Arms heal. So do legs, hips, collarbones, wrists, fingers and the myriad other bones that make up your skeleton. But your spinal cord is not so resilient. (At least not yet.) And until the day regenerative technologies exist to repair your ripped spinal cord, you're going to want to protect it.
Start with the cervical spine (neck) protector described earlier. Then consider additional armor to beef up the rigidity of your back. And the key word is rigidity. Don't mess around with soft armor -- go for the hard stuff.
A 2002 study found that the thoracic spine is the most common spinal injury point for motorcycle crashes [source: Robertson et al]. This stretches from your upper back to just below the rib cage. So consider additional armor here.
This back armor can be a strap-on backpack, or it can be built directly into a jacket.