Quantifying scapula orientation and its influence on maximal hand force capability and shoulder muscle activity

BackgroundNon-neutral scapular orientations are often implicated as potential causes of shoulder pathologies. However, their specific influence on shoulder functional capabilities is largely unknown. This study objectively measured scapular orientation and quantified its influence on shoulder muscle activity levels and hand force capabilities during vertical and horizontal manual exertions.MethodsTen healthy male university students performed 24 exertions in combinations of scapular orientation (protracted, neutral and retracted), exertion direction (up, down, medial, lateral) and intensity (maximal or 40 N). Scapular orientation was quantified using an acromion marker cluster method. An orientation by intensity repeated measures ANOVA identified differences in quantified scapular orientation. A two-way multivariate ANOVA identified the influence of scapular orientation and hand force direction on muscle activity and hand force capability.FindingsParticipants assumed consistent retracted, neutral, and protracted scapular orientations during exertions, and these three orientations were different from each other (F(2, 99) = 158.57; P-value: 0.0001). Scapular orientation and exertion direction influenced muscle recruitment almost universally (P-value: 0.05). Scapular orientation did not influence hand force capability (F(2, 99)=1.34; P-value: 0.05), but a hand force direction effect on force existed (F(3, 99)=144.19; P-value: 0.0001).InterpretationThese findings support recommendations of health practitioners who advocate neutral scapular orientations to reduce injury risk, as a neutral orientation achieved a balanced overall muscle use pattern between retraction and protraction. Also, lowered muscle activity and higher maximal forces suggest that downward exertion forces may be preferable when possible.