This paper deals with the application of interval analysis for outer bounding the physical parameters of parallel robots. The robot is modeled with classical Lagrange equation which leads to an inverse dynamic model linear with respect to the parameters. Assuming the error additive on input (motor torque), the problem is termed as a constraint propagation one and the solution is performed with a preconditionned Gauss-Seidel contractor modified in order to be used with over-determined linear systems. Experimental results are exhibited for a fully parallel robot with 4 degrees of freedom.