Comparison of an EMG-based and a stress-based method to predict shoulder muscle forces

The estimation of muscle forces in musculoskeletal shoulder models is still controversial. Two different methods are widely used to solve the indeterminacy of the system: electromyography (EMG)-based methods and stress-based methods. The goal of this work was to evaluate the influence of these two methods on the prediction of muscle forces, glenohumeral load and joint stability after total shoulder arthroplasty. An EMG-based and a stress-based method were implemented into the same musculoskeletal shoulder model. The model replicated the glenohumeral joint after total shoulder arthroplasty. It contained the scapula, the humerus, the joint prosthesis, the rotator cuff muscles supraspinatus, subscapularis and infraspinatus and the middle, anterior and posterior deltoid muscles. A movement of abduction was simulated in the plane of the scapula. The EMG-based method replicated muscular activity of experimentally measured EMG. The stress-based method minimised a cost function based on muscle stresses. We compared muscle forces, joint reaction force, articular contact pressure and translation of the humeral head. The stress-based method predicted a lower force of the rotator cuff muscles. This was partly counter-balanced by a higher force of the middle part of the deltoid muscle. As a consequence, the stress-based method predicted a lower joint load (16% reduced) and a higher superior–inferior translation of the humeral head (increased by 1.2 mm). The EMG-based method has the advantage of replicating the observed cocontraction of stabilising muscles of the rotator cuff. This method is, however, limited to available EMG measurements. The stress-based method has thus an advantage of flexibility, but may overestimate glenohumeral subluxation.     

Shoulder muscle function depends on elbow joint position: an illustration of dynamic coupling in the upper limb

Shoulder muscle function has been documented based on muscle moment arms, lines of action and muscle contributions to contact force at the glenohumeral joint. At present, however, the contributions of individual muscles to shoulder joint motion have not been investigated, and the effects of shoulder and elbow joint position on shoulder muscle function are not well understood. The aims of this study were to compute the contributions of individual muscles to motion of the glenohumeral joint during abduction, and to examine the effect of elbow flexion on shoulder muscle function. A three-dimensional musculoskeletal model of the upper limb was used to determine the contributions of 18 major muscles and muscle sub-regions of the shoulder to glenohumeral joint motion during abduction. Muscle function was found to depend strongly on both shoulder and elbow joint positions. When the elbow was extended, the middle and anterior deltoid and supraspinatus were the greatest contributors to angular acceleration of the shoulder in abduction. In contrast, when the elbow was flexed at 90°, the anterior deltoid and subscapularis were the greatest contributors to joint angular acceleration in abduction. This dependence of shoulder muscle function on elbow joint position is explained by the existence of dynamic coupling in multi-joint musculoskeletal systems. The extent to which dynamic coupling affects shoulder muscle function, and therefore movement control, is determined by the structure of the inverse mass matrix, which depends on the configuration of the joints. The data provided may assist in the diagnosis of abnormal shoulder function, for example, due to muscle paralysis or in the case of full-thickness rotator cuff tears.     

Towards a model for force predictions in the human shoulder.

In this paper the concept of a three-dimensional biomechanical model of the human shoulder is introduced. This model is used to analyze static load sharing between the muscles, the bones and the ligaments. The model consists of all shoulder structures, which means that different positions and different load situations may be analyzed using the same model. Solutions can be found for the complete range of shoulder motion. However, this article focuses only on elevation in the scapular plane and on forces in structures attached to the humerus. The intention is to expand the model in future studies to also involve the forces acting on the other shoulder bones: the scapula and the clavicle. The musculoskeletal forces in the shoulder complex are predicted utilizing the optimization technique with the sum of squared muscle stresses as an objective function. Numerical results predict that among the muscles crossing the glenohumeral joint parts of the deltoideus, the infraspinatus, the supraspinatus, the subscapularis, the pectoralis major, the coracobrachialis and the biceps are the muscles most activated during this sort of abduction. Muscle-force levels reached values of 150 N when the hand load was 1 kg. The results from the model seem to be qualitatively accurate, but it is concluded that in the future development of the model the direction of the contact force in the glenohumeral joint must be constrained.     

