Patient's with Parkinson's disease move very slowly, if at all.
However, during an emergency ( such as fire in the building) they
sometimes move rapidly and vigorously. What is the possible
explanation?

i know this,'
when i was a child, i saw my teacher one day can carry a stone what over 200kg,
the store roll to him, he said he suddenly fell he will die, he need
to daff the store! he said why he so powerful, because there are
appetence at human's life.try to think about this: when human lived in
the field, fligt with many many strong animals,if our human do not
have this appetence ,how we can win?
my teacher study KONGFU from he was a child. some ofhis teachers also
have this performents too.
this is their website:http://www.guijiquan.com/?
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Further to the previous comment, I am no expert by a long shot but I
am a biology student so will add my knowledge as I see it.
In an emergency, when we are scared or frightened etc, we can do
certain things better than we normally can. In the past, as humans or
what we have descended from, we needed to run or fight in some
circumstances. Our adrenal (I think more correctly called suprarenal)
glands secrete adrenaline which is a hormone. It diverts blood away
from the gut ('non-vital' organs) and leads it to the heart, lungs,
muscles etc, things that are vital to fight or run. Adrenaline is
often thought of as the 'fight or flight' hormone. It helps us fight
better, or fly (run away) better or faster.
Perhaps adrenaline, and maybe other hormones etc, can cause someone
with Parkinsons's disease to be better equipped to fight or run.