# 3D-Printed Helmet for Electromagnetic Articulograph Applied in the Study of Oral Physiology
## 4 Discussion
The helmet group completed all Session 2 recordings with a single head correction. Due to the positioning of the reference sensors on the mastoid processes and the glabella, misalignment of the reference sensors may occur. The sensors located on the mastoid processes are affected by mandibular movements, as these movements displace the skin in these areas. The sensor on the glabella may be influenced by head movements and involuntary actions, such as eyebrow raising, which also shift the skin. In the case of the helmet, it is primarily affected by head movements that may pull on the cables and cause displacement; however, it isolates the skin movements. This may explain why the need to perform multiple head corrections during recording sessions persists, albeit with a reduced frequency  
Skin motion artifacts have been described in literature [39]. This is a result of skin elasticity and depends on factors, such as age, body mass index and movement extent. In other motion capture studies, this problem is addressed by using simple fixation methods, such as tape [40] or elastic warps [34]. Due to the nature of the head correction procedure, these methods are insufficient, since the limit is on the order of 1.5 mm for the three sensors. If the three sensors are loose, even small displacements contribute to an increase in the taxonomic distance, and head correction cannot be performed. If sensors are placed on an elastic surface, such as elastic wraps, any change in the tension of the wrap modifies the relative positions of the reference sensors and increases the taxonomic distance. In previous studies analyzing mastication or Posselt's envelope of motion, the samples consisted of subjects aged between 20 and 30 years; therefore, skin laxity was not an issue and skin fixation was sufficient. In our study, tests were conducted on subjects aged between 60 and 80 years, and [PERSON]'s envelope of motion was analyzed, which requires the subject to move the mandible to extreme positions. Consequently, skin movement may have increased due to the age-related laxity. Moreover, participants were asked to chew a variety of foods, whereas in previous studies only peanuts and carrots were used [26; 27; 28]. Another consideration in the design of the present helmet in the context of dental research was the use of sEMG to record anterior temporal muscle activity, along with electromagnetic articulography [38]. To allow this function, the anterior temporal area must be free for electrode placement. This could be an inconvenience if goggles, lens or elastic wraps are used as in the studies by [PERSON] et al. [20] and [PERSON] et al. [35; 36]. Although the reduction in the average number of HCs seems to suggest a benefit of the helmet, statistical analysis indicates that this difference could be attributed to chance, possibly due to variability in the helmet's fit. A possible solution to this problem is the design of a set of helmets that adapt to different head sizes or to design an adjustable fixation system. On the other hand, the use of the helmet allows preparation prior to the subject's arrival, which saves recording time. A limitation of this study is the small sample size. For this reason, statistical analyses comparing male and female participants, as well as those with functional dentition and complete denture wearers, were not performed. In addition, no method was established to verify the level of sensor fixation. The only similar device found in the literature was the use of plastic glasses by [PERSON] et al. [20] and [PERSON] et al. [35; 36], but no comparison between the use of glasses and sensors on skin was made by these authors.