Ultrasound gives the opportunity to look at muscles and observe their change in length. This tool has increased the knowledge about muscle-tendon dynamics and sometimes revealed surprising muscle stretch behaviour. System identification experiments use robots to disturb the ankle and measure its torque and angle. Muscle movement is derived from the measured joint angle and their assumed relationship. This study uses plane-wave ultrasound imaging to investigate the relation between ankle angle and muscle length during system identification experiments. The first goal is to determine the feasibility of using ultrasound measurements for system identification.  The second goal is to investigate the validity of the assumption that muscle stretch is proportional to ankle angle and the effect of this assumption on the prediction of reflex size. Transient and continuous disturbances were applied to the ankle, while images of the soleus and gastrocnemius were recorded with ultrasound and processed with an image tracking algorithm. For small (1° SD) continuous perturbations ankle angle and muscle length can be assumed proportional during a relax task. However, a conclusion cannot be drawn for the position task due to the low coherence of the muscle length measurements. For transient perturbations with a high velocity (>90°/s) the muscle length showed oscillations that were not present in the ankle angle, demonstrating a non-proportional relationship. The gastrocnemius velocity predicted the size of the short latency reflex better than the perturbation velocity of the ankle robot. Using plane-wave ultrasound imaging for system identification experiments was feasible.