Yep, a seismograph. Mine was a vertical version, but yeah, the same thing. You suspended the muscle, with a pen attached somehow (I don't remember, it was decades ago), and when you shocked it, the muscle would spasm, and write on the rolling paper. As the spasms increased, the markings would be smaller, until it seized up, and drew a straight line. Then you'd douse the muscle with saline, and you could do it again, although the results were less dramatic on subsequent runs. It seemed like the muscle would eventually just fatigue, and not be as revivable by saline. So we only had to repeat it a few times.

And it wasn't just one experiment, being done by the teacher in front of everybody. We all had our own seismographs, and had to kill and dissect our own frogs. It was a real hand-on class, and was one of the best high school classes I've ever taken. It was a college level pre-med class, and I've used the info I learned in that class every day throughout my entire life.