Effect of external load on scapular upward rotation during arm elevation: the knot concept

BACKGROUND Failure of the scapulohumeral rhythm (SHR) is observed in patients with shoulder joint dysfunction. The SHR reportedly has a 2:1 ratio during scapular upward rotation with arm elevation. However, three-dimensional scapular motion analysis has indicated variations in this ratio according to the arm elevation angle. We observed 2 distinct patterns: the scapular upward rotation decreased after knot formation (type I) or increased after knot formation (type II) during arm elevation. In the present study, we aimed to identify the knot and investigate the influence of varying external loads on this kinesiological change point. METHODS We evaluated 35 healthy adult men (35 dominant-side shoulders) with a mean age of 20 ± 1.7 years (mean height: 172 ± 6.4 cm, mean weight: 65.7 ± 5.8 kg). Participants performed scapular plane elevation with no load or with an external load (1-5 kg) while sitting on a chair. The measured scapular upward rotation values were interpolated using the spline function and fitted to line graphs, and the change in these values was compared for various loads. RESULTS The estimated knot angles (standard error) in the no load condition, and with external loads of 1, 2, 3, 4, and 5 kg were 83.5 (2.9°), 81.2 (2.9°), 81.0 (2.9°), 76.1 (2.9°), 73.4 (3.1°), and 75.8 (3.1°), respectively. No significant difference was noted in the knot position at 1-2 kg (vs. unloaded), although the knot was significantly lower at 3-5 kg (3 kg: p = 0.01, 4 kg: p = 0.001, and 5 kg: p = 0.02). Moreover, we observed that participants either exhibited increased or decreased upward rotational momentum after knot formation. CONCLUSION Our results confirm that the kinesiological change point (the knot) during scapular upward rotation occurred at lower angles in cases of increasing external loads.