really good stuff, love the case studies. I was surprised to hear that squats are easier on the back compared to deadlifts.
I imagine that the key variables are:
1: The moment arm between load and the intervertebral joints (longer moment arm = more counter torque that the spinal erectors have to produce, less compression due to the load, and more compression due to the action of the erectors. Also more shear force).
2: Degree of hip flexion. More hip flexion => more hamstring tension => spinal erectors have to work harder to overcome hamstring tension.
I would think that the squat taxes the back more than the deadlift on the first count (more hip flexion). And I think (but not certain), that the moment arm is, on average, greater with the deadlift, due to the more horizontal back angle. Maybe it's my own anthropometry (short torso, long legs & arms), but I've always found squats tougher on my lower back than deadlifts, given the same weight on bar. Perhaps this is because the long arms allow me to establish a more vertical back angle in the deadlift compared to the average, thus reducing the moment arm and the amount of hip flexion.