  Onsager-Machlup functionals are used to describe the dynamics of a continuous
stochastic process. For a stochastic process taking values in a Riemannian
manifold, they have been studied extensively. We describe the Onsager-Machlup
functional with respect to the sup norm for a hypoelliptic Brownian motion on a
Heisenberg group. Unlike in the Riemannian case we do not rely on the tools
from differential geometry such as comparison theorems or curvature bounds as
these are not easily available in the sub-Riemannian setting. In addition, we
study fine properties of trajectories of the hypoelliptic Brownian motion,
including a new notion of horizontal continuous curves.
