Representations of the muscles of the vertebral column as per Spigelius (1685). Note the attention to detail in the presentation
As early as 1685, Stenonis  described the geometry of the fasciculi: its diaphragm, although simplified, gave a clear picture of its organization at the tendon which becomes progressively thicker. Borelli  noted that “the length of a muscle was proportional to its degree of motion, which depends on the shortening” and admitted seven forms: prismatic, rhomboidal, orbicular, crossed, penniforme, radiate, helical. Stenonis noted  that the length of fascicles was constant for each type of muscle (Fig. 2).
In 1892, Trolard, describing the spinal muscles, noted that the transversospinalis was most often reduced to a few fascicles. “Despite its numerous dissections, it refused to allow a classification and adopted a topographic description from superficial to deep” .
The papers on the strength of the muscle were carried by the German anatomists Fick , von Lanz, and Wachsmuth  at the end of the nineteenth and twentieth centuries. Fick  noted the relations between size and power and characterized the importance of the notion of all fascicles in relation to their maximum strength (“Cross Section Area”).
The conventional descriptions were based on the complex muscle-tendon with 2 parts, a red central muscle and a tendinous terminal. The histological and physiological evaluation must be modified and has not been developed in anatomy textbooks. Dissections that we have done since 1985 have led to a macroscopic dissection on the disposition of the fascicles with the pennation, the aponeurosis, and the fascia to support innovative therapies .
Embryologically, the skeletal muscles derive from the mesoderm, whose cells are organized into regular groups with anteroposterior distribution with the somites. There are several species of myoblasts during ontogenesis, those who remain in the somitic territory, at the origin of the trunk muscles, and those who migrate to the limb bud. The differentiation of mesoderm into myoblastic cells is dependent on four regulatory genes : myoD, myogenin, myf5, myf6, plus a fifth less well identified: myd . They are organized into 42–44 somites.
Some (e.g., rectus abdominis) derive from the fusion of several myotomes . The muscular precursors of a myotome give rise to separate muscles in the horizontal plane (trapezius, sternocleidomastoid) or in the frontal plane (external oblique and transversus abdominis) and migrate at a distance from their original myotome (trapezius, latissimus dorsi). Before birth, the number of fascicles increases and between 2 and 16 years, according to a multiple of 14 with a regular rhythm up to 50 years, their size is greater in men than in women, measuring 3–10 cm long and less than 0.1 mm thick (Figs. 3, 4, and 5).
By the fifth week, the myotubes appear; at 8 weeks (30 mm vc), all the muscles have their mature form and change their orientation with the exception of the axial longitudinal muscles.
General and Semantic Organization of the Muscle
Curvatures: Anatomical and Biomechanical Fundamentals
The human column has undergone a double evolution with adaptation for bipedal stance and recumbency. Throughout its height, it is unstable in the three planes of space, the regulation of which is ensured by all the structures constituting two essential mechanical principles with variable curvatures and the articular tripod. The notion of curvature is old, Fick  integrated the hip and pelvis entirely and the functional consequences of other types of curvature. At the end of growth, we consider that its morphology is stable. If it were straight and rigid, we would have difficulty maintaining our balance and in the face of mechanical stress, the deformation would be unpredictable (buckling).
To adapt, each segment has a curvature that ensures a prestressed state of defined shape, organized to oppose precise predictive forces so as to avoid breakage.
Thus, during loading, the deformation will occur by accentuating the curvature, and under such conditions, it is easier to resist the deformation by concentrating the muscular insertions into the concavity. The existence of curvatures and the flexibility of the intervertebral joints contribute to its stability in an erect position. There are so-called physiological curvatures which provide greater resistance according to the laws governing the elastic columns with alternating curvatures. For these, the resistance to the pressure is expressed by the relation N2 + 1, where N is the number of curvatures (4 × 4) and 1 the resistance of a rectilinear column of the same diameter. In the case of the column with 4 curvatures: cervical, thoracic, lumbar, and sacral, the resistance is 17 but it is not appropriate to consider the rigid sacral curvature thus resulting in 3 curvatures (3 × 3 + 1 = 10) (Figs. 6 and 7). It is excessive to establish a strict relationship in a structure where bone density, power of muscles, and ligaments intervene in strength as much as the number of curvatures. Some vertebrae or “keystones” have a special position with respect to these curvatures, neutral, and passive vertebrae during movements. The keystone of the cervical spine is C5 or C6, the thoracic and lumbar spine in T7 and L3. The transition vertebrae T1, T12, and L5 are not very mobile during the movements between the different segments. The first and last are protected, but the thoracolumbar junction is not and forms the frequent traumatic focus that extends from T10 to L1. Cervical lordosis is in harmony with thoracic kyphosis and lumbar lordosis specific to humans. Standing exists only in part for semi-bipeds (semi-brachiators) and more fragile extent in other primates. It hardly exists in infants and only forms when the trunk is erect and then when walking.
