Muscles

Muscle is made of bundles of contractile elementary units - the striated muscle fibers - with their major axis lying along the contraction direction. The muscular fibers are multinuclear cellular units derived, during embryonal development, from mesodermal cells of the primitive segments. The fibers have a cylindrical or polyhedral shape with smoothed angles; they have a considerable length, varying from a few millimeters to several centimeters, and a width between 10 and 100 mm. Considerable differences between different muscle fibers can be observed and, even within the same muscle, the fibers' diameter can vary according to work, nutritional conditions and other causes.

Muscular fibers are arranged parallel to one another and they are supported by a structure of connective tissue. Muscle is externally surrounded by a thick connective sheath called the epimy-sium; from the internal aspect of this sheath several septa depart to constitute the perimysium, which surrounds diverse bundles of muscular fibers, named fascicles. Blood vessels and nerves run within the perimysium, which also contains the neuromuscular spindles.Very light and thin septa arising from the perymysium spread into the fascicles to surround every single muscular fiber and thus form the endomysium. The endomysium, a network made of reticular fibers, blood capillaries, a few connective cells together with some small nervous bundles, constitutes the framework found right around the striated muscle fibers, and it represents the site of metabolic exchange between striated muscle fibers and blood (Fig. 3.19) [14,23,33].

The epimysial, perimysial and endomysial coverings come together where muscles connect to adjacent structures: the extremity of the muscle may continue as a tendon or insert onto the periosteum, aponeurosis or the dermis; this structure is extremely resistant, since the tensile forces turn into tangential forces that are more easily born. At a submicroscopic level, the muscular fibers end in a conical shape and adapt to the connective tissue just like fingers adapt to a glove; at the two endpoints of the muscular fiber, the myofibrils are attached to the sarcolemma. By means of these devices, the muscular fibers are strongly connected to the terminal insertion and the force developed by contraction does not lose any efficiency at the passage from muscle to tendon and

Anatomical drawing of a muscle with magnification of a fascicle showing its endomysial framework

there is no risk of detachment. From a clinical point of view, such detachments occur only in rare situations, for it is much easier for a tendon to detach from a bone fragment at its insertion, in the case of an exceptionally strong contraction.

The macroscopic shape of muscles varies according to their function. Each muscle presents at least one muscular belly and two tendons, one at the origin and the other at the insertion. In some cases, like the rectus abdominis muscle, the muscle consists of different bellies united together with fibrous insertions. Another possible structure is observed for example in the biceps, triceps and quadriceps muscles, consisting of multiple origins and insertions on a single muscular belly.

The most frequent setting is the semipennate type for both extremities of the muscle. In this case, tendons are wide, flattened and oriented in the opposite direction, originating respectively from one side and the other of the muscle. The muscular bundles are directed from tendon to tendon and the shorter and more numerous they are, the wider the tendon insertion is. This setting influences the biomechanics, because the degree of muscular shortening depends on the fibers' length and the energy of contraction depends on the number of fibers constituting the muscle; two muscles with the same length, width and thickness, and therefore the same volume, but with different number and length of fibers, will also have different shortening capability and contraction energy. Therefore, when assessing the biomechanical characteristics of a muscle, not only the volume should be taken into account, but also the type of insertion, whose width influences the number and length of the fibers. The internal structure of skeletal muscle varies according to its specific function. Muscles with fibers parallel to the longitudinal axis (muscles of the abdomen, head and neck) are made for bearing reasonable weights for long distance activities. On the other hand, the uni-, bi- and circum-pennate settings (muscles of the limbs) can bear greater weights for a shorter period of time.

The internal structure of muscles can be easily assessed by ultrasound imaging. The external connective sheath of the muscle (epimysium) appears as a hyperechoic external band measuring a maximum of 2-3 mm of thickness and, on longitudinal US sections, continues without interruption along the corresponding tendon profile (Fig. 3.20).

The fibro-adipose septa (perimysium) are seen as hyperechoic lines separating the contiguous hypoechoic muscular bundles (fascicles) from one another (Fig. 3.21).

The typical pennate structure of muscles can be easily assessed in longitudinal axis views (Fig. 3.22 a), where the hyperechoic fibro-adipose septa converge, with a mainly parallel course, on a cen tral aponeurosis, appearing as a thin, highly reflective band [32,34].

