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The complexity of shoulder mechanics combined with the movement of skin relative to the scapula makes it difficult to measure shoulder kinematics with sufficient accuracy to distinguish between symptomatic and asymptomatic individuals. Multibody skeletal models can improve motion capture accuracy by reducing the space of possible joint movements, and models are used widely to improve measurement of lower limb kinematics. In this study, we developed a rigid-body model of a scapulothoracic joint to describe the kinematics of the scapula relative to the thorax. This model describes scapular kinematics with four degrees of freedom: 1) elevation and 2) abduction of the scapula on an ellipsoidal thoracic surface, 3) upward rotation of the scapula normal to the thoracic surface, and 4) internal rotation of the scapula to lift the medial border of the scapula off the surface of the thorax. The surface dimensions and joint axes can be customized to match an individual's anthropometry. We compared the model to \"gold standard\" bone-pin kinematics collected during three shoulder tasks and found modeled scapular kinematics to be accurate to within 2 mm root-mean-squared error for individual bone-pin markers across all markers and movement tasks. As an additional test, we added random and systematic noise to the bone-pin marker data and found that the model reduced kinematic variability due to noise by 65% compared to Euler angles computed without the model. Our scapulothoracic joint model can be used for inverse and forward dynamics analyses and to compute joint reaction loads. The computational performance of the scapulothoracic joint model is well suited for real-time applications; it is freely available for use with OpenSim 3.2, and is customizable and usable with other OpenSim models.

While several biomechanical investigations have measured acromion and scapular spine strains for various pathological conditions to better understand the risk factors for fracture, no study has measured strains in the native shoulder. The objective of this study was to use an ex-vivo shoulder motion simulator to measure principal strain during continuous, unconstrained, muscle-driven motion of the native shoulder. Eight cadaveric specimens (57 ± 6 years) were used to simulate scapular plane abduction (27.5 to 80° of humerothoracic elevation), forward flexion (27.5 to 72.5° of humerothoracic elevation), external rotation (0 to 40° of external rotation), and circumduction (elliptical path) with glenohumeral rotation speeds of 10°/s. Principal strain was measured throughout motion in four clinically relevant regions of the scapular spine and acromion according to the Levy classification using tri-axial strain gauge rosettes. Increases in humeral elevation during scapular plane abduction and forward flexion were associated with increases in deltoid force and scapula strain. However, above approximately 60° of humerothoracic elevation, strains plateaued while deltoid forces continued to increase indicating that scapula strain patterns are influenced by deltoid force magnitude and direction. Scapula strain was higher during scapular plane abduction than forward flexion in all regions but was only significantly higher in Levy 3B (p = 0.038). The highest strains were observed in Levy regions 2 and 3A (p ≤ 0.01) which correspond to regions with the highest clinically observed fracture rates demonstrating that the shape of the acromion and scapular spine may influence strain distribution irrespective of the joint condition.

Myofascial trigger points (MTrPs) and body postural misalignment, including the position of the scapula, can contribute to the onset and persistence of musculoskeletal pain. However, the relationship in asymptomatic cases remains unclear. Therefore, this study aimed to investigate the relationship between the asymmetry of the resting scapular position and latent MTrPs in the upper trapezius muscle (UTM) in asymptomatic adults. A total of 32 asymptomatic adult men (mean age, 26.28 ± 1.1 years) were included in this study. Full-body photographs were taken from the posterior view, with the participants resting in a standing position. To determine the degree of asymmetry of the resting scapular position, the horizontal scapular alignment angle (HSAA) was analyzed from the photographs. The assessor identified the presence of latent MTrPs in the right and left UTMs. The HSAA was significantly lower in the group with latent MTrPs in the right UTM than in those without latent MTrPs. The results showed that the right scapula was more depressed than the left scapula in the group with latent MTrPs in the right UTM. Furthermore, multiple regression analysis indicated that the dominant arm and presence of latent MTrPs in the right UTM significantly contributed to the prediction of the HSAA. The results of this study demonstrated a close relationship between the asymmetry of the resting scapular position and latent MTrPs in the UTM in asymptomatic adults, which may contribute to the onset and persistence of musculoskeletal pain.

