I came across this paper [1], published in 2003, while looking for something to explain these observations about myself:

If I'm sitting in a restaurant, say, I get stiff. The more densely packed the worse. The act of getting up from the table is particularly difficult. But within a few minutes of leaving, walking home, I lose most of my stiffness.

Over at least the last 5 years my side to side tap test scores have been slowly improving, even though I subjectively feel that my PD has got worse.

Goerendt et al. tested the hypothesis that "during the execution of repetitive sequential movements, there would be increased release of striatal endogenous [dopamine]".

A part of the pathogenesis of PD is the loss of cells in the substantia nigra. These neurons project into the striatum, and it is there that the loss of dopamine is felt.

Goerendt et al. report: "These results support the existence of dopaminergic adaptive processes in Parkinson's disease and suggest that parkinsonian impairments are related to increasing demands on a dysfunctional dopaminergic system."

As I see it, the important word here is "adaptive". What can we do to get the best possible adaptations? What part of the benefits of exercise is due to this?

Delving deeper I found this [2]:

"We have chosen to label this motor improvement "paradoxical kinesia"
because even severely disabled PD patients are able to repeatedly catch a ball thrown at them. ... Three mechanisms are proposed to be involved in paradoxical kinesia including: noradrenergic augmentation; compensatory activation of cerebellar circuits; and activation of basal ganglia reserves."

Ball catching is something that I still do well.

References

[1] "Dopamine release during sequential finger movements in health and Parkinson's disease: a PET study"
Ines K. Goerendt, Cristina Messa, Andrew D. Lawrence, Paul M. Grasby, Paola Piccini, David J. Brooks
Brain, Volume 126, Issue 2, February 2003, Pages 312-325
Dopamine release during sequential finger movements in health and Parkinson’s disease: a PET study | Brain | Oxford Academic

[2] Reply from the authors | Neurology

John