阵孪性状态，简称阵挛，为一系列由于肌肉突然的拉扯产生非自主性的肌肉节律性收缩。阵挛是一种特定的神经现象，特别是上运动神经元疾病（UMNL），如渐冻人（ALS）、中风、多发性硬化、脊髓损伤或是肝性脑病变（hepatic encephalopathy）。不像局部肌纤维抽动（fasciculations）现象（通常是下运动神经元疾病造成），阵挛通常是由一反射造成的大的肢体动作。

造成阵挛的原因是因为深层肌腱反射（DTR）发生后，中枢神经没有办法抑制它。

阵挛最常发生在脚踝的地方，临床上可以用快速的Dorsiflexion来诱发。另一个常测试阵挛的地方是膝盖骨，测试方法是快速的把膝盖骨向下推。五下以上的阵挛就被视为不正常。

Clonus (from the Greek for "violent, confused motion") is a series of involuntary, rhythmic, muscular contractions and relaxations. Clonus is a sign of certain neurological conditions, particularly associated with upper motor neuron lesions involving descending motor pathways, and in many cases is, accompanied by spasticity (another form of hyperexcitability). Unlike small, spontaneous twitches known as fasciculations (usually caused by lower motor neuron pathology), clonus causes large motions that are usually initiated by a reflex. Studies have shown clonus beat frequency to range from 3–8 Hz on average, and may last a few seconds to several minutes depending on the patient’s condition.