An energy efficient joint-locking mechanism that works in conjunction with the main actuator of a robot module is presented. The mechanism will enable chain-style modular reconfigurable robots to perform a wide array of tasks such as dynamic motion and bio-inspired locomotion while consuming less power. The design process for developing this mechanism is presented, and analysis is provided. This mechanism is ideal for modular reconfigurable robot systems, but can be modified to suit many applications. A prototype is developed that outperforms comparable devices such as those that utilize piezoelectrics, magnetic particles, and electromagnetically-actuated disc and drum brakes in terms of power consumption and specific torque.