There is often a difficulty in ADHD children connecting the input the muscles give to the brain. This makes otherwise simple tasks incredibly difficult. Imagine for a moment that you had to look for your hands every time you wanted to take hold of something. You would need to be paying much more attention than you do now, and would have difficulty every time you stopped paying attention.
An unusual feature in many ADHD patients is that , while they have difficulty with writing legibly, they are often skilled artistically and able to make intricate pictures with litttle difficulty. The problem appears to be in the conversion of symbolic information.
A dysgraphia joke : " What do you call someone with bad handwriting?" Answer: A DOCTOR!
Remediation of handwriting, though often attempted, is often futile. Use of bypass methods, such as typewriters, computers, and tape recorders are often much more effective. A key goal is to allow the person to process information as fast as they can think. Trying to write if you are dysgraphic is like trying to pour an ocean through a garden hose.