lost motion/ low humidity/plastic action

[Tvak@AOL.COM]

I had always thought that lost motion that can develop in a low-humidity 
environment (i.e. the winter months here in Chicago) was due to the wood 
action parts drying out and shrinking.  I guess that's an incorrect 
assumption.  I just came across a Kawai vertical that had quite alot of lost 
motion, but all the action parts were plastic, except the key itself.  
Plastic wippen, plastic jack, and plastic hammer butt.  I didn't think 
plastic is affected by humidity.  My guess is that the felt and leather (on 
the wippen and hammer butt) must have shrunk due to low humidity creating the 
lost motion.

Or is there some other reason that pianos develop lost motion in low 
humidity?  This piano has not been played much since I saw it last year, so I 
don't think it's due to wear.

Tom Sivak