Rebecca L. Brookham

On the suitability of using surface electrode placements to estimate muscle activity of the rotator cuff as recorded by intramuscular electrodes

BACKGROUND Electromyography (EMG) is commonly used to assess muscle activity. Although previous studies have had moderate success in predicting individual intramuscular muscle activity from surface electrodes, extensive data does not exist for the rotator cuff. This study aimed to determine how reliably surface electrodes represent rotator cuff activity during 20 maximal exertions. METHODS Five channels of EMG were recorded on the following rotator cuff muscles: supraspinatus and infraspinatus intramuscular and surface recordings, and teres minor intramuscular recordings. An additional 3 surface electrodes were placed over the upper and middle trapezius and posterior deltoid. Subjects performed ramped maximal voluntary contractions (MVCs) for each muscle, followed by 20 isometric maximal exertions. Linear least squares best fit regressions (unconstrained and constrained with zero-intercept) were used to compare: intramuscular and surface supraspinatus and infraspinatus signals, respectively, and intramuscular teres minor and surface infraspinatus signals. FINDINGS Relationships existed between wire and surface electrode measurements for all rotator cuff muscles: supraspinatus (r(2)=0.73); teres minor (r(2)=0.61); infraspinatus (r(2)=0.40), however prediction equations indicated large overestimations and offsets. INTERPRETATION When appropriate multiplicative coefficients are considered, surface supraspinatus and infraspinatus electrodes may be used to estimate intramuscular supraspinatus and teres minor activations, respectively, in maximal exertions similar to those tested. However, until these relationships are better defined in other postures, intensities and exertion types, the use of surface electrodes to estimate indwelling rotator cuff activity is cautioned against.

Construct Validity of Muscle Force Tests of the Rotator Cuff Muscles: An Electromyographic Investigation

Background Manual muscle tests (MMTs) are used in clinical settings to evaluate the function and strength (force-generating capacity) of a specific muscle in a position at which the muscle is believed to be most isolated from other synergists and antagonists. Despite frequent use of MMTs, few electromyographic evaluations exist to confirm the ability of MMTs to isolate rotator cuff muscles. Objective This study examined rotator cuff isolation during 29 shoulder muscle force tests (9 clinical and 20 generic tests). Design An experimental design was used in this study. Participants and Measurements Electromyographic data were recorded from 4 rotator cuff muscles and 10 additional shoulder muscles of 12 male participants. Maximal isolation ratios (mean specific rotator cuff muscle activation to mean activation of the other 13 recorded muscles) defined which of these tests most isolated the rotator cuff muscles. Results Three rotator cuff muscles were maximally isolated (obtained highest isolation ratios) within their respective clinical test groups (lateral rotator test group for the infraspinatus and teres minor muscles and abduction test group for the supraspinatus muscle). The subscapularis muscle was maximally isolated equally as effectively within the generic ulnar force and clinical medial rotation groups. Similarly, the supraspinatus and teres minor muscles were isolated equally as effectively in some generic test groups as they were in their respective clinical test groups. Limitations Postural artifact in the wire electrodes caused exclusion of some channels from calculations. The grouping of muscle force tests based on test criteria (clinical or generic tests and muscle action) may have influenced which groups most isolated the muscle of interest. Conclusions The results confirmed the appropriateness of 9 commonly used clinical tests for isolating rotator cuff muscles, but suggested that several other muscle force tests were equally appropriate for isolating these muscles.

HIGH-PASS FILTERING SURFACE EMG IN AN ATTEMPT TO BETTER REPRESENT THE SIGNALS DETECTED AT THE INTRAMUSCULAR LEVEL

Surface electromyography (EMG) is often used to represent activation profiles of the underlying musculature. The purpose of this study was to assess the potential of high‐pass (HP) filtering to improve the matching of surface EMG signals to those signals recorded intramuscularly. EMG was recorded at the skin surface over the infraspinatus and supraspinatus muscles as well as from fine‐wire electrodes placed in the infraspinatus, supraspinatus, and teres minor muscles. The surface EMG signals were HP‐filtered at 18 cutoff frequencies (0–510 HZ in 30 HZ increments), and the time‐histories were correlated with the signals from the wire electrodes. HP filtering did not significantly alter the correlated muscle activation waveform relationship between the surface and wire signals until cutoffs reached 240 HZ. HP filtering of the surface signals did not improve the representation of the muscle fiber‐level activation profile, but the results suggest that enough information resides in the high‐frequency components of the signal to reproduce the activation time–history profile of the muscle. Muscle Nerve, 2010

The utility of an empirically derived co-activation ratio for muscle force prediction through optimization

