[RogerL]

I went to grad school with a guy w/o arms. Sure, he could write with his feet. It was extraordinarily hard and difficult. Nothing changes the physical facts - we neither have the nerves or fine control in our legs/feet that are present in our hands.

We also build fine motor control in very young childhood; children can't color within the lines (for example) because the nerves just aren't there yet. I would expect someone born without arms to have better fine motor control of their legs than an adult that tries to develop the ability. And it takes them years - 7 or 8 years to get pretty decent control. Even if I had the neuroplasticity of a young child (I surely don't) I wouldn't want to spend years trying to develop that control.

I face this every day on the piano. You distribute motion between fingers, wrist, forearm, elbow, and shoulder based on the amount of fine control needed. Sure, you can use your shoulder to trill, but it is always going to be very slow and clumsy. finges are best, but you face fatigue, so you usually use wrist rotation to get muscles with more endurance suppling the gross motions, and then fingers for the fine control of dynamics. You can't change this physical reality, at the most you can compensate (e.g. someone with a fused wrist would have to find alternatives).