452 total squats in 21 minutes. Could definitely feel that it'd been more than a month since I tried this.
"Suicide By Squats", a brutal "How long can you last?" exercise.
Blog writeup: http://regex.info/blog/2017-09-23/2825
Set a timer. During the first minute, do two deep, proper-form squats. Do four during the second minute, six during the third, and so on, adding two for each minute. When you fail to be able to do the squats for that minute, you're done.
Did some push-ups and a 4-minute plank as a warmup.
VERY ODD: there's a short spike in my heart rate at about the 13-minute mark. Had the same kind of thing at about the same time last time I did this (https://www.strava.com/activities/1232720549). Weird.