Jaw-muscle pain associated with temporomandibular disorders (TMD) has traditionally been linked to hyperactivity or abnormal contraction of masticatory muscles such as “bruxism”. A frequent clinical observation has been that many patients with TMD exhibit a tendency to clench or grind their teeth during sleep. It has been considered for a long time that unaccustomed or abnormal contractions of the muscles causes this pain, and once the pain had developed, it causes more muscle hyperactivity, setting up a vicious cycle. However, this theory has not been clarified scientifically and the relationship between various types of orofacial motor activity and TMD is still unclear. This review will discuss the association between bruxism and jaw-muscle pain. All data is based on narrative evidence. The results showed that jaw-muscle activity would evoke significant levels of pain but it is not as prolonged as other temporomandibular disorders.