Is there any exercise for finger extensors based on bodyweight?
I'm looking for something that can be done anywhere without any special equipment. I've heard, for example, about opening hand in a bucket of rice, but I don't have a bucket of rice anywhere near, and I don't want to waste food...
Elastic bands are commonly used for this purpose. Commercial products (e.g. photo below) will typically come in a variety of different tensions, with the idea that you progress through them as you get stronger, but you can also just use ordinary elastic bands from a stationary supply store, and increase the tension by adding more bands at the same time.
