A series elastic actuator puts a spring (elastic) in front (series) of a motor (actuator). This gives the actuator shock resistance to unexpected collisions as well as the ability to measure force by applying Hooke's law to the measured spring delfection. At the ReNeu Robotics Lab, we used them in almost all of the robots, including upper body and hand exoskeletons. In fact, the undergraduates made a video for the lab YouTube channel which got 3rd place in a national research video competition. Although my only involvement in this was narration, I still got to go to a research conference in Washington DC to see all kinds of inspiring research.