The essential role in the maintenance of this attitude is devolved to the muscular tone of posture, in particular to small permanent contractions of the muscles of the paravertebral gutters and the pelvic girdle. The first biomechanical experiment on stability with incorporated muscles was presented by Bergmark  with an elastic function. In a histological study, Nitz  determined the number of neuromuscular spindles in the muscles of the lumbar portion with, in the interlamellar and interspinous muscles, where a much larger percentage of neuromuscular spindles exist than in multifidus and semispinalis.
These intersegmental muscles , in addition to their mechanical role, are regulators of tension (Fig. 8).
Musculo-Aponeurotic Static and Dynamic Mechanical Imperatives
The muscles and their fascia provide stabilization in an automatic control mode via the extrapyramidal system. Each vertebral body is positioned statically and dynamically, not only in relation to its neighbors, by the play of the short intersegmentary muscles placed in the depth of the vertebral gutter, but also in relation to the whole of the spine by the action of the superficial long muscles, which supposes complex muscular attachments. The imperative is the maintenance of a symmetry, with a rigorous adjustment of ligamentary traction and tension with 3 muscular groups: posterior vertebral, oblique latero-vertebral (vertebral rotation) in case of deflection and anterior, so that any imbalance may cause a deviation (Fig. 9).
The general systematization of the posterior muscles is based on the direction of their bundles which comprise different groups, for example, at the lumbar level: a sacro-transverse group (ilio-costal and longissimus dorsi), transverso-spinal group (transverse spinous with several long lamellar chevrons, short and long spinous), interspinal group (7 pairs of interspinous muscle), and intertransverse group (7 pairs of intertransverse muscles). The laterovertebral muscles form 2 powerful muscular columns at the lateral lumbar part (quadratus lumborum and ilio-psoas). The anterior abdominal muscles counterbalance the action of the posterior muscles along with those at the cervical level (longis coli, rectus abdominis) and by a continuous superficial set from the head to the pelvis (sterno-cleidomastoid, sternum, rectus abdominis, and 3 lateral oblique muscles with, from surface to deep, the external oblique, internal oblique, and transversus of the abdomen, whence their importance in lumbar posture (Fig. 10).
Comparative Anatomy: Curves and Muscles
The studies of comparative anatomy by Winkler  were a response to the relationship between the notion of curvature and the anatomy of the muscles: “We did not hesitate to extrapolate this study in the field of comparative anatomy , knowing that it would be of great interest by observing simpler cases, less evolved than in humans.” One of the main factors that oppose the thoracic vertebrae to the lumbar is the anticline with the orientation of their spinous processes in relation to the intermediate vertebra (approx. T12). Those of the thoracic vertebrae are inclined towards the intermediate vertebra, and they generally diminish in height as they approach it. At the level of the lumbar vertebrae, we find approximately these same features provided that they are observed in the caudal-cranial direction. The spinous processes gradually incline towards the limiting vertebra and diminish in value as we get closer to it. Those of the thoracic region are in retroversion, while the lumbar are in anteversion with respect to the intermediate vertebra. Anticlinia does not uniformly exist in the mammalian series, which is a function of the degree of mobilization of the trunk subdivided into two: cervico-thoracic or anterior and lumbosacral or posterior chains. In quadrupeds, the ilio-costal muscle is subdivided into two segments, one lumbar and the other thoraco-cervical. The lumbar part may be lacking (insectivores), while the other two still exist. The muscle is all the more flat and tendinous as the convexity of the column is accentuated. In the latter case, when the curvature is too strong, there is no muscle (insectivores), when it is neutral, neither convex nor concave, the muscle does not exist (ungulates). It is observed that, in quadrupeds, the ilio-costal muscle is generally weak and tending to its initial lumbar part, that it is more and more “strong and fleshy” at its thoraco-cervical terminal part. In semi-orthostatic simians, the ilio-costal muscle is consistent in size from one end to the other. In anthropoids and in man, on the other hand, it is highly developed and fleshy in the lumbar region and progressively weakens in the cranial direction (Figs. 11 and 12).
