The resulting heritability index showed genetic factors to be the main cause of the variability of the acromiohumeral distance, with shared and unique environmental factors contributing only slightly to the variability.

The role of genetic factors is also supported by the results of the acromiohumeral distance comparisons of the three groups of workers. No significant differences were found among groups who performed or had performed different types of labor. This was confirmed both in the whole study cohort and within the monozygotic and dizygotic subjects. These data appear to be partially in contrast to those of Frost and Andersen [51], who observed that shoulder-intensive work was a risk factor for impingement syndrome. Analogously, van Rijn et al. [52] noted that highly repetitive work was associated with the occurrence of subacromial impingement, and Roquelaure et al. [53] observed that skilled blue-collar workers were more likely to develop subacromial impingement, especially if forced to abduct the arm repeatedly. Finally, in a longitudinal study, Svendsen et al. [54] showed that forceful work, work with elevated arms, and repetitive work each doubled the risk of surgery for subacromial impingement.

Our study suggests that the anatomical features that influence the width of the subacromial space are mainly genetically determined. However, if the subacromial space is already constitutionally narrow, external factors would strongly contribute to further reduction of the space, making it too tight. This might occur as a consequence of the ossification of the acromial insertion of the coracoacromial ligament [9]; of contracture of the posterior capsule of the glenohumeral joint, which would lead to upward migration of the humeral head [55–57]; or of scapular muscle performance deficits [58].