Development of a novel index of shoulder's mobility based on the configuration space volume and its link to mono-axial amplitudes.

At first sight, shoulder mobility is frequently evaluated through mono-axial amplitude. Interestingly, for diagnosing shoulder hyperlaxity or frozen shoulder, external rotation of the arm whilst at the side (ER1) is commonly used. However, by definition, a mono-axial amplitude does not fully reflect shoulder global mobility. Our goal was to propose a novel index for measuring shoulder global mobility and secondly to evaluate the link between main mono-axial amplitudes and this new index. Twenty-eight female subjects (mean age 24.8 years) without upper limb pathology participated in the study. The movements of their right dominant arm were measured with an opto-electronic motion capture system. They performed 5 mono-axial maximal amplitude motions (axial rotations in three different postures, flexion/extension and abduction from rest) and a global range of motion exploring all the reachable space around the three axes of rotation. From this, we computed the correlation coefficient between the volume of the reachable space and each possible linear combination of the 5 mono-axial amplitudes. Even though ER1 is often chosen to assess global mobility, it demonstrated the lowest correlation with measured joint mobility. To assess shoulder global mobility, clinical routine examination should more take into account external/internal rotation with the shoulder abducted, then abduction and finally flexion/extension. However, further clinical testing in other populations has to be done to evaluate the potential generalization of this result.