Clonus is the repeating contraction of a stretched muscle. Normally, if a muscle is stretched rapidly, it will contract, which then puts a stretch on the antagonistic muscle, which contracts. For example, rapidly dorsiflexing the ankle stretches the gastrocnemius, causing it to contract, resulting in plantar flexion, which stretches the anterior tibialis, causing dorsiflexion, which stretches the gastrocnemius, which may cause another planar flexion. In UMN disease this process may continue for several beats (non-sustained) or indefinitely (sustained) so long as steady stretch is applied to the gastrocnemius by dorsiflexing the ankle because of disinhibition that normally quells the excessive contractions. To elicit clonus, for example in the ankle, briskly dorsiflex the ankle, and maintain some pressure on the foot.