Fiber-type traps: revisiting common misconceptions about skeletal muscle fiber types with application to motor control, biomechanics, physiology, and biology.

Skeletal muscle is a highly complex tissue that is studied by scientists from a wide spectrum of disciplines, including motor control, biomechanics, exercise science, physiology, cell biology, genetics, regenerative medicine, orthopedics, and engineering. Although this diversity in perspectives has led to many important discoveries, historically, there has been limited overlap in discussions across fields. This has led to misconceptions and oversimplifications about muscle biology that can create confusion and potentially slow scientific progress across fields. The purpose of this synthesis paper is to bring together research perspectives across multiple muscle fields to identify common assumptions related to muscle fiber type that are points of concern to clarify. These assumptions include 1) classification by myosin isoform and fiber oxidative capacity is equivalent, 2) fiber cross-sectional area (CSA) is a surrogate marker for myosin isoform or oxidative capacity, and 3) muscle force-generating capacity can be inferred from myosin isoform. We address these three fiber-type traps and provide some context for how these misunderstandings can and do impact experimental design, computational modeling, and interpretations of findings, from the perspective of a range of fields. We stress the dangers of generalizing findings about "muscle fiber types" among muscles or across species or sex, and we note the importance for precise use of common terminology across the muscle fields.

Cosimulation of the index finger extensor apparatus with finite element and musculoskeletal models.

Musculoskeletal modeling has been effective for simulating dexterity and exploring the consequences of disability. While previous approaches have examined motor function using multibody dynamics, existing musculoskeletal models of the hand and fingers have difficulty simulating soft tissue such as the extensor mechanism of the fingers, which remains underexplored. To investigate the extensor mechanism and its impact on finger motor function, we developed a finite element model of the index finger extensor mechanism and a cosimulation method that combines the finite element model with a multibody dynamic model. The finite element model and cosimulation were validated through comparison with experimentally derived tissue strains and fingertip endpoint forces respectively. Tissue strains predicted by the finite element model were consistent with the experimentally observed strains of the 9 postures tested in cadaver specimens. Fingertip endpoint forces predicted using the cosimulation were well aligned in both force (difference within 0.60 N) and direction (difference within 30°with experimental results. Sensitivity of the extensor mechanism to changes in modulus and adhesion configuration were evaluated for ± 50% of experimental moduli, presence of the radial and ulnar adhesions, and joint capsule. Simulated strains and endpoint forces were found to be minimally sensitive to alterations in moduli and adhesions. These results are promising and demonstrate the ability of the cosimulation to predict global behavior of the extensor mechanism, while enabling measurement of stresses and strains within the structure itself. This model could be used in the future to predict the outcomes for different surgical repairs of the extensor mechanism.

The influence of induced gait asymmetry on joint reaction forces.

Chronic injury- or disease-induced joint impairments result in asymmetric gait deviations that may precipitate changes in joint loading associated with pain and osteoarthritis. Understanding the impact of gait deviations on joint reaction forces (JRFs) is challenging because of concurrent neurological and/or anatomical changes and because measuring JRFs requires medically invasive instrumented implants. Instead, we investigated the impact of joint motion limitations and induced asymmetry on JRFs by simulating data recorded as 8 unimpaired participants walked with bracing to unilaterally and bilaterally restrict ankle, knee, and simultaneous ankle + knee motion. Personalized models, calculated kinematics, and ground reaction forces (GRFs) were input into a computed muscle control tool to determine lower limb JRFs and simulated muscle activations guided by electromyography-driven timing constraints. Unilateral knee restriction increased GRF peak and loading rate ipsilaterally but peak values decreased contralaterally when compared to walking without joint restriction. GRF peak and loading rate increased with bilateral restriction compared to the contralateral limb of unilaterally restricted conditions. Despite changes in GRFs, JRFs were relatively unchanged due to reduced muscle forces during loading response. Thus, while joint restriction results in increased limb loading, reductions in muscle forces counteract changes in limb loading such that JRFs were relatively unchanged.

