clonus

Imagine pressing the front of someone's foot to bend the ankle upward, holding it there, and watching the foot start to bounce — beat, beat, beat — in a steady rhythm against your hand, as if a tiny motor had switched on. That repeating, involuntary tap is clonus. It is one of the most striking signs that an upper motor neuron pathway has been injured, and the ankle is where it shows up most often.

Clonus is a runaway stretch reflex. Normally, when a muscle is suddenly lengthened, a single quick reflex contraction follows and then it settles. After upper motor neuron damage, the reflex loop is overexcitable and poorly damped: the contraction stretches the opposing tendon, which triggers another reflex, which triggers another, locking into a self-sustaining oscillation of roughly five to eight beats per second. A few beats that fade away can occur in tense but healthy people; sustained clonus that keeps going as long as the stretch is held is a clear sign of pathology. It is examined by giving the relaxed ankle a brisk push up and holding the stretch.

In rehabilitation, clonus is more than a curiosity. It can be set off by everyday triggers — standing up, a sock catching the toes, the footrest of a wheelchair — and a foot that judders uncontrollably interferes with standing, transfers, and putting on shoes, and can disturb sleep. It is usually managed as part of the broader spasticity picture: stretching, careful positioning, sometimes oral antispasticity medication or a focal injection, rather than treated as a separate disease.