"That's not ironic it's just coincidental."

Muscle spasms.  Pretty much the bane of my existence after stairs.

How they work is that muscles, though unconnected from the brain, are still designed to move.  The same is true of anyone, even non-paras or quads.  Have you been sitting, bored, and your leg just started bouncing?  It's not a conscious action, it just happens.  That's basically what happens when a para or quad's muscles start spasming, but they can't stop it by just... trying to.

Muscle spasms don't start immediately after injury, and some paras and quads don't get them at all.  During rehab I would have a meeting with my whole "team" (doctors, therapists, etc.)  We would get together and compare notes and prognoses.  Well once, maybe a month in, I was in one of these meetings and my doctor first told me about spasms.  He said that they generally happen with leg muscles but could really happen with any muscle group that the paralysis effects.  So for me, any muscle from the chest down.

The funny thing is, that within and hour of that meeting I had my first spasm.  Let me tell you; it's bizarre.  For a month the only muscles I could properly control where in my shoulders and arms.  Even my neck was out of commission due to the brace I was wearing.  Then, suddenly, my legs are bouncing up and down.  The sensation went from bizarre, to awesome, to irritating quite quickly.

Anything can cause a muscle spasm.  The most common cause for me is an uneven sidewalk.  Or a brick sidewalk.  I hate those.  I know that they're pretty, but a bump every few inches means I really can't carry anything on my lap because my legs are GOING to spasm and I'll drop it.  Concrete slab sidewalks are my best friends.

Other causes are: shift a covering bed sheet, bumping into something, or the briefest touch.  The most terrifying is when my abs spasm while I'm in the shower or something like that.  The chair I use in the shower is not the most stable of platforms, it's even less comfortable.  But the water hitting my skin can sometimes, rarely, make my abs spasm which almost throws me from my chair.  My legs and waist straighten extremely fast and my legs then start shaking.  Add to that the not super stable chair and my heart starts hammering quickly due to the adrenaline.

Spasms do have a plus side.  They help maintain muscle tone and blood flow.  They help prevent blood clots and muscle atrophy.  So they're annoying, but extremely important.