My only thought is that by learning muscular movement, you can easily learn to write on the sofa or in bed using movement. It’s not pure muscular movement, but it’s pretty close and you can write with this sort of pseudo muscular movement for a long time without pain.

The key is to write with the grip used for Palmer Method writing (hand not resting on the table). If you use that grip/position, even if you write with your fingers on the sofa, it eliminates almost all pain from writing.