A musculoskeletal shoulder simulation of moment arms and joint reaction forces after medialization of the supraspinatus footprint in rotator cuff repair

A non-anatomical reinsertion of the supraspinatus medially to the original footprint to avoid over-tensioning of the tendon in large and retracted tears is one surgical option in rotator cuff (RC) repair. The purpose of the study was to determine the biomechanical effects on the glenohumeral joint with regard to this surgical technique. A modified musculoskeletal computational shoulder model was used to evaluate the change in moment arms and muscle forces of the RC and the co-contracting muscles and the alteration of the joint reaction forces (compressive and shear forces) after reinsertion of the supraspinatus 5?mm, 10?mm, 15?mm and 20?mm medially to the original footprint. A medialization of the supraspinatus reduces its moment arm in glenohumeral abduction. In case of a medialization of the attachment of 15?mm and 20?mm, the supraspinatus restricts glenohumeral abduction at 54° and 68°. In glenohumeral forward flexion and in lower degrees of internal rotation the moment arm of the supraspinatus increases for a medialized tendon attachment and decreases in external rotation in relation to the anatomical condition. A medialization of the supraspinatus insertion point yields in an increase in muscle force for abduction, internal and external rotation. In the present model a medially non-anatomic reinsertion reduces significantly the compressive glenohumeral joint reaction and the glenohumeral stability. Moreover, the results show that a medialization of the supraspinatus leads to a reduction of the supraspinatus moment arm especially in abduction. This leads to an increase of a compensatory supraspinatus load for stabilization the humerus in space, which may potentially cause a postoperative overload of the tendon-bone-complex.     

EMG and force production of some human shoulder muscles during isometric abduction

Surface EMG was recorded in four subjects on three different occasions from the three parts of the deltoid, the clavicular part of the pectoralis major and from the infraspinatus muscles at different angles of abduction, in the frontal and scapular plane. The integrated EMG was related to the maximum values found for each muscle or muscle part during test contractions (%EMG). Linear relations can be seen for abduction angle vs %EMG. During abduction in the scapular plane the middle and posterior parts of the deltoid muscle showed significantly less activity than in the frontal plane. A simple two dimensional model to calculate the deltoid force out of total external moment at the shoulder is presented. For the middle part of the deltoid an EMG-force relation is presented. The maximal deltoid forces found during test contractions are compared with the absolute muscle force. Also, the length-force relation for the middle part of the deltoid muscle is given between 30° and 90° of abduction.     

Automatic external rotation during passive glenohumeral abduction

A study of human cadaveric glenohumeral joints was undertaken to investigate the mechanism of external rotation during abduction. Previously explored extra-articular influences such as the coracoacromial arch and the rotator cuff muscles were removed from the specimens. A specially designed device was used to measure abduction in the scapular plane as well as any rotation of the humerus. The findings show that external rotation is a consistent feature of passive abduction in the plane of the scapula and could take place in the absence of extra-articular influences. The mechanism appears to lie either in the nature of the capsule or of the articular contours. A role for the intra-articular part of the long head of the biceps cannot be excluded.     

Glenohumeral stability in simulated rotator cuff tears

Rotator cuff tears disrupt the force balance in the shoulder and the glenohumeral joint in particular, resulting in compromised arm elevation torques. The trade-off between glenohumeral torque and glenohumeral stability is not yet understood. We hypothesize that compensation of lost abduction torque will lead to a superior redirection of the reaction force vector onto the glenoid surface, which will require additional muscle forces to maintain glenohumeral stability. Muscle forces in a single arm position for five combinations of simulated cuff tears were estimated by inverse dynamic simulation (Delft Shoulder and Elbow Model) and compared with muscle forces in the non-injured condition. Each cuff tear condition was simulated both without and with an active modeling constraint for glenohumeral stability, which was defined as the condition in which the glenohumeral reaction force intersects the glenoid surface. For the simulated position an isolated tear of the supraspinatus only increased the effort of the other muscles with 8%, and did not introduce instability. For massive cuff tears beyond the supraspinatus, instability became a prominent factor: the deltoids were not able to fully compensate lost net abduction torque without introducing destabilizing forces; unfavorable abductor muscles (i.e. in the simulated position the subscapularis and the biceps longum) remain to compensate the necessary abduction torque; the teres minor appeared to be of vital importance to maintain glenohumeral stability. Adverse adductor muscle co-contraction is essential to preserve glenohumeral stability.     

