Simbody is a high-performance, open-source toolkit for science- and
engineering-quality simulation of articulated mechanisms, including
biomechanical structures such as human and animal skeletons, mechanical
systems like robots, vehicles, and machines, and anything else that
can be described as a set of rigid bodies interconnected by joints,
influenced by forces and motions, and restricted by constraints.

Simbody includes a multibody dynamics library for modeling motion
in generalized/internal coordinates in O(n) time. This is sometimes
called a Featherstone-style physics engine.