Williamson demonstrated the precise effect of occlusal interferences on muscle coordination and normal muscle ac- tivity. Using electromyographic procedures, he showed that interfering contacts on posterior teeth in any eccentric posi- tion caused hyperactivity of the elevator muscles. But if the anterior guidance was allowed to disclude all posterior teeth from any contact other than centric relation, the elevator muscles either stopped active contraction or noticeably re- duced it the moment the posterior teeth were discluded. If heavy contact on any posterior tooth in any eccentric posi- tion causes a response of muscle hyperactivity, it has the ef- fect of loading the tooth or teeth with the occlusal interfer- ences, but the elevator muscle hypercontraction also loads the joint with the same hyperactivity.