  Fitts' law is a linear equation relating movement time to an index of
movement difficulty. The recent finding that Fitts' law applies to voluntary
movement of the vocal tract raises the question of whether the theory of speech
production implies Fitts' law. The present letter establishes a theoretical
connection between Fitts' law and the Task Dynamics model of speech production.
We derive a variant of Fitts' law where the intercept and slope are functions
of the parameters of the Task Dynamics model and the index of difficulty is a
product logarithm, or Lambert W function, rather than a logarithm.