Evaluating anthropometric scaling of a generic adult model to represent pediatric shoulder strength.

The structure of the developing musculoskeletal system during childhood and adolescence influences tissue loading and function. Anatomical features important for musculoskeletal loading such as muscle volume and limb proportion vary with age but limited available anatomical data for the developing limb makes predicting loads challenging. Our aim was to evaluate whether anthropometric scaling of an existing adult musculoskeletal upper limb model is sufficient to accurately represent pediatric strength. An adult upper limb model was scaled using two scale factors based on length features and max isometric force (MIF). Length features (e.g. limb and muscle length) were scaled based on linear regression for available literature reports of forearm length vs. height (N = 366 Pediatric, N = 107 Adults), while MIF was scaled based on relating body mass vs. total shoulder muscle volume (N = 6). Children-specific models were developed for 6 pediatric individuals whose height, body mass, and shoulder moment-generating capacity (a common measure of strength) were previously reported. These models were used to predict isometric shoulder moments for flexion/extension, internal/external rotation, and ad/abduction and compared with physical measurements previously reported. The predicted isometric shoulder moments were significantly correlated to measured moments for these same individuals (p < 0.04, r2 > 0.7). However, predicted moments tended to underestimate measured values; shoulder external rotation was most accurately predicted (slope: 1.1234) while shoulder adduction was most underestimated (slope: 0.4624). This work provides an initial basis for pediatric scaling but illustrates the important need for additional direct measures of muscle size and limb strength and function in a pediatric population.

Location of brachial plexus birth injury affects functional outcomes in a rat model.

Brachial plexus birth injury (BPBI) results in shoulder and elbow paralysis with shoulder internal rotation and elbow flexion contracture as frequent sequelae. The purpose of this study was to develop a technique for measuring functional movement and examine the effect of brachial plexus injury location (preganglionic and postganglionic) on functional movement outcomes in a rat model of BPBI, which we achieved through integration of gait analysis with musculoskeletal modeling and simulation. Eight weeks following unilateral brachial plexus injury, sagittal plane shoulder and elbow angles were extracted from gait recordings of young rats (n = 18), after which rats were sacrificed for bilateral muscle architecture measurements. Musculoskeletal models reflecting animal-specific muscle architecture parameters were used to simulate gait and extract muscle fiber lengths. The preganglionic neurectomy group spent significantly less (p = 0.00116) time in stance and walked with significantly less (p < 0.05) elbow flexion and shoulder protraction in the affected limb than postganglionic neurectomy or control groups. Linear regression revealed no significant linear relationship between passive shoulder external rotation and functional shoulder protraction range of motion. Despite significant restriction in longitudinal muscle growth, normalized functional fiber excursions did not differ significantly between groups. In fact, when superimposed on a normalized force-length curve, neurectomy-impaired muscle fibers (except subscapularis) accessed regions of the curve that overlapped with the control group. Our results suggest the presence of compensatory motor control strategies during locomotion following BPBI. The clinical implications of our findings support emphasis on functional movement analysis in treatment of BPBI, as functional and passive outcomes may differ substantially.

A Direct Comparison of Node and Element-Based Finite Element Modeling Approaches to Study Tissue Growth.

Finite element analysis is a useful tool to model growth of biological tissues and predict how growth can be impacted by stimuli. Previous work has simulated growth using node-based or element-based approaches, and this implementation choice may influence predicted growth, irrespective of the applied growth model. This study directly compared node-based and element-based approaches to understand the isolated impact of implementation method on growth predictions by simulating growth of a bone rudiment geometry, and determined what conditions produce similar results between the approaches. We used a previously reported node-based approach implemented via thermal expansion and an element-based approach implemented via osmotic swelling, and we derived a mathematical relationship to relate the growth resulting from these approaches. We found that material properties (modulus) affected growth in the element-based approach, with growth completely restricted for high modulus values relative to the growth stimulus, and no restriction for low modulus values. The node-based approach was unaffected by modulus. Node- and element-based approaches matched marginally better when the conversion coefficient to relate the approaches was optimized based on the results of initial simulations, rather than using the theoretically predicted conversion coefficient (median difference in node position 0.042 cm versus 0.052 cm, respectively). In summary, we illustrate here the importance of the choice of implementation approach for modeling growth, provide a framework for converting models between implementation approaches, and highlight important considerations for comparing results in prior work and developing new models of tissue growth.

