What term describes sustained involuntary contraction of agonist and antagonist muscles?

The term that best describes sustained involuntary contractions of both agonist and antagonist muscles is dystonia. Dystonia is characterized by abnormal, sustained muscle contractions that can lead to twisting or repetitive movements and abnormal postures. These contractions often involve agonist (the muscle that causes the movement) and antagonist (the muscle that opposes the movement) muscles simultaneously, resulting in a combination of movement and tension.

In contrast to dystonia, hemiballismus is a condition marked by violent, flinging movements typically affecting one side of the body but does not involve sustained contractions of opposing muscle groups. Hyperkinesia refers generally to an increase in movement and can include various types of excessive movements, but does not specifically denote sustained contractions. Hypokinesia involves reduced movement, as seen in conditions like Parkinson's disease, and is not related to the sustained involuntary contractions that characterize dystonia.