Abstract
BackgroundFailure 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.MethodsWe 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.ResultsThe 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.ConclusionOur results confirm that the kinesiological change point (the knot) during scapular upward rotation occurred at lower angles in cases of increasing external loads.
Highlights
Failure of the scapulohumeral rhythm (SHR) is observed in patients with shoulder joint dysfunction
There was no interaction between the scapular upward rotation angle and the amount of external load, and no main effect was observed for the amount of external load on scapular upward rotation (Fig. 3)
With regard to scapular movements before and after knot formation, we observed 2 distinct patterns: the scapular upward rotation decreased after knot formation or increased after knot formation during arm elevation (Fig. 4)
Summary
Failure of the scapulohumeral rhythm (SHR) is observed in patients with shoulder joint dysfunction. We aimed to identify the knot and investigate the influence of varying external loads on this kinesiological change point. Various studies have reported that scapular upward rotation is influenced by external loads (Forte et al 2009; Kon et al 2008; McQuade et al 1998). Studies comparing loaded and unloaded scapular movement during arm elevation have reported that the scapular upward rotation decreased (Kon et al 2008; McQuade et al 1998), increased (Forte et al 2009), or was not influenced (de Castro et al 2014; Pascoal et al 2000) by the external load. In the present study, we aimed to identify this knot during continuous scapular upward rotation, and the influence of external loads on this kinesiological change point
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