Reduced joint motion supersedes asymmetry in explaining increased metabolic demand during walking with mechanical restriction.

Recent research has highlighted the complex interactions among chronic injury- or disease-induced joint limitations, walking asymmetry, and increased metabolic cost. Determining the specific metabolic impacts of asymmetry or joint impairment in clinical populations is difficult because of concurrent neurological and physiological changes. This work investigates the metabolic impact of gait asymmetry and joint restriction by unilaterally (asymmetric) and bilaterally (symmetric) restricting ankle, knee, and combined ankle and knee ranges of motion in unimpaired individuals. We calculated propulsive asymmetry, temporal asymmetry, and step-length asymmetry for an average gait cycle; metabolic rate; average positive center of mass power using the individual limbs method; and muscle effort using lower limb electromyography measurements weighted by corresponding physiological cross-sectional areas. Unilateral restriction caused propulsive and temporal asymmetry but less metabolically expensive gait than bilateral restriction. Changes in asymmetry did not correlate with changes in metabolic cost. Interestingly, bilateral restriction increased average positive center of mass power compared to unilateral restriction. Further, increased average positive center of mass power correlated with increased energy costs, suggesting asymmetric step-to-step transitions did not drive metabolic changes. The number of restricted joints reduces available degrees of freedom and may have a larger metabolic impact than gait asymmetry, as this correlated significantly with increases in metabolic rate for 7/9 participants. These results emphasize symmetry is not by definition metabolically optimal, indicate that the mechanics underlying symmetry are meaningful, and suggest that available degrees of freedom should be considered in designing future interventions.

Isolating the energetic and mechanical consequences of imposed reductions in ankle and knee flexion during gait.

BACKGROUND: Weakness of ankle and knee musculature following injury or disorder results in reduced joint motion associated with metabolically expensive gait compensations to enable limb support and advancement. However, neuromechanical coupling between the ankle and knee make it difficult to discern independent roles of these restrictions in joint motion on compensatory mechanics and metabolic penalties. METHODS: We sought to determine relative impacts of ankle and knee impairment on compensatory gait strategies and energetic outcomes using an unimpaired cohort (N = 15) with imposed unilateral joint range of motion restrictions as a surrogate for reduced motion resulting from gait pathology. Participants walked on a dual-belt instrumented treadmill at 0.8 m s-1 using a 3D printed ankle stay and a knee brace to systematically limit ankle motion (restricted-ank), knee motion (restricted-knee), and ankle and knee motion (restricted-a + k) simultaneously. In addition, participants walked without any ankle or knee bracing (control) and with knee bracing worn but unrestricted (braced). RESULTS: When ankle motion was restricted (restricted-ank, restricted-a + k) we observed decreased peak propulsion relative to the braced condition on the restricted limb. Reduced knee motion (restricted-knee, restricted-a + k) increased restricted limb circumduction relative to the restricted-ank condition through ipsilateral hip hiking. Interestingly, restricted limb average positive hip power increased in the restricted-ank condition but decreased in the restricted-a + k and restricted-knee conditions, suggesting that locking the knee impeded hip compensation. As expected, reduced ankle motion, either without (restricted-ank) or in addition to knee restriction (restricted-a + k) yielded significant increase in net metabolic rate when compared with the braced condition. Furthermore, the relative increase in metabolic cost was significantly larger with restricted-a + k when compared to restricted-knee condition. CONCLUSIONS: Our methods allowed for the reproduction of asymmetric gait characteristics including reduced propulsive symmetry and increased circumduction. The metabolic consequences bolster the potential energetic benefit of targeting ankle function during rehabilitation. TRIAL REGISTRATION: N/A.

