Muscle coordination and scapular kinematic patterns in individuals post rotator-cuff repair
Creators: name: Sharma, Sapna
Publisher: name: University of Iowa
Year: 2022
Subjects: Muscle activity ratios
Rotator-cuff repair
Scapular upward rotation
public abstract

description: Rotator-cuff (RC) tears often present with pain, weakness and limited range of motion in the shoulder, thus affecting the functional capacity and quality of life. Most commonly, RC tears involve the supraspinatus and the infraspinatus tendons. External-rotation (ER) exercises are generally used to restore the strength in the RC muscles, especially the infraspinatus. However, the RC muscles do not work in isolation but instead coordinate with one another and with other should muscles such as deltoid during various shoulder movements. The relative muscle activity ratios for infraspinatus (IS) to other shoulder muscles such as middle deltoid (MD) in individuals post RC-repair is not known. Gaining information about IS/MD muscle activity ratio will contribute to understanding the muscular component of GH joint stability post-RC cuff repair since these muscles form a force couple at the glenohumeral joint. In addition, evaluating the muscle coordination patterns between other scapular muscles such as the upper trapezius (UT) and lower trapezius (LT) would further help to understand the imbalances in scapulohumeral rhythm in individuals post-RC repair. This foundational knowledge may be useful in designing rehabilitation protocols in this patient population. The aim of this thesis is to characterize the relative muscle coordination of shoulder muscles bilaterally at three different angles of humeral abduction (0⁰, 30⁰ and 60⁰) during an active an ER movement task in individuals post rotator-cuff (RC) repair as compared to healthy controls. The secondary objective of this thesis was to evaluate scapular upward rotation angles at three static positions of humeral abduction (0⁰, 30⁰ and 60⁰) in this patient population. Individuals post rotator-cuff repair and healthy controls (n=13 in each group) were recruit5ed in this study. Surface electromyographic (EMG) muscle activity was recorded from the UT, LT, IS and MD muscles during an ER task at three different angles of humeral abduction, 0⁰, 30⁰ and 60⁰, bilaterally in each group (healthy controls: dominant and non-dominant sides; RC repair group: repaired and non-repaired sides). Scapular upward rotation was measured at three static humeral abduction positions 0⁰, 30⁰ and 60⁰, bilaterally in each group using a digital inclinometer. Normalized EMG UT/LT and IS/MD muscle activity ratios, and scapular upward rotation were the main outcome measures. A 2x3 mixed-model ANOVA yielded a group main effect for the IS/MD ratio (F(1,24) =6.9, p=0.015) for the comparison between the dominant side (healthy) and the repaired side (RC repair group), with a lower ratio in individuals post-RC repair at 0⁰ (mean difference healthy – RC repair = 2.28, p=0.009) and 30⁰ (mean difference healthy – RC repair = 0.46, p=0.040) abduction positions. The IS/MD ratio at 0⁰ abduction was nearly 2 times greater in the healthy controls than those in the RC repair group. There was also a significant main effect of the group (F (1,24) =8.8, p=0.007) for the IS/MD ratio for the comparison between the non-dominant side (healthy) and the non-repaired side (RC repair group), with a lower ratio in individuals post-RC repair at 0⁰ (mean difference healthy – RC repair = 1.9, p=0.014) and 30⁰ (mean difference healthy – RC repair = 0.52, p=0.001) abduction positions. The IS/MD ratio at 0⁰ abduction was about 1.5 times greater in the healthy controls than those in the RC repair group. The IS/MD ratio at 60⁰ abduction angle did not differ between the two groups. The lower IS/MD ratios the RC repair group were primarily due to increased MD activity levels in both the repaired and non-repaired (presumably healthy) shoulders compared to the healthy controls. There were no differences in the UT/LT ratios for the two side-to-side comparisons between the two groups (p&gt;0.05). However, the IS/MD ratio was found to be significantly higher on the non-repaired side in females than in males in the RC repair group, at all the three angles of humeral abduction (p&lt;0.05). Also, within the RC repair group, there were no differences in the UT/LT and IS/MD muscle activity ratios between subjects over and under the age 56 years (p=0.10). Similarly, there were no differences in the UT/LT and IS/MD ratios between subjects who were 1-year or less post-surgery as compared to those who were more than 1-year post-surgery (p=0.08). For the scapular upward rotation, there was a significant group main effect for the comparison between the dominant side (healthy) and the repaired side (RC repair group), with slightly greater scapular upward rotation in individuals post-RC repair as compared to healthy (p = 0.03). There was also a significant main effect of group for the comparison between the non-dominant side (healthy) and the non-repaired side (RC repair group), with individuals post-RC repair demonstrating greater scapular upward rotation on their non-repaired sides as well (p = 0.03). Data suggested that the mean scapular upward rotation angles differed by about 0.5-1⁰ between the two groups at each humeral abduction position, and these relatively small differences in scapular upward rotation angles limit the clinical and physiological relevance of this measure. Clinically, these findings suggest that IS muscle may not be providing sufficient activation to counteract the superior translation of the humerus due to increased deltoid activation in individuals post-RC repair, especially at lower angles of humeral abduction (0⁰ and 30⁰). Future research should determine if exercises aimed at restoring the altered IS/MD force couple and scapular upward rotation are beneficial to improve shoulder function in individuals post-RC repair.