Physiotherapy vs. capsular shift and physiotherapy in multidirectional shoulder joint instability

PurposeThe aim of the study was to compare the kinematic parameters and the on–off pattern of the muscles of patients with multidirectional instability (MDI) treated by physiotherapy or by capsular shift and postoperative physiotherapy before and after treatment during elevation in the scapular plane.ScopeThe study was carried out on 32 patients with MDI of the shoulder treated with physiotherapy, 19 patients with MDI of the shoulder treated by capsular shift and postoperative physiotherapy, and 25 healthy subjects. The motion of skeletal elements was modeled by the range of humeral elevation, scapulothoracic angle and glenohumeral angle, scapulothoracic (ST) and glenohumeral (GH) rhythms, and relative displacement between the rotation centers of the humerus and scapula. The muscle pattern was modeled by the on–off pattern of muscles around the shoulder, which summarizes the activity duration of the investigated muscles.ResultsThe different ST and GH rhythms and the increased relative displacement between the rotation centers of the scapula and the humerus were observed in MDI patients. The physiotherapy strengthened the rotator cuff, biceps brachii, triceps brachii, deltoid muscles, and increase the neuromuscular control of the shoulder joints. Capsular shift and physiotherapy enabled bilinear ST and GH rhythms and the normal relative displacement between the rotation centers of the scapula and humerus to be restored. After surgery and physiotherapy, the duration of muscular activity was almost normal.ConclusionThe significant alteration in shoulder kinematics observed in MDI patients cannot be restored by physiotherapy only. After the capsular shift and postoperative physiotherapy angulation at 60° of ST and GH rhythms, the relative displacement between the rotation centers of the scapula and humerus and the duration of muscular activity were restored.     

A comprehensive and volumetric musculoskeletal model for the dynamic simulation of the shoulder function

We present a volumetric and extensive finite element model of the shoulder usable in the context of inverse control, in which the scapula is left unconstrained on the ribcage. Such a model allows for exploring various shoulder movements, which are essential for making patient-specific decisions. The proposed model consists of 23 volumetric muscles parts modelled using the finite element method. The glenohumeral, acromioclavicular and sternoclavicular joints are modelled with soft ball-socket constraints. The musculoskeletal model can be controlled by a tracking-based algorithm, finding the excitations values in the muscles needed to follow some target points. The moment arms obtained during abduction and rotation are compared with the literature, which includes results from cadaveric data and a fine FE model of the rotator cuff and the deltoid. We simulated the paralysis of serratus anterior, a main reason of scapular winging, and compared it with its physiological counterpart. A deficiency in the range of motion as well as a reduction in upward rotation were observed, which both corroborate clinical observations. This is one of the most comprehensive model of the shoulder, which can be used to study complex pathologies of the shoulder and their impact on functional outcome such as range-of-motion.     

Suprascapular nerve block results in a compensatory increase in deltoid muscle activity

A balance exists between the deltoid and rotator cuff contribution to arm elevation. Both cadaver and computer models have predicted an increase in deltoid muscle force with dysfunction of the rotator cuff. The goal of the present study was to verify this phenomenon in vivo by examining the effects of paralysis of the supraspinatus and infraspinatus muscles with a suprascapular nerve block on the electrical activity of seven shoulder muscles. Electromyographic data were collected before and after the administration of the block. The block resulted in a significant increase in muscle activity for all heads of the deltoid, with a higher percentage increase noted at lower elevation angles. Although the deltoid activity was reduced as the subjects recovered from the block, even low levels of cuff dysfunction were found to result in increased deltoid activity. These results suggest that even small disruptions in the normal function of some rotator cuff muscles (e.g., due to fatigue or impingement syndrome), may result in an increase in deltoid activity. It is possible that such compensation may result in higher superior loads at the glenohumeral joint, possibly increasing the risk of tendon damage.     

