Summary

This study compared shoulder motion and muscle activation during abduction and flexion between individuals with large-to-massive rotator cuff tears and an asymptomatic group, revealing compensatory increases in trapezius muscle activity in the tear group, particularly during abduction, and highlighting the importance of evaluating extreme ranges of motion for detecting deficits.

Abstract

Introduction

Rotator cuff tears can significantly impair daily activities. Biomechanical and neuromuscular compensatory mechanisms during shoulder elevation remain unclear, particularly in individuals with large-to-massive rotator cuff tears. The purpose of this study was to compare shoulder motion and muscle activation levels during elevation in the frontal (i.e., abduction) and sagittal (i.e., forward flexion) planes between individuals who have sustained a rotator cuff tear and an asymptomatic cohort.

Methods

Twenty asymptomatic participants (>50 years old) were recruited from the local community, all free of past or present shoulder pain. Ten patients with two-tendon tears (>3cm) confirmed by MRI were recruited from a local clinic (cuff group). Surface electromyography was used to monitor the supraspinatus, upper (UFT) and lower (LFT) fibres of trapezius and anterior and lateral deltoids while optical motion capture tracked torso, thorax, scapular and arm movement during maximal abduction and flexion. Scapulothoracic (ST) rotation, and glenohumeral (GH) and thoracohumeral (TH) flexion and abduction were calculated using Cardan rotation sequences, along with peak percent maximum voluntary isometric contraction (%MVIC). Outcomes were compared using 2-factor ANOVAs or independent samples t-tests, with significance set at 0.05.

Results

The groups were comparable in age and BMI (kg/m²), with greater distribution of males in the cuff group. Significantly less peak maximal abduction (TH) between the cuff and asymptomatic population (difference of ~24) emerged, identified through decrease in GH motion (~28 p<0.05) but not ST motion (p>0.05). Maximum shoulder flexion was less in the cuff group, measured through the TH joint, but did not demonstrate between group significance.

Overall, the cuff group demonstrated increased relative peak muscle activity of >100% MVIC compared to asymptomatic group for flexion and abduction. Particularly, during abduction, the cuff group showed greater UFT activation (136% peak %MVIC) compared to the asymptomatic group (82% peak %MVIC), indicating increased reliance on the UFT in the presence of a rotator cuff tear. Between group peak muscle differences were not found to be significantly different during flexion. However, in flexion, both groups demonstrated significantly greater LFT compared to SSP activation, highlighting increased relative activity of the scapular muscles compared to the cuff.

Conclusions

These findings reveal compensatory mechanisms in muscle activation and joint angles during shoulder movements, highlighting the impact of rotator cuff issues on functional performance. Although the groups had similar peak scapular rotation, differences were observed in peak humeral angles relative to the glenoid and thorax during abduction. This highlights the importance of evaluating extreme ranges of motion to detect deficits. The relative trapezius muscle activation peaks underscore the role of scapular muscle activity in patients with rotator cuff tears. This has possible clinical relevance for addressing and/or identifying neck and high scapular pain in individuals with rotator cuff problems.

This research sets the stage for a deeper understanding of kinematic and neuromuscular deficits seen in rotator cuff pathology, but further investigation with greater a sample size is required. It provides information for continued muscle use in large tear populations and future possibilities for identification of activation patterns following repair or reconstruction.