Introduction. Temporomandibular disorders (TMD) are a group of conditions that are multifactorial in nature, and it primarily presents with symptoms of pain and decreased functional capacity. Therapeutic ultrasound is a method of physical therapy that has shown clinically to provide immediate relief of pain in patients with different musculoskeletal disorders. In this study, for the purpose of immediate management of painful masticatory muscle and tissues, the investigators attempt to understand the mechanism of therapeutic ultrasound by exploring the physiologic changes peripherally and centrally. In addition, the investigators aim to determine if there is a dose response relationship between two different intensities (0.4 W/cm2 and 0.8 W/cm2) and between two sets of duty cycles (50% and 100%). A third purpose is to determine if therapeutic ultrasound applied to the masseter muscle is capable of eliciting segmental effects in the temporalis muscle. Methods. A total of 28 adult females with bilateral myalgia according to Diagnostic criteria for TMD (DC/TMD) will be recruited for this investigational trial. Each subject will be randomly allocated to one of the four ultrasound intervention groups each of which has different settings. Using Sonicator®740, therapeutic ultrasound intervention will be performed on both masseters for five minutes on each side of the face. A template will be used to confine the borders of the muscle and to determine the sites for the repeated outcome measures. Outcome parameters of: self-reported pain scale, pressure pain threshold, electromyographic muscle amplitudes and intraoral muscle temperatures will be measured for masseter and temporalis muscles on each side of the face, and will be assessed at baseline and after intervention on each side.

The "Brief Summary" above encompasses the detailed description.

Inclusion Criteria:

• Adult females who have bilateral myalgia based on Diagnostic Criteria for TMD (DC-TMD) criteria and current pain intensity (≥4 out of 10, where 0 is no pain and 10 is the worst pain ever) on both sides.

Exclusion Criteria:

• Those who have a history or were diagnosed with systemic musculoskeletal disorders or rheumatologic diseases (e.g. fibromyalgia, muscular atrophy).
• Certain conditions such as neoplasms or fractures.
• Neuropathies or neurological disorders.
• Participants who are currently taking muscle relaxants or analgesics.
• Those who have undergone any form of physical therapy within the last 60 days.

One in group C and two in group D did not meet inclusion criterion of reporting current pain intensity of ≥4 on the Diagnostic Criteria for TMD.

Throughout the trial of one hour.

Three subjects who met the inclusion criteria on recruitment (bilateral masseter pain of four or more) did not meet the criteria at the time of the experiment so were not analyzed.