Using Reinforcement Learning to Estimate Human Joint Moments From Electromyography or Joint Kinematics: An Alternative Solution to Musculoskeletal-Based Biomechanics.

Reinforcement learning (RL) has potential to provide innovative solutions to existing challenges in estimating joint moments in motion analysis, such as kinematic or electromyography (EMG) noise and unknown model parameters. Here, we explore feasibility of RL to assist joint moment estimation for biomechanical applications. Forearm and hand kinematics and forearm EMGs from four muscles during free finger and wrist movement were collected from six healthy subjects. Using the proximal policy optimization approach, we trained two types of RL agents that estimated joint moment based on measured kinematics or measured EMGs, respectively. To quantify the performance of trained RL agents, the estimated joint moment was used to drive a forward dynamic model for estimating kinematics, which was then compared with measured kinematics using Pearson correlation coefficient. The results demonstrated that both trained RL agents are feasible to estimate joint moment for wrist and metacarpophalangeal (MCP) joint motion prediction. The correlation coefficients between predicted and measured kinematics, derived from the kinematics-driven agent and subject-specific EMG-driven agents, were 98% ± 1% and 94% ± 3% for the wrist, respectively, and were 95% ± 2% and 84% ± 6% for the metacarpophalangeal joint, respectively. In addition, a biomechanically reasonable joint moment-angle-EMG relationship (i.e., dependence of joint moment on joint angle and EMG) was predicted using only 15 s of collected data. In conclusion, this study illustrates that an RL approach can be an alternative technique to conventional inverse dynamic analysis in human biomechanics study and EMG-driven human-machine interfacing applications.

Influence of Brachial Plexus Birth Injury Location on Glenohumeral Joint Morphology.

PURPOSE: Patient presentation after brachial plexus birth injury (BPBI) is influenced by nerve injury location; more contracture and bone deformity occur at the shoulder in postganglionic injuries. Although bone deformity after postganglionic injury is well-characterized, the extent of glenohumeral deformity after preganglionic BPBI is unclear. METHODS: Twenty Sprague-Dawley rat pups received preganglionic or postganglionic neurectomy on a single forelimb at postnatal days 3 to 4. Glenohumeral joints on affected and unaffected sides were analyzed using micro-computed tomography scans after death at 8 weeks after birth. Glenoid version, glenoid inclination, glenoid and humeral head radius of curvature, and humeral head thickness and width were measured bilaterally. RESULTS: The glenoid was significantly more declined in affected compared with unaffected shoulders after postganglionic (-17.7° ± 16.9°) but not preganglionic injury. Compared with the preganglionic group, the affected shoulder in the postganglionic group exhibited significantly greater declination and increased glenoid radius of curvature. In contrast, the humeral head was only affected after preganglionic but not postganglionic injury, with a significantly smaller humeral head radius of curvature (-0.2 ± 0.2 mm), thickness (-0.2 ± 0.3 mm), and width (-0.3 ± 0.4 mm) on the affected side compared with the unaffected side; changes in these metrics were significantly associated with each other. CONCLUSIONS: These findings suggest that glenoid deformities occur after postganglionic BPBI but not after preganglionic BPBI, whereas the humeral head is smaller after preganglionic injury, possibly suggesting an overall decreased biological growth rate in this group. CLINICAL RELEVANCE: This study expands understanding of the altered glenoid and humeral head morphologies after preganglionic BPBI and its comparisons with morphologies after postganglionic BPBI.

Preganglionic and Postganglionic Brachial Plexus Birth Injury Effects on Shoulder Muscle Growth.