The relation between increased deltoid activation and adductor muscle activation due to glenohumeral cuff tears

In patients with rotator cuff tears lost elevation moments are compensated for by increased deltoid activation. Concomitant proximal directed destabilizing forces at the glenohumeral joint are suggested to be compensated for by ‘out-of-phase’ adductor activation, preserving glenohumeral stability. Aim of this study was to demonstrate causality between moment compensating deltoid activation and stability compensating ‘out-of-phase’ adductor muscle activation.A differential arm loading with the same magnitude of forces applied at small and large moment arms relative to the glenohumeral joint was employed to excite deltoid activation, without externally affecting the force balance. Musculoskeletal modeling was applied to analyze the protocol in terms of muscle forces and glenohumeral (in)stability. The protocol was applied experimentally using electromyography (EMG) to assess muscle activation of healthy controls and cuff tear patients.Both modeling and experiments demonstrated increased deltoid activation with increased moment loading, which was higher in patients compared to controls. Model simulation of cuff tears demonstrated glenohumeral instability and related ‘out-of-phase’ adductor muscle activation which was also found experimentally in patients when compared to controls.Through differential moment loading, the assumed causal relation between increased deltoid activation and compensatory adductor muscle activation in cuff tear patients could be demonstrated. ‘Out-of-phase’ adductor activation in patients was attributed to glenohumeral instability. The moment loading protocol discerned patients with cuff tears from controls based on muscle activation.     

EMG of scapulohumeral muscles in the chimpanzee during reaching and "arboreal" locomotion

Current views on the function of the deltoid and rotator cuff muscles emphasize their roles in arm-raising as participants in a scapulohumeral force "couple." The acceptance of such a mechanism is based primarily on a 1944 EMG study of human shoulder muscle action. More recently, it has been suggested that shoulder joint stabilization constitutes a second and equally important function of the cuff musculature, especially in nonhuman primates which habitually use their forelimbs in overhead postural and locomotor activities. Few comparative data exist, however, on the actual recruitment patterns of these muscles in different species. In order to assess the general applicability of a scapulohumeral force couple model, and the functional significance of the differential development of the scapulohumeral musculature among primate species, we have undertaken a detailed study of shoulder muscle activity patterns in nonhuman primates employing telemetered electromyography, which permits examination of unfettered natural behaviors and locomotion. The results of our research on the chimpanzee, Pan troglodytes, on voluntary reaching and two forms of "arboreal" locomotion reveal four ways in which previous perceptions of the function of the scapulohumeral muscles must be revised: 1) the posterior deltoid is completely different in function from the middle and anterior regions of this muscle; 2) the integrity of the glenohumeral joint during suspensory postures is not maintained solely by osseoligamentous structures; 3) the function of teres minor is entirely different from that of the other rotator cuff muscles and is more similar to the posterior deltoid and teres major; and 4) each remaining member of the rotator cuff plays a distinct, and often unique, role during natural behaviors. These results clearly refute the view that the muscles of the rotator cuff act as a single functional unit in any way, and an alternative to the force couple model is proposed.     

Static optimization underestimates antagonist muscle activity at the glenohumeral joint: A musculoskeletal modeling study

Static optimization is commonly employed in musculoskeletal modeling to estimate muscle and joint loading; however, the ability of this approach to predict antagonist muscle activity at the shoulder is poorly understood. Antagonist muscles, which contribute negatively to a net joint moment, are known to be important for maintaining glenohumeral joint stability. This study aimed to compare muscle and joint force predictions from a subject-specific neuromusculoskeletal model of the shoulder driven entirely by measured muscle electromyography (EMG) data with those from a musculoskeletal model employing static optimization. Four healthy adults performed six sub-maximal upper-limb contractions including shoulder abduction, adduction, flexion, extension, internal rotation and external rotation. EMG data were simultaneously measured from 16 shoulder muscles using surface and intramuscular electrodes, and joint motion evaluated using video motion analysis. Muscle and joint forces were calculated using both a calibrated EMG-driven neuromusculoskeletal modeling framework, and musculoskeletal model simulations that employed static optimization. The EMG-driven model predicted antagonistic muscle function for pectoralis major, latissimus dorsi and teres major during abduction and flexion; supraspinatus during adduction; middle deltoid during extension; and subscapularis, pectoralis major and latissimus dorsi during external rotation. In contrast, static optimization neural solutions showed little or no recruitment of these muscles, and preferentially activated agonistic prime movers with large moment arms. As a consequence, glenohumeral joint force calculations varied substantially between models. The findings suggest that static optimization may under-estimate the activity of muscle antagonists, and therefore, their contribution to glenohumeral joint stability.     

