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If this sounds interesting, then you can learn more about this in the recommended further reading. Type Ic, IIc, Iac, IIab fibers have all been identified, consequently meaning that there is a total of 7 different types of muscle fiber found in human skeletal muscle. And because of this feature we now recognise that a number of hybrid fiber types also exist. However, muscle fibers can actually contain multiple myosin heavy chain isoforms. We can classify muscle fibers due to the presence of myosin heavy chain (MHC) isoforms, the three original isoforms identified are MHC I, IIa and IIb. It is these myofilaments that slide across one another to allow the muscle to contract and generate force. Myofibrils contain functional units called sarcomeres which are defined by Z-discs and contain thick (myosin) and thin (actin) myofilaments which gives muscle its striated appearance. Each fiber is surrounded by a plasma membrane called the sarcolemma, and inside this sits the sarcoplasm, a gelatin-like substance which separates myofibrils and helps maintain the shape of the cell. These fibers are singular protein dense cells which contain many nuclei, mitochondria and endoplasmic reticulum. Skeletal muscle is therefore made up of hundreds, if not thousands of muscle fibers. Within each muscle fiber we have a number of myofibrils which are aligned in parallel, each individual fiber is surrounded by endomysium – the smallest of the connective tissue. Within this is a number of muscle fibers which are bundled together to form a fascicle, which are held in place by the perimysium. Each muscle is wrapped in a thick connective tissue called the epimysium. To help understand the role of muscle fibers, it is useful to be aware of the structure of skeletal muscle itself. 8 Further Reading: What are muscle fibers?.7 Why understanding fiber type is important for coaches, athletes and practitioners.
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5 What affects muscle fiber composition.4 Differences between muscle fiber types:.