We present a general framework for the modelling of a class of mechanical systems for robotic manipulation, consisting of articulated limbs with redundant tendinous actuation and unilateral constraints. Such systems, that include biomorphically designed devices, are regarded as a collection of rigid bodies, interacting through connections that model both joints and contacts with virtual sprin s. Methods previously d o veloped for the analysis odforce distribution in multiple whololimb manipulation are generalised to this broader class of mechanisms, and are shown to provide a basis for the control of co-contraction and internal forces that guarantee proper operation of the system.