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Skeletal muscles, skeletal striated muscles or plainly muscles are integral to the locomotor system responsible for movements. The human body's musculoskeletal system has more than 600 muscles 1 making up around 40% of the body weight. They are heterogeneous and have different architectures and other features, some of which are reflected in their names.
Skeletal muscles can be actively or voluntarily controlled by the central nervous system.
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Skeletal muscles comprise hundreds to thousands of muscle fibers grouped in muscle bundles or fascicles 2,3.
Each muscle unit (fibers, fascicles, and whole muscles) is surrounded by an extracellular matrix layer consisting of connective tissue of different strengths 2,3:
endomysium: surrounds each muscle fiber
perimysium: surrounds the muscle fasciculi
epimysium: covers the whole muscle
The different connective tissue layers provide continuity with tendons and aponeuroses, support the individual muscle fibers and fascicles to resist contractile forces, and provide passageways for nerves and blood vessels 1.
Skeletal muscles function is related to their contractibility and includes various tasks such as the following 4:
continuous low-intensity activity (e.g. posture, joint stability, heat production)
repeated submaximal contractions (e.g. respiration, eye movements, locomotion, writing)
fast maximal contractions (e.g. sprinting, jumping, kicking, throwing)
storage source for amino acids
Muscle power depends on different factors related to the muscle fibers as composition, fiber length and size as well as factors related to the whole muscle such as muscle length, size lever and muscular architecture 5.
Muscle fibers can be classified into the following types 1,2:
type 1 (slow-twitch): peak tension in approximately 110 ms; slow oxidative
type 2 (fast-twitch): peak tension in approximately 50 ms
type 2a: fast oxidative/glycolytic
type 2x: glycolytic
type 2c: rare (only in 1-2% of the population)
With regard to their architecture skeletal muscles can be subdivided into the following subtypes 5:
longitudinal: parallel attachment of muscle fibers
fusiform (e.g. biceps brachii muscle)
strap (e.g. rectus abdominis, pronator quadratus)
pennate muscles: oblique attachment of muscle fibers to the tendon, allowing a higher force production
unipennate (e.g. soleus muscle, vastus lateralis)
bipennate (e.g. rectus femoris muscle)
multipennate (e.g. deltoid muscle, gluteus medius)
convergent (e.g. pectoralis major, temporalis muscle)
circular/concentric (e.g. orbicularis oculi)
Skeletal muscles are usually attached to bones via tendons or aponeuroses. The connection between muscle bellies and tendons is formed by the myotendinous junction 4.
The main arteries run parallel to the longitudinal axis of the muscular fascicles and give rise to feeding arteries that branch into different arterioles and eventually terminal arterioles that supply arterial blood to capillaries, which run again parallel to the longitudinal axis of the muscle fibers within the endomysium.
The main draining veins of skeletal muscles run parallel to the longitudinal axis of the muscular fascicles, usually close to the - often similarly named - arteries.
The lymphatic vessels within the skeletal muscle do not have any contractile properties and thus depend on the inherent contractility of the skeletal muscle and arteriolar pulsations. Lymphatic drainage originates from lymph capillaries within the endomysium merging into smaller lymphatic vessels that pass through the perimysium connecting to larger vessels.
The muscle fibers are the basic cellular units of skeletal muscles and every muscle fiber consists of a single multinucleated muscle cell or myocyte 1-3. They have a size of 10-120 µm and are covered by a plasma membrane, the plasmalemma 1.
Myocytes consist of myofibrils composed of sarcomeres, that feature the basic contractile properties of muscle and are the simplest functional unit of the myofibrils 1. Microscopically the arrangement of sarcomeres creates a striated or striped appearance, which can be seen in skeletal and cardiac muscles and contribute to their classification 1. Sarcomeres consist of thick and thin filaments composed of different proteins, namely myosin (thick filaments), actin, troponin and tropomyosin (thin filaments) and are arranged parallel to the muscle fiber axis 1-3.
In addition to myocytes, skeletal muscles contain satellite cells, a population of adult stem cells, which are found within the basal lamina and the plasmalemma or sarcolemma and play an important role in muscular growth and repair due to their ability to differentiate into new myocytes 1-3.
On plain radiographs muscles are characterized by a non-specific soft tissue density, generally indistinguishable from adjacent soft tissues, unless separated from them by fat.
On ultrasound, skeletal muscles usually display a hypoechoic appearance with hyperechoic structures reflecting the interspersed perimysium. On longitudinal view, some of the muscles might demonstrate a pennate pattern and on a transverse or short-axis view, the appearance has been described as a ‘starry night’ pattern 8.
On CT, skeletal muscles are characterized by soft tissue density with a fibrillar feathery structure.
On high-resolution MRI, muscles have a feathery structure with delicate interspersed connective tissue structures.
On MRI normal skeletal muscle is considered as a reference tissue for the signal characteristics of other tissues and organs and normal muscles, therefore, have an isointense signal. The signal characteristics of muscle compared to fat, fluid, and tendons are listed below:
T1: hypointense (fat), isointense (fluid), hyperintense (tendon)
T2: hypointense (fat), hypointense (fluid), mildly hyperintense (tendon)
PD: hypointense (fat), hypointense (fluid), hyperintense (tendon)
STIR/PDFS/T2FS: hyperintense (fat), hypointense (fluid), hyperintense (tendon)
DWI/ADC: no diffusion restriction
T1 C+(Gd): hypointense (fat), isointense (fluid), hyperintense (tendon)
Myofibrils are formed by the fusion of multiple embryonic cells 1.
Pathologies associated with skeletal muscles include the following 1,3:
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