Clark R. Dickerson

A mathematical musculoskeletal shoulder model for proactive ergonomic analysis

Occupational shoulder musculoskeletal injuries and disorders are common. Generally available shoulder work analysis tools do not offer insight into specific muscle load magnitudes that may indicate increased risk, nor do they address many concerns germane to job analysis. To address these issues, a biomechanical model of the shoulder was developed to include several critical components: the systematic inclusion of kinematic and kinetic effects, population scalability, geometric realism, an empirical glenohumeral constraint, and integration with digital ergonomics analysis software tools. This unique combination of features in a single model was explored through examination of both experimental and simulated data with the developed analysis tool. The utility of the model is discussed together with a review of its specific strengths and weaknesses, and the potential for its future use in proactive ergonomic analyses and workplace simulations.

Superior humeral head migration occurs after a protocol designed to fatigue the rotator cuff: A radiographic analysis

HYPOTHESIS Awkward postures and repetitive work have been suggested to lead to shoulder fatigue, which may in turn decrease the subacromial space. The aim of this study was to quantify changes in humeral head position relative to the glenoid after rotator cuff fatigue. We hypothesized that the humeral head would migrate superiorly with fatigue due to the inability of the rotator cuff muscles to balance the upward pull of the deltoid. MATERIALS AND METHODS Four anterior-posterior radiographs (at 0°, 45°, 90°, and 135° of elevation in the scapular plane) of the glenohumeral joint were taken before and after a fatiguing task. The fatiguing task was a simulated job task intended to exhaust the entire rotator cuff. RESULTS The position of the humeral head with respect to the glenoid cavity was significantly affected both by arm elevation angle and fatigue state. In the prefatigued state, increasing arm elevation angle was related to superior translation until 90°, after which the humeral head moved inferiorly to a more central position. In the postfatigued state, the inability of the rotator cuff to centralize the humeral head led to increasing translations with higher elevations. DISCUSSION Superior humeral head migration was associated with the fatigued state. This implies that overhead or repetitive work, or both, may accelerate the development of subacromial impingement through reduction of the subacromial space. CONCLUSIONS Continuous overhead work demonstrably created rotator cuff fatigue, which apparently inhibited the ability of the shoulder musculature to resist upward humeral translation.

The relationship between shoulder torques and the perception of muscular effort in loaded reaches

The objective of this study was to define the quantitative relationship between external dynamic shoulder torques and calibrated perceived muscular effort levels for load delivery tasks, for application in job analyses. Subjects performed a series of loaded reaches and, following each exertion, rated their perceived shoulder muscular effort. Motion and task physical requirements data were processed with a biomechanical upper extremity model to calculate external dynamic shoulder torques. Calculated torque values were then statistically compared to reported calibrated perceived muscular effort scores. Individual subject torque profiles were significantly positively correlated with perceived effort scores (r2 = 0.45–0.77), with good population agreement (r2 = 0.50). The accuracy of the general regression model improved (r2 = 0.72) with inclusion of factors specific to task geometry and individual subjects. This suggests two major conclusions: 1) that the perception of muscular shoulder effort integrates several factors and this interplay should be considered when evaluating tasks for their impact on the shoulder region; 2) the torque/perception relationship may be usefully leveraged in job design and analysis.

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.

The influence of providing feedback on force production and within-participant reproducibility during maximal voluntary exertions for the anterior deltoid, middle deltoid, and infraspinatus.

The normalization of electromyographic signals to a maximum voluntary reference contraction is common practice within the ergonomics research paradigm. However, there is a lack of support for a common protocol for obtaining maximum repeatable exertions. Specifically, there is minimal evidence to support the use of providing force magnitude feedback during the production of voluntary maximum exertions (MVE) in terms of both signal amplitude and repeatability. Therefore, the purpose of this study was to determine (1) if an MVE force magnitude feedback protocol increased both the force exerted and corresponding muscle activity, (2) if force magnitude feedback improved the within-participant reproducibility of the force or activity observed, and (3) if the surface electromyography (sEMG) signal processing method affected the repeatability of determining peak muscle activity. Seventeen participants completed a series of MVEs; first without feedback of the forces they produced, then with feedback of the forces they were producing, and again without feedback to determine if providing force feedback influenced their ability to produce a maximum force. Hand force and sEMG from the anterior deltoid, middle deltoid, and infraspinatus were measured during each exertion. The results showed that the highest forces and muscle activities were achieved when force feedback was provided. Force magnitude feedback resulted in a 7-22% increase in magnitude (for force and activity) and a decrease of 11-46% in the coefficient of variation specifying an improvement in the within-participant reproducibility. Signal processing techniques also affected within-participant reproducibility, however to a much lesser extent. The peak value from a 500-ms moving window average of the linear enveloped or root mean squared sEMG was the most reproducible technique tested.

The specificity of fatiguing protocols affects scapular orientation: Implications for subacromial impingement.

