The EMG activity in the temporal muscle, masseter muscle, and in the lower lip was recorded during 27 minutes use of three makes of mouth-pieces for air supply during diving. For comparison, recordings were made also of the muscle activity with the mandible in the postural position and during maximal bite. Five male amateur divers, aged 22–35, took part in the investigation.

The muscle activity necessary to hold the mouthpiece in position varied from one make of mouthpiece to the other. Compared with the activity with the mandible in the postural position, the activity in the lip increased most, and then considerably, during use of a mouthpiece. Two of the mouthpieces also raised the activity in the masseter muscle. The temporal muscle was affected least and showed an increase in activity during use of only one of the mouthpieces.

In the laboratory experiments the muscle activity necessary for retention of the mouthpieces was only a fraction of that during maximal bite. It is stressed, however, that the force with which the mouthpiece is gripped when diving may, owing to emotional factors, be much greater. To avoid undue muscle fatigue and pain it is therefore recommended to use the mouthpiece which, according to the laboratory studies, was shown to require the least muscle activity for retention.