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What Spasticity Is — and Isn’t

"Spasticity" gets used as a catch-all for any stiff, tight, or jumpy limb — but it has a precise meaning, and three look-alikes hide inside that loose habit. Learn the velocity-dependent definition, how to tell it apart from rigidity, dystonia, and a fixed contracture, and how the Ashworth and Tardieu scales pin a slippery feeling to a number.

One word doing four jobs

In the first guide of this rung you met the upper motor neuron syndrome — the bundle of changes that appears when the brain or spinal cord stops sending its normal, modulated commands down to the muscles. Spasticity is one member of that bundle, and it is easily the most talked-about. But it is also the most misused word in tone management. On a busy ward you will hear a tight calf, a clenched hand, a jerky knee, and a stiff shoulder all called "spastic." Often three of those four are something else entirely. Getting the word right is not pedantry; it is the difference between a treatment that helps and one that wastes effort, money, and sometimes the patient's remaining movement.

Remember from earlier in the ladder that muscle tone is the quiet, background resistance a relaxed muscle gives when you move the limb for the patient. In a healthy person that resistance is light, smooth, and the same whether you move fast or slow. The four conditions we are about to separate all raise that resistance — but each raises it in its own characteristic way. The trick to telling them apart is not how stiff the limb feels at the end. It is what the resistance does while you are moving: does it depend on speed, stay constant, build up as a posture, or simply refuse to budge at all?

Spasticity: resistance that depends on speed

The classic definition, from the neurophysiologist James Lance, is worth memorizing because it does so much work: spasticity is a velocity-dependent increase in resistance to passive stretch. Decode that and you have the whole idea. "Passive stretch" means you, not the patient, are doing the moving. "Velocity-dependent" is the heart of it: the faster you stretch the muscle, the more it fights back. Move slowly and the limb may give way easily; move quickly and you hit a wall. That wall comes from an over-eager stretch reflex — the same reflex a doctor triggers with a tendon hammer — that has lost the brain's restraining hand and now fires hard at any brisk pull.

Picture a man six months after a stroke. As you gently bend and straighten his elbow at a slow, conversational pace, it moves through nearly its full range. Then you do it once more, briskly — and a third of the way through, the biceps suddenly catches and grabs the forearm. Hold steady against it for a moment and, often, the resistance melts and the arm gives the rest of the way. That sudden catch followed by release is the hallmark, and it has a name worth knowing: the clasp-knife response, because it feels like opening a folding pocket-knife that resists and then snaps open. When the reflex is very brisk you may also feel a rhythmic beating against your hand — that is clonus, the same circuit firing over and over.

The three look-alikes

Now meet the impostors, all of which you must learn to set apart — the glossary gathers them under spasticity vs rigidity, dystonia, and contracture. Rigidity also raises resistance, but it is the mirror image of spasticity in one decisive way: it is *not* velocity-dependent. The limb resists you evenly throughout the range, the same whether you move fast or slow, and there is no catch-and-release — it can feel smooth ("lead-pipe") or ratcheting ("cogwheel"). Rigidity comes from the basal ganglia, as in Parkinson disease, not from a runaway stretch reflex. So the bedside test that separates the two is simply this: change your speed and feel whether the resistance changes with you.

Dystonia is different again. Here the muscle contracts on its own, pulling the body part into a twisted, sustained posture — a turned neck, a curled hand, an inturned foot. The resistance you feel is not a passive stretch reflex but the patient's own active, involuntary muscle pulling against you, and it often comes and goes with position, action, or even emotion. A child with cerebral palsy may have a foot that turns inward and points down whenever she tries to reach for a toy, then relaxes when she rests — that fluctuating, posture-driven pull is dystonic, and it can sit right alongside spasticity in the same child. Telling which is which matters because they respond to different treatments.

The fourth is the most easily confused and the most consequential to miss: a fixed contracture. A joint contracture is no longer a nervous-system problem at all — it is a mechanical one. Over weeks of a limb sitting in one shortened position, the muscle, tendon, and the tissue around the joint physically remodel and shorten, so the joint simply will not open to its full range no matter how slowly you move or how long you wait. Spasticity left untreated is one of the things that *causes* a contracture, but once the contracture is set, relaxing the nerves does nothing — there is no reflex to quiet, only short tissue to lengthen. This is exactly why the spasticity track sits next to contracture prevention: the line between a problem you can quiet and one you must stretch out or release surgically is the line between two completely different treatment plans.

Putting a number on it: Ashworth and Tardieu

A feeling in your hand cannot be charted or compared, so — exactly as you learned when grading strength — we need a scale. The most widely used is the modified Ashworth scale. You move the joint through its range once, fairly briskly, and grade the resistance you meet from 0 (none) up to 4 (the limb is rigid in flexion or extension). It is quick, needs no equipment, and everyone has heard of it. Its weakness, though, sits right at the heart of our definition: the modified Ashworth scale mixes together the velocity-dependent reflex (true spasticity) and any plain mechanical stiffness (a budding contracture) into a single number. A high Ashworth grade tells you the limb is hard to move — but not *why*.

The Tardieu scale was built to fix exactly that gap, and it does so by taking the velocity-dependent definition seriously. Instead of one stretch, you do two: one very slow, and one fast. The slow stretch reveals the joint's true mechanical end-range — how far it goes when no reflex is provoked. The fast stretch provokes the reflex and you note the angle at which the muscle suddenly catches. The *difference* between those two angles is the part that is genuinely reflex-driven — the part a nerve-targeted treatment can actually help. When the slow and fast angles are nearly the same, the stiffness is mostly mechanical, and you are looking at contracture, not spasticity. In one elegant move, the Tardieu scale pulls apart the two things Ashworth blurs together.

MODIFIED ASHWORTH SCALE (one brisk pass through range)
  0   No increase in tone
  1   Slight catch at end of range, then minimal resistance
  1+  Slight catch, then minimal resistance through < half the range
  2   More marked tone through most of range; limb still moves easily
  3   Considerable tone; passive movement difficult
  4   Limb rigid in flexion or extension

TARDIEU (separates reflex from mechanics)
  V1  stretch as slow as possible  -> mechanical end-range (R2)
  V3  stretch as fast as possible  -> angle of catch    (R1)
  R2 - R1  =  the reflex ("dynamic") component worth treating
  R2 = R1  =  stiffness is mostly contracture, not spasticity
Two everyday tone scales. Ashworth gives one fast number; Tardieu uses a slow and a fast stretch so the gap between them isolates the reflex component.

Why the right word changes the plan

Run the four conditions back through one bedside maneuver and you can sort most of them yourself. Move the limb slowly, then quickly, and ask what the resistance does.

  1. Resistance grows the faster you move, with a catch-and-release through the range — that is spasticity, a velocity-dependent stretch reflex.
  2. Resistance is even and the same at any speed, with no catch — that is rigidity, pointing toward the basal ganglia, not the stretch reflex.
  3. The patient's own muscle actively pulls the part into a twisting posture that shifts with action or position — that is dystonia.
  4. The joint will not reach full range even moved slowly and held — that is a fixed contracture, a mechanical problem, no longer about the nerves.

Each verdict opens a different door, which you will explore later in this rung. A velocity-dependent reflex might respond to oral agents, to focal injections that quiet an overactive muscle, or to a careful stretching program. A fixed contracture answers to sustained stretch, serial casting, or surgery — not to a drug that calms reflexes. Mislabel a contracture as "spasticity" and you may inject a muscle that has no reflex left to quiet, achieve nothing, and delay the stretch the joint actually needed. The precise word is not the end of the work; it is the fork in the road that decides which work to do.