That's an idea that predates series elastic actuators. It's been used in tendon robots, where there are two winding drums pulling on two cables, with a spring between the end of each cable and the load. The cables oppose each other. Tighten up both, and the joint becomes stiff. Loosen up both, and the joint becomes flexible. Like muscles.
CWRU robotics liked this idea. Lightweight robot arms and painting robots have used it. Snake robots often work that way. Robot hands are often tendon driven.
Tendon robots are not popular for industrial use. The tendon cables wear out. Robots which do the same thing over and over wear their tendons at the same points. Not good in factory settings.
A useful model of a muscle is a spring and damper in parallel, like an auto suspension. The spring's neutral point, the spring's spring constant, and the damper's damping constant are all controllable. An actuator built that way can provide both force and position control, and can absorb shock loads. This is roughly how biological muscles behave.
The most practical actuator like that is a double-ended air cylinder. Each end has two proportional valves, to let air in from the air supply and to let air out to exhaust.
Such devices can be tuned from stiff to rigid, held at any position, and can absorb shock loads, which just compress the air trapped in the cylinder. Festo, the German automation equipment manufacturer, is a leading promoter of that approach.[1] They've popularized precision pneumatic control with a computer managing the valves. Works great in factories where you can plug into a compressed air line. Festo has lots of videos online, if you like looking at pneumatic actuators.
A series elastic actuator is a different concept. It's a stiff spring on the end of a stiff screw-type linear actuator. Shock loads compress the spring, a sensor at the spring detects this, and the linear actuator is commanded to quickly spin up the motor and unload the spring. Force can be measured from the length of the spring. It's a way to fake a real force actuator with a cheap positional one.
This was a popular idea in research robotics, because it's something that can be put together from off the shelf components. It's a useful research tool. But it's not a very good actuator for large loads. There's a race between the applied load squeezing the spring and the motor spinning up to take off the load before the spring bottoms out. It's only useful for a limited load range. If the spring is too stiff, you still take a high shock load at the gear train. If the spring is too soft, the spring bottoms out before the motor can catch up. You can buy series elastic actuators from academic and hobbyist suppliers [2], but they're rarely seen in factories.
There's been much robot actuator progress in the last decade. Not miracles, just money and good specialized mechanical engineering.
[1] https://www.youtube.com/watch?v=GedJiaz_E1I
[2] https://www.hebirobotics.com/actuators