PURPOSE: Brachial plexus birth injury can differ in presentation, depending on whether the nerve ruptures distal to, or avulses proximal to, the dorsal root ganglion. More substantial contracture and bone deformity at the shoulder is typical in postganglionic injuries. However, changes to the underlying muscle structure that drive these differences in presentation are unclear. METHODS: Seventeen Sprague-Dawley rats received preganglionic or postganglionic neurectomy on a single limb on postnatal days 3 and 4. Muscles crossing the shoulder were retrieved once the rats were sacrificed at 8 weeks after birth. External rotation range of motion, muscle mass, muscle length, muscle sarcomere length, and calculated optimal muscle length were measured bilaterally. RESULTS: Average shoulder range of motion in the postganglionic group was 61.8% and 56.2% more restricted at 4 and 8 weeks, respectively, compared with that in the preganglionic group, but affected muscles after preganglionic injury were altered more severely (compared with the unaffected limb) than after postganglionic injury. Optimal muscle length in preganglionic injury was shorter in the affected limb (compared with the unaffected limb: -18.2% ± 9.2%) and to a greater extent than in postganglionic injury (-5.1% ± 6.2%). Muscle mass in preganglionic injury was lower in the affected limb (relative to the unaffected limb: -57.2% ± 24.1%) and to a greater extent than in postganglionic injury (-28.1% ± 17.7%). CONCLUSIONS: The findings suggest that the presence of contracture does not derive from restricted longitudinal muscle growth alone, but also depends on the extent of muscle mass loss occurring simultaneously after the injury. CLINICAL RELEVANCE: This study expands our understanding of differences in muscle architecture and the role of muscle structure in contracture formation for preganglionic and postganglionic brachial plexus birth injury.

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.

Integrated iterative musculoskeletal modeling predicts bone morphology following brachial plexus birth injury (BPBI).

Brachial plexus birth injury (BPBI) is the most common nerve injury among children. The glenohumeral joint of affected children can undergo severe osseous deformation and altered muscle properties, depending on location of the injury relative to the dorsal root ganglion (preganglionic or postganglionic). Preganglionic injury results in lower muscle mass and shorter optimal muscle length compared to postganglionic injury. We investigated whether these changes to muscle properties over time following BPBI provide a mechanically-driven explanation for observed differences in bone deformity between preganglionic and postganglionic BPBI. We developed a computational framework integrating musculoskeletal modeling to represent muscle changes over time and finite element modeling to simulate bone growth in response to mechanical and biological stimuli. The simulations predicted that the net glenohumeral joint loads in the postganglionic injury case were nearly 10.5% greater than in preganglionic. Predicted bone deformations were more severe in the postganglionic case, with the glenoid more declined (pre: -43.8°, post: -51.0°), flatter with higher radius of curvature (pre: 3.0 mm, post: 3.7 mm), and anteverted (pre: 2.53°, post: 4.93°) than in the preganglionic case. These simulated glenoid deformations were consistent with previous experimental studies. Thus, we concluded that the differences in muscle mass and length between the preganglionic and postganglionic injuries are critical mechanical drivers of the altered glenohumeral joint shape.

Effect of Mechanically Passive, Wearable Shoulder Exoskeletons on Muscle Output During Dynamic Upper Extremity Movements: A Computational Simulation Study.

