Abstract: A Body and/or Caudal Fin (BCF) fish modulate its body stiffness by mechanisms consisting of antagonistic muscles. The mecha-nisms can be considered as Redundant Planar Rotational Parallel Mechanisms (RPRPM) with antagonistic flexible elements. For a typical RPRPM, its stiffness consists of the adjustable stiffness resulting from internal forces and the inherent stiffness caused by inherent com-pliances of flexible elements. In order to decouple the adjustable stiffness from the inherent stiffness and expand the range of stiffness variation, a variable-stiffness decoupled mechanism based on the Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator (MACCEPA) is presented and used to construct a soft robotic fish with large stiffness variation. According to the analysis of the evolution from RPRPM to MACCEPA, it can be found that MACCEPA is just a special type of RPRPM with only an adjustable stiffness. In addition, MACCEPA existed before RPRPM mechanism. The prototype of the soft robotic fish with variable-
stiffness decoupled mechanisms is built to explore the relationships between the body stiffness and the swimming performance. It is validated experimentally that the stiffness variation multiple of the robotic fish is raised, the swimming performance of the robotic fish is improved when the stiffness is modulated to match the driving frequency.

Key words: decoupled, bionic soft robotic fish, pretension, variable stiffness, MACCEPA