• The arthrokinetic reflex is an orthopedic concept that states that internal derangement can cause muscle spasm in the muscles supporting that joint and that when the internal derangement is resolved the muscles cease to spasm. For example, in the setting of disc displacement with reduction, increased EMG activity of the temporalis and masseter muscles occurs when the condyle slides over the posterior band of the disc during closure. Muscle activity decreases when the disc position is normalized with mouth opening.