By and large, children with selective mutism lacks facial expression in kindergarten, school, etc.. They spend hours with expressionless face every day. Some of them may overcome the disorder soon. But others may continue to be mute several years.
The problem is the latter. If they suffer from selective mutism for years and spend hours with expressionless face every day in school or kindergarten, what effect do facial muscles have?
I guess they don't use their facial muscles very much. If facial muscles aren't used very much in childhood or adolescence, what happens?
I don't know about facial muscles well.
But I guess one possibility is that it becomes more difficult for them to create facial expressions. Of course, many of them are forced to have blank facial expressions by anxiety. But if their anxiety decreases, I guess their undeveloped facial muscles may prevent them from creating facial expressions. That can influence communication. In general, the less expressive face a person shows in face-to-face contact, the less he or she can communicate successfully. They may need to train their facial muscles if they want to communicate with other people successfully.
Speaking of facial muscles, some Japanese women train their facial muscles for cosmetic purpose. It is said that people tend to look old for their age if their facial muscles are weakened. If so, do children or people with selective mutism look old ? ... I don't know.
