Recent research on exoskeletons and braces has examined the ways of improving flexibility, wearability or overall weight-reduction. Part of the challenge arises from the significant loading requirements, while the other part comes from the inflexibilities associated with traditional (rigid link-moving joint) system architectures. Compliant mechanisms offer a class of articulated multibody systems that allow creation of lightweight yet adjustable-stiffness solutions for exoskeletons and braces, which we study further. In particular, we will introduce the parallel coupled compliant plate (PCCP) mechanism and pennate elastic band (PEB) spring architecture as potential candidates for brace development. PCCP/PEB system provides adjustable passive flexibility and selective stiffness to the user with respect to posture of knee joint, without need for mediation by active devices and even active sensors. In addition to the passive mode of operation of the PCCP/PEB system, a semi-active design variant is also explored. In this semi-active design, structural stiffness reconfigurability is exploited to allow for changes of preload of the PEB spring to provide force and torque customization capability. The systematic study of both aspects (passive and semi-active) upon the performance of PCCP/PEB system is verified by a lightweight 3D printed physical brace prototype within a ground-truth (optical motion tracking and six degrees-of-freedom (6DOF) force transducer) measurement framework.