Our current understanding of healthy scapula motion is mainly based on studying the shoulder when it is generating an abduction torque against gravity. However, the shoulder can perform diverse tasks beyond abduction. In particular, little attention has been given to how scapula motion contributes to concentric adduction despite its involvement in high-demand tasks such as rock climbing and wheelchair transfers. Investigating scapular kinematics during concentrically loaded arm-lowering can provide insight into the mechanical demands underlying healthy scapula motion. In this study, we combined biplanar videoradiography and optical motion capture with a controllable cable machine to compare the three-dimensional humerothoracic, glenohumeral, and scapulothoracic kinematics between a weighted pull-down task (involving concentric shoulder adduction) and a weighted press-up task (involving concentric shoulder abduction) in ten healthy adults. We observed significantly more scapulothoracic upward rotation and less glenohumeral abduction during concentric adduction than concentric abduction. Our findings indicate that scapula upward rotation is not simply a function of overall humerothoracic elevation, but instead varies in a load-specific manner – potentially to orient the glenoid in a way that facilitates glenohumeral joint stability. We also observed substantial inter-individual variability in scapular kinematics within a task, and in how individuals responded to the different tasks. Our findings help provide a more well-rounded understanding of healthy scapular kinematics such that we can better identify and treat unhealthy motion (i.e., dyskinesis). Our findings can also inform musculoskeletal models that simulate scapulothoracic kinematics.

Many daily activities involve hand-behind-back (HBB) movements, which are often difficult for patients with chronic shoulder pathologies. HBB movements involve upper-limb and scapular motions. However, thoracic spine motions during the HBB movements and the effects of thoracic spine alignment on the HBB movement kinematics remain uninvestigated. This study aimed to examine the upper-limb, scapular, and thoracic spine motions during HBB movements by using a three-dimensional motion analysis system. Twenty healthy young adults were included (10 women and 10 men, mean age = 22.8 years). HBB movement measurements started while the participants stood with their palms facing forward. They then placed their hands behind their buttocks, with their palms facing backward. Almost all participants exhibited elbow flexion, shoulder extension, abduction, internal rotation, and scapular anterior tilt during the HBB movement. The scapular upward/downward rotation differed between the participants. Scapular, shoulder, and elbow motions showed significant correlations, suggesting that the HBB movements from the arms positioned on the side to the hands placed behind the buttocks involve complex coordinated motions of the shoulder, elbow, and scapula. Several participants exhibited little thoracic motion, whereas the others flexed their thoracic spine. Individuals with a smaller thoracic flexion motion tended to have a larger initial thoracic flexion angle before the HBB movement. The initial thoracic flexion angle and thoracic flexion motion did not correlate with the scapular, shoulder, or elbow motion; thus, healthy young adults with little thoracic flexion did not use specific compensation strategies during the HBB movements until the hands reached the buttocks.

Musculoskeletal models of the shoulder are needed to understand the mechanics of overhead motions. Existing models implementing the shoulder rhythm are generic and might not accurately represent an individual’s scapular kinematics. We introduce a method to personalize the shoulder rhythm of a computational model of the upper body that defines the orientations of the clavicle and scapula based on glenohumeral joint angles. During five static calibration poses, we palpate and measure the orientation of the scapula. We explore the importance of representing shoulder elevation by introducing clavicle elevation as a degree of freedom that is independent of the glenohumeral angles. For ten subjects, we record the five calibration poses, ten additional static poses, and dynamic arm raises covering the participants’ full range of motion in each body plane using optical motion capture. We examine the data using a dynamically-constrained inverse kinematics analysis. Shoulder rhythm personalization, independent clavicle elevation, and both in combination reduce the average upper body marker tracking error compared to the generic model in the static poses (26 mm to 17–20 mm) and in the dynamic trials (22 mm to 14–17 mm). Only personalization reduces the average scapula marker error (51 mm to 36–38 mm) and scapula axis-angle error (15° to 10°) compared with the palpated ground truth measurements in the static poses, and in the dynamic trials at instances that best match the static poses (53 mm to 37–40 mm, 15° to 9°). Our results show that personalizing upper body models improves kinematic tracking. We provide our experimental data, model, and methods to allow researchers to reproduce and build upon our results.

