This chapter presents the conceptual design, prototype development, and testing of the ASSIST-FEE devices, which are cable-driven assisting devices for elbow flexion and extension. Three successive versions of the ASSIST-FEE are developed as proof-of-concept prototypes, each progressively refined to achieve a lightweight, portable, and user-friendly solution suitable for both clinical and home-based exercise. The experimental validation of these devices followed a structured testing protocol, including controlled exercise trials with real-time monitoring and data acquisition, to assess operational performance and evaluate their effectiveness in assisting motion. Beyond the elbow application, this work also presents the extension of the proposed conceptual solution to assist movements of the shoulder and knee joints, highlighting the versatility and scalability of the modular ASSIST devices.