Muscle function in glenohumeral joint stability during lifting task

Simulation Study of the Upper-Limb Isometric Wrench Feasible Set With Glenohumeral Joint Constraints

The aim of this work is to improve musculoskeletal-based models of the upper-limb wrench feasible set (WFS), i.e., the set of achievable maximal wrenches at the hand for applications in collaborative robotics and computer aided ergonomics. In particular, a recent method performing wrench capacity evaluation called the iterative convex hull method (ICHM) is upgraded in order to integrate nondislocation and compression limitation constraints at the glenohumeral (GH) joint not taken into account in the available models. Their effects on the amplitude of the force capacities at the hand, glenohumeral joint reaction forces and upper-limb muscles coordination in comparison to the original iterative convex hull method are investigated in silico. The results highlight the glenohumeral potential dislocation for the majority of elements of the wrench feasible set with the original iterative convex hull method and the fact that the modifications satisfy correctly stability constraints at the glenohumeral joint. Also, the induced muscles coordination pattern favors the action of stabilizing muscles, in particular the rotator-cuff muscles, and lowers that of known potential destabilizing ones according to the literature.

Does enforcing glenohumeral joint stability matter? A new rapid muscle redundancy solver highlights the importance of non-superficial shoulder muscles

The complexity of the human shoulder girdle enables the large mobility of the upper extremity, but also introduces instability of the glenohumeral (GH) joint. Shoulder movements are generated by coordinating large superficial and deeper stabilizing muscles spanning numerous degrees-of-freedom. How shoulder muscles are coordinated to stabilize the movement of the GH joint remains widely unknown. Musculoskeletal simulations are powerful tools to gain insights into the actions of individual muscles and particularly of those that are difficult to measure. In this study, we analyze how enforcement of GH joint stability in a musculoskeletal model affects the estimates of individual muscle activity during shoulder movements. To estimate both muscle activity and GH stability from recorded shoulder movements, we developed a Rapid Muscle Redundancy (RMR) solver to include constraints on joint reaction forces (JRFs) from a musculoskeletal model. The RMR solver yields muscle activations and joint forces by minimizing the weighted sum of squared-activations, while matching experimental motion. We implemented three new features: first, computed muscle forces include active and passive fiber contributions; second, muscle activation rates are enforced to be physiological, and third, JRFs are efficiently formulated as linear functions of activations. Muscle activity from the RMR solver without GH stability was not different from the computed muscle control (CMC) algorithm and electromyography of superficial muscles. The efficiency of the solver enabled us to test over 3600 trials sampled within the uncertainty of the experimental movements to test the differences in muscle activity with and without GH joint stability enforced. We found that enforcing GH stability significantly increases the estimated activity of the rotator cuff muscles but not of most superficial muscles. Therefore, a comparison of shoulder model muscle activity to EMG measurements of superficial muscles alone is insufficient to validate the activity of rotator cuff muscles estimated from musculoskeletal models.

Developing a predictive equation of muscular age to evaluate muscle health in Chinese community-dwelling older women

Muscle health plays an important role in most common age-related diseases and injuries. To date, a standardized quantitative method to assess muscle health has not yet been developed. Principal component analysis was employed to develop a predictive equation of muscular age based on muscle health variables including skeletal muscle mass of lower limb, grip strength and maximal gait speed. Then validity of muscular age was tested by comparing chronological age to muscular age of the elderly. A predictive equation of muscular age was developed. Muscular age = 0.690 × chronological age - 1.245 × skeletal muscle mass of lower limb + 0.453 × grip strength - 1.291 × maximal walking speed + 40.547. Cross-sectional validity test showed that the predictive equation of muscular age is a valid method to assess muscle health. It is applicable not only to the normal elderly but to the elderly with pre-sarcopenia or sarcopenia.

