The opposing muscle on the other side of the bone that straightens the joint when it contracts is called an extensor e. Skeletal muscles are attached to the bones with tendons which are very tough tissue that do not stretch much.
An example of this is the Achilles tendon, which you can feel behind your ankle. Support and movement systems: Muscles Voluntary and involuntary muscles Types of muscles Muscle action. Bending of an arm. Used with Permission. I don't think about walking. Why is this action a voluntary action?
I need to do arm curls to exercise my biceps to make them big and strong, do I have to exercise my heart too? Can we control our involuntary muscles? Related Topics:. There they help to push the baby out of the mother's body when it's time to be born. You'll find smooth muscles at work behind the scenes in your eyes, too. These muscles keep the eyes focused. The muscle that makes up the heart is called cardiac muscle. It is also known as the myocardium say: my-uh-KAR-dee-um.
The thick muscles of the heart contract to pump blood out and then relax to let blood back in after it has circulated through the body. Just like smooth muscle, cardiac muscle works all by itself with no help from you. A special group of cells within the heart are known as the pacemaker of the heart because it controls the heartbeat. Now, let's talk about the kind of muscle you think of when we say "muscle" — the ones that show how strong you are and let you boot a soccer ball into the goal.
These are your skeletal muscles — sometimes called striated say: STRY-ay-tud muscle because the light and dark parts of the muscle fibers make them look striped striated is a fancy word meaning striped. Skeletal muscles are voluntary muscles, which means you can control what they do. Your leg won't bend to kick the soccer ball unless you want it to. These muscles help to make up the musculoskeletal say: mus-kyuh-low-SKEL-uh-tul system — the combination of your muscles and your skeleton, or bones.
Together, the skeletal muscles work with your bones to give your body power and strength. In most cases, a skeletal muscle is attached to one end of a bone. It stretches all the way across a joint the place where two bones meet and then attaches again to another bone.
Skeletal muscles are held to the bones with the help of tendons say: TEN-dunz. Tendons are cords made of tough tissue, and they work as special connector pieces between bone and muscle. The tendons are attached so well that when you contract one of your muscles, the tendon and bone move along with it.
Skeletal muscles come in many different sizes and shapes to allow them to do many types of jobs. Some of the biggest and most powerful muscles are your calf and thigh muscles.
They give your body the power it needs to lift and push things. Muscles in your neck and the top part of your back aren't as large, but they are capable of some pretty amazing things: Try rotating your head around, back and forth, and up and down to feel the power of the muscles in your neck. These muscles also hold your head high. As post-mitotic tissues, skeletal muscles depend on satellite cells to adapt and regenerate throughout life.
These stem cells reside in specialized niches between the sarcolemma of muscle fibers and their encapsulating basement membranes. Satellite cell maintenance, activation and differentiation are governed by complex cascades of transcription factors. For an extensive review of these cellular circuitries, readers are referred to the recent review by Almada and Wagers Of particular relevance to this manuscript, a growing body of evidence suggests that satellite cell fate is also strongly influenced by the interactions with the ECM niche in which they reside.
Indeed, as a dynamic environment, the stem cell niche transmits mechanical and chemical signals that act to protect quiescent stem cells or induce activation, proliferation, and differentiation. In the quiescent state, satellite cells express the canonical cell regulator paired box protein 7 PAX7 Olguin and Olwin, In vitro studies have demonstrated that a greater portion of satellite cells express PAX7 when cultured on matrigel, a mixture of ECM proteins and growth factors Wilschut et al.
Further support for the notion that the ECM is actively involved in the maintenance of satellite cell quiescence comes from reports that satellite cells removed from their niche quickly enter the cell cycle and lose their capacity for myogenic differentiation Gilbert et al. Intriguingly, satellite cells appear to also be able to sense and respond to different ECM mechanical properties. In fact, PAX7 expression and satellite cell survival are greater when cultured on hydrogels that mimic the physiological stiffness of muscle Gilbert et al.