Best practices for scapular motion tracking are still being determined. The repeatability of different scapular kinematic procedures needs to be evaluated. The purpose of this study was to assess the test-retest reliability of two scapular kinematic procedures: double calibration with AMC (D-AMC) and individualized linear modelling (LM). Ten healthy participants had their upper body movement tracked with optical motion capture in two identical sessions. Five scapular calibration poses were performed, and seven dynamic functional tasks were tested. Scapular angles were calculated from both procedures (D-AMC vs LM). The D-AMC approach uses two poses (neutral and maximum elevation) and tracks the scapula with a rigid cluster, while the LM approach predicts scapular positioning from humeral angles based on equations built from the calibration pose data. Angle waveforms and repeatability outcomes were compared. Internal and upward rotation angle waveforms were significantly different (p < 0.05) between kinematic procedures for some tasks, with maximum mean differences up to 17.3° and 23.2°, respectively. Overall, repeatability outcomes were similar between procedures, but the LM approach was slightly better for tilt and the D-AMC approach was notably improved for upward rotation in certain tasks. For example, minimal detectable changes during the Forward Transfer ranged from 6.9° to 11.9° for the D-AMC and 8.9° to 25.3° for the LM. Discrepancies between procedures may be a function of the calibration poses chosen. Additional calibration poses may improve the comparisons between procedures.

To determine whether rotator cuff strength, glenohumeral joint range of motion and scapular control are associated with shoulder injuries among elite male handball players. A total of 206 players in the Norwegian elite handball league for men were tested prior to the 2011-2012 season. Measures included: (1) glenohumeral internal and external rotation range of motion, (2) isometric internal rotation, external rotation and abduction strength and (3) assessment of scapular dyskinesis. Players were followed prospectively for the entire regular season (30 weeks), with shoulder problems registered bi-weekly using the Oslo Sports Trauma Research Center Overuse Injury Questionnaire. A cumulative severity score was calculated for each player based on their questionnaire responses. This was used as the outcome measure in risk factor analyses. The average prevalence of shoulder problems throughout the season was 28% (95% CI 25% to 31%). The prevalence of substantial shoulder problems, defined as those leading to moderate or severe reductions in handball participation or performance, or to time loss, was 12% (95% CI 11% to 13%). Significant associations were found between obvious scapular dyskinesis (OR 8.41, 95% CI 1.47 to 48.1, p<0.05), total rotational motion (OR 0.77 per 5° change, 95% CI 0.56 to 0.995, p<0.05) and external rotation strength (OR 0.71 per 10 Nm change, 95% CI 0.44 to 0.99, p<0.05) and shoulder injury. Injury prevention programmes should incorporate interventions aimed at improving glenohumeral rotational range of motion, external rotation strength and scapular control.

Scapular morphology is predictive of locomotor adaptations among primates, but this skeletal element is scarce in the hominin fossil record. Notably, both scapulae of the juvenile Australopithecus afarensis skeleton from Dikika, Ethiopia, have been recovered. These scapulae display several traits characteristic of suspensory apes, as do the few known fragmentary adult australopith representatives. Many of these traits change significantly throughout modern human ontogeny, but remain stable in apes. Thus, the similarity of juvenile and adult fossil morphologies implies that A. afarensis development was apelike. Additionally, changes in other scapular traits throughout African ape development are associated with shifts in locomotor behavior. This affirms the functional relevance of those characteristics, and their presence in australopith fossils supports the hypothesis that their locomotor repertoire included a substantial amount of climbing.

Upper crossed syndrome (UCS) refers to the altered muscle activations and movement patterns in scapulae along with some abnormal alignment in the upper quarter, which may contribute to the dysfunction of the cervicothoracic and glenohumeral joints. The present study aimed to investigate the effectiveness of a comprehensive corrective exercise program (CCEP) and subsequent detraining on alignment, muscle activation, and movement pattern in men with the UCS. This randomized controlled trial included 24 men. The intervention group conducted CCEP (8 weeks), followed by four weeks of detraining and the control group maintained normal daily activities. Electromyography of selected muscles, scapular dyskinesis test, head, shoulder, and thoracic spine angle were measured at baseline, post-test, and follow-up. There were significant differences for Group x time interaction and also for within-group from pre-test to post-test and follow-up in all outcomes. Also, significant differences were observed in three outcomes at post-test and follow-up between the CCEP and control group in favor of the CCEP. In Conclusion, the present study demonstrates that the CCEP for individuals with UCS is feasible and effective, improving muscle activation imbalance, movement patterns, and alignment. Importantly, these improvements were maintained after four weeks of detraining, suggesting lasting neuromuscular re-training adaptations.