Neuromuscular control: from a biomechanist's perspective

Understanding neural control of movement necessitates a collaborative approach between many disciplines, including biomechanics, neuroscience, and motor control. Biomechanics grounds us to the laws of physics that our musculoskeletal system must obey. Neuroscience reveals the inner workings of our nervous system that functions to control our body. Motor control investigates the coordinated motor behaviours we display when interacting with our environment. The combined efforts across the many disciplines aimed at understanding human movement has resulted in a rich and rapidly growing body of literature overflowing with theories, models, and experimental paradigms. As a result, gathering knowledge and drawing connections between the overlapping but seemingly disparate fields can be an overwhelming endeavour. This review paper evolved as a need for us to learn of the diverse perspectives underlying current understanding of neuromuscular control. The purpose of our review paper is to integrate ideas from biomechanics, neuroscience, and motor control to better understand how we voluntarily control our muscles. As biomechanists, we approach this paper starting from a biomechanical modelling framework. We first define the theoretical solutions (i.e., muscle activity patterns) that an individual could feasibly use to complete a motor task. The theoretical solutions will be compared to experimental findings and reveal that individuals display structured muscle activity patterns that do not span the entire theoretical solution space. Prevalent neuromuscular control theories will be discussed in length, highlighting optimality, probabilistic principles, and neuromechanical constraints, that may guide individuals to families of muscle activity solutions within what is theoretically possible. Our intention is for this paper to serve as a primer for the neuromuscular control scientific community by introducing and integrating many of the ideas common across disciplines today, as well as inspire future work to improve the representation of neural control in biomechanical models.

Humanoid robots to mechanically stress human cells grown in soft bioreactors

For more than 20 years, robotic bioreactor systems have facilitated the growth of tissue-engineered constructs using mechanical stimulation. However, we are still unable to produce functional grafts that can translate into clinical use. Humanoid robots offer the prospect of providing physiologically-relevant mechanical stimulation to grafts and implants which may expedite their clinical deployment. To investigate the feasibility of a humanoid bioreactor, we have designed a flexible bioreactor chamber that can be attached to a modified musculoskeletal (MSK) humanoid robot shoulder joint. We demonstrate that fibroblast cells can be grown in this chamber while undergoing physiological adduction-abduction on the robotic arm. A preliminary evaluation of the transcriptome of the cells after 14 days indicated a clear influence of the loading regime on the gene expression profile. These early results will facilitate the exploration of MSK humanoid robots as a biomechanically more realistic platform for tissue engineering and biomaterial testing applications.

Shoulder muscle activity and perceived comfort of industry workers using a commercial upper limb exoskeleton for simulated tasks

We compared the effects of using a commercial exoskeleton on shoulder muscle activity, task completion time, perceived effort and comfort while performing four tasks in different shoulder positions. Fourteen automotive industry workers performed four simulated tasks with shoulder at A≈0°, B ≈ 45°, C ≈ 90° and D ≈ 115° flexion. The electromyographic activity of the Medial Deltoid (MD) and the Anterior Deltoid (AD) decreased when wearing the exoskeleton. The effect sizes (ES) were, for MD: ES = 0, ES = -0.2, ES = -0.6, ES = -0.3; and for AD: ES = 0.3, ES = -0.6, ES = -0.8, ES = -0.6; for tasks A, B, C and D, respectively. We also found increased Anterior Deltoid/Triceps Brachii co-contraction, a typical joint stabilization mechanism. Wearing the exoskeleton increased the completion time of task B and reduced the perceived effort of tasks A and C, improving overall comfort. These findings are useful to organize the logistics of the workstations that use upper limb exoskeletons to improve the effectiveness of this equipment.

