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The phenomenon that describes alternating muscle contractions and relaxations in response to a sustained stretch is clonus. Clonus is characterized by rhythmic, repetitive contractions that occur as a response to an external stimulus, such as a stretch. This involuntary neuromuscular response signifies an upper motor neuron lesion and can be observed in conditions such as multiple sclerosis, spinal cord injuries, or after a stroke.

In contrast, muscle spasticity refers to increased muscle tone and exaggerated deep tendon reflexes, leading to stiff and jerky movements, but it does not describe the rhythmic alternating contractions seen in clonus. Myokymia is the term used for involuntary muscle contractions that appear as slow, wave-like movements in muscle fibers, which differ from the rhythmic patterns associated with clonus. Muscle cramping involves sustained involuntary contractions but does not involve the alternating pattern characteristic of clonus; instead, cramps often result in pain and a prolonged tightness in the muscle involved. Therefore, the correct identification of clonus aligns with its definitive pattern of alternation during sustained stretch responses.