The number of muscles presumed to be 400 varies: for Chaussier  368, Theile  346, Sappey  501 distributed into 190 truncal, 63 head-neck, 98 upper limbs, 104 lower limbs, and 46 of the abdomino-pelvic system.
Their denominations were based on a single principle. Before Sylvius , they were designated by the numerical names of first, the second, following the order of their superposition or their use. The order of Galen  was entirely topographical. Vésale  substituted a physiological order according to their use, true or supposed, and corrected many errors.
For Cruveilhier , their shape was determined: from their resemblance either with geometric forms, with generally known objects , according to their symmetry or their lack of symmetry. Under this last report, there was a very great difference between the osseous and muscular systems. “The topographical order was preferable in many respects, in that it was essentially anatomical and offered the advantage of appreciating the relations of the muscles between them and the various regions between them; it had, from the point of view of the economy of the subjects and the facility of the preparations, an incontestable advantage on the physiological order, with which, moreover, could be reconciled for a few regions. He adopted the topographical order with some modifications which made it possible to study all myology on the same subject.”
Since the twentieth century, anatomists had wanted to use a vocabulary that is understood by everyone with a universal language with Latin, which was that of Nomina Anatomica. Since the adoption in principle of Nomina Anatomica at Oxford in 1950, they were recognized and legitimized, then recognized by the International Federation of Associations of Anatomists (IFAA).
The modalities of their insertions are one of the characteristics that allow their distinction into several groups. Those with insertions of extended origin having variable shapes (rectilinear, curved) or with multiple digitations (levator scapulae) (Figs. 13, 14, and 15).
Another group is distinguished by the presence of the single or double origin tendon with two or more muscular bodies, with all of these musculo-aponeurotic complexes grouping together on a single tendon at its termination. The longissimus dorsi in its thoracic portion is detached from the sacral crest by a fascial layer which extends with the ilio-costal and ends with two chevrons: costal on the last five ribs and transverse on the transverse processes of T12–T7. Conversely, it may have insertions of osteo-periosteal origin as after constitution of the muscular body; it divides into several distinct tendons such as the two bundles of the dorsal part of the ilio-costal strand of the longissimus dorsi which consists of 6 tense strands of the posterior arch of the 1st and 2nd ribs towards the posterior arches of the ribs of the 5th to the 11th.
Intrinsic and Semantic Morphology
It is worth distinguishing the “anatomical muscle fascicles of the functional muscular fibers.” The new anatomical approach benefits from details of anatomical architectural parameters with overall muscular length, length of anatomical fascicles, length of muscle fiber, and angle of pennation. Its actual length is defined as the distance between the proximal insertion of the muscle fiber and its distal insertion on the fascia . This measurement is variable according to the torsion of the fibers. Their length consisting of 500–10,000 parallel myofibrils can only be determined by micro-dissection after fixation. The fascicle is the sum of several muscle fibers, difficult to isolate by microdissection and which can contain from 5 to 50 fibers. Fundamentally, the anatomical fascicle is not necessarily representative of the real function of the muscle, which can only be identified histologically . The determination of long or short fibers depends on the method of fixation, the measurement of the length of the sarcomere being difficult to interpret.
Sarcomere and Myofibrils
The sarcomere forms the histological functional unit and the fascicle the macroscopic functional unit. The universal organization consists of sarcomeres, which group together in the form of myofibrils , which in turn constitute the fascicles. The sarcomere consists of actin and myosin interconnected contractile proteins arranged to slide relative to each other. The energy allowing their movement comes from the hydrolysis of the adenosine triphosphate fixed on the head of the myosin which can then tilt. This tilting pulls the action on the M band and causes shortening of up to 1 μm which can then create shortening of up to 10 cm .
Each myofibril is composed of 1000 to 2 million sarcomeres in series, 2–3 μm in length and 1 μm in diameter.
The muscular fascicle of the brachial biceps contains more than 100,000 sarcomeres that can be arranged in parallel or in series to increase their power and movement. Its reactivity during growth or a period of immobilization can only be interpreted to the extent that one knows the type of fascicles and the disposition of the sarcomeres. Experimentally, Huijing  studied the degree of atrophy in rats (soleus, gastrocnemius) in these two situations. The results showed that their intrinsic structure was different. For the soleus, the sarcomeres were arranged in parallel and well adapted in their reactivity. Conversely for the gastrocnemius, only sarcomeres in parallel adapted to the circumstances. He concluded that the architecture had a considerable role in explaining these different behaviors without changing the length of the fascicles (Figs. 16 and 17).
