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Ginn K.1, Cathers I.2, Boettcher C.3, Halaki M.4
1University of Sydney, Sydney Medical School, Sydney, Australia, 2Australian Catholic University, Sydney, Australia, 3Regent Street Physiotherapy, Newcastle, Australia, 4University of Sydney, Faculty of Health Sciences, Lidcombe, Australia
Background: Graduated exercise to restore normal muscle recruitment patterns and improve muscle strength is the cornerstone of conservative management of shoulder pain. However, optimal exercise therapy is unknown because there is limited information regarding specific changes in shoulder muscle function associated with shoulder pain. Assessing changes in shoulder muscle activity levels in the presence of pain using electromyography (EMG) is problematic due to the effect of pain on a patient's ability to produce maximum contractions in order to normalise EMG data to enable valid comparisons with asymptomatic subjects. Timing of muscle activity with respect to movement can provide information about the consistency of muscle recruitment patterns without requiring EMG data to be normalised. This would provide valid information regarding motor patterning in patients with shoulder pain.
Purpose: The aim of this study was to compare the timing of glenohumeral and axioscapular muscle recruitment in patients with shoulder pain and asymptomatic control subjects during cyclical shoulder movement.
Methods: Twenty-eight subjects were recruited - 14 with shoulder pain and 14 control subjects. Electromyography from nine shoulder muscles (pectoralis major, latissimus dorsi, middle deltoid, supraspinatus, infraspinatus, subscapularis, upper and lower trapezius and serratus anterior) was recorded using a combination of indwelling and surface electrodes. A draw-wire sensor was used to measure shoulder movement. A small range (30°-45°) rapid (as fast as possible) shoulder flexion/extension task was performed in standing with the elbow extended and the arm by the side. Data was collected continuously during approximately 20 repetitions of the flexion/extension task. A cross-correlation and spectrographic analysis provided a measure of the phase (relative timing of muscle activity with respect to movement) at the predominant frequency of movement for each muscle for each subject. Student t-tests were used to compare mean phase values between subjects with and without shoulder pain.
Results: All symptomatic subjects had chronic shoulder pain ranging from 2 to 20 years duration. All subjects with shoulder pain demonstrated a positive Hawkins-Kennedy impingement sign and five also demonstrated positive apprehension signs. Eight subjects with painful shoulders experienced pain during the flexion/extension task. There was a greater variability in the relative timing of muscle activity with respect to movement in subjects with shoulder pain compared to control subjects. There were significant differences between subjects with shoulder pain and control subjects in the relative timing in all muscles (p 0.05) except subscapularis (p=0.64) and latissimus dorsi (p=0.11) regardless of whether subjects had instability signs and symptoms associated with their shoulder pain or whether they experienced pain on the rapid task.
Conclusion(s): People suffering from chronic shoulder pain recruit axioscapular and glenohumeral (including rotator cuff) muscles in patterns which differ from asymptomatic subjects. As synchrony of humeral and scapula movement is crucial to painfree, full range shoulder joint function these altered movement patterns are likely to be hindering the resolution of symptoms in chronic shoulder pain conditions.
Implications: Exercise strategies focusing on the restoration of normal motor patterning and not simply muscle strengthening should form part of the rehabilitation of chronic shoulder pain conditions.
Funding acknowledgements: This study did not receive external funding.
Topic: Musculoskeletal: upper limb
Ethics approval: This study was approved by the Human Research Ethics Committee at the University of Sydney Project Numbers 02-2005/1/7910 & 07-2007/10138
All authors, affiliations and abstracts have been published as submitted.
