## 4 Discussion  
The present study compared head and trunk kinematics during two experimental tasks. The heading task explicitly required subjects to orientate their head towards designated targets. For the pointing task, movements of the head were implicitly required as subjects gestured towards the same targets with their outstretched right arm. The posture of the head in space was observed to be finely tuned to target direction, irrespective of the task. Pointing movements with the dominant right arm were associated with a posterior tilt of the trunk. Moreover, the systematic left axial rotation was indicative of participation of the trunk to the upper-limb reaching synergy, even in this relatively simple gesture. The combination of the target related spatial constraints on the head and the synergetic trunk movement in pointing induced distinct combinations of rotation (flexion-extension, lateral bending and axial rotation) at the level of the cervical spine. Our results highlight complex reciprocal movement constraints imposed upon the cervical and thoracic spine during functional upper limb movement. These findings have potential implications in the management of sports related musculoskeletal disorders of the neck and shoulder. Target dependent head orientation in space. Participants were observed to orientate their heads consistently toward the different targets across both heading and pointing tasks. Being able to adjust the position of one's head effectively with respect to a given target is an important component for success in various motor skills (e.g., bat and ball sports). Combined movement of the head, trunk and lower limbs for maintaining the orientation of the head in space has been previously documented in studies examining whole body locomotor tasks (e.g., walking, running, hopping...) [30; 31; 32; 33] or different postural constraints involving varying trunk orientation [34]. The present study demonstrated similar tuning of the orientation of the head in space when fixating targets (heading) and when completing functional arm movement towards those targets (pointing). But the amplitude of the angles was smaller for pointing. Of course, orientation of the head with respect to the target may be assumed to support the line of sight. Shifting one's gaze generally implies rotation of the head accompanying eye movements so as to place the image on the fovea (review in [17]). During the heading task, we found that the maximal amplitude of head axial rotation was comparable to target eccentricity (i.e., \(20^{\circ}\)), suggesting that gaze orientation was mainly ensured by the head, with small eye-in-head deviations. This is not surprising given the explicit instruction for alignment of the head with the target. Amplitude of head movements was smaller during pointing and infers that foveation involved comparatively greater deviation of the visual axis with respect to the head. These observations are consistent with the known regulation of gaze direction in parallel with hand movements as a function of task requirements [16; 18; 19; 35]. The difference we observedwas quantitatively consistent with previous studies analyzing the contribution of head orientation to gaze direction [36; 37]. Neck movement offsets trunk contribution to upper limb activity. During movements of the head independent of the arm, the trunk was found to remain relatively stable. In contrast to this, the pointing task was associated with marked leftward axial rotation with more moderate posterior incline and leftward lean. This trunk rotation is most probably part of a synergy with the upper limb [38; 39]. This phenomenon had a corresponding effect for the kinematics of the head with respect to the trunk. That is to say, during the heading task, variations in amplitude of the head with respect to the trunk were similar to those for the head in space. During pointing, however, the head was globally rotated toward the right relative to the trunk owing to the leftward rotation of the trunk.