Muscle architecture

Last updated March 27, 2024

Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle's mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force production and gearing vary depending on the different muscle parameters such as muscle length, fiber length, pennation angle, and the physiological cross-sectional area (PCSA).^[1]

Architecture types

Parallel and pennate (also known as pinnate) are two main types of muscle architecture. A third subcategory, muscular hydrostats, can also be considered. Architecture type is determined by the direction in which the muscle fibers are oriented relative to the force-generating axis. The force produced by a given muscle is proportional to the cross-sectional area, or the number of parallel sarcomeres present.^[2]

Parallel

The parallel muscle architecture is found in muscles where the fibers are parallel to the force-generating axis.^[1] These muscles are often used for fast or extensive movements and can be measured by the anatomical cross-sectional area (ACSA).^[3] Parallel muscles can be further defined into three main categories: strap, fusiform, or fan-shaped.

Strap

Strap muscles are shaped like a strap or belt and have fibers that run longitudinally to the contraction direction.^[4] These muscles have broad attachments compared to other muscle types and can shorten to about 40–60% of their resting length.^[3]^[4] Strap muscles, such as the laryngeal muscles, have been thought to control the fundamental frequency used in speech production, as well as singing.^[5] Another example of this muscle is the longest muscle in the human body, the sartorius.

Fusiform

Fusiform muscles are wider and cylindrically shaped in the center and taper off at the ends. This overall shape of fusiform muscles is often referred to as a spindle. The line of action in this muscle type runs in a straight line between the attachment points which are often tendons. Due to the shape, the force produced by fusiform muscles is concentrated into a small area.^[3] An example of this architecture type is the biceps brachii in humans.

Convergent

The fibers in convergent, or triangular muscles converge at one end (typically at a tendon) and spread over a broad area at the other end in a fan-shape.^[3]^[6] Convergent muscles, such as the pectoralis major in humans, have a weaker pull on the attachment site compared to other parallel fibers due to their broad nature. These muscles are considered versatile because of their ability to change the direction of pull depending on how the fibers are contracting.^[3]

Typically, convergent muscles experience varying degrees of fiber strain. This is largely due to the different lengths and varying insertion points of the muscle fibers. Studies on ratfish have looked at the strain on these muscles that have a twisted tendon. It has been found that strain becomes uniform over the face of a convergent muscle with the presence of a twisted tendon.^[7]

Pennate

Unlike in parallel muscles, fibers in pennate muscles are at an angle to the force-generating axis (pennation angle) and usually insert into a central tendon.^[3]^[8] Because of this structure, fewer sarcomeres can be found in series, resulting in a shorter fiber length.^[2]^[3] This further allows for more fibers to be present in a given muscle; however, a trade-off exists between the number of fibers present and force transmission.^[3]^[8] The force produced by pennate muscles is greater than the force produced by parallel muscles.^[3] Since pennate fibers insert at an angle, the anatomical cross-sectional area cannot be used as in parallel fibered muscles. Instead, the physiological cross-sectional area (PCSA) must be used for pennate muscles. Pennate muscles can be further divided into uni-, bi- or multipennate.

Unipennate

Unipennate muscles are those where the muscle fibers are oriented at one fiber angle to the force-generating axis and are all on the same side of a tendon.^[1] The pennation angle in unipennate muscles has been measured at a variety of resting length and typically varies from 0° to 30°.^[1] The lateral gastrocnemius is an example of this muscle architecture.

Bipennate

Muscles that have fibers on two sides of a tendon are considered bipennate.^[1] The stapedius in the middle ear of humans, as well as the rectus femoris of the quadriceps are examples of bipennate muscles.

