This study was designed to investigate the attitude of a positioner (Finition) or an Hawley's retention appliance to create a symmetric function of the masticatory, neck and trunk muscles during the orthodontic retention phase. Sixteen patients (18.5 +/- 2.2 years) were included in the sample. All the patients received an orthodontic therapy for the treatment of an Angle's class I malocclusion and skeletal class I, for a period of 2 years. After removal of orthodontic brackets, eight patients wore a teeth positioner (group I) as retention appliance and eight subjects wore an Hawley's appliance as upper retention (group II). Surface electromyographic (sEMG) activity was recorded from the bilateral anterior temporal, posterior temporal, masseter, sternocleidomastoid (SCM), upper and lower trapezius and cervical muscles at mandibular rest position and during maximal voluntary clenching (MVC) at T0, i.e. soon after the removal of orthodontic brackets and at T1, i.e. after 3 months of retention treatment. No significant difference was observed between the two groups at T0. At T1, at mandibular rest position, the group I showed a significantly lower sEMG activity of anterior temporal and SCM muscles compared with the group II. During MVC, at T1, the group I showed a significantly lower sEMG activity of the SCM and posterior cervicals, compared with the group II. The positioner seems to show a relevant effect on the sEMG activity of masticatory, neck and trunk muscles.