The trunk as a part of the kinematic chain for arm elevation in healthy subjects and in patients with frozen shoulder.

During arm elevation, the trunk may have both a postural and synergic role, but few 3-D experimental studies exist of the phenomenon, and the contribution of trunk rotations to arm elevation has not been studied in patients with frozen shoulder. Thirty healthy volunteers performed maximal dominant arm elevation in 2 planes, sagittal (anteflexion) and frontal (abduction), and 13 patients with unilateral frozen shoulder performed arm elevation on the unaffected then affected side. Trunk rotations and humeral elevation were measured with use of an electromagnetic system (Polhemus Fastrak). Flexion/extension, inclination (lateral bending) and torsion (rotation around the main axis) of the trunk were measured at intermediate (45 degrees and 60 degrees ) and maximal levels of arm elevation. For patients, trunk rotations were also measured during elevation on the unaffected side at a level corresponding to maximal arm elevation of the contralateral affected side. Healthy volunteers made a small (4 degrees -9 degrees ) but consistent pattern of trunk rotations characterized mainly by extension during anteflexion and torsion during abduction associated with biphasic inclination (ipsilateral then contralateral). As expected, patients showed restricted arm elevation of the affected shoulder but performed larger trunk extension and torsion at intermediate levels of elevation with a similar pattern as above. Inclination range was limited during elevation of the affected shoulder, with no initial ipsilateral inclination on any side. Our results suggest that the trunk contributes to the kinematic chain for arm elevation in both groups. Trunk extension and torsion may compensate for impaired arm elevation. Conversely, the irregularities in trunk inclination may contribute to the impairment and be a target for rehabilitative management.