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.     

Development of an Arthroscopic Joint Capsule Injury Model in the Canine Shoulder

Background

The natural history of rotator cuff tears can be unfavorable as patients develop fatty infiltration and muscle atrophy that is often associated with a loss of muscle strength and shoulder function. To facilitate study of possible biologic mechanisms involved in early degenerative changes to rotator cuff muscle and tendon tissues, the objective of this study was to develop a joint capsule injury model in the canine shoulder using arthroscopy.

Methods

Arthroscopic surgical methods for performing a posterior joint capsulectomy in the canine shoulder were first defined in cadavers. Subsequently, one canine subject underwent bilateral shoulder joint capsulectomy using arthroscopy, arthroscopic surveillance at 2, 4 and 8 weeks, and gross and histologic examination of the joint at 10 weeks.

Results

The canine subject was weight-bearing within eight hours after index and follow-up surgeries and had no significant soft tissue swelling of the shoulder girdle or gross lameness. Chronic synovitis and macroscopic and microscopic evidence of pathologic changes to the rotator cuff bony insertions, tendons, myotendinous junctions and muscles were observed.

Conclusions

This study demonstrates feasibility and proof-of-concept for a joint capsule injury model in the canine shoulder. Future work is needed to define the observed pathologic changes and their role in the progression of rotator cuff disease. Ultimately, better understanding of the biologic mechanisms of early progression of rotator cuff disease may lead to clinical interventions to halt or slow this process and avoid the more advanced and often irreversible conditions of large tendon tears with muscle fatty atrophy.     

Isolated ruptures of the supraspinatus muscle

We rarely encounter isolated ruptures of the supraspinatus muscle. At the Clinic of Orthopedics at the Faculty Hospital in Olomouc, we encountered only 21 cases out of 385 arthroscopic operation cases from October 1998 to October 2003. The patients were examined by USG, 5 patients were examined arthrographically and 3 patients underwent MRI examination. Of these 21 patients, only 3 were operated for acute post-injury haemarthrosis of the shoulder joint. During arthroscopic operation, an isolated rupture of the supraspinatus muscle was discovered in all these patients. The remaining 40 patients were only treated at our clinic for problems associated with impingement syndrome after an interval of 3-11 months and were indicated for operational therapy for the rupture of the supraspinatus muscle, verified sonographically and by MRI. Surgically we performed end to end sutures in 12 patients, in 9 cases we performed refixation using 1-2 titanium MITEK anchors. We supplemented the work by a detailed anatomical study of the supraspinatus muscle on 27 cadaverous anatomical preparations. It was noted that the supraspinatus muscle may be divided into three parts, with a superficial and deep layer of muscle fascicles. An aponeurotic insertion tendon runs through the center, to which part of the superficial muscle fascicles are attached. The muscle fascicles, including the central attachment tendon, run across the superior margin of the shoulder joint and by broad tendon are attached to the superior surface of the greater tubercle of the humerus. Together with the long head of the biceps muscle, they act as a significant shoulder stabiliser. The authors believe that due to the course of the muscle fascicles, this muscle acts as a significant shoulder stabiliser and a powerful abductor and elevator in the shoulder joint. The inferior portion of the muscle fascicles acts as an external rotator of the shoulder.     

Ultrastructural aspects of the skeletal striated muscle, macrophages and fibroblasts during the regression of tadpole tails

Striated muscle fibres and fibroblasts observed at electron microscope were entirely developed when the tail of tadpoles reached its maximum size. However, during resorption of the tail, striated muscle fibres showed signs of degeneration: rupture and disorganization of myofibrils, altered mitochondria and lipid droplets in the cytoplasm. A great amount of macrophages phagocyting myofibrils and fibroblasts containing collagen fibrils in several breakdown stages were also observed among degenerated muscle fibres.     

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.     

Transglutaminases expression in human supraspinatus tendon ruptures and in mouse tendons

The ethiopathogenesis of rotator cuff disease remains poorly understood. Many studies advocate the importance of extra cellular matrix for the homeostasis of connective tissue. Transglutaminase enzymes family has been studied in the context of connective tissue formation and stabilisation. Here, we investigated transglutaminases expression pattern in biopsies of normal and injured supraspinatus tendons of human shoulders and in the Achilles tendons of transglutaminase 2 knock-out and wild-type mice. Our results show that different transglutaminase family members are differentially expressed in human and mouse tendons, and that transglutaminase 2 is down-regulated at mRNA and protein levels upon human supraspinatus tendon ruptures.     

