Calculation of reaction forces is an essential task in the simulation of mechanical systems with unilateral constraints, like bipedal robots. Most methods are based on the rigid body assumption. These methods can be categorized into constraint-based and penalty-based approaches. In the first ones, reaction forces are computed by solving an optimization problem based on linear or nonlinear complementarity conditions. Constrained-based approaches can be based on acceleration-force formulations or on velocity-impulse formulations. In the second type of approaches, spring-damper models are used to minimize interpenetration between the bodies in contact. In this paper we apply the three above-mentioned approaches to the cal- culation of ground reaction forces of a 3D bipedal robot with no trivial feet. The numerical results are compared from three different points of view: numerically, implementation complex- ity and calculation time. Author's contributions for event-driven and penalty approaches are presented.