I've got perfect squat form, I video myself doing them regularly and check. Maybe that lulled me into a false sense of security.
I do one-leg leg presses. A set with the right leg, then a set with the left. Here's what the Stronglifts guy has to say about leg presses:
The Leg Press is a machine where you push weight away with your feet. Some Leg Press machines are 45° incline, others are horizontal. But they’re never substitutes for Squats. The weight moves, you don’t. You don’t balance the weight, the machine does. Go too deep and your lower back will round at the bottom and squeeze your spinal discs. Unless you have no arms to hold the bar on your back, stick with free weight Squats.
And of course that's exactly what I've been doing, trying to get the full range of motion.
My lower back has been sore, which is new to me. I know lots of people have back pain from cycling, I've never experienced it before, until now.