Wearable passive (ie, spring powered) shoulder exoskeletons could reduce muscle output during motor tasks to help prevent or treat shoulder musculoskeletal disorders. However, most wearable passive shoulder exoskeletons have been designed and evaluated for static tasks, so it is unclear how they affect muscle output during dynamic tasks. The authors used a musculoskeletal model and Computed Muscle Control optimization to estimate muscle output with and without a wearable passive shoulder exoskeleton during 2 simulated dynamic tasks: abduction and upward reach. To an existing upper extremity musculoskeletal model, the authors added an exoskeleton model with 3-dimensional representations of the exoskeleton components, including a spring, cam wheel, force-transmitting shoulder cable, and wrapping surfaces that permitted the shoulder cable to wrap over the shoulder. The exoskeleton reduced net muscle-generated moments in positive shoulder elevation by 28% and 62% during the abduction and upward reach, respectively. However, muscle outputs (joint moments and muscle effort) were higher with the exoskeleton than without at some points of the movement. Muscle output was higher with the exoskeleton because the exoskeleton moment opposed the muscle-generated moment in some postures. The results of this study highlight the importance of evaluating muscle output for passive exoskeletons designed to support dynamic movements to ensure that the exoskeletons assist, rather than impede, movement.

Mechanics and energetics of post-stroke walking aided by a powered ankle exoskeleton with speed-adaptive myoelectric control.

BACKGROUND: Ankle exoskeletons offer a promising opportunity to offset mechanical deficits after stroke by applying the needed torque at the paretic ankle. Because joint torque is related to gait speed, it is important to consider the user's gait speed when determining the magnitude of assistive joint torque. We developed and tested a novel exoskeleton controller for delivering propulsive assistance which modulates exoskeleton torque magnitude based on both soleus muscle activity and walking speed. The purpose of this research is to assess the impact of the resulting exoskeleton assistance on post-stroke walking performance across a range of walking speeds. METHODS: Six participants with stroke walked with and without assistance applied to a powered ankle exoskeleton on the paretic limb. Walking speed started at 60% of their comfortable overground speed and was increased each minute (n00, n01, n02, etc.). We measured lower limb joint and limb powers, metabolic cost of transport, paretic and non-paretic limb propulsion, and trailing limb angle. RESULTS: Exoskeleton assistance increased with walking speed, verifying the speed-adaptive nature of the controller. Both paretic ankle joint power and total limb power increased significantly with exoskeleton assistance at six walking speeds (n00, n01, n02, n03, n04, n05). Despite these joint- and limb-level benefits associated with exoskeleton assistance, no subject averaged metabolic benefits were evident when compared to the unassisted condition. Both paretic trailing limb angle and integrated anterior paretic ground reaction forces were reduced with assistance applied as compared to no assistance at four speeds (n00, n01, n02, n03). CONCLUSIONS: Our results suggest that despite appropriate scaling of ankle assistance by the exoskeleton controller, suboptimal limb posture limited the conversion of exoskeleton assistance into forward propulsion. Future studies could include biofeedback or verbal cues to guide users into limb configurations that encourage the conversion of mechanical power at the ankle to forward propulsion. TRIAL REGISTRATION: N/A.

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 glenohumeral 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 four horizontal targets (planes of elevation: 0 deg, 45 deg, 90 deg, and 135 deg) at 90 deg thoracohumeral elevation and three elevation targets (thoracohumeral elevations: 20 deg, 90 deg, 170 deg) at 90 deg plane of elevation. The 45 deg 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 135 deg horizontal target was particularly unstable for unimanual pushing (143% less stable than the 45 deg target), and the applied force for this task acts parallel to the glenoid, likely creating shear forces or limiting compressive forces. Pushing was less stable than pulling (all targets except sagittal 170 deg for both task types and horizontal 45 deg for bimanual) (p < 0.01), which is consistent with prior reports. For example, unimanual pushing at the 90 deg 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 tasks from push to pull rather than optimizing task layout. There was no difference in stability between bimanual and unimanual tasks, suggesting no stability benefit to bimanual operation.

Computational analysis of glenohumeral joint growth and morphology following a brachial plexus birth injury.