Titin: The Primary Elastic Protection of the Sarcomere
Titin is the most abundant “connective” protein in striated muscle, representing 10% of the myofibrillar mass, playing a role in the organization of sarcomere components essential for their growth.
It is involved in the control of the assembly of sarcomeric proteins regulating the elasticity of sarcomere related to thick myosin filaments and extends from the Z disc to the M band (almost half a sarcomere, i.e., more than a micron in length). With a length of 1 μm, about 30,000 amino acids and a molecular weight of 4000 kDa, it is the largest known polypeptide chain controlling integrity and ensuring the mechanical stability of the sarcomere. During traction, it generates a force sufficient to oppose the stretching tension of the sarcomere and plays a role in myofibrillogenesis , thus its degradation weakens the muscle. At its termination, it constitutes an element of information at the level of the Z and M bands as a factor of stabilization of passive stretching, a true spring of protection of the sarcomere during extension. Friden and Lieber  showed that passive stretching imposed strains equivalent to painful maximal muscle tension that degraded titin. By force or equivalent elongation, the extensibility of the elastic component of a rat soleus was about half that of a rat’s rectus femoris. Such a difference would come from the existence of the different isoforms of titin and from its length of the extensible part. According to the type of muscle, it has different mechanical properties.
Its role is fundamental in the protection of sarcomeres during excessive traction, mainly in the vertebral column where its constraints are central to its stability.
The control of displacement during contraction is controlled by titin, which has a finely tuned passive tension setting (Fig. 18).
When stretching the sarcomere, it generates a force capable of opposing the physiological tension of stretching. Exceeding this causes a rupture of the myosin actin bridge with trauma to the sarcomere . The length of the sarcomeres varies according to the position of the joint.
Muscles and Aponeurosis
When these models join together, they result in a complex muscular organization of effective functionality. The length of fascicles of a long muscle should not be judged by that of the fleshy aspect but should encompass its associated fascial elements.
The generic term of fascicle is a functional set of sarcomeres as discussed by Huxley  and Alexander . The particular fusiform shape of many muscles is a false image of the true length of the fascicles. The observation before dissection gives the impression that a fascicle extends from one end of the muscle to the other . After a specific dissection, the particular disposition of each muscular fascicle whose muscular constitution is based on fundamental models adapted to their function with parallel fascicles, convergent with three-dimensional arrangements [13, 33–35]. In man, the length of fascicles is on average 4–5 cm, the longest being 10–15 cm. The axis of the muscle being not the same as that of the fascicles which compose it, we must study, for each, their direction with respect to the aponeuroses and the tendon.
Muscle Fasciculi and Aponeurosis
Muscle Fasciculi and Pennation : Topographical Economics
Muscles requiring considerable efforts have a large cross section with parallel fascicles, which would require an extensive and non-punctiform bone insertion. But there being no room for a muscle too large and thick, a solution has been found by an oblique arrangement of the fascicles (pennate) over the whole length of the bone and the fascia, which continues with a tendon, an organ of transmission.
The dissections show in all cases the presence of an aponeurosis with two types of fasciculi which are inserted into the main axis of the aponeurosis or alternatively, laterally according to the principle of a unipennate or bipennate muscle . On a global level, the insertions of the fascicles are arranged in the three planes of space (Fig. 19). The justification for this provision has two functions. The first is the constant development of a rotational movement with a certain amplitude according to the length of the fascicles. The second is based on the principle of permanently maintaining the tension of the aponeurosis which allows the fascicles to maintain a permanent tension for a maximum yield , whatever the position of the joint.
For the muscles of the pelvis, the central fascia is important and the fascicles which are involved are of the unipennate type. For the extensors, the aponeurosis are in the form of two resilient layers on which short fascicles fit into the category of unipennate antigravity muscles. In this case, the two fasciae are broad, flattened, and mirrored, are very powerful, less rapid, and have limited displacement. For the flexors, the constant central fascia occupies between 90 and 95% of the total length of the muscle, from its origin to its termination. In the case of the intrinsic ones, the arrangement is proportional to those of the girdles with a central fascia and a comparable arrangement of the fascicles (Fig. 20). Each muscle has two extremities, mostly with two tendons, one of origin, and the other of terminals with fascicles having a particular disposition, we can predict innumerable varieties in their constitution.
The fascicles are implanted directly or obliquely on the aponeurosis, like the barbs of a feather on their common stem: (penna: feather), as per Borelli  and Stenonis .
