The masseter muscle: the silent period and its clinical implications.

Standardized solenoid chin taps were delivered downward and upward as parallel as possible to the fibers of the superficial part of the masseter muscle and backward at a right angle to this direction. Each one of the 10 healthy subjects received 10 taps in each direction during isometric masseter muscle activity (clenching the teeth). Taps in all three directions evoked the classic jaw jerk response, M1, a short latency (mean, 8.3 msec) excitation followed by an SP (mean latency, 15.9 msec; mean duration, 42.6 msec). The SP duration varied considerably not only between subjects but also within individuals. Forty-nine percent of the SPs were divided into two parts by an EMG peak, M2, with mean latency 42 msec and mean amplitude 78 microV. Sometimes this peak marked the end of the SP. The SP duration has been claimed to be of diagnostic significance because it is longer in patients with mandibular dysfunction. It is concluded that the large variations of the parameter found in this as well as in other studies make it impossible to establish criteria for a healthy or pathologic SP duration. A carefully obtained history and thorough clinical examination are probably more reliable means to establish a correct diagnosis.