## 2 Materials and Methods  
A convenience sample of 10 healthy adult participants (5 men and 5 women) was recruited. Mean age was 28.6 \(\pm\) 7.1 years (range 21 to 43), with mean height 1.69 \(\pm\) 0.9m (range 1.57 to 1.83), mean weight 65.9 \(\pm\) 12.4 kg (range 47-89) and mean body mass index (BMI) 22.9 \(\pm\) 3 (range 18.8-30.8). All were right-handed and had normal or corrected-to-normal vision. Clinical examination by an experienced physiotherapist (AR) was carried out prior to the experimental procedure to rule out asymptomatic anomalies of the arm or spine. Persons having an antecedent of orthopedic or neurological pathology affecting the shoulder or cervical spine were not eligible for this study. The study protocol was approved by the committee for the protection of persons Ile-de-France III (CPP-IDFIII, no. 2013-A00660-45). All participants provided informed written consent prior to commencement. A white panel with eight colored targets (0.015m diameter) presented in a circular arrangement (0.32m radius) around a central black cross was mounted on the wall. Each target was placed at 45\({}^{\circ}\) intervals corresponding to trigonometric directions (0\({}^{\circ}\) for East, increasing counterclockwise). According to this configuration each target was designated according to the directions: North, Northeast, East, Southeast, South, Southwest, West, Northwest. Panel height was adjusted to the level of the eyes of each person on an individual basis (Figure 1a). Three electromagnetic sensors (Polhemus Fastrak) (SPACE FASTRAK, Colchester, VT, USA) were placed on each participant: one was fixed to the head using a customized helmet, another attached to the skin over the manubrium, and the third strapped to the right arm with Velcro below the insertion of the deltoid in order to monitor humero-thoracic arm motion. A fourth sensor was used to digitize the bony landmarks. Consistent with the International Shoulder Group (ISB) recommendations [27], the trunk was bounded by the xiphoid process, suprasternal notch with the C7 and T8 spinous processes. Finally, the position of the tragus and external corner of each eye were used to define canthomeatl plane (Figure 1b). The Polhemus Fastrak(r) transmitter, which provided the general frame of reference, was fitted to a height adjustable tripod placed in front of the participant, close to the level of the navel. Bony landmarks of the trunk and head were calibrated by a preliminary digitization of their position using a digital stylus, and then computed in the local coordinate system of the sensor fixed on the corresponding segment. The 3D position in space of the bony landmarks of the trunk, head and arm were computed by projecting their local coordinates with the 3D position and orientation of the respective sensor. Then, reference frames were calculated for the head and trunk (with X from left to right tragus, Z from the middle between eye corners to the middle between the tragus and Y perpendicular, Figure 1b), consistent with the ISB protocol [27]. Rotation matrices between frames were calculated, then the 3D rotations were expressed by using Euler angle sequences XZY. For the purposes of this study, the Euler angle reference frame for head and trunk orientation are referred to in terms of anterior/posterior tilt (AP), lateral tilt (LA) and axial rotation (RO). By definition, the Euler angles are zero in the reference posture, positive values indicating posterior tilt, left lateral tilt, left axial rotation. As it is known that the canthomeatl plane is inclined by reference to the horizontal [28] we aligned the reference frames of the head and trunk for each participant so that they were parallel to the global reference frame in the baseline posture, with X right, Y up and Z backward. In the starting position, participants were required to stand upright with their trunk parallel to the panel of targets, arms by their sides.