Also, satellite cells cultured on soft hydrogel feature greater functional capacity after transplantation into recipient muscle Cosgrove et al. In addition, ECM components have been shown to influence stem cell division. Specifically, the proteins fibronectin Bentzinger et al.
Upon muscle trauma or in response to increased loading, the usually mostly quiescent satellite cells become activated and differentiate into myoblasts to finally fuse into mature myofibers. While this process requires the timely expression of various transcription factors, such as myogenic factor 5, myogenic determination protein or myogenin Almada and Wagers, , several studies point to the influence of the ECM on each of these steps.
Experiments with mouse Grefte et al. The contributions of single proteins are still poorly understood, however, the concomitant presence of poly- D -lysine and laminin Boonen et al. In mice, it has been shown that muscle satellite cells produce ECM collagens to maintain quiescence in a cell-autonomous manner with collagen V being a critical component of the quiescent niche, as depletion leads to anomalous cell cycle entry and gradual diminution of the stem cell pool Baghdadi et al.
Just as for the maintenance of quiescence, adequate mechanical properties of the ECM niche may also be important for satellite cell maturation. Indeed, myotubes have been found to differentiate optimally on substrates with muscle-like stiffness Engler et al. At older age, skeletal muscles typically demonstrate fibrotic morphology Lieber and Ward, As opposed to fascial densification, where the general structure of collagens may be preserved Pavan et al. Also, absolute collagen content and non-enzymatic cross-linking of collagen fibers may be increased Haus et al.
However, this increase is associated with a shift toward a higher ratio of type I to type III collagen Hindle et al. Furthermore, collagen type IV concentration is enhanced in the basal lamina of slow twitch muscles, whereas laminin concentration seems to decrease with age Kovanen et al. The increased deposition of basal lamina proteins has also been shown to expel satellite cells from their niches, which affects the regulation of satellite cell divisions Snow, and may explain the lower numbers of satellite cells typically counted in old as compared to young muscle Brack et al.
A review including an extensive summary of the effects of aging on skeletal muscle ECM has recently been published by Etienne et al. These data support the hypothesis that age-associated changes in the ECM might be driven by a decreased degradation capacity rather than by increased synthesis of collagenous structures.
This is further supported by findings that suggest a diminished resistance exercise-induced remodeling capacity of ECM structures in aged muscles Wessner et al. While the mechanisms are not yet fully understood, these changes are also believed to directly impair muscle function by hindering fiber contractility Azizi et al. It is well known that skeletal muscle plays an important role for the insulin-stimulated uptake of glucose Richter and Hargreaves, The role of the ECM in this context might be less clear.
Increased amounts of type I and III collagen were found in both type 2 diabetic and also non-diabetic obese subjects Berria et al. Whether this might also be true in the context of diabetes has been investigated in an animal study. Interestingly, the genetic depletion of MMP9 did not induce insulin resistance in lean mice despite resulting in an increase of collagen IV. However, when mice were fed a high-fat diet the deletion caused a profound state of insulin resistance.
These results further strengthen the role of IMCT components in the progress of muscle insulin resistance, especially in a state of overfeeding Kang et al.
Finally, hyaluronan, a major constituent of the ECM is increased in high-fat diet-induced obesity in mice. Treatments with PEGPH20, which dose-dependently reduces hyaluronan in muscle ECM is suggested for the treatment of insulin-resistance with a concomitant decrease in fat mass, adipocyte size, as well as hepatic and muscle insulin resistance Kang et al.
To summarize, various components of the ECM have been shown to be affected in various stages of diabetes. Studies on whether diabetes is linked to muscle weakness are controversial Leong et al. The first evidence to indicate the malleability of IMCT in response to physical activity was published as early as in the s, when Suominen and Heikkinen and Suominen et al.
The effect of endurance exercise on the pro-collagenous enzymatic activity was later found to be more prominent in red as compared to white muscle Takala et al.
