The authors identify an important phenomenon they call "formulation stiffness" in the numerical simulation of tree-structured multibody systems such as robot manipulators. The numerical simulation problem is usually treated as two separate problems: (i) the forward dynamics problem for computing system accelerations, and (ii) the numerical integration problem far advancing the state in time. The authors show that the interaction of these two problems leads to new conclusions about the overall efficiency of multibody simulation algorithms; in particular the fastest forward dynamics methods are not necessarily best when considered in conjunction with the popular adaptive stepsize integration methods. Specifically, the authors show that the articulated-body method is better suited to deal with certain types of numerical problems than the composite rigid body method. The authors present examples of simulations and discuss the practical implications of these results.