This paper evaluates the reliability of the use of muscle activation during unuttered (silent) vowel by an individual and reports the study of repeating of the experiments over several days. Surface electromyogram has been used as an indicator of muscle activity and independent component analysis (ICA) has been used to separate the electrical activity from different muscles. The results demonstrate that there is 'reasonable' relationship between muscle activities of the corresponding muscles when the experiments are repeated. The results also indicate that there is a variation when the same sound is spoken at different speeds of utterances. This can be attributable to the lack of audio feedback when the same sound is uttered. This analysis will be useful for the research undertaking facial surface electromyography (sEMG) for speech analysis, which is being considered as a potential human computer interface (HCI) for people suffering from motor disabilities.