Which best defines clonus?

• A. Rhythmic, involuntary muscle contractions following a sudden stretch
• B. A single brisk reflex
• C. A resting tremor
• D. A voluntary repetitive movement

Answer: (A)

Clonus is a sign of upper motor neuron hyperexcitability, where a sudden passive stretch triggers a chain of rhythmic, involuntary muscle contractions. When a quick stretch is applied (for example, rapid ankle dorsiflexion), instead of a single brief reflex, the muscle contracts and then relaxes in repeated cycles for several beats. This happens because inhibitory control from the brain over spinal reflex circuits is reduced, making the alpha motor neurons more easily discharging in response to stretch (via the Ia afferents).

This distinguishes clonus from a single brisk reflex, which is just an exaggerated but non-rhythmic muscle contraction. It also differs from a resting tremor, which occurs at rest due to basal ganglia problems, and from a voluntary repetitive movement, which is intentional rather than involuntary and stimulus-driven.