Aponeurosis: Fundamental Structure
The aponeurosis is the indissociable complementary element of the muscle. The fascicles are inserted either along the axis of the fascia or obliquely to distribute the mechanical stresses avoiding rupture during muscular contraction, all along its length.
Small posterior serrated muscles are often joined by a continuous fascia. This disposition is present in mammals (rodents), which plays a role in respiration (spino-costal muscle) (Figs. 21, 22, and 23).
A Historical Conceptual Error
For Cruveilhier  the term aponeurosis was used, a name whose etymology displayed a great anatomical error (aponeurosis, from the Greek apo neuron as the ancients regarded the nerve as the white parts). Today they are referred to as fascia (fascia: band), applied by extension to all the aponeuroses as the dedicated name in the form of a broad band. In 1847, thanks to his observation  and his reflection, he gave a macroscopic description of a remarkable precision “the most general disposition is as follows:
It is along the faces and edges of this aponeurosis that the fascicles are born (insert). It is again on a membranous surface or aponeurosis that they terminate. This aponeurosis, gathering on itself, constitutes a terminating tendon which the fleshy fibers (fascicles) leave at a more or less considerable distance from its extremity. It results from this provision: (1) a considerable development of surface for the insertion of the fascicles which the tendon collects, so to speak, in order to concentrate their efforts on the same point; (2) the obliquity of insertion or incidence of the fasciculi in relation to the tendon which represents the axis of the muscle, that is, the direction. It is conceivable that this obliquity is of the greatest relevance in the dynamic relation of the force of action of the muscles and leads necessarily to a great loss of forces. There is a great variety in these angles of insertion or incidence of fascicles on the aponeurosis. It is conceivable that the facility of multiplying the fascicles by arranging them in this way obliquely outweighs the disadvantage of their direction.
The original tendon extends as a form of membrane or aponeurosis in the thickness or surface of the muscle.
Intrinsic Structure of the Aponeurosis: Endomysium and Perimysium Aponeurosis
Electron microscopy of the fascicles shows that the endomysium is a continuous collagenous tissue that allows it to be individualized. It forms an important part of the isolation mechanism of the fascicles or constitutes the outer part or reticular leaflet of the sarcolemma. The epimysium sends fine septae of connective tissue called perimysium determining primary, secondary, and tertiary bundles. Within these fasciculi, each fascicle is surrounded by a thin layer of connective tissue consisting mainly of an outer lamina (basal lamina) and reticulin fibers, the endomysium. It is the whole of this connective tissue that transmits the constraints of the fascicles to the fascia (Fig. 24).
The different muscular fascicles are not randomly grouped, but constitute regular bundles surrounded by an outer layer of dense connective tissue, limiting the muscles as a whole called epimysium.
In 1989, Ettema and Huijing , on the physiological level, led to an interpretation and a comprehensive mechanical synthesis of the internal structures of the fascicle and aponeurosis. For the sarcolemma of isolated fibers, the endomysium contributes to the elastic static function as it can be stretched to 140–150% and contributes to the elasticity and viscosity of the muscular complex. The sum of the connective tissue increases the compliance of the muscle in addition to that of the fascicles and the tendon.
Due to the tension that is transmitted through the aponeurosis to the endomysium, the mechanical properties of this connective tissue are a factor contributing to improve the mechanical properties of the muscle, transferring the stresses between the discontinuous fascicles through transendomysial bundles. In contrast to the high tensile forces of the collagenous fibers on which the fascicles attach themselves, the translaminar leaflets undergo shear forces of the endomysium . Each muscle fiber is surrounded by a fibrous tissue (perimysium ) which is organized into cells which, by their adhesion, undergo progressive elastic transmission of mechanical shear stresses.
In certain configurations, the intramuscular fascial segments may be subdivided into any number of small segments or grouped into a whole (law of commutativity and associativity) (Figs. 25 and 26). When bending the head, the muscle stretching is such that the sarcomeres may be destroyed. To combat this eventuality, there are multiple aponeuroses in the muscle to distribute the constraints and protect them. In addition, depending on the length of the lever arm, the muscle has a resistant and elastic aponeurosis structure giving it a “digastric” appearance. The semi-spinalis cervicis responds to these objectives (transverse processes of C3–T5 with two bundles, medial (T5–T3), lateral (T3–C3), terminating at the occipital nuchal line) with a fascial intersection.
