This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
The Holodeck has a force field treadmill. If its occupants get too
close to the walls, they are shifted away. Since the Holodeck can
modify its gravity in 3 dimensions, the occupants won't notice any
inertial change. The novel "Reunion" [TNG] (while non-canon) goes into
details about this.
 
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