I used to do crossfit, yeah bust my chops, I was in the best shape of my life...but I got hurt a lot. One workout of the day was all squats. From regular air squats to partner squats where you have a person on your back. I had a thick but attractive lady with bug boobs as my partner so way better than having some dudes sack resting on your lower back. (She was a hardcore crossfitter and squat my skinny ass like I weighted a pound)
Anyway, we did 250 different reps of squats with a 400 meter run between sets. My legs have never been so tired and the soreness 2 days later was epic in a back way.