Our posture can make it easier or harder when using a keyboard. Alan Kogosowski (Genius of the Piano) provides a great description that resonates with me.
He describes using the seat as a fulcrum on which to pivot the body, so that our energy ascends even as our weight is taken by the chair. Our upper back, between the shoulder blades, can then provide support to the arms. Our arms are no longer carrying themselves alone, but the whole of us is poised for action
I suggest you play with this initially by finding your sit bones and pivoting gently back and forward (see my blog Towards a better way to sit). Notice what difference it makes to your upper back, and then to your arms.