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Skeletal Muscle
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
The cell membrane of the muscle fiber is the sarcolemma and has the ionic properties characteristic of excitable cells, manifested as a resting membrane voltage of about –90 mV and the ability to generate and propagate a muscle action potential. The sarcolemma is coated on the outside by a basement membrane formed largely of glycoproteins, and each muscle fiber is surrounded by a delicate layer of connective tissue, the endomysium (Figure 9.1). Groups of about 10 to more than 100 muscle fibers are bundled together into fascicles, the number of muscle fibers in a fascicle being larger in muscles that produce greater force, with less fineness of control. Fascicles are surrounded, in turn, by another layer of connective tissue, the perimysium. The whole muscle is ensheathed by a dense layer of irregular connective tissue, the epimysium.
Aponeurosis behaviour during muscular contraction: A narrative review
Published in European Journal of Sport Science, 2018
The aponeurosis and tendon are seamlessly connected within and outside the muscle belly through a connective tissue network, which surrounds muscle fibres (endomysium), groups of muscle fibres (perimysium) and whole muscles (epimysium) (Purslow, 2010). The means by which transverse forces are transmitted to the aponeurosis and surrounding muscles (Maas & Finni, 2018) are presumably through this connective tissue network (Street, 1983), which is stressed as muscle fibres are activated and fluid pressures within and outside the muscle’s fascial compartment increase during contraction (Aratow et al., 1993; Ateş et al., 2018; Reinhardt, Siebert, Leichsenring, Blickhan, & Böl, 2016). Longitudinal force transmission seemingly occurs through the same network and mechanism, as increasing muscle fluid volume has been shown to increase passive force by interacting with surrounding collagen fibres (Sleboda & Roberts, 2017). It has been shown that the arrangement of the collagen fibres within the endomysium and perimysium allows their geometry to vary as a function of muscle length, such that radial expansion of muscle fibres can be resisted at short lengths (Azizi, Deslauriers, Holt, & Eaton, 2017; Gindre, Takaza, Moerman, & Simms, 2013) and longitudinal stretch of the muscle fibres can be resisted at long lengths (Gindre et al., 2013; Purslow, 1989; Purslow & Trotter, 1994). Collagen fibres of the endomysium are connected to collagen fibres of the perimysium and are expected to be constituents of the tendon (Huijing, 1999), which emphasises the continuity of the connective tissue network and force transmission pathways from the muscle fibres to the tendon. Collagen fibres at the ends of muscle fibres can also transmit force to the tendon through the myotendinous junction, whereby the sarcolemma folds extensively in the direction of muscle fibres (Huijing, 1999) and collagen fibres from both the muscle fibres and aponeurosis are linked in an interdigitated fashion (Huijing, 1999; Maas & Finni, 2018).