Median frequency of the myoelectric signal in cervical paraspinal muscles.

Objective measurements of the function of cervical paraspinal muscles (CPM) are lacking. Changes in the frequency spectrum of the myoelectric (ME) signal have been used in the limb muscles as an objective measurement of muscle function. Based on our previous investigational work on limb muscles, we attempted to measure parameters related to median frequency (MF) of the ME signal to measure localized muscle fatigue in cervical paraspinal muscles in 14 healthy subjects. The ME signals were recorded bilaterally using surface electrodes placed posteriorly at the C3 to C6 level. Sustained isometric neck extensions were performed at 20%, 50%, 80%, and 100% of maximum voluntary contractions (MVC). Initial median frequency (IMF) and slope of the MF during sustained isometric contraction were obtained by analyzing the ME signal using an IBM-XT computer, a laboratory interface system, and custom-designed software. The results indicated that the MF of the CPM behave somewhat differently from limb muscles, and that the CPM are less fatigable than limb muscles. The test-retest reliability, using our system, was found to be high, and data processing showed a variability of 4 to 8Hz. These results imply that neck muscles appear to behave differently at high and low force levels in response to daily functional demand. The MF measurements appear to be a useful objective technique for evaluating muscle function of cervical musculature.