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Skeletal Muscle
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
Skeletal muscle architecture refers to the arrangement of fibers in a muscle and has an important bearing on muscle function. Whereas the muscle fibers in fascicles (Figure 9.1) are parallel to one another, the fascicles themselves can have different orientations relative to one another and to tendons, which lie on the line of action of the muscles. There are basically four different muscle architectures: Parallel muscles, in which the fascicles run parallel to the line of action of the muscle and generally extend from one end of the muscle to the other (see Section 10.4). A good example is the sartorius muscle, a ribbon-shaped muscle, of about 40 cm length in humans, that runs obliquely along the thigh and is involved in flexion of the knee as well as flexion, abduction, and lateral rotation of the hip. Parallel muscles could also be spindle shaped, or fusiform, as in the biceps brachii muscle of the upper arm.Convergent muscles, in which the fascicles extend over a fairly wide area at one end of the muscle and converge to a common attachment site at the other end, as in the pectoralis muscles of the upper chest. The distinguishing feature of a convergent muscle is that it can pull in different directions, depending on the parts of the muscle that are activated.Pennate muscles, in which the fascicles are oblique to the force-generating axis of the muscle as a whole. In unipennate muscles, the muscle fibers are oriented at a single angle relative to the force-generating axis, this angle being generally between 0°–30°. An example is the extensor digitorum longus muscle of the leg, which extends the small toes and dorsiflexes the foot. Typically, the fascicles extend from an aponeurosis on one side to a tendon on the other side, as illustrated diagrammatically in Figure 9.9a. In bipennate muscle, the fascicles converge toward a central tendon from both sides (Figure 9.9b), as in the rectus femoris, a large muscle in the quadriceps group of muscles that extend the knee. In multipennate muscle, the fascicles are oriented at several angles relative to the axis of force generation (Figure 9.9c), as in the deltoid muscle that controls shoulder movement. The important characteristics of pennation are discussed in Section 10.4.Sphincter muscles, which are circular muscles that surround an opening or recess and perform some controlling function upon contracting. An example is the orbicularis oculi muscle around the eye, which closes the eyelids and whose contraction can be involuntary, as in sleeping and blinking.
Aponeurosis behaviour during muscular contraction: A narrative review
Published in European Journal of Sport Science, 2018
Since the magnitude of transverse aponeurosis strains might modulate the longitudinal stiffness of animal aponeurosis (Azizi & Roberts, 2009), it is possible that variable aponeurosis widths at the same muscle force, but at different muscle lengths, might result in differing longitudinal aponeurosis strains. This would alter the amount of energy that is stored and returned for a given muscle force and the corresponding muscle fascicle behaviour. There is some indirect evidence from the rat extensor digitorum longus muscle that aponeurosis strain depends on both the active state of the muscle and muscle length (Ettema & Huijing, 1989); however, it was not clear from this study if aponeurosis stiffness was influenced by muscle length. An elegant follow-up study by Scott and Loeb (1995) showed that the apparent proximal aponeurosis stiffness of the cat soleus muscle consistently increased with muscle length at low force levels, but the increase in aponeurosis stiffness became less dependent on muscle length as force increased. The authors suggested that the decrease in the apparent aponeurosis stiffness during contraction at short muscle lengths was because of increased widths of the longitudinal collagen bands, which perhaps increased the crimp of collagen compared with longer muscle lengths (Scott & Loeb, 1995). However, only the direction, and not the magnitude, of transverse aponeurosis strains was assessed in this study, due to limitations with the measurement technique.