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(Light) sparring can in fact be helpful for people with Parkison's.

interesting do you know if any aspect of grappling training was researched ?

Not that I know of. But the positive effect of sparring is well known, and you can even find scientific papers https://www.ncbi.nlm.nih.gov/pubmed/21088118. But repeatedly heavy punches to the head has been linked to Parkinsons, so the role of boxing and PD is Janus-faced:)

I wonder if it might benefit to limit sparring to body blows
for Parkinsons I mean