Introduction: To increase the accuracy of analyzing the impact of muscle activity on Electrical Impedance Myography (EIM) signals, we introduced an automated test-bench system for seated leg extension exercises that targets the quadriceps femoris muscle. Methods: To maintain a constant angle during EIM measurements, we extended a leg extension device with a motor-gear combination to enable freely adjustable positions with high accuracy, facilitating isometric muscle conditions. Besides, we implemented a multi-layer safety concept and a digital twin to develop the testing scenario before placing either the subject’s knee joint or motor components under stress. Results: The system is robust in terms of stability and performance, which enables the compensation of varying lower leg weights of \(5 \ {\text {kg}}\) to \(15 \ {\text {kg}}\) and maintains the target angle during active force development. Discussion: Motor guided testing scenarios show a large potential to enable an accurate analysis of the effect of eccentric, concentric, or isometric muscle contraction on EIM signals and to analyze the performance of the quadriceps femoris muscle, which strength is an indicator of fall risk in older adults.