Previous work suggests a relationship between sustained low-level tooth clenching and the aetiology of myogenous temporomandibular disorder (TMD) pain. This study aimed to establish a reliable system with which to evaluate low-level electromyographic (EMG) activity related to low-level tooth clenching while discriminating speech activity, which is one of the most common facial muscle activities to be discriminated from low-level clenching. This device should facilitate the clinical evaluation of awake muscle activity in TMD patients. Eight female and eight male subjects (38 center dot 9 +/- 11 center dot 3years) participated in the study to evaluate the validity of estimation of speech duration. Actual speech duration was defined by one examiner by pointing out the timing of beginning and end point of each speech on wave-editing software. Speech duration, as detected by a voice sensor system, which was activated by a voice loudness of 54 center dot 71 +/- 5 center dot 00 dB, was significantly correlated with the above actual speech duration (P<0 center dot 01, R-2=0 center dot 9935). An actual recording with the system was carried out in one TMD patient and one healthy volunteer and revealed that the duration of diurnal EMG activity higher than 5% MVC was 1649 center dot 16s and 95 center dot 99s, respectively. As the voice sensor system adopted in this study could define the exact onset and offset of each segment of speech, EMG activity during speech could be precisely discriminated. The results of this study demonstrate that the EMG system with voice sensor system would be an effective tool for the evaluation of low-level masticatory muscle activity.