10 X 200 meters on 2:00.
I did this workout two years ago (at sea level) and it ended my season. Today it felt like a recovery session where I felt better at the end than I did before. I was recovering easily at 2:00.
Here's a pic of my HRV before the workout and at 90 minutes post workout. Had this been "intense" for me the HRV should have dropped. Not entirely sure though.