Skeletal muscle tissue is voluntary. Cardiac muscle tissue is involuntary. (We don't have to direct our heart to beat, thus it is involuntary muscle.)
To do a voluntary action means to do it without pay, or without having to be asked or forced. Example: Martin's voluntary cleanup of the rubbish surprised them since nobody had asked him to do it.
Voluntary
Encopresis can be either involuntary or voluntary
mastication is simply chewing, so yes, it is voluntary