Quantifying extensibility of rotator cuff muscle with tendon rupture using shear wave elastography: A cadaveric study

Surgical repair for large rotator cuff tear remains challenging due to tear size, altered muscle mechanical properties, and poor musculotendinous extensibility. Insufficient extensibility might lead to an incomplete reconstruction; moreover, excessive stresses after repair may result in repair failure without healing. Therefore, estimates of extensibility of cuff muscles can help in pre-surgical planning to prevent unexpected scenarios during surgery. The purpose of this study was to determine if quantified mechanical properties of the supraspinatus muscle using shear wave elastography (SWE) could be used to predict the extensibility of the musculotendinous unit on cadaveric specimens. Forty-five fresh-frozen cadaveric shoulders (25 intact and 20 with rotator cuff tear) were used for the study. Passive stiffness of 4 anatomical regions in the supraspinatus muscle was first measured using SWE. After detaching the distal edge of supraspinatus muscle from other cuff muscles, the detached muscle was axially pulled with the scapula fixed. The correlation between the SWE modulus and the extensibility of the muscle under 30 and 60 N loads was assessed. There was a significant negative correlation between SWE measurements and the experimental extensibility. SWE modulus for the anterior-deep region in the supraspinatus muscle showed the strongest correlation with extensibility under 30 N (r = 0.70, P < 0.001) and 60 N (r = 0.68, P < 0.001). Quantitative SWE assessment for the supraspinatus muscle was highly correlated with extensibility of musculotendinous unit on cadaveric shoulders. This technique may be used to predict the extensibility for rotator cuff tears for pre-surgical planning.     

Does overfeeding enhance genotype effects on energy metabolism and lipid deposition in breast muscle of ducks?

We evaluated the effects of genotype (Muscovy, Pekin and their crossbreed hinny and mule ducks) and feeding levels (overfeeding between 12 and 14 weeks of age vs ad libitum feeding) on energy metabolism and lipid deposition in breast muscle of ducks. Samples of breast muscle (Pectoralis major) were collected at 14 weeks of age from 8 birds per group. Overfeeding induced an accumulation of lipids in breast muscle (1.5- to 1.7-fold, depending on genotype) mainly induced by triglyceride deposition. It also induced a considerable increase in the amounts (expressed as g/100 g of tissue) of saturated and mono-unsaturated fatty acids (SFA, MUFA), while the amounts of poly-unsaturated fatty acids (PUFA) remained unchanged in hinny and Muscovy ducks or slightly increased in Pekin and mule ducks. In breast muscle, overfeeding decreased the activity of the main enzymes involved in lipogenesis from glucose (glucose-6-phosphate dehydrogenase, G6PDH, malic enzyme, ME, acetyl CoA carboxylase, ACX). Lipoprotein lipase (LPL) activity in Pectoralis major muscle was also significantly decreased (-21%). The ability of muscle tissues to catabolize long-chain fatty acids, as assessed by beta-hydroxyacyl CoA dehydrogenase (HAD) activity, was increased in Pectoralis major muscle, as was cytochrome-c oxidase (COX) activity. Hybrid and Pekin ducks exhibited higher levels of ACX and LPL activity in Pectoralis major muscle than Muscovy ducks, suggesting a greater ability to synthesise lipids in situ, and to take up circulating lipids. Total lipid content in breast muscle of hybrid and Pekin ducks was higher than in that of Muscovy ducks. In hybrid and Pekin ducks, lipid composition of breast muscle was characterized by higher amounts of triglycerides, SFA and MUFA than in Muscovy ducks. Finally, oxidative metabolism was greater in Pectoralis major muscles of hybrid and Pekin ducks than in Muscovy ducks, suggesting an adaptative strategy of muscle energy metabolism according to lipid level.     

Role of fatty acid uptake and fatty acid β-oxidation in mediating insulin resistance in heart and skeletal muscle