Biomechanical optimization models that apply efficiency-based objective functions often underestimate or negate antagonist co-activation. Co-activation assists movement control, joint stabilization and limb stiffness and should be carefully incorporated into models. The purposes of this study were to mathematically describe co-activation relationships between elbow flexors and extensors during isometric exertions at varying intensity levels and postures, and secondly, to apply these co-activation relationships as constraints in an optimization muscle force prediction model of the elbow and assess changes in predictions made while including these constraints. Sixteen individuals performed 72 isometric exertions while holding a load in their right hand. Surface EMG was recorded from elbow flexors and extensors. A co-activation index provided a relative measure of flexor contribution to total activation about the elbow. Parsimonious models of co-activation during flexion and extension exertions were developed and added as constraints to a muscle force prediction model to enforce co-activation. Three different PCSA data sets were used. Elbow co-activation was sensitive to changes in posture and load. During flexion exertions the elbow flexors were activated about 75% MVC (this amount varied according to elbow angle, shoulder flexion and abduction angles, and load). During extension exertions the elbow flexors were activated about 11% MVC (this amount varied according to elbow angle, shoulder flexion angle and load). The larger PCSA values appeared to be more representative of the subject pool. Inclusion of these co-activation constraints improved the model predictions, bringing them closer to the empirically measured activation levels.

Comparing surface and indwelling electromyographic signals of the supraspinatus and infraspinatus muscles during submaximal axial humeral rotation

This study quantified the relationship between EMG signals recorded by surface and indwelling electrodes for the infraspinatus and supraspinatus during submaximal axial humeral rotation. Muscular activity was measured on 20 participants during 82 submaximal isometric internal or external axial humeral rotations in a range of postures and intensities. Equations to predict indwelling magnitudes from surface data were generated and the effects of humeral angle and intensity on this relationship were also evaluated. Supraspinatus surface data explained 72-76% of the variance in the indwelling data. Surface data overestimated indwelling data by up to 30% of maximal voluntary contraction (MVC). Infraspinatus surface data explained 62-64% of the variance in the indwelling data, but overestimated by 72% and 400% MVC in external and internal axial humeral rotation trials, respectively. Humeral abduction angle and exertion intensity both altered the relationship between electrode types modestly (p < 0.01) for most muscles and exertions. Better variance explanation was achieved for these submaximal exertions than previously reported values for maximal exertions. These results help inform electrode type selection for the recording of supraspinatus and infraspinatus EMG. Caution is recommended when interpreting surface recordings as indicators of indwelling recordings for exertions where the muscle studied is not a primary mover.

The working shoulder: assessing demands, identifying risks, and promoting healthy occupational performance

Abstract Background: Occupational shoulder injuries are a concern for society and clinicians. To best treat and assess the influence of physical work tasks on shoulder health, it is necessary to first understand shoulder function and dysfunction, common occupational pathologies, and existing methods to assess and prevent work-related shoulder injuries. Objectives: The objectives of this paper are to present fundamental shoulder biomechanics, discuss known occupational risk factors for shoulder injury, review assessment and prevention techniques for shoulder injuries, and describe the range of tools available to evaluate work tasks for potential physical shoulder stressors. Major findings: The complex shoulder mechanism has a unique role for performing work. Recognition of the delicate balance between creating arm mobility and maintaining joint stability is crucial to understanding how the region can be injured during work. Tools exist to aid clinicians in thinking critically about occupational shoulder injury, and can provide information about both specific and general physical exposures. Conclusions: The shoulder is a complicated assembly of many neuromusculoskeletal elements, and these tissues can be injured in a variety of ways. This paper provides background on the nature of these injuries and discusses preventative strategies available to pragmatically assist clinicians.

Comparison of humeral rotation co-activation of breast cancer population and healthy shoulders.

Upper limb morbidities are common amongst the breast cancer population (BCP) and have a direct impact on independence. Comparing muscle co-activation strategies between BCP and healthy populations may assist in identifying muscle dysfunction and promote clinical interpretation of dysfunction, which could direct preventative and therapeutic interventions. The purposes of this study were to define humeral rotation muscle co-activation of a BCP and to compare it with a previously defined co-activation relationship of a healthy population. Fifty BCP survivors performed 18 isometric internal and external rotation exertions at various postures and intensities. Surface and intramuscular electrodes recorded shoulder muscle activity. BCP co-activation was predicted at r(2)=0.77 during both exertion types. Humeral abduction angle and task intensity were important factors in the prediction of co-activation in both populations. Comparisons made between populations identified differing muscle strategies used by BCP to maintain postural control. Compared to healthy co-activation, the BCP demonstrated greater activation of internal (IR) and external rotator (ER) type muscles during their respective rotation type. The BCP demonstrated increased (⩾8.7%) activation of pectoralis major. This study has provided insight into how BCP muscles compensate during dysfunction. Continued advancement of this knowledge can provide more understanding of dysfunction, promote generation of evidence-based therapies, and can be useful in biomechanical modeling.

