In this study, we proposed a new approach for determining the dynamic knee joint range of motion according to the external load applied during leg extension exercise. One elderly participant volunteered. The dynamometer task was performed to develop a subject-specific model using maximum voluntary contractions. EMG signals were also measured simultaneously. After the dynamometer task, 3D motion data were captured during leg extension exercise. The data obtained from the dynamometer task were used to develop the subject-specific model using an EMG-driven model, and then the developed model was used to estimate joint moment during leg extension exercise. Adjusting the model parameters positively affected the correlation and RMS error. The correlation between the model prediction and the measured joint moment increased with decreasing RMS error. The predicted knee joint moments during the leg extension exercise showed the usual inverse parabolic shapes in terms of time. These results implied that the dynamic range of motion should vary according to the external load applied to the joint. In this paper, we proposed a novel method to determine the dynamic knee joint range of motion using an EMG-driven model. We expect that the proposed approach will be employed to design exercise/rehabilitation protocols for the elderly.