Multipennate

The third type of pennate subgroup is known as the multipennate architecture. These muscles, such as the deltoid muscle in the shoulder of humans, have fibers that are oriented at multiple angles along the force-generating axis.^[1]

Hydrostats

Muscular hydrostats function independently of a hardened skeletal system. Muscular hydrostats are typically supported by a membrane of connective tissue which holds the volume constant. Retaining a constant volume enables the fibers to stabilize the muscle's structure that would otherwise require skeletal support.^[9] Muscle fibers change the shape of the muscle by contracting along three general lines of action relative to the long axis: parallel, perpendicular and helical. These contractions can apply or resist compressive forces to the overall structure.^[10] A balance of synchronized, compressive and resistive forces along the three lines of action, enable the muscle to move in diverse and complex ways.^[10]

Contraction of helical fibers causes elongation and shortening of the hydrostat. Unilateral contraction of these muscles can cause a bending movement. Helical fibers can oriented into either left or right-handed arrangements. Contraction of orthogonal fibers causes torsion or twisting of the hydrostat.

Force generation

Muscle architecture directly influences force production via muscle volume, fiber length, fiber type and pennation angle.

Muscle volume is determined by the cross-sectional area. Anatomical cross-sectional area is

$CSA={\frac {V}{l}}$

where

$\textstyle V$ stands for volume
$\textstyle l$ stands for length

In muscles, a more accurate measurement of CSA is physiological CSA (PCSA) which takes into account fiber angle.

$PCSA={\frac {m\cdot \cos \theta }{l\cdot \rho }}$

where

$\textstyle m$ stands for muscle mass
$\textstyle \theta$ stands for the fiber angle
$\textstyle l$ stands for fiber length
$\textstyle \rho$ stands for muscle density

PCSA relates the force produced by the muscle to the summation of the forces produced along the force generating axis of each muscle fiber and is largely determined by the pennation angle.^[3]^[8]

Fiber length is also a key variable in muscle anatomy. Fiber length is the product of both the number of sarcomeres in series in the fiber and their individual lengths. As a fiber changes length, the individual sarcomeres shorten or lengthen, but the total number does not change (except on long timescales following exercise and conditioning). To standardize fiber length, length is measured at the peak of the length-tension relationship (L0), ensuring all sarcomeres are at the same length. Fiber length (at L0) does not affect force generation, much as the strength of a chain is unaffected by the length. Similarly, increased fiber cross-section or multiple fibers increase the force, like having multiple chains in parallel. Velocity is affected in the reverse manner – because sarcomeres shorten at a certain percentage per second under a certain force, fibers with more sarcomeres will have higher absolute (but not relative) velocities.^[11] Muscles with short fibers will have higher PCSA per unit muscle mass, thus greater force production, while muscle with long fibers will have lower PCSA per unit muscle mass, thus lower force production. However, muscles with longer fibers will shorten at greater absolute speeds than a similar muscle with shorter fibers.^[2]

The type of muscle fiber correlates to force production. Type I fibers are slow oxidative with a slow rise in force and an overall low force production. The type I fibers have a smaller fiber diameter and exhibit a slow contraction. Type IIa fibers are fast oxidative which exhibit fast contraction and a fast rise in force. These fibers have fast contraction times and maintain some, though not a great amount of their force production with repeated activity due to being moderately fatigue resistant. Type IIb fibers are fast glycolytic which also exhibit fast contraction and fast rise in force. These fibers display extremely large force production, but are easily fatigued and therefore unable to maintain force for more than a few contractions without rest.

Pennation angle

The pennation angle is the angle between the longitudinal axis of the entire muscle and its fibers. The longitudinal axis is the force generating axis of the muscle and pennate fibers lie at an oblique angle. As tension increases in the muscle fibers, the pennation angle also increases. A greater pennation angle results in a smaller force being transmitted to the tendon.^[9]

Muscle architecture affects the force-velocity relationship. Components of this relationship are fiber length, number of sarcomeres and pennation angle. In pennate muscles, for example, as the fibers shorten, the pennation angle increases as the fibers pivot which effects the amount of force generated.^[2]

