## 4 Discussion
Importantly, then, as movement of the trunk participates in upper limb activity, the cervical spine rotates with complex 3D adjustments required to compensate trunk movement by counter-rotation [24]. At the same time, head movement with respect to the trunk was highly dependent upon target location during the pointing task. Broadly speaking, these variations in the amplitude of axial rotation and anterior head tilt were comparatively smaller than those observed in the heading task (Figure 3). For example, when pointing toward the West target, axial rotation of the head with respect to the trunk was negligible. In such an instance the movement of the trunk propelling the upper limb equally provides the necessary axial rotation for position of the head with respect to the target. The kinematic constraints on the neck are evidenced by the sinusoidal fitting analysis. For heading, the angles describing the orientation of the head relative to the trunk varied with large periodic amplitude as a function of target direction, with a small offset. In contrast, the amplitude of the periodic variation was smaller for pointing and associated by a greater offset. This suggests a greater functional rigidity of the cervical spine in this condition. The biomechanical constraints exerted on the neck during pointing are further illustrated by the decoupling between rotation and lateral bending observed in half of the participants. The coupling between rotation and lateral bending most clearly observed during heading can be mathematical (due to Euler angles formalism), anatomical or functional. The mathematical coupling can be neglected for the relatively small angles that we observed [40]. The anatomical shape and orientation of the posterior articular facets of the cervical spine constrain the rotation of vertebrae, as demonstrated by imagery and anatomical studies [41; 42; 43]. The constraints between vertebrae vary according to the level in the cervical spine. According to [44], they lead to a homolateral coupling between rotation and lateral bending in the lower cervical spine and hetero-lateral in the upper cervical spine. Because of differences in methodologies, concluding on the effect of anatomical constraints on the global movements of the cervical spine is difficult [45; 46; 47; 48; 49]. It is likely that the coupling between rotation and lateral bending we observed was a functional coupling linked to the neural control of gaze direction as formalized by the Listing law [50; 51]. Another likely explanation is that this movement is the result of simple biomechanical effects of the sternocleidomastoid. In effect, the different heads of this muscle establish moment arms which preferentially generate torque across these two axes [52]. The fact that coupling between axial rotation and lateral bending of the neck differed across subjects during pointing indicates that muscle activation at the level of the neck changes as one integrates upper limb movement into the process reorienting towards a target in the environment. Implications for management of musculoskeletal complaints. This study highlighted that the cervical spine compensates for trunk rotation during functional upper limb activity. Issues with cervical position sense are a common symptom for people experiencing neck pain or persistent symptoms after neck trauma [53; 54]. We would, thus, anticipate that people with musculoskeletal neck disturbances might, therefore, experience difficulties with accommodating movements of their head when the arm is solicited, particularly for complex sporting movements. Problems in either the timing or amplitude of reciprocal head movement may be expected to further aggravate neck injuries. Our findings thus reinforce the importance of specific cervical proprioceptive retraining for musculoskeletal neck injuries [55].