It's a terrible, and common, mistake to think that the hands are the only thing responsible for playing an instrument. The whole body is an integrated unit with everything relying on and affecting everything else.
Below are all 7 parts of Michael Bonner's video series on posture for guitarists.
Part 1: The Spine: What Makes It Happy?
Part 2: Sitting: You're Doing It Wrong
Part 3: Standing: You're Doing It Wrong
Part 4: The SHOCKING Truth About The Footstool
Part 5: Sitting To Strum: You're Doing It Wrong
Part 6: Practical Guidelines For Guitaring: Standing
Part 7: Practical Guidelines For Guitaring: Sitting