Direct measurements of collagen content first performed in the late s confirmed that the type IV collagen content increased in the fatigue-resistant soleus muscle of rats following lifelong endurance training Kovanen et al.
The exercise-induced increase in collagen notwithstanding, Gosselin et al. The effects of immobilization on the skeletal muscle ECM are not entirely unequivocal. Early studies by Karpakka et al. Changes in collagen content in response to short-term immobilization or disuse were later found to be rather small Savolainen et al. A more recent study, by contrast, found the content of collagen I and the biomechanical properties elastic modulus, max stress and yield stress of crural fascia ensheathing the rat triceps surae muscle to be significantly increased after as little as 21 days of hindlimb unloading Huang et al.
In non-exercising humans, immunohistochemical staining suggested no changes in the density of the collagen I network after 60 days of bed rest. In subjects performing a countermeasure exercise protocol consisting of reactive jumps on a sledge system, by contrast, collagen I immunoreactivity was reduced as compared to baseline levels Schoenrock et al. In one of the first respective studies, Williams and Goldspink severed the tendons of the plantaris and gastrocnemius muscles of male rats to overload the soleus muscles.
Histological analyses further suggested that the increase in IMCT was mostly due to a thickening of the endomysium. Focusing on the myotendinous junction, Zamora and Marini performed similar experiments and isolated the rat plantaris muscle through tenotomy of the soleus and ablation of the gastrocnemius muscles.
In comparison with control animals, the fibroblasts located at the myotendinous junction developed a higher degree of activation of cytoplasm, nucleus and nucleolus after as little as one to two weeks of functional overload.
While the gains in myofiber cross-sectional area were similar after 21 days of functional overload, the increases in muscle wet weight were significantly larger in ILknockout mice. Histological analyses confirmed that this surplus gain in muscle weight could be explained by significantly larger increases in non-contractile tissue content and hydroxyproline concentration, which is indicative of collagen content and fibrosis.
Conversely, mRNA expression of MyoD, a transcription factor required for myo- rather than fibrogenic differentiation of satellite cells Zammit, , was significantly attenuated in animals lacking IL Jointly, these results indicate that synergist elimination induces an increase in IMCT content and, specifically, a thickening of endomysial structures in overloaded muscles.
IGF-1 appears to play an important role in the regulation of this process, as lack of IGF-1 has been shown to lead to excessive accumulation of IMCT and, potentially, impaired muscle regenerative potential. One of the first studies to test and compare different forms of resistance-like exercise in men was performed by Brown et al.
These results were confirmed in two later studies similarly using high-intensity eccentric exercise that found both increased procollagen processing and type IV collagen content as well as higher MMP and TIMP activities Crameri et al.
Interestingly, Crameri et al. The transient upregulation of tenascin C and other ECM glycoproteins e. These findings suggest that an acute bout of resistance exercise triggers a catabolic response in young muscle but that this effect may be impaired at older age. The subsequent anabolic reaction, characterized by the upregulation of structural collagens I, III, IV and laminin, has been found to occur with a significant delay, thus suggesting that muscle repair consequent to an acute bout of damaging lengthening contractions follows a biphasic nature Mackey et al.
Interestingly, a recent study by Sorensen et al. This observation supports the notion that dysregulated ECM cues may be responsible for the increased ECM deposition and reduced stem cell activity typically seen in older muscle Grounds, One of the first studies to directly compare different forms of muscular contraction in terms of their acute ECM remodeling potential was published by Heinemeier et al.
These authors performed a study in rodents and found that the activity of genes associated with collagen biosynthesis e. In humans, collagen protein fractional synthesis rates have also been proposed to be more increased following an acute bout of eccentric as compared to concentric training Holm et al.
In fact, diminished MMP activity after prolonged training consisting of electrically evoked isometric contractions in rats may reflect successful ECM reinforcement Ogasawara et al. In addition to contraction mode, skeletal muscle ECM may also be sensitive to exercise intensity. Carmeli et al. In humans, by contrast, one study by Holm et al. In this study, collagen fractional synthesis rates were evenly increased following both interventions.