Fatty acids are a major fuel source used to sustain contractile function in heart and oxidative skeletal muscle. To meet the energy demands of these muscles, the uptake and β-oxidation of fatty acids must be coordinately regulated in order to ensure an adequate, but not excessive, supply for mitochondrial β-oxidation. However, imbalance between fatty acid uptake and β-oxidation has the potential to contribute to muscle insulin resistance. The action of insulin is initiated by binding to its receptor and activation of the intrinsic protein tyrosine kinase activity of the receptor, resulting in the initiation of an intracellular signaling cascade that eventually leads to insulin-mediated alterations in a number of cellular processes, including an increase in glucose transport. Accumulation of fatty acids and lipid metabolites (such as long chain acyl CoA, diacylglycerol, triacylglycerol, and/or ceramide) can lead to alterations in this insulin signaling pathway. An imbalance between fatty acid uptake and oxidation is believed to be responsible for this lipid accumulation, and is thought to be a major cause of insulin resistance in obesity and diabetes, due to lipid accumulation and inhibition of one or more steps in the insulin-signaling cascade. As a result, decreasing muscle fatty acid uptake can improve insulin sensitivity. However, the potential role of increasing fatty acid β-oxidation in the heart or skeletal muscle in order to prevent cytoplasmic lipid accumulation and decrease insulin resistance is controversial. While increased fatty acid β-oxidation may lower cytoplasmic lipid accumulation, increasing fatty acid β-oxidation can decrease muscle glucose metabolism, and incomplete fatty acid oxidation has the potential to also contribute to insulin resistance. In this review, we discuss the proposed mechanisms by which alterations in fatty acid uptake and oxidation contribute to insulin resistance, and how targeting fatty acid uptake and oxidation is a potential therapeutic approach to treat insulin resistance.     

Histochemical investigations of the accumulation of lipids in heart papillary muscle

Synopsis The lipid composition and fatty change of the papillary muscle, arterioles and chordae tendineae of the left cardiac ventricle of sixty-two patients who had died of various diseases in hospital were analysed qualitatively by histochemical techniques and thin-layer chromatography.The histochemical techniques revealed considerable differences in the fatty change of the myocardium, arterioles and chordae tendineae. The myocardial change seems to be due solely to accumulation of triglycerides. The fatty change in the chordae tendineae appears to be the result mainly of accumulation of cholesterol esters and, to some extent, of free fatty acids. The arteriolar fatty change arises mainly from accumulation of cholesterol esters and some triglycerides. The myocardial and arteriolar fatty change is greatest at the tip of the papillary muscle.The results of thin-layer chromatography showed that the lipid composition of the papillary muscle was qualitatively highly constant. The triglycerides and free fatty acid content varied.The osmium tetroxide--naphthylamine technique, which indicates phospholipids, stained the cardiac and arteriolar muscle fibres uniformly. No focal staining, suggestive of fatty change, was noted.All three tissue sites studied reacted with Baker's acid Haematein test. Staining was focal and clearly associated with fatty change. The results observed suggested that Baker's acid Haematein test is not specific for phospholipids, but it may stain neutral fats and cholesterol esters whenever large amounts of these are present in a tissue.     

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.     

Adiponectin reduces lipid content in chicken myoblasts by activating AMPK signaling pathway

Studies in mammals have shown that adiponectin is secreted mainly by adipocytes, and it plays a crucial role in glucose and lipid metabolism in muscles. Clarifying the cross-talk role of adiponectin between adipose tissue and skeletal muscle tissue is very important for internal homeostasis. The glucose and lipid metabolism of chicken is different from that of mammals, and the role of adiponectin in chickens is unclear. Therefore, it is of great significance to study the effect and mechanism of adiponectin on lipid metabolism in chickens. In the present study, the regulating effect of adiponectin on lipid metabolism in chicken myoblasts was explored by adding a certain concentration of exogenous recombinant adiponectin. Results showed that adiponectin reduced intracellular lipid content, increasing the mRNA expression of adiponectin receptor and cellular uptake of glucose and fatty acids. In addition, adiponectin activated the 5′ adenosine monophosphate activated protein kinase (AMPK) signaling pathway. The above results suggested that adiponectin reduced intracellular lipid content, mainly by binding to adiponectin receptor, activating AMPK pathway, increasing cellular uptake of glucose and fatty acids and promoting lipid oxidation.     

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.     

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.     

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.     

Morphology of the melon and its tendinous connections to the facial muscles in bottlenose dolphins (Tursiops truncatus)

The melon is a lipid‐rich structure located in the forehead of odontocetes that functions to propagate echolocation sounds into the surrounding aquatic environment. To date, the melon's ability to guide and impedance match biosonar sounds to seawater has been attributed to its unique fatty acid composition. However, the melon is also acted upon by complex facial muscles derived from the m. maxillonasolabialis. The goal of this study was to investigate the gross morphology of the melon in bottlenose dolphins (Tursiops truncatus) and to describe how it is tendinously connected to these facial muscles. Standard gross dissection (N = 8 specimens) and serial sectioning (N = 3 specimens) techniques were used to describe the melon and to identify its connections to the surrounding muscles and blubber in three orthogonal body planes. The dolphin forehead was also thin‐sectioned in three body planes (N = 3 specimens), and polarized light was used to reveal the birefringent collagen fibers within and surrounding the melon. This study identified distinct regions of the melon that vary in shape and display locally specific muscle‐tendon morphologies. These regions include the bilaterally symmetric main body and cone and the asymmetric right and left caudal melon. This study is the first to identify that each caudal melon terminates in a lipid cup that envelopes the echolocation sound generators. Facial muscles of the melon have highly organized tendon populations that traverse the melon and insert into either the surrounding blubber, the connective tissue matrix of the nasal plug, or the connective tissue sheath surrounding the sound generators. The facial muscles and tendons also lie within multiple orthogonal body planes, which suggest that the melon is capable of complex shape change. The results of this study suggest that these muscles could function to change the frequency, beam width, and directionality of the emitted sound beam in bottlenose dolphins. The echolocation sound propagation pathway within the dolphin forehead appears to be a tunable system. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Sub-regional activation of supraspinatus and infraspinatus muscles during activities of daily living is task dependent

