This paper proposes a novel mechanism for orthotic devices, which has an active joint actuator with a biarticular elastic mechanism that connects two joints simultaneously. The biarticular elasticity in the proposed mechanism is adopted to utilize the energy transfer function of the biarticular muscles so that (1) the torque from the active joint actuator is efficiently transmitted, and (2) the forces required to propel the upper body forward are assisted in a more efficient and sophisticated way. In recent years, many researchers have discovered that the biarticular elasticity is a key mechanism of humans' efficient movement and implemented the biarticular principle into various types of robots. However, the application of the biarticular mechanism has been hindered by the coupling and redundancy which the mechanism induces. This paper provides theoretic fundamentals for analyzing the kinematics and dynamics of the biarticular elasticity to help engineers design assistive mechanisms and controllers using the biarticular elasticity. The effectiveness of the proposed methods are verified by simulation studies in this paper. (C) 2014 Elsevier B.V. All rights reserved.