The effect of glenohumeral plane of elevation on supraspinatus subacromial proximity

Shoulder pain is a common clinical problem affecting most individuals in their lifetime. Despite the high prevalence of rotator cuff pathology in these individuals, the pathogenesis of rotator cuff disease remains unclear. Position and motion related mechanisms of rotator cuff disease are often proposed, but poorly understood. The purpose of this study was to determine the impact of systematically altering glenohumeral plane on subacromial proximities across arm elevation as measures of tendon compression risk. Three-dimensional models of the humerus, scapula, coracoacromial ligament, and supraspinatus were reconstructed from MRIs in 20 subjects. Glenohumeral elevation was imposed on the humeral and supraspinatus tendon models for three glenohumeral planes, which were chosen to represent flexion, scapular plane abduction, and abduction based on average values from a previous study of asymptomatic individuals. Subacromial proximity was quantified as the minimum distance between the supraspinatus tendon and coracoacromial arch (acromion and coracoacromial ligament), the surface area of the supraspinatus tendon within 2 mm proximity to the coracoacromial arch, and the volume of intersection between the supraspinatus tendon and coracoacromial arch. The lowest modeled subacromial supraspinatus compression measures occurred during flexion at lower angles of elevation. This finding was consistent across all three measures of subacromial proximity. Knowledge of this range of reduced risk may be useful to inform future studies related to patient education and ergonomic design to prevent the development of shoulder pain and dysfunction.     

Load transfer across the scapula during humeral abduction

Stress analysis in the individual parts of the scapula under normal physiological conditions is necessary to understand the load transfer mechanism, its relation with morphology of bone and to analyse the deviations in stress patterns due to implantation of the glenoid. The purpose of this study was to obtain stress distribution in the scapula during abduction of the arm and to obtain a qualitative estimate of the function of coracoacromial ligament. An accurate three-dimensional (3D) finite element (FE) model of the natural scapula has been developed for this purpose, using computed tomography data and shell-solid modelling approach. The model was experimentally validated. A musculoskeletal shoulder model of forces that calculates all muscle, ligament and joint reaction forces, in six load cases (30-180 degrees) during unloaded humeral abduction was used as applied loading conditions for the 3D FE model. High tensile and compressive stresses (15-60 MPa) were generated in the thick bony ridges of the scapula, like the scapular spine, lateral border, glenoid and acromion. High compressive stresses (45-58 MPa) were evoked in the glenoid and at the connection of glenoid-scapular spine-infraspinous fossa. The stresses in the infraspinous fossa and supraspinous fossa were low (0.05-15 MPa). These results indicated that the transfer of major muscle and joint reaction take place predominantly through the thick bony ridges, whereas the fossa area act more as attachment sites of large muscles. During humeral abduction, coracoacromial ligament was stretched, and presumably will be under tension.     

Expression of Myosin Heavy Chain Isoforms in the Supraspinatus Muscle of Different Primate Species: Implications for the Study of the Adaptation of Primate Shoulder Muscles to Different Locomotor Modes

The supraspinatus muscle is a key component of the soft tissues of the shoulder. In pronograde primates, its main function, in combination with the other rotator cuff muscles (subscapularis, infraspinatus, and teres minor), is to stabilize the glenohumeral joint, whereas in orthograde primates it functions together with the deltoid, to elevate the upper extremity in the scapular plane. To determine whether these functional differences are also reflected in the molecular biochemistry of the supraspinatus muscles involved in these different locomotor modes, we used real-time polymerase chain reaction (RT-PCR) to analyze the expression of the myosin heavy chain (MHC) isoforms in supraspinatus muscles from modern humans and 12 species of pronograde and orthograde primates. The MHC expression pattern in the supraspinatus muscle of pronograde primates was consistent with its function as a tonic and postural muscle, whereas the MHC expression pattern observed in the supraspinatus muscle of nonhuman orthograde primates was that of a muscle that emphasizes speed, strength, and less resistance to fatigue. These findings are consistent with the role of the supraspinatus in the posture and locomotor modes of these groups of nonhuman primates. The humans included in the study had an expression pattern similar to that of the nonhuman orthograde primates. In conclusion, molecular analysis of skeletal muscles via RT-PCR can contribute to a better understanding of the morphological and functional characteristics of the primate musculoskeletal system.     