The supraspinatus and infraspinatus muscles each have multiple sub-regions that may activate differentially in activities of daily living. Awareness of these differential demands critically informs rehabilitation of rotator cuff muscle following injury, particularly if centered on recovering and strengthening the rotator cuff to perform daily tasks. This study quantified muscle activation of supraspinatus and infraspinatus sub-regions during the performance of six activities of daily living. Twenty-three participants (mean: 22.6 ± 2.6 years) completed the following tasks: opening a jar, reaching at shoulder height, overhead reaching, pouring water from a pitcher, eating with a spoon, and combing hair. Indwelling electromyography was collected from the anterior and posterior supraspinatus and superior, middle, and inferior infraspinatus. Tasks requiring high arm elevations (e.g. reaching at shoulder and overhead height) activated anterior supraspinatus between 21 and 28% MVC. The posterior supraspinatus consistently activated between 10 and 30% MVC across all tasks. All sub-regions of infraspinatus activated highly (between 18 and 25% MVC) in tasks requiring high arm elevations in flexion. These findings may be leveraged to define effective measures to increase rotator cuff function in daily tasks.     

Comparison of the phospholipid and triacylglycerol fatty acid profile of rat serum, skeletal muscle and heart

Although several studies have analyzed the fatty acid profile of phospholipids (PL) and, to a lesser degree, triacylglycerols (TG) in one or more tissues concurrently, a systematic comparison of the fatty acid composition of different tissues and/or lipid classes is lacking. The purpose of the present study was to compare the fatty acid composition of major lipid classes (PL and TG) in the rat serum, soleus muscle, extensor digitorum longus muscle and the heart. Lipids were extracted from these tissues and analyzed by a combination of thin-layer chromatography and gas chromatography. We found many significant differences in various tissues and lipid classes. Serum had the most distinct fatty acid profile in PL but this "uniqueness" was less apparent in TG, where differences among tissues were in general less frequent than in PL. These two skeletal muscles exhibited similar fatty acid composition in both lipid classes despite their different muscle fiber type composition, denoting that fiber type is not a major determinant of the fatty acid composition of rat skeletal muscle. The fatty acid profile of heart PL was the most different from that of the other tissues examined. PL were rich in polyunsaturated fatty acids, whereas TG were rich in monounsaturated fatty acids. Although the reasons for the differences in fatty acid profile among the tissues examined are largely unknown, it is likely that these differences have an impact upon numerous biological functions.     

Filamin C plays an essential role in the maintenance of the structural integrity of cardiac and skeletal muscles, revealed by the medaka mutant zacro

Filamin C is an actin-crosslinking protein that is specifically expressed in cardiac and skeletal muscles. Although mutations in the filamin C gene cause human myopathy with cardiac involvement, the function of filamin C in vivo is not yet fully understood. Here we report a medaka mutant, zacro (zac), that displayed an enlarged heart, caused by rupture of the myocardiac wall, and progressive skeletal muscle degeneration in late embryonic stages. We identified zac to be a homozygous nonsense mutation in the filamin C (flnc) gene. The medaka filamin C protein was found to be localized at myotendinous junctions, sarcolemma, and Z-disks in skeletal muscle, and at intercalated disks in the heart. zac embryos showed prominent myofibrillar degeneration at myotendinous junctions, detachment of myofibrils from sarcolemma and intercalated disks, and focal Z-disk destruction. Importantly, the expression of γ-actin, which we observed to have a strong subcellular localization at myotendinous junctions, was specifically reduced in zac mutant myotomes. Inhibition of muscle contraction by anesthesia alleviated muscle degeneration in the zac mutant. These results suggest that filamin C plays an indispensable role in the maintenance of the structural integrity of cardiac and skeletal muscles for support against mechanical stress.