STANDARDIZED ELECTROMYOGRAPHIC INDEXES ALLOW A RELIABLE MEASUREMENT OF MASTICATORY MUSCLES FUNCTION decreases and the conduction velocity is biased towards higher values 27 . Moreover we have to underline that surface electrodes , in many cases , capture electrical signals generated from several muscles -crosstalk effect- preventing a single muscle measurement . In facial cutaneous areas , the mimic muscles and sometimes anatomical variability , could complicate the univocal sEMG signal-muscle matching ( as previously reported platysma to masseter partial coverage ) 32 . Indeed it is well known that some muscles functional compartmentalization due to their anatomical and innervation complexity thwarts a simple function assessment ; in particular Temporalis muscles is a composite morphologic-functional entity not yet fully understood 33 , 34 . In any case , taking into account the technical complications , it appears reasonable to apply sEMG analysis to understand the functional areas central management instead of focusing on single muscle function measurement . Notwithstanding the amount of experimental and theoretical studies , no global consensus was achieved in the sEMG raw signal interpretation . For this reason , studies that could increase the understanding of sEMG advantages and limits in order to allow a more aware clinical application are required . The most revealing standardization effort occurred in 1997 – 1999 within the European Project on ‘‘ Surface EMG for Non Invasive Assessment of Muscles ” where a fine analysis was presented for a number of sEMG acquisition technical factors 35 , 36 . Yet , when comparing several EMG studies published in the year 2004 , Armijo-Olivo et al . stated that 60 % of the analysed studies did not report a normalization procedure , making comparisons of the EMG signal difficult due to anthropomorphic differences between recording sites and individuals 37 . They underlined that comparison of muscle function evaluating the absolute values of the recorded potentials ( that is , microvolt values ) provides inaccurate results . As a consequence of this inaccuracy , comparing the EMG activities between subjects and under different conditions requires a normalization process . The efficacy of intra-subjects standardization procedure was investigated also for the jaw stretch reflex . In fact , Koutris et al . demonstrated that when the amplitude of the masseter muscle during the jaw stretch reflex is normalized relative to the prestimulus EMG amplitude , it becomes independent from the location of the electrodes over the muscle 38 . In the present study , the repeatability of standardized ( normalized ) indexes calculated in independent sEMG acquisitions was evaluated . Indeed , as suggested by Ferrario et al ., sEMG could be usefully employed for the control of the muscular modifications induced by variations in the occlusal conditions , either natural ( for instance , different occlusal classes , crossbite occlusion ), or artificial ( for instance , dental prostheses , fillings , or even orthodontic treatment ) 26 , 39 . The use of sEMG can help in the correct construction of oral devices 40 . To simplify the understanding of occlusal induced muscular modifications , easy-tointerpret indexes could be helpful in the clinical device / prosthesis-adapting procedures . In order to perform a correct evaluation of these indexes in the everyday clinical practice , their reliability should be accurately understood . No significant differences in the standardized indexes elaborated from sEMG signals of T1 and T2 acquisition sessions were found , strengthening the effectiveness of the standardization procedure . The small sample analysed in the present study does not allow the evaluation of the values in an epidemiologic perspective . The average POC of Temporalis Anterior and Masseter , obtained during this study , resulted slightly lower than the ones reported by De Felicio et al . and by Ferrario et al ., probably due to differences in the selection of the sample 9 , 16 , 24 . As regards the standardization of masticatory muscles sEMG signal , a unique agreement within the scientific community does not exist 28 . Different sEMG studies have made use of the protocol and related indexes proposed by Ferrario et al . to evaluate the masticatory muscles 16 , 24 , 41-44 . The only difference between two acquisitions ( on cotton rolls and in IP ) is dental contact , thus limiting the technical and biological noise inherent in this instrumental analysis . This way , through the estimate of standardized indexes , it is possible to evaluate the muscular reaction to dental proprioception . Other stimuli , that can influence the muscular activity in both tests ( for instance , intramuscular pain ), cannot always be identified by evaluating the standardized indexes ( supposing that they can influence the standardization task and the teeth clenching in the same way ). For this reason , an effective dental occlusion instrumental evaluation applying sEMG protocol needs healthy supporting tissues ( bones , teeth , periodontium ). Applying these clinical recommendations , the proposed standardization approach allows dental proprioception effect on functional areas management isolation , reducing the crosstalk and technical signal confounding factors relevance . Anyway , instrumental information about muscular functional adaptation at dental contact can be useful for the classification of dental patients , and the functionalization of therapeutic procedures as removable and fixed prostheses . As an example , patient L . D ., a woman aged 26 , needed rehabilitation of incongruous composite fillings ( Fig . 2 ). An initial sEMG test could aid the Dentist to estimate if the reduced posterior vertical dimension , due to filling wear , can cause altered muscular performance . As shown in Figure 3 , the initial sEMG test revealed that the pre-operative occlusal condition did not allow a maximal muscular activity . Masseter POC was lower than 80 % as the result of a no-synchronous and asymmetric Masseter
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