Potatopolis OP t1_j23yop3 wrote on December 29, 2022 at 2:17 PM

Reply to comment by marjan21 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

That still doesn't quite add up in my head - when I bend a pencil, I'm over-flexing a single, rigid structure. The spine is lots of smaller rigid structures, held together by soft, bendy stuff.

Moskau50 t1_j24s3gw wrote on December 29, 2022 at 5:38 PM

You're assuming that the soft, flexy tissue will break before the bone will. But the tissue, being soft and flexy, specifically won't break, whereas the bone, being rigid, cannot flex at all. Both materials being subjected to the same load, it's not a given that the tissue will break before the bone will.

Potatopolis OP t1_j24sfil wrote on December 29, 2022 at 5:40 PM

Not quite - I'm assuming that soft tissue will bend/flex before the bone breaks - in fact, it must to allow what spinal flex we have as it is.

If you pull on your hamstring too hard due to flexing joints around it too much, the hamstring tears rather than your femur breaking or similar. This is different to how the spine and its connective tissue/muscles works (in that it seems it's the bones that come apart, rather than anything softer), and it's the explanation of that difference that I'm looking for.

Moskau50 t1_j24vcn6 wrote on December 29, 2022 at 5:58 PM

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.