Surface electromyography is very useful in the evaluation and treatment of TMD patients. Hypertonicity of masticatory muscles at rest is indicative of TMD. To determine hypertonicity, we need to know the normative data. “Normal” values, in microvolts, for each muscle group comes from the work of Dr. Jeffrey Cram Ph.D. who did the research and published the normative data in his book “Clinical EMG for Surface Recordings: Volume 2” Aspen Publication 1998. These values are:
Anterior Temporalis (TA) 2.8 uV
Masseter (MM) 2.0 uV
Digastric (DA) 1.7 uV
Posterior Temporalis (TP) 2.2 uV
Sternocleidomastoid (SM) 1.5 uV
Some have misinterpreted how these EMG norms are used by a clinician. They are, in themselves, not diagnostic of TMD. After all the clinician is the one who diagnoses the patient’s condition using many different metrics including EMG data.
It needs to be understood by the clinician that these normal values are similar to other physiologic metrics such as the ‘normal’ blood pressure of 80/120 mm of Hg. Of course this blood pressure value is not applicable for all humans. Many physically fit people have much lower ‘normal’ blood pressure and there are healthy individuals with a slightly higher normal blood pressure.
On K7 scan 9, resting electromyography of muscle Groups A & G, or A & B, the normal values are shown on the left and actual values on the right. In this example below, LSM is hypertonic. Even though the RTA value is within the norm, it seems to be hypertonic compared to LTA.
In the example below, both TA’s are hypertonic, with the LTA clearly more so. Even though RSM value is only slightly over the norm of 1.5 uV, it is nearly twice that of the LSM and so would be considered hypertonic.