Children affected with brachial plexus birth injury (BPBI) undergo muscle paralysis. About 33% of affected children experience permanent osseous deformities of the glenohumeral joint. Recent evidence suggests that some cases experience restricted muscle longitudinal growth in addition to paralysis and reduced range of motion at the shoulder and elbow. It is unknown whether altered loading due to paralysis, muscle growth restriction and contracture, or static loading due to disuse is the primary driver of joint deformity after BPBI. This study uses a computational framework integrating finite element analysis and musculoskeletal modeling to examine the mechanical factors contributing to changes in bone growth and morphometry following BPBI. Simulations of 8 weeks of glenohumeral growth in a rat model of BPBI predicted that static loading of the joint is primarily responsible for joint deformation consistent with experimental measures of bone morphology, whereas dynamic loads resulted in normal bone growth. Under dynamic loading, glenoid version angle (GVA), glenoid inclination angle (GIA), and glenoid radius of curvature (GRC) (-1.3°, 38.2°, 2.5 mm respectively) were similar to the baseline values (-1.8°, -38°, 2.1 mm respectively). In the static case with unrestricted muscle growth, these measures increased in magnitude (5.2°, -48°, 3.5 mm respectively). More severe joint deformations were observed in GIA and GRC when muscle growth was restricted (GVA: 3.6°, GIA: -55°, GRC: 4.0 mm). Predicted morphology was consistent with literature reports of in vivo glenoid morphology following postganglionic BPBI. This growth model provides a framework for understanding the most influential mechanical factors driving glenohumeral deformity following BPBI.

Modeling a rotator cuff tear: Individualized shoulder muscle forces influence glenohumeral joint contact force predictions.

BACKGROUND: Rotator cuff tears in older individuals may result in decreased muscle forces and changes to force distribution across the glenohumeral joint. Reduced muscle forces may impact functional task performance, altering glenohumeral joint contact forces, potentially contributing to instability or joint damage risk. Our objective was to evaluate the influence of rotator cuff muscle force distribution on glenohumeral joint contact force during functional pull and axilla wash tasks using individualized computational models. METHODS: Fourteen older individuals (age 63.4 yrs. (SD 1.8)) were studied; 7 with rotator cuff tear, 7 matched controls. Muscle volume measurements were used to scale a nominal upper limb model's muscle forces to develop individualized models and perform dynamic simulations of movement tracking participant-derived kinematics. Peak resultant glenohumeral joint contact force, and direction and magnitude of force components were compared between groups using ANCOVA. FINDINGS: Results show individualized muscle force distributions for rotator cuff tear participants had reduced peak resultant joint contact force for pull and axilla wash (P ≤ 0.0456), with smaller compressive components of peak resultant force for pull (P = 0.0248). Peak forces for pull were within the glenoid. For axilla wash, peak joint contact was directed near/outside the glenoid rim for three participants; predictions required individualized muscle forces since nominal muscle forces did not affect joint force location. INTERPRETATION: Older adults with rotator cuff tear had smaller peak resultant and compressive forces, possibly indicating increased instability or secondary joint damage risk. Outcomes suggest predicted joint contact force following rotator cuff tear is sensitive to including individualized muscle forces.

Spatial dependency of shoulder muscle demand during dynamic unimanual and bimanual pushing and pulling.

Work involving extensive pushing and pulling is associated with higher frequency of shoulder complaints. While reports of shoulder muscle demand during submaximal isometric tasks are abundant, dynamic submaximal push-pull exertions are not well understood. We evaluated how muscle demand (weighted EMG average) of surface glenohumeral muscles varies with task type and target. Seventeen healthy young adults performed seated unimanual and bimanual pushes and pulls to 3 thoracohumeral elevations (20°, 90°, 170°) and 4 elevation planes (0°, 45°, 90°, 135°) with loading at 15% of isometric push-pull capacity. Pulling required less demand than pushing (p < 0.0001). Muscle demand varied more with elevation than elevation plane. The lowest target had highest demand for pulling (p < 0.01), and the most elevated target had highest demand for pushing (p < 0.0001). Working above the shoulder is known to increase demand during isometric tasks, however, these results suggest that for dynamic tasks working against gravity has a larger effect on demand than task target.