Moskau50 t1_j24vcn6 wrote
Reply to comment by Potatopolis in ELI5: If I "break my back" due to over-flexion (as opposed to sheer impact), what's actually happening? Given the vertebrae are connected by soft tissue, shouldn't that tear before any bones do? by Potatopolis
At no point in flexing your joints are you putting any significant stress on your femur or other straight-bone in the limb.
The tendons and ligaments are providing support in other directions/axes that your bone isn't providing. If you twist an ankle, your tibia or fibula isn't bearing the major stress; you could twist someone's ankle 360 degrees around, completely destroying the entire tendon structure, and the tibia/fibula will be structurally intact. Likewise, if someone hits you in the shin with a 2x4, your ACL or Achilles isn't bearing the major stress; completely smash the tibia and fibula, and the ACL and Achilles, while now useless, won't tear.
In your back, the discs and vertebra are basically in-line. They are sharing the load between the two whenever you're bending the spine. So that force gets distributed across/applied to both, which means either can break from the same load.
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