Spasticity is an involuntary muscle contraction caused by a hyperexcitability of the reflex arc that occurs due to damage to the brain or the spinal cord. Huh?? Yeah, agreed. Ok, I get it, keep it simple. Did you ever wonder why your leg seems to spaz out for no apparent reason? Or why your hand sometimes hits you in the face at the most unplanned time? That is spasticity. And if you suffer from this, it is really something that is critical to understand to get the most out of your post-stroke rehab.

 What is spasticity?

Let me try this again without attempting to boost my ego by giving a super complicated “medical definition”.  Spasticity is a movement disorder caused by damage to the brain or spinal cord that causes certain muscles to contract without your consent. Ugh. I know, I hear that sentiment almost every day from my patients.

What causes spasticity after a stroke?

So, the verdict has not totally come in on this one. Meaning, researchers still can’t agree on what exactly causes this to happen. But the best guess (most consensus amongst the literature) is that the nerves that go to the muscles are not getting clear “instructions” from the brain. Causing on overactive stretch relfex

What is the stretch reflex?

The stretch reflex is a muscle contraction in response to lengthening.  It is kind of a protective mechanism so that muscles do not get overstretched or lengthened beyond their elastic capabilities. This happens quickly, almost automatically. A sensor in the spinal cord senses the muscle is lengthening and immediately, almost instantaneously, triggers a motor nerve to make that muscle contract.  Thus, “protecting the muscle”, right? Not necessarily…….

vector diagram of the stretch reflex involved in spasticity


Let’s say you are reaching for a child who is falling. Well, of course, you do not want this stretch reflex to be activated.  That is when the brain steps in and stops (inhibits) the stretch reflex from being activated.  A way to look at this is the stretch reflex is like a child and the brain is the parent. The child may know how to walk, but the parent must step in if walking (or any activity) is “not appropriate” for the given situation.

What causes an overactive stretch reflex after a stroke?

The most common theory amongst the literature (to date), after a stroke this stretch reflex is not getting that “inhibition” input from the brain. Therefore, anytime certain muscles are lengthening, the stretch reflex senses the lengthening and kicks in.

The most common muscles that we see this, are in the muscles that:

Bend the elbow (bicep): The bicep can be “lengthened quickly” just by gravity.  Therefore, you might notice that the bicep “involuntary contracts” when you stand up. 

vector image of bicep spasticity

Point the foot (gastroc soleus and posterior tibialis): The muscles that point the foot can become lengthened just by standing. Therefore you might notice that the foot points harder in standing.