Extrapolation of an empirical elbow muscle co-activation relationship to a novel task set: implications for predictions of individual muscle demands

Biomechanical optimisation models applying efficiency-based objective functions often underestimate antagonist contributions. Previous work has quantified an empirical co-activation relationship in the elbow musculature, demonstrating that implementing this relationship as a constraint in an elbow muscle force prediction model improves muscle force predictions. The current study evaluated this modified model by extrapolating the co-activation relationship to 36 novel isometric unilateral, right-handed exertions, including those requiring greater intensity of effort and performed in different postures. Surface electromyography was recorded from the elbow flexors and extensors. Novel extrapolative co-activation relationships were developed and used as constraints in a muscle force prediction model. Model predictions using both constraints were compared with empirical biophysical data. Predictions by the modified model were more consistent with biophysical data than those by the original model for the novel exertions. Novel co-activation relationships did not further enhance predictions when compared with the previous relationship, suggesting that extrapolation of the previous relationship is feasible.

Quantification of upper limb electromyographic measures and dysfunction of breast cancer survivors during performance of functional dynamic tasks

Background: Upper limb morbidities within the breast cancer population can interfere with completing daily life activities. Current knowledge of upper limb capabilities is limited; previous increases in muscle activation on the affected cancer side suggest this population works at a higher fraction of their capability. The purposes of this study were to describe upper limb capabilities and dysfunction of breast cancer survivors through muscle activation monitoring via surface electromyography and muscle‐specific strength tests during functional tasks. Methods: Fifty survivors performed 88 dynamic tasks (divided into range of motion‐reach or rotate, activities of daily life and work tasks). Muscle activation was examined for functional and strength testing tasks. Findings: Total muscle effort (summation of integrated electromyography across measured muscles) was up to 5.1% greater on the affected side during work tasks (p = 0.0258). Increased activations existed in posterior deltoid, supraspinatus, upper trapezius and serratus anterior (p < 0.05) for several tasks, including daily living tasks. Reduced activation occurred in affected pectoralis major sternal during all tasks (p < 0.0001–0.0032), and affected infraspinatus in all but daily living tasks (p = 0.0002–0.0328). The affected side infraspinatus, supraspinatus and upper trapezius muscles demonstrated significant reductions in targeted strength testing (p = 0.0001–0.0057). Interpretation: Both primary and secondary muscles (outside surgery and radiation fields) were affected. In general, this population works at higher levels of muscle effort for the affected side yet demonstrates weakness in strength testing, which may reflect tissue damage. Strengthening exercises for the posterior rotator cuff and upper trapezius may be the most beneficial.

Examining upper limb kinematics and dysfunction of breast cancer survivors in functional dynamic tasks

Background: Comorbidities within the breast cancer population can reduce quality of life. Current breast cancer survivor upper limb kinematic strategies unfortunately lack robust connection with performing important activities of daily living. Methods: Accordingly, fifty breast cancer survivors performed 88 dynamic tasks (divided into range of motion‐reach, range of motion‐rotate, activity of daily living, and work tasks). Humerothoracic and scapulothoracic angles were extracted from motion capture data. Bilateral differences existed for range of motion, and maximal and minimal scapulothoracic and humerothoracic angles. Findings: Generally, the affected side used less range of motion across task types. Humerothoracic angles on the affected side experienced 6.7° less range of motion in plane of elevation in range of motion‐reach (p < 0.01), 2.3° less elevation angle range of motion in range of motion‐rotate (p = 0.01), and 7.1° more internal rotation range of motion in range of motion‐rotate (p < 0.01). Scapulothoracic angles on the affected side had 2° more anterior/posterior tilt range of motion in work tasks (p = 0.03), 3.4° less maximal protraction in activity of daily living tasks (p = 0.01), and 3.5° less minimum downward rotation in range of motion‐rotate (p < 0.01). Interpretation: A reduced range of motion on the affected side suggests the breast cancer population had less varied movement strategies, keeping movements in narrower ranges to avoid disability, pain, or subacromial impingement. This investigation produced an unprecedentedly diverse collection of three‐dimensional humerothoracic and scapulothoracic kinematics for a breast cancer population. Documentation of physical capability, dysfunction, and adaptive strategies is a crucial step towards developing targeted strategies for enhancing functional recovery in breast cancer survivors. HighlightsUpper extremity motion in 88 tasks in breast cancer survivors is examined.Bilateral differences existed, with the affected side using less range of motion.Differences in capability between sides were not universal across task types.These suggest the breast cancer population has less varied movement strategies.These findings are crucial to develop targeted strategies for enhancing recovery.