Targeted muscle effort distribution with exercise robots: Trajectory and resistance effects

The objective of this work is to relate muscle effort distributions to the trajectory and resistance settings of a robotic exercise and rehabilitation machine. Muscular effort distribution, representing the participation of each muscle in the training activity, was measured with electromyography sensors (EMG) and defined as the individual activation divided by the total muscle group activation. A four degrees-of-freedom robot and its impedance control system are used to create advanced exercise protocols whereby the user is asked to follow a path against the machine's neutral path and resistance. In this work, the robot establishes a zero-effort circular path, and the subject is asked to follow an elliptical trajectory. The control system produces a user-defined stiffness between the deviations from the neutral path and the torque applied by the subject. The trajectory and resistance settings used in the experiments were the orientation of the ellipse and a stiffness parameter. Multiple combinations of these parameters were used to measure their effects on the muscle effort distribution. An artificial neural network (ANN) used part of the data for training the model. Then, the accuracy of the model was evaluated using the rest of the data. The results show how the precision of the model is lost over time. These outcomes show the complexity of the muscle dynamics for long-term estimations suggesting the existence of time-varying dynamics possibly associated with fatigue.

Real-time optimization of an ellipsoidal trajectory orientation using muscle effort with Extremum Seeking Control

We present an approach for real-time model-free optimization of the orientation of the elliptical trajectory. The performance is evaluated in simulation and experimental stages. Our model-free approach is based on the use of Extremum Seeking Control (ESC) as the real-time optimizer. The experimental stage is performed using a 4 degrees-of-freedom robot and its impedance control system to create advanced exercise protocols whereby the user is asked to follow a path against the machine's neutral path and resistance. Another model-free approach based on the use of the global optimizer Biogeography-based optimization (BBO) was previously reported for simulation results. This last framework has a good performance as a result of exhaustive searches but with a high computational cost limiting its use on real-time experiments. The performance of the ESC approach was validated by comparing the results with those of BBO using five different arm models representing real human arms. In the real-time experiments, muscle activations representing the participation of each muscle in the training activity were measured with electromyography sensors (EMG) and real-time processed from raw signals. The muscle objective can be professionally selected by a therapist to emphasize or de-emphasize certain muscle groups. The robot establishes a zero-effort circular path, and the subject is asked to follow an elliptical trajectory. The control system produces a user-defined stiffness between the deviations from the neutral path and the force/torque applied by the subject. The results show that the framework was able to successfully find the optimal ellipsoidal orientation converging to similar solutions in short period trials of 50 s.

Joint-line medialization after anatomical total shoulder replacement requires more rotator cuff activity to preserve joint stability

BACKGROUND

The biomechanical effects of joint-line medialization during shoulder surgery are poorly understood. It was therefore the purpose of this study to investigate whether medialization of the joint line especially associated with total shoulder arthroplasty leads to changes in the rotator cuff muscle forces required to stabilize the arm in space.

METHODS

A validated computational 3-D rigid body simulation model was used to calculate generated muscle forces, instability ratios, muscle-tendon lengths and moment arms during scapular plane elevation. Measurements took place with the anatomical and a 2 mm and 6 mm lateralized or medialized joint line.

RESULTS

When the joint line was medialized, increased deltoid muscle activity was recorded throughout glenohumeral joint elevation. The rotator cuff muscle forces increased with medialization of the joint line in the early phases of elevation. Lateralization of the joint line led to higher rotator cuff muscle forces after 52° of glenohumeral elevation and to higher absolute values in muscle activity. A maximum instability ratio of >0.6 was recorded with 6 mm of joint-line medialization.

CONCLUSION

In this biomechanical study, medialization and lateralization of the normal joint line during total shoulder arthroplasty led to substantial load changes on the shoulder muscles used for stabilizing the arm in space. Specifically, medialization does not only lead to muscular shortening but also to increased load on the supraspinatus tendon during early arm elevation, the position which is already most loaded in the native joint.

Diagnosis and Monitoring of Osteoporosis with Total-Body 18F-Sodium Fluoride-PET/CT

In recent years, ¹⁸F-Sodium Fluoride (NaF)-PET/CT has seen its role in the detection and management of osteoporosis increase. This article reviews the extent of this application in the literature, its efficacy compared with other comparable imaging tools, and how total-body PET/CT combined with global disease assessment can revolutionize measurement of total osteoporotic disease activity. NaF-PET/CT eventually can be the modality of choice for metabolic bone disorders, especially with these advances in technology and computation.

