Comments on standardization of reflex measurements in human masseter muscle, including silent periods

The following comments are given on the guidelines suggested by Türker (1988) for studies on oral reflexes using the surface EMG of the masseter muscles in man, including the silent period. (i) Attention should not be confined to electrical stimulation since mechanoreceptors in the periodontium, especially in the periodontal ligament, can only be activated by mechanical stimulation of a tooth. Furthermore, different modes of stimulation and stimulation sites do not yield equivalent reflex complexes. Weak and transient mechanical stimulation of a tooth while clenching at a low level of 5 or 10% MVC is a selective model (89%) to study the influence of periodontal receptors. (ii) Statistical criteria should be applied on rectified and averaged EMG records to prevent any subjective bias in the measurements of reflex variables. Whether a period of increased EMG activity is due to clustering of action potentials of motor units firing of which was delayed by a preceding inhibition, or to a real excitatory influence, can be assessed by applying statistical criteria on averaged and subsequently rectified records. (iii) The clenching level should be less than 25% of MVC to avoid muscle fatigue, and to elicit pronounced reflexes with a weak and therefore more selective stimulus. (iv) The stimulus intensity should be much less than six times the threshold if selectivity for mechanoreceptors is desired. Furthermore, periods of increased activity in surface EMG resulting from a weak stimulus are probably due to real excitatory influences and not to clustering of motor unit action potentials after an inhibition. (v) Jaw separation and the use of a force transducer are not always advantageous. If mechanical stimulation is applied to a tooth, clenching in full habitual occlusion causes a complete suppression of the influence of muscle spindles. A shift in activity between the various elevator muscles, and hence a change in activity of the muscle studied, may occur during an experimental session if biting force is used as a feedback. Such a shift is not relevant if feedback is carried out on rectified and low-pass filtered (1.6 Hz) EMG of the muscle studied.