I always thought it was superficial muscle.
Did it on squats.
Still have full mobility, no issues on leg press.
Just need to ice it up and shut down squats for a week or two, might be time to invest in squat shoes.
That sucks, man.
Any guess as to the exact cause?
Did it trying to establish a new max for 5*5, was on my way down after set 4 rep 1. Best guess is I was a little too wide to be going so low. Lucky no tear, just strained it, I immediately just shrugged the bar when I felt an odd sensation +1 for going high back on squat, I could have done something serious if I was a low back guy.