Woke up yesterday with my back much worse. Very debilitating. Today it's better and I was able to do a Tabata circuit of calisthenics and low-weight squats. I basically switched from 50 lb goblet squats, decline push-ups, and chin-ups, done 5 reps each over 10 minutes. I definitely underestimated myself. Even though I was breathing plenty hard the time seemed to speed by and I had a lot left in the tank. Will go for 15 next time. I wasn't exactly rushing to the set-up of each exercise, though, because of my back.