In lower-limb rehabilitation programs, patients that suffer from neuromuscular disorders with manual muscle test (MMT) level 2 are able to perform voluntary muscle contraction and visible limb movement provided that a therapist assists the patient to eliminate the weight of his/her leg. In addition, the physical therapist is clinically needed to guide the patient performing a hip-only or knee-only motion during rehabilitation. The objective of this paper is to present a new assistive training device that replaces the function of the therapist in helping the MMT-level-2 patients self-training their hip and knee flexion/extension motions under an antigravity environment. First, we will present a novel reconfigurable mechanism, which can possess two working configurations for guiding the knee-only and hip-only training, respectively. Then, based on the theory of static balancing, two linear springs are attached to the device to generate an antigravity training environment in both configurations for the patient. The static balance design is verified by a numerical example with the support of software simulation. A prototype is built up and tested on healthy subjects. By using the electromyography (EMG) measurement, the myoelectric signals of four major muscles for the subject with/without the aid of the device are analyzed. The results show that the myoelectric voltages of the stimulated muscles are significantly reduced when the subject is assisted with the device. It further demonstrates that moving the fixation positions of the limb segments to other positions could distinctly reduce the assistive force from the device, which suggests multiple training modes to the patients in strengthening the training intensity. In conclusion, this paper presents a successful pioneering work on the design of rehabilitation devices via the integration of the principles of reconfigurable mechanisms and static balancing.