It's not the height of the fall, it's the sudden start and stop that get you. The startle response puts your body in the worst possible state for the inevitable sudden stop. When an immovable object meets an unstoppable force, shit gets broken. I fell 30" off a ladder. Landed on my heels with my knees straight. Compressed 3 vertibrae, crushed 3 discs, and up slipped and rotated my left sacroiliac. That was nearly 25 years ago. I still feel it every day.