Jaclyn N. Chopp

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 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.

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.

On the feasibility of obtaining multiple muscular maximal voluntary excitation levels from test exertions: A shoulder example

Currently, contrasting views exist regarding which body and arm postures are most effective for eliciting maximal voluntary exertions in the shoulder muscles. Informed exertion standardization may improve comparisons between subjects and muscle groups for normalized electromyography values. Additionally, identifying exertions that can produce equivalent maximal electrical activity values can reduce experimental setup time and reduce the likelihood of fatigue development. This research study examined twelve posture and force direction defined test exertions to identify those that elicited maximal electrical activity from the deltoid (anterior and middle fibres) and pectoralis major (clavicular and sternal heads). Further, the question of whether a single test exertion could obtain maximal electrical activity from multiple muscle fascicles was explored. Maximal activation was demonstrated for the deltoid during several exertions that incorporated an upward force exertion and the pectoralis major for multiple exertions that included an inward force direction. Finally, two test exertions produced maximal electrical activity from both muscles of interest. This research supports the notion that a range of exertions can elicit maximal electrical activity from a muscle, rather than one specific exertion. This suggests that researchers may be able to leverage a smaller set of test exertions to evaluate multiple muscles simultaneously without loss of data quality, and thereby decrease overall experimental data collection time while maintaining high fidelity data.

Resolving the contributions of fatigue-induced migration and scapular reorientation on the subacromial space: An orthopaedic geometric simulation analysis

Subacromial impingement occurs when tissues interposed between the humeral head and the acromion become compressed between the bones. Two fatigue-related mechanisms are suggested to contribute to this impingement: superior humeral head migration and scapular reorientation, specifically downward rotation, anterior tilt and protraction. Although both mechanisms have received attention, controversy exists regarding their relative contributions to reducing subacromial space width. This simulation study presents a three-dimensional orthopedic model for assessing the spatial relationship between the humerus and the scapula. Through incorporating empirically generated fatigue data as inputs, changes in the subacromial space associated with each mechanism acting independently and in combination were quantified. These changes were examined for three scenarios (average, impingement sparing, impingement causing) and at three humeral elevation angles (0°, 45° and 90°) in a total of 36 unique simulations. Humeral head migration generally decreased the subacromial space and scapular reorientation generally increased the space across scenarios. This strongly implies that superior humeral migration is the dominant fatigue-related mechanism associated with increasing the likelihood of impingement. The results suggest that future research should target efforts to reduce migration in industry, primarily through avoidance or modification of situations known to induce rotator cuff fatigue, such as overhead work.

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.

Three-dimensional comparison of static and dynamic scapular motion tracking techniques

The shoulder is complex and comprised of many moving parts. Accurately measuring shoulder rhythm is difficult. To classify shoulder rhythm and identify pathological movement, static measures have been the preferred method. However, dynamic measures are also used and can be less burdensome to obtain. The purpose of this paper was to determine how closely dynamic measures represent static measures using the same acromion marker cluster scapular tracking technique. Five shoulder angles were assessed for 24 participants using dynamic and static tracking techniques during humeral elevation in three planes (frontal, scapular, sagittal). ANOVAs were used to identify where significant differences existed for the factors of plane, elevation angle, and tracking technique (static, dynamic raising, dynamic lowering). All factors were significantly different for all shoulder angles (p<0.001), except for elevation plane in scapulothoracic protraction/retraction (p=0.955). Tracking techniques were influential (p<0.001), but the grouped mean differences fell below a clinically relevant 5° benchmark. There was large variation in mean differences of the techniques across individuals. While population averages are similar, individual static and dynamic shoulder assessments may be different. Caution should be taken when dynamic shoulder assessments are performed on individuals, as they may not reflect those obtained in static scapular motion tracking.

Scapular orientation following repetitive prone rowing: Implications for potential subacromial impingement mechanisms

While fatigue of the rotator cuff demonstrably causes superior humeral head migration and concomitant risk of impingement, the relationship between specific muscular fatigue, scapular dyskinesis and impingement risk is less clear. The purpose of this study was to examine changes in scapular orientation following a simulated prone rowing fatiguing protocol that targeted the scapula stabilizing muscles while attempting to alleviate rotator cuff muscular demands. Scapular orientation and muscle activity were collected from participants before and immediately after the fatiguing task. This task fatigued both the stabilizing (upper and middle trapezius, and latissimus dorsi) and rotator cuff (supraspinatus, and infraspinatus) muscles. The upper extremity muscle fatigue pattern caused by the protocol did not elicit any significantly changes in three-dimensional scapular position with all post-fatigue changes being ≤ 1° (p = 0.17-0.58). These results indicated that scapular reorientation is likely not the dominant mechanism of fatigue-induced subacromial impingement development. However, the substantial variability present in the kinematics prevents complete exclusion of scapular dyskinesis as a secondary causal mechanism of impingement.

Investigating shoulder muscle loading and exerted forces during wall painting tasks: Influence of gender, work height and paint tool design

The task of wall painting produces considerable risk to the workers, both male and female, primarily in the development of upper extremity musculoskeletal disorders. Insufficient information is currently available regarding the potential benefits of using different paint roller designs or the possible adverse effects of painting at different work heights. The aim of this study was to investigate the influence of gender, work height, and paint tool design on shoulder muscle activity and exerted forces during wall painting. Ten young adults, five male and five female, were recruited to perform simulated wall painting at three different work heights with three different paint roller designs while upper extremity muscle activity and horizontal push force were recorded. Results demonstrated that for female participants, significantly greater total average (p = 0.007) and integrated (p = 0.047) muscle activity was present while using the conventional and curly flex paint roller designs compared to the proposed design in which the load was distributed between both hands. Additionally, for both genders, the high working height imposed greater muscular demands compared to middle and low heights. These findings suggest that, if possible, avoid painting at extreme heights (low or high) and that for female painters, consider a roller that requires the use of two hands; this will reduce fatigue onset and subsequently mitigate potential musculoskeletal shoulder injury risks.

Evaluating risks for the upper extremity during the installation of electrical meters

Installation of electrical meters is an occupational task that may place utility employees at risk of upper extremity discomfort or disorders. This study focused on identifying the most preferable installation technique from several alternatives using a combination of professional utility employees and experimental subjects. Factors considered included variation in installation height, the use of a hand-held mallet, lubrication applied to the meter and the use of one vs. two hands for installation. Installing meters located above shoulder height resulted in a two-fold increase in both peak and cumulative hand acceleration and pressure. Further, the use of a force-absorbing striking mallet showed significantly lowered peak pressure (60%) compared to other techniques. Peak acceleration and cumulative pressure were significantly lower than other techniques for both experienced and inexperienced subjects when using the mallet. Additionally, the mallet installations had amongst the lowest ratings of perceived exertion across installation types. Thus, the primary recommendations for meter installation with respect to the analyzed scenarios are to use a force-absorbing striking mallet and avoid installations at high locations.