In this paper the authors deal with the problem of force/tracking control of constrained manipulators without velocity measurements interacting with an infinitely stiff environment. They use a reduced-order model for which the coordinates space is restricted to a subset Omega where the Jacobian constraint is guaranteed to be nonsingular. In contrast to other similar approaches, the authors do not assume that the generalized trajectories belong to the subset Omega all the time, thus our contribution is to prove that if the trajectories start within the subset Omega, they remain in it. Furthermore, we prove uniform asymptotic stability considering only position and force measurements.