1. Introduction
In laboratory studies, Electromyography (EMG) can be used to evaluate chewing. The EMG analysis makes it possible to identify muscle behaviors in different situations such as chewing and swallowing, and during maximal voluntary isometric contraction [12]. Studies have reported the prevalence of feeding problems in children with down syndrome as 50-80% [15]. For specialists dealing with people with Down syndrome, it is important to use instrumental assessments in addition to observation to better understand the features of dysphagia in this population.
Considering that no study has been conducted on EMG analysis of masseter and temporalis muscles during chewing in children with Down syndrome compared to healthy peers, this study was conducted with this purpose so that it can greatly help the specialists engaged in the treatment and intervention of these patients.
2. Materials & Methods
In this descriptive-analytical and case-control study, 15 children with Down syndrome aged 2-12 years in Arak, Iran were selected using a convenience sampling method as well as 15 healthy children aged 2-12 years matched for age and gender who were selected by a conveience sampling method. After meeting the criteria to enter the study and explaining the study process to the parents of the children and obtaining written informed consent from them, the children were asked to sit comfortably in a chair with no head resting and look at the computer screen in front of them. The EMG activity of the masseter and temporalis muscles was evaluated by an 8-channel EMG device at a sampling frequency of 1000 Hz and a calibrated 24 BIT device (PartoDanesh Co., Iran). After placement of the electrodes, the EMG activity was recorded during voluntary chewing of a seedless fruit (apple fruit). The obtained information was then entered into SPSS for statistical analysis.
3. Results
Tables 1 and 2 show the mean amplitude and frequency of EMG for the masseter and temporalis muscles while chewing in children with Down syndrome and healthy peers, respectively.




4. Discussion & conclusion
Among studies similar to our study, one study was conducted in 1989 on EMG analysis of the masseter muscle in people with Down syndrome, whose results are consistent with the current study. In a study by Hennequin et al. [8] to examine differences in chewing indices between 11 adults with Down Syndrome and 12 people without Down Syndrome. The ability to chew was evaluated by means of video recordings taken during a standardized meal that included 10 natural foods. Their results are also consistent with the results of the present study. The claim that people with Down syndrome have chewing problems and have less muscle strength and fatigue was highlighted by this study. The use of treatment methods to increase the strength of the muscles responsible for chewing in these patients should be assessed with follow-up. In a study in 2021 on masticatory function in children with Down syndrome aged 8-10 years, the results also showed that these children had approximately 50% chewing performance and maximum occlusal force compared to healthy age-matched peers [28], which also support the results of the present study. In other study aimed at evaluating the temporomandibular joint in 40 people with Down syndrome, it was found that 77.5% of these people had temporomandibular disorders [30]. Their results can be related to the results of our study; the disorder in the temporomandibular joint may be due to the dysfunction of the muscles responsible for chewing.

Ethical Considerations
Compliance with ethical guidelines
The protocol of this study was approved by the ethics committee of Arak University of Medical Sciences (Code: IR.ARAKMU.REC.1396.76). The personal information of participants was kept confidential, and written informed consent was obtained from the parents

Funding
This study was funded by Arak University of Medical Sciences (Grant No.:2794).

Authors' contributions
Conceptualization, design, and writing: Abdolreza Yavari, Mohammad Reza Arab; preparing initial draft, data analysis and interpretation: All authors; editing and review: Marzieh Babaei and Farhad Fatehi.

Conflicts of interest
The authors declare no conflict of interest.

Acknowledgements
The authors would like to thank the Department of General Education, Department of Welfare, and Rehabilitation Centers in Arak as well as participants for their cooperation.

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