spasticity

Try to straighten a spastic elbow slowly and it gives way with little fuss. Try to straighten the same elbow quickly and it suddenly grabs and resists, sometimes with a catch like the click of a folding pocket-knife, then often eases once you push past it. This odd quality — the muscle fights harder the faster you move it, but relaxes if you go slowly — is the heart of spasticity. It is the most familiar of the overactive features of the upper motor neuron syndrome.

The classic definition is a velocity-dependent increase in resistance to passive stretch, arising from an overactive stretch reflex. Here is the mechanism in plain terms: stretch receptors inside the muscle (muscle spindles) report how fast the muscle is being lengthened. Normally the brain damps their signal. After upper motor neuron injury that damping is lost, so a fast stretch triggers an exaggerated reflex contraction — the limb resists. Because the trigger is the speed of stretch, slow movement provokes little reaction while fast movement provokes a lot. The 'catch' followed by release as you keep pushing is called the clasp-knife phenomenon.

Spasticity matters in rehabilitation because it sits at a fork. On the harmful side it can make dressing, washing, and hand use difficult, cause pain and spasms, disturb sleep, and over months pull joints toward fixed contractures. On the helpful side, leg stiffness can let a weak person stand and pivot. So the practical question is never 'how do we get rid of it?' but 'is this particular spasticity helping or hurting, and where?' — which is why it is measured (modified Ashworth, Tardieu) and treated selectively rather than blanket-suppressed.