thing I have noticed is that when I’m playing some new and difficult piece, my thumb easily stiffens again. When I learn the piece, it’s easier to keep it in the right position.