Study design: Controlled laboratory study.
Objectives: To examine the effects of altering posture on the subacromial space (SAS) in subjects with rotator cuff disease and subjects without shoulder pain.
Background: Poor upper quadrant posture has been linked to altered scapular mechanics, which has been theorized to excessively reduce SAS. However, no study has examined the direct effects of altering upper quadrant posture on SAS. We hypothesized that upright posture would increase and slouched posture would decrease the SAS, as compared to a normal posture, when measured both with the shoulder at rest along the side of the trunk and when maintained in 45° of active shoulder abduction.
Methods: Participants included 2 groups: the subjects with shoulder pain and rotator cuff disease, as diagnosed via magnetic resonance imaging (n = 31), and control subjects without shoulder pain (n = 29). The SAS was imaged with ultrasound using a 7.5-MHz linear transducer placed in the coronal plane over the posterior to midportion of the acromion. The SAS was measured on ultrasound images using the acromiohumeral distance (AHD), defined as the shortest distance between the acromion and the humerus. The AHD was measured in 2 trials at 2 arm angles (at rest along the trunk and at 45° of active abduction) and across 3 postures (normal, slouched, and upright), and averaged for data analysis.
Results: Two mixed-model analyses of variance, 1 for each arm angle, were used to compare AHD across postures and between groups. There was no interaction between group and posture, and no significant main effect of group for either arm position. There was no significant main effect of posture for the arm at rest (P = .26); however, there was a significant main effect of posture on AHD at the 45° abduction arm angle (P = .0002), with a significantly greater AHD in upright posture (mean AHD, 9.8 mm), as compared to normal posture (mean AHD, 8.6 mm).
Conclusion: The effect of posture on SAS, as measured by the 2-dimensional AHD using ultrasound of the posterior to middle aspect of the SAS, is small. The AHD increased with upright posture by 1.2 mm compared to normal posture, when the arm was in 45° active abduction.