Study design

All examinations were performed at Department of Dentistry and Oral Health, Aarhus University including clinical evaluations combined with self-reported and objective measures of mastication.

Participants were asked to fill in an Oral Health Impact Profile (OHIP) concerning perception of own oral health and its impact on quality of life. The OHIP contains 49 questions concerning oral function in chewing, eating, talking, as well as oral pain and discomfort (Gabardo, Moyses & Moyses, 2013). Answers are rated from 0-4 according to the amount of time the participant experienced the problem or discomfort in question from “never / don’t know” (0), “almost never” (1), “sometimes” (2), “often” (3), and “very often” (4). The total score ranges from 0 to 196.

The maximum bite force was measured using a U-shaped bite force transducer displaying force ranging from 0 –1000 (Kgf). The participants were asked to clench their teeth for 5 s and the highest bite force displayed was noted as their peak value. The test was repeated four times (Svensson & Arendt-Nielsen, 1996).

Maximum bite force was measured initially in the session and repeated after each of the following functional tests. After the static and functional tests the participants were given a two-minute rest before the next test was commenced. A bite force transducer provided the result of measured maximum bite force directly on the A/D converter display expressed in kgf as described by Svensson & Arendt-Nielsen (1996).

Electromyography (EMG) measurements were performed with bipolar EMG surface electrodes placed along the central part of the masseter muscle, the anterior temporalis muscles, and the suprahyoid muscles. The placement of the electrodes was made by palpation of the electrodes during maximum contraction. The Amplification of the EMG data was differentiated, filtered by 20–200 Hz and sampled at 512 Hz (Svensson, Arendt-Nielsen & Houe, 1996).

In one of the functional tests, participants were asked to chew test food for a maximum of one minute or until the food was swallowed completely. In the soft and brittle food test, a standardised cookie (half an “Oreo” mini biscuit without the cream) was used. In the non-brittle and hard food test, a piece of carrot was used measuring 25 mm in diameter, cut in squares and with a thickness of five mm. Finally, to evaluate muscle fatigue, the participants were asked to chew a chewing gum for five minutes. The number of chewing cycles was recorded with EMG and electrognathography (EGG).

During the functional tests, jaw movements were recorded at the lower incisor in the anterior, lateral and anterior-posterior axis with a sirognatograph (Svensson, Arendt-Nielsen & Houe, 1996). Participants were placed in upright position on a chair during measurements, and the recordings were stored as EGG signals.