Mechanism of the Aponeurosis
At the level of the aponeurosis , there is no uniform extension zone during passive or active movements . These tension variations are according to the axial stresses that have already been observed for tendons, ligaments, and aponeurosis. For the aponeurosis, it appears that the tension forces are higher at the two extremities of the muscle than in the middle part. Its behavior varies between passive movements and active movements. In passive movement, it deforms more in the direction of the length and less in the direction of the width. Knowledge of its elastic properties is essential for understanding changes in fascicle length or of the sarcomere. When modeling the muscle–tendon complex, the variations in length, the difference observed in the changes between the fascicles and the muscular length were attributed to the elastic compliance component of this fascia in the muscle according to Hill’s conceptual model, created with contractile and elastic structures [2, 37]. This concept is reinforced by the junction between the aponeurosis and the tendon, which has a conical shape to distribute the harmonious transmission of the stresses (Fig. 27).
Mechanical tests on the elastic properties of a muscle focus on the tendinous structures isolated from the rest of the muscular element, ignoring the role of the aponeurosis [2, 37, 38]. Comparative studies of the reaction pattern during muscular contraction or passive stretching showed that fascial characteristics differ significantly: in passive movement, there is more fascial length involved in weak forces than comparatively for muscle contraction activities. The difference in length may be greater than 1.25 mm for approximately 6% of the length of aponeurosis. During an isometric contraction of a muscle, the contraction of the fascia is limited to about 3.5% of the total fascial length. During this isomeric contraction, the energy expenditure of the aponeurosis is evaluated at 2.8 microJoules (μJ). As a passive element, this finding may seem paradoxical. Although passive, any structure during a displacement is subject to an intimate cellular change that is accompanied by energy-releasing cellular metabolic variations, either in the form of a loss or a recovery. According to a more affordable model, the fascia can be compared to a piece of elastic which, when tensioned beyond its length, stores a force (requiring an energy expenditure for its tensioning) which, when stopped will regain its initial length and as a result, restore the energy stored during tensioning (e.g., archery and pole vaulting) (Fig. 28).
When the force decreases from the maximum force (optimum length) to a force equal to its passive resistance (initial state), the aponeurosis decreases in length by 9%, resulting in work of 4.8 μJ. Dissection of the lumbar portion of the longissimus dorsi (antigravitary) shows a aponeurosis that occupies 95% of its length. The aponeurosis mass occupies the lateral part of the lumbar curvature which inserts on the deep medial aspect of the rough surface of the iliac bone between the articular surface and the medial lip of the iliac crest. Each bundle ends with a lateral chevron on the costiform processes and a medial chevron on the accessory tubercles of the lumbar vertebrae.
When standing, the fascia behaves like a passive rope that maintains joint balance without involving muscular contraction resulting in a significant saving of energy expenditure.
During one movement, its length variation is considered an important factor in explaining the difference in muscle length changes. For Lieber , during passive movements in the frog, the elasticity of the aponeurosis was 8% and for the tendon of the semi-tendinosis it was 2%. Huijing  showed a greater extension of the aponeurosis of gastrocnemius in the rat when the muscle was in contraction. For the same mechanical effect, the tendon had a tension of 3 to 4%, which indicated that the compliance of the aponeurosis was different from that of the muscle. The lengthening of the muscle depends on the properties of the aponeurosis which is an important element as a factor of storage of energy and its release during muscle contraction. The elastic energy stored during the passive elongation is very low (about 0.04 μJ) because of the small levels of force, if the muscle is activated to a certain length, the aponeurosis is stretched by about 2% resulting in 0.56 μJ elastic energy storage. For an active muscle, if the muscular force decreases, the force exerted on the aponeurosis is also reduced; this results in a release of energy of 3.15 μJ. The consequence is that the relaunched energy of 3.5 μJ can be obtained if the fascia follows the force/length displacement curve during the isometric contraction of a muscle. The properties of the fascia appear to be an important field for architectural modeling applications of a muscle. For muscular modeling, therefore, the stiffness of the fascia must be incorporated for elongations of less than 10%. The origin of the restitution of energy is still obscure, but may be dependent on the aponeurosis-tendon junction. All these fundamental notions find their application in the structure of the posterior muscles of the vertebral column, where the aponeuroses occupy a prominent place.
Muscle with Parallel Fascicles: Pseudo-Penniform
The fascicles have the same direction between the two insertions with parallel fascicles . This arrangement is mechanically ideal and has maximum efficiency requiring simultaneous contraction of all sarcomeres in parallel or series. This anatomical arrangement is met for some muscles: rhomboid, serratus posterior (Fig. 29).