Ultrasound evaluation of the direction of the muscle fibers represents an important parameter for the measurement of the pennation angle; this angle is measured between the muscular fibers' direction and the central aponeurosis axis (usually corresponding to the longitudinal muscular axis). The value of the angle varies depending on the function of the muscle and, within the same muscle, on the functional state (contraction/relaxation). In transverse views, the muscle is sectioned according to a plane that is orthogonal to the muscular longitudinal axis, with a typical US structure appearance; the 1st and 2nd order fascicles show an irregu

Fig.3.20

EFV scan of the sural triceps. MG = medial gastrocnemius; S = soleus; E = epimysium.The hyperechoic appearance of the epimysium wrapping the muscle bellies and continuing to the aponeurosis is shown

Fig.3.20

EFV scan of the sural triceps. MG = medial gastrocnemius; S = soleus; E = epimysium.The hyperechoic appearance of the epimysium wrapping the muscle bellies and continuing to the aponeurosis is shown

Fig.3.21

'In vitro' scan of bovine muscle. The hyperechoic appearance of perimysial septa (P) wrapping the muscle bellies (MB) is shown

Fig.3.21

'In vitro' scan of bovine muscle. The hyperechoic appearance of perimysial septa (P) wrapping the muscle bellies (MB) is shown lar polygonal shape, defined by thin, elongated, hyperechoic septa corresponding to the perimysial fibro-adipose septa (Fig. 3.22 b) [32,34,35].

When studying both muscles and tendons, it is fundamental that the ultrasound beam is correctly tilted so that it is always perpendicular to the examined muscular plane, in order to avoid the appearance of hypoechoic artifactual zones that can be misinterpreted by inexperienced operators [36,37].

In some body regions the sonographer can observe accessory muscles, not to be misinterpreted as pathologic masses. The most common described "pseudomasses" are the palmaris longus muscle at the wrist [38], the accessory soleus and the peroneus quartus at the ankle.

Fig.3.22 a, b

a Longitudinal US scan of a pennate muscle.The characteristic pennate appearance is given by the convergence of perimysial septa. b The transverse US scan shows the polygonal arrangement of the muscular fascicles and hyperechoic perimysial septa a Longitudinal US scan of a pennate muscle.The characteristic pennate appearance is given by the convergence of perimysial septa. b The transverse US scan shows the polygonal arrangement of the muscular fascicles and hyperechoic perimysial septa

Color Doppler longitudinal (a, c) and transverse (b, d) scans of the quadriceps (vastus lateralis) before (a, b) and after exercise (c, d). A diffuse intramuscular hypervascularization is shown after intense activity. This is related to the physiological hyperemia

Color Doppler longitudinal (a, c) and transverse (b, d) scans of the quadriceps (vastus lateralis) before (a, b) and after exercise (c, d). A diffuse intramuscular hypervascularization is shown after intense activity. This is related to the physiological hyperemia

US examination should always be performed as a comparative technique with the contralateral muscle and in an active and passive dynamic way, both during contraction and during relaxation. This allows a functional evaluation of the muscle.

The degree of muscular contraction affects the oblique direction of the echoic septal pattern; in particular a higher obliquity of the fibers is observed when the muscle is relaxed. The images obtained during an isometric contraction may show an apparent increase of the muscular mass and of the hypoechogenicity depending on the muscular bundles' thickening during contraction. Hypertro phy of the muscular bundles, typically observed in athletes, can be associated with increased muscular hypoechogenicity [39].

Finally, physical exercise is associated with an increase in muscular vascularization (blood flow is 20 times higher than in standard conditions) and a consequent increase in the muscular mass volume, up to 10-15%. The muscular volume gets back to standard conditions after about 10-15 minutes of rest [18].

Doppler techniques and, more specifically, power Doppler, can demonstrate the physiological muscular hyperemia after contraction (Fig. 3.23 a-d).

Relaxation Audio Sounds Dusk At The Oasis

Relaxation Audio Sounds Dusk At The Oasis

This is an audio all about guiding you to relaxation. This is a Relaxation Audio Sounds with sounds from Dusk At The Oasis.

Get My Free MP3 Audio


Post a comment