Muscle compensation strategies to maintain glenohumeral joint stability with increased rotator cuff tear severity: A simulation study

Rotator cuff tear (RCT) in older adults may cause decreased muscle forces and disrupt the force balance at the glenohumeral joint, compromising joint stability. Our objective was to identify how increased RCT severity affects glenohumeral joint loading and muscle activation patterns using a computational model. Muscle volume measurements were used to scale a nominal upper limb model’s peak isometric muscle forces to represent force-generating characteristics of an average older adult male. Increased RCT severity was represented by systematically decreasing peak isometric muscle forces of supraspinatus, infraspinatus, and subscapularis. Five static postures in both scapular and frontal planes were evaluated. Results revealed that in both scapular and frontal planes, the peak glenohumeral joint contact force magnitude remained relatively consistent across increased RCT severity (average 1.5% and −4.2% change, respectively), and a relative balance of the transverse force couple is maintained even in massive RCT models. Predicted muscle activations of intact muscles, like teres minor, increased (average 5–30% and 4–17% in scapular and frontal planes, respectively) with greater RCT severity. This suggests that the system is prioritizing glenohumeral joint stability, even with severe RCT, and that unaffected muscles play a compensatory role to help stabilize the joint.     

Does supraspinatus initiate shoulder abduction?

PurposeIt is commonly stated that supraspinatus initiates abduction; however, there is no direct evidence to support this claim. Therefore, the aims of the present study were to determine whether supraspinatus initiates shoulder abduction by activating prior to movement and significantly earlier than other shoulder muscles and to determine if load or plane of movement influenced the recruitment timing of supraspinatus.MethodsElectromyographic recordings were taken from seven shoulder muscles of fourteen volunteers during shoulder abduction in the coronal and scapular planes and a plane 30° anterior to the scapular plane, at 25%, 50% and 75% of maximum load. Initial activation timing of a muscle was determined as the time at which the average activation (over a 25 ms moving window) was greater than three standard deviations above baseline measures.ResultsAll muscles tested were activated prior to movement onset. Subscapularis was activated significantly later than supraspinatus, infraspinatus, deltoid and upper trapezius, while supraspinatus, infraspinatus, upper trapezius, lower trapezius, serratus anterior and deltoid all had similar initial activation times. The effects of load or plane of movement were not significant.ConclusionsSupraspinatus is recruited prior to movement of the humerus into abduction but not earlier than many other shoulder muscles, including infraspinatus, deltoid and axioscapular muscles. The common statement that supraspinatus initiates abduction is therefore, misleading.     

3D finite element models of shoulder muscles for computing lines of actions and moment arms

Accurate representation of musculoskeletal geometry is needed to characterise the function of shoulder muscles. Previous models of shoulder muscles have represented muscle geometry as a collection of line segments, making it difficult to account for the large attachment areas, muscle–muscle interactions and complex muscle fibre trajectories typical of shoulder muscles. To better represent shoulder muscle geometry, we developed 3D finite element models of the deltoid and rotator cuff muscles and used the models to examine muscle function. Muscle fibre paths within the muscles were approximated, and moment arms were calculated for two motions: thoracohumeral abduction and internal/external rotation. We found that muscle fibre moment arms varied substantially across each muscle. For example, supraspinatus is considered a weak external rotator, but the 3D model of supraspinatus showed that the anterior fibres provide substantial internal rotation while the posterior fibres act as external rotators. Including the effects of large attachment regions and 3D mechanical interactions of muscle fibres constrains muscle motion, generates more realistic muscle paths and allows deeper analysis of shoulder muscle function.