Sensitivity of Neuromechanical Predictions to Choice of Glenohumeral Stability Modeling Approach

Most upper-extremity musculoskeletal models represent the glenohumeral joint with an inherently stable ball-and-socket, but the physiological joint requires active muscle coordination for stability. The authors evaluated sensitivity of common predicted outcomes (instability, net glenohumeral reaction force, and rotator cuff activations) to different implementations of active stabilizing mechanisms (constraining net joint reaction direction and incorporating normalized surface electromyography [EMG]). Both EMG and reaction force constraints successfully reduced joint instability. For flexion, incorporating any normalized surface EMG data reduced predicted instability by 54.8%, whereas incorporating any force constraint reduced predicted instability by 43.1%. Other outcomes were sensitive to EMG constraints, but not to force constraints. For flexion, incorporating normalized surface EMG data increased predicted magnitudes of joint reaction force and rotator cuff activations by 28.7% and 88.4%, respectively. Force constraints had no influence on these predicted outcomes for all tasks evaluated. More restrictive EMG constraints also tended to overconstrain the model, making it challenging to accurately track input kinematics. Therefore, force constraints may be a more robust choice when representing stability.

Sex differences in upper limb musculoskeletal biomechanics during a lifting task

Women experience higher prevalence of work-related upper limb musculoskeletal disorders compared to men. Previous studies have investigated the biological, kinematic and electromyographic sex-related differences during a lifting task but the actual differences in musculoskeletal loads remain unknown. We investigated the sex differences in three musculoskeletal indicators: the sum of muscle activations, the sum of muscle forces and the relative time spent beyond a shear-compression dislocation ratio. A musculoskeletal model was scaled on 20 women and 20 men lifting a 6 or 12kg box from hip to eye level. Women generated more muscle forces and activations than men, regardless of the lifted mass. Those differences occurred when the box was above shoulder level. In addition, women might spend more time beyond a shear-compression dislocation ratio. Our work suggests higher musculoskeletal loads among women compared to men during a lifting task, which could be the result of poor technique and strength difference.

Effect of rotator cuff muscle activation on glenohumeral kinematics: A cadaveric study

Healthy shoulder function requires the coordination of the rotator cuff muscles to maintain the humeral head's position in the glenoid. While glenohumeral stability has been studied in various settings, few studies have characterized the effect of dynamic rotator cuff muscle loading on glenohumeral translation during shoulder motion. We hypothesize that dynamic rotator cuff muscle activation decreases joint translation during continuous passive abduction of the humerus in a cadaveric model of scapular plane glenohumeral abduction. The effect of different rotator cuff muscle activity on glenohumeral translation was assessed using a validated shoulder testing system. The Dynamic Load profile is a novel approach, based on musculoskeletal modeling of human subject motion. Passive humeral elevation in the scapular plane was applied via the testing system arm, while the rotator cuff muscles were activated according to the specified force profiles using stepper motors and a proportional control feedback loop. Glenohumeral translation was defined according to the International Society of Biomechanics. The Dynamic load profile minimized superior translation of the humeral head relative to the conventional loading profiles. The total magnitude of translation was not significantly different (0.805) among the loading profiles suggesting that the compressive forces from the rotator cuff primarily alter the direction of humeral head translation, not the magnitude. Rotator cuff muscle loading is an important element of cadaveric shoulder studies that must be considered to accurately simulate glenohumeral motion. A rotator cuff muscle activity profile based on human subject muscle activity reduces superior glenohumeral translation when compared to previous RC loading profiles.