In terms of ECM adaptations to prolonged resistance training, only data from animal studies exist. To summarize, several studies investigating the acute effects of physical activity in both rodents and men have indicated that exercise may stimulate both the degradation and synthesis of collagen in skeletal muscle. The repair of exercise-induced microtrauma follows a biphasic pattern, in which glycoproteins first create a transitional matrix to guide catabolic processes, and anabolic processes to reinforce the IMCT structure occur with a significant delay.
The potential of exercise to induce ECM remodeling seems to be dependent on contraction mode with eccentric contractions triggering a greater response than either concentric or isometric muscle action. Few studies testing the results of different exercise intensities are available, with so-far results suggesting that protein breakdown but not synthesis may be provoked more strongly by higher intensities. Disuse acutely downregulates the activity of enzymes related to the biosynthesis of collagens, although at the protein-level changes occur at a slow rate.
Cross-sectional comparisons involving mostly endurance- trained rodents suggest that chronic physical activity may result in a reinforced IMCT phenotype. The only long-term longitudinal training studies available to date have been performed in rodents and suggest that prolonged resistance training may be useful in countering excessive IMCT accumulation at older age.
The physiological and functional consequences of training-induced IMCT remodeling require further investigation. The present review aimed to provide an overview over the current state of knowledge concerning the skeletal muscle ECM, which plays an essential, albeit frequently underestimated role in the maintenance of muscle homeostasis, influences muscle function and adaptation and may be a key for the treatment of muscular and metabolic disorders consequent to aging or disease.
As a complex meshwork of various collagens, glycoproteins, proteoglycans and elastin, the ECM embeds contractile muscle fibers and serves via integrins and the dystrophin-associated glycoprotein complex, respectively, as biochemical and mechanical interface between muscle cells and their surroundings.
Functionally, the ECM serves as medium for the transmission of contractile force, which may not only serve to increase the efficiency of muscular contraction but also to protect muscle fibers from excessive stress and facilitate healing of microtrauma. Specific ECM components, such as fibronectin, collagen VI and different proteoglycans, may additionally promote stem cell division.
Conversely, laminin, glycosaminoglycans and other proteoglycans have been shown to promote satellite cell differentiation and their fusion into mature myofibers.
Scientific evidence further demonstrates that the ECM of skeletal muscles is a malleable tissue that may undergo remodeling processes consequent to aging, disease, physical training or disuse. Specifically, aging typically leads to overall increased deposition of collagenous tissue, changes in collagen composition shift toward higher type I to type III collagen and increased non-enzymatic collagen cross-linking through advanced glycation end products.
Extracellular matrix remodeling may also be associated with metabolic disorders, such as diabetes. In turn, such remodeling may impair integrin signaling, thus reducing insulin sensitivity.
Further ECM components potentially representing targets for insulin resistance are hyaluronan, the dystrophin-dystroglycan complex as well as MMP9. Finally, ECM remodeling may be triggered by physical exercise. Cross-sectional studies in humans and longitudinal studies in rodents further suggest that such increased collagen turnover may lead to reinforced collagenous structures in chronically trained subjects and prevent excessive collagen deposition i.
Studies investigating the consequences of prolonged disuse have shown controversial results. While early studies reported decreased hydroxylase activity and hydroxyproline content after short-term immobilization, more recent works found increased collagen I content after 21 days of hindlimb unloading in rats but no change after 60 days of bed rest in humans.
Further research and particularly human training studies are required to investigate the influence of different training modalities on ECM structure and composition. RC contributed to the literature research and drafted the manuscript. MG and BW contributed to the literature research and revised the manuscript. All authors have approved the final version of the manuscript and agreed to be accountable for all aspects of the work.
All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Aagaard, P. A mechanism for increased contractile strength of human pennate muscle in response to strength training: changes in muscle architecture. Adams, G. Skeletal muscle myosin heavy chain composition and resistance training.
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