Normalization reduces the spatial dependency of the jaw-stretch reflex activity in the human masseter muscle

The jaw-stretch reflex is the short-latency response in the jaw-closing muscles after a sudden stretch. The hypothesis whether normalization of the jaw-stretch reflex amplitude with respect to prestimulus electromyographic (EMG) activity will make the amplitude more independent of the location of the electrodes over the masseter muscle was tested. A 5 × 6 electrode grid was used to record the jaw-stretch reflex from 25 sites over the right masseter muscle of 15 healthy men. The results showed that there was a significant site dependency of the prestimulus EMG activity and the reflex amplitude. High cross-correlation coefficients were found between the spatial distribution of mean prestimulus EMG activities and reflex amplitude. When the reflex amplitude was normalized with respect to the prestimulus EMG activity, no site dependency was found. In conclusion, normalization of the jaw-stretch reflex amplitude by the prestimulus EMG activity strongly reduces its spatial dependency.