EMG-Assisted Algorithm to Account for Shoulder Muscles Co-Contraction in Overhead Manual Handling

Glenohumeral stability is essential for a healthy function of the shoulder. It is ensured partly by the scapulohumeral muscular balance. Accordingly, modelling muscle interactions is a key factor in the understanding of occupational pathologies, and the development of ergonomic interventions. While static optimization is commonly used to estimate muscle activations, it tends to underestimate the role of shoulder’s antagonist muscles. The purpose of this study was to implement experimental electromyographic (EMG) data to predict muscle activations that could account for the stabilizing role of the shoulder muscles. Kinematics and EMG were recorded from 36 participants while lifting a box from hip to eye level. Muscle activations and glenohumeral joint reactions were estimated using an EMG-assisted algorithm and compared to those obtained using static optimization with a generic and calibrated model. Muscle activations predicted with the EMG-assisted method were generally larger. Additionally, more interactions between the different rotator cuff muscles, as well as between primer actuators and stabilizers, were predicted with the EMG-assisted method. Finally, glenohumeral forces calculated from a calibrated model remained within the boundaries of the glenoid stability cone. These findings suggest that EMG-assisted methods could account for scapulohumeral muscle co-contraction, and thus their contribution to the glenohumeral stability.

Sex differences in glenohumeral muscle activation and coactivation during a box lifting task

Manual material handling is associated with shoulder musculoskeletal disorders, especially for women. Sex differences in glenohumeral muscle activity may contribute to women's higher injury risk by affecting shoulder load and stability. We assessed the effects of sex (25 women vs 26 men) and lifting load (6 kg vs 12 kg) on muscle activation during box lifting from hip to eye level. Surface and intramuscular electromyography were recorded from 10 glenohumeral muscles. Most muscles were more activated for the heavier box and for women. These effects were larger for 'prime movers' than for stabilisers and antagonists. Despite their apparently heterogeneous effects on muscle activity, sex and mass did not affect Muscle Focus, a metric of coactivation. This may be partly related to the limited sensitivity of the Muscle Focus. Nevertheless, sex differences in strength, more than in coactivation patterns, may contribute to the sex imbalance in the prevalence of musculoskeletal disorders. Practitioner summary: We studied sex differences in glenohumeral muscle activity in a lifting task to eye level. Women lifting a 6-kg box activated their muscles similarly to men lifting a 12-kg box, i.e. up to 48% of their maximum capacity. Interventions minimising shoulder load should be implemented, especially for women. Abbreviations: BB: biceps brachii; DeltA: anterior deltoid; DeltL: lateral deltoid; DeltP: posterior deltoid; DoF: degrees of freedom; ED: effect duration; EMG: electromyography; ES: effect size; Infra: infraspinatus; Lat: latissimus dorsi; MF: muscle focus; MMH: manual material handling; MVA: maximal voluntary activation; Pect: pectoralis major; Subscap: subscapularis; Supra: supraspinatus; TB: triceps brachii.

Spatial Dependency of Glenohumeral Joint Stability during Dynamic Unimanual and Bimanual Pushing and Pulling

Degenerative wear to the glenoid from repetitive loading can reduce effective concavity depth and lead to future instability. Workspace design should consider glenohumeral stability to prevent initial wear. While stability has been previously explored for activities of daily living including push-pull tasks, whether stability is spatially dependent is unexplored. We simulated bimanual and unimanual push-pull tasks to 4 horizontal targets (planes of elevation: 0º, 45º, 90º, and 135º) at 90º thoracohumeral elevation and 3 elevation targets (thoracohumeral elevations: 20º, 90º, 170º) at 90º plane of elevation. The 45º horizontal target was most stable regardless of exertion type and would be the ideal target placement when considering stability. This target is likely more stable because the applied load acts perpendicular to the glenoid, limiting shear force production. The 135º horizontal target was particularly unstable for unimanual pushing (143% less stable than the 45º target), and the applied force acts parallel to the glenoid, likely creating shear forces. Pushing was less stable than pulling (all targets except sagittal 170º for both task types and horizontal 45º for bimanual) (p<0.01), which is consistent with prior reports. For example, unimanual pushing at the 90º horizontal target was 197% less stable than unimanual pulling. There were limited stability benefits to task placement for pushing, and larger stability benefits may be seen from converting pushing to pulling rather than optimizing task layout. There was no difference in stability between bimanual and unimanual tasks, suggesting no stability benefit to bimanual operation.