BACKGROUND shoulder impairments are often associated with altered scapular kinematics. As muscles control scapular movement, functionally altering muscle performance through fatigue may produce scapular kinematics that mimic those of injured patients. The aim of this study was to examine if changes in scapular tilt, rotation and pro/retraction following two different upper extremity fatiguing protocols have any implications with respect to subacromial impingement. METHODS scapular orientation was monitored during posturally constrained static holds (at 0°, 45° and 90° of humeral elevation) before and after two fatiguing protocols, one global and one local. Both protocols are associated with producing changes in shoulder skeletal arrangement. FINDINGS following the global fatiguing protocol, there was significantly more scapular posterior tilt (P<0.01) and upward rotation (P<0.02), particularly at 90° humeral elevation. No changes in scapular orientation occurred following the local fatiguing protocol. INTERPRETATION scapular orientation changes following muscle fatigue acted to increase the subacromial space. Thus, the rotator cuff muscles, not the scapular stabilizers, have more influence on actively preventing mechanical subacromial impingement. The lack of evidence of reduction of the subacromial space thus implicates superior humeral head translation as a more likely primary mechanism of the initiation of subacromial impingement.

Experimental evaluation of a computational shoulder musculoskeletal model

BACKGROUND Many evaluations of shoulder biomechanical models have focused on static exertions in constrained postures, but few have considered tasks that are more complex. This study examines model performance in load delivery tasks for a range of target locations. METHODS The study evaluated an optimization-based muscle force prediction model used to assess dynamic load transfer tasks. Model predictions were compared with experimental electromyographic data for two task phases: (1) static hold and (2) dynamic reach. FINDINGS Predictions correlated positively over all subjects with electromyographic data for prime movers (deltoid [r=0.53]; infraspinatus [r=0.63]; biceps [r=0.61]), though variations in the correlation existed across subjects and tasks. Conversely, the model predicted electromyographic activity of secondary muscles somewhat less accurately. The model also predicted inactivity for electromyographic inactive muscles. INTERPRETATION The model provides important insights into activity levels muscles that most actively respond to external moments during manual load transfer tasks.

Predictors of perceived effort in the shoulder during load transfer tasks

The mechanism of muscular effort perception in the shoulder was examined in this experiment. Two shoulder biomechanical models and experimental muscle activity data were used to assess physical exposure for a series of reaching tasks. Effort perception was quantitatively correlated to these measures of physical loading, both at the resultant torque (r2 = 0.50) and muscle activity model-based muscle force predictions (MFPs): r2 = 0.42, electromyography (EMG): r2 = 0.26) levels. Muscle data did not explain variation in effort perception more fully than torque data. The inclusion of subject and task variables improved the ability of each model to explain variability in effort perception (torque: r2 = 0.74; MFP: r2 = 0.67, EMG: r2 = 0.64). These results suggest that effort perception may not be fully explained by only an image of the motor command, but is rather a complex integrative quantity that is affected by other factors, such as posture and task goals, which may be dependent on sensory feedback.

Shoulder Strength of Females While Sitting and Standing as a Function of Hand Location and Force Direction

This study evaluated single-handed isometric push strength capabilities of females working at or above-shoulder level. We examined the influence of force exertion direction (vertical, horizontal and lateral), angle of shoulder flexion from horizontal (0 degrees, 30 degrees, 60 degrees and 90 degrees) and gross body posture (standing and sitting), on maximal volitional shoulder strength. Force exertion direction had the greatest affect on shoulder strength (p<0.0001). Strength was greatest in the vertical axis pushing downwards and weakest in the horizontal plane pushing forwards. Angle influenced shoulder strength when considered together with direction (p<0.0001). However, these effects were dominated by direction results. Marginal differences in strength existed between sitting and standing (p>0.05). These results can be used to design workspaces that consider individual strength limitations and their dependence on force direction, work orientation, and gross body posture.

The impact of work configuration, target angle and hand force direction on upper extremity muscle activity during sub-maximal overhead work

Overhead work has established links to upper extremity discomfort and disorders. As many jobs incorporate working overhead, this study aimed to identify working conditions requiring relatively lower muscular shoulder load. Eleven upper extremity muscles were monitored with electromyography during laboratory simulations of overhead work tasks. Tasks were defined with three criteria: work configuration (fixed, stature-specific); target angle (−15°, 0°, 15°, 30° from vertical); direction of applied hand force (pulling backwards, pushing forwards, downwards, sideways, upwards). Normalised electromyographic activity was greater for fixed configurations, particularly when pulling in a backward direction (total activity = 108.3% maximum voluntary exertion (MVE)) compared to pushing down or forward (total activity ranging from 10.5 to 17.3%MVE). Further, pulling backwards at angles of –15° and 0° showed the highest muscular demand (p < 0.05). These results suggest that, if possible, positioning overhead work in front of the body with exertions directed forwards will result in the lowest upper extremity muscle demand. Statement of Relevance: Overhead work pervades occupational settings and is associated with risk of upper extremity musculoskeletal disorders. The muscular intensity associated with performing overhead work was assessed in several combinations of work placement and hand force direction. These findings should have utility for designing overhead work tasks that reduce muscular exposure.