Hi Joopoele,
Hoe gaat U? Heel moi veer vindt je niet?
I think that acetyl choline and dopamine need to be in balance, acetyl choline contracts muscles, dopamine releases the contraction. In PD, the balance is disturbed by an inability to make dopamine so we need to add the precurser levodopa or block acetyl choline to some extent to get the balance right.
I am not sure why you think a slow movement has more friction than a fast movement, or what evidence there is for this. When I am "off", my leg muscles are stretched and I can't release them to walk due to lack of sufficient dopamine, (and excess acetyl choline). When I take levodopa and it moves to the brain and boosts my level of dopamine, getting the balance back to normal with acetyl choline, I can walk.
When I try to cross a busy road, the stress causes an increase in the blood-brain barrier (BBB) permeability, and my small supply of dopamine is dumped to the bloodstream. The result, I freeze. Dopamine does not pass a healthy BBB, but it is a smaller molecule than levodopa which does pass the BBB, so it does not need much of an increase in the permeability in order to pass. The result, a freeze is the same situation as when I am off, ie an imbalance in the dopamine/acetyl choline level, with high acetyl choline and low dopamine.
I think this explains what we observe rather than the "friction" idea.
There isn't really friction in moving muscles, it is the signal to move that is lost. If there was muscle friction, you would not easily pull the tendons on your chicken leg and move the claw. It would be rigid.
Ron