Architectural gear ratio

Architectural gear ratio (AGR) relates the contractile velocity of an entire muscle to the contractile velocity of a single muscle fiber. AGR is determined by the mechanical demands of a muscle during movement. Changes in pennation angle allow for variable gearing in pennate muscles.^[12] Variable pennation angle also influences whole-muscle geometry during contraction. The degree of fiber rotation determines the cross-sectional area during the course of the movement which can result in increases of the thickness or width of the muscle.^[12] Pennation angle can be modified through exercise interventions.^[13]

	High gear ratio	Low gear ratio
Contraction velocity ratio (muscle/fiber)	Whole muscle ≫ muscle fiber	Approximately 1:1 ratio
Force developed by whole muscle	Low-force contractions	High-force contractions
Velocity developed by whole muscle	High-velocity contractions	Low-velocity contractions
Pennation angle (fiber rotation)	Increase in pennation angle	Minute or no decrease in pennation angle
Cross-sectional variance	Increase thickness (increase distance between aponeuroses)	Decrease thickness (decrease distance between aponeuroses)

A high gear ratio occurs when the contraction velocity of the whole muscle is much greater than that of an individual muscle fiber, resulting in a gear ratio that is greater than 1. A high gear ratio will result in low force, high velocity contractions of the entire muscle. The angle of pennation will increase during contraction accompanied by an increase in thickness. Thickness is defined as the area between the aponeuroses of the muscle. A low gear ratio occurs when the contraction velocity of the whole muscle and individual fibers is approximately the same, resulting in a gear ratio of 1. Conditions resulting in a low gear ratio include high force and low velocity contraction of the whole muscle. The pennation angle typically shows little variation. The muscle thickness will decrease.

Related Research Articles

The muscular system is an organ system consisting of skeletal, smooth, and cardiac muscle. It permits movement of the body, maintains posture, and circulates blood throughout the body. The muscular systems in vertebrates are controlled through the nervous system although some muscles can be completely autonomous. Together with the skeletal system in the human, it forms the musculoskeletal system, which is responsible for the movement of the body.

Skeletal muscles are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle tissue, and are often known as muscle fibers. The muscle tissue of a skeletal muscle is striated – having a striped appearance due to the arrangement of the sarcomeres.

A myofibril is a basic rod-like organelle of a muscle cell. Skeletal muscles are composed of long, tubular cells known as muscle fibers, and these cells contain many chains of myofibrils. Each myofibril has a diameter of 1–2 micrometres. They are created during embryonic development in a process known as myogenesis.

<span class="mw-page-title-main">Sarcomere</span> Repeating unit of a myofibril in a muscle cell

A sarcomere is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells which are formed during embryonic myogenesis. Muscle fibers contain numerous tubular myofibrils. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as alternating dark and light bands. Sarcomeres are composed of long, fibrous proteins as filaments that slide past each other when a muscle contracts or relaxes. The costamere is a different component that connects the sarcomere to the sarcolemma.

A hydrostatic skeleton or hydroskeleton is a type of skeleton supported by hydrostatic fluid pressure, common among soft-bodied invertebrate animals colloquially referred to as "worms". While more advanced organisms can be considered hydrostatic, they are sometimes referred to as hydrostatic for their possession of a hydrostatic organ instead of a hydrostatic skeleton, where the two may have the same capabilities but are not the same. As the prefix hydro- meaning "water", being hydrostatic means being fluid-filled.

A muscle cell, also known as a myocyte, is a mature contractile cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal, smooth, and cardiac (cardiomyocytes). A skeletal muscle cell is long and threadlike with many nuclei and is called a muscle fiber. Muscle cells develop from embryonic precursor cells called myoblasts.

The Frank–Starling law of the heart represents the relationship between stroke volume and end diastolic volume. The law states that the stroke volume of the heart increases in response to an increase in the volume of blood in the ventricles, before contraction, when all other factors remain constant. As a larger volume of blood flows into the ventricle, the blood stretches cardiac muscle, leading to an increase in the force of contraction. The Frank-Starling mechanism allows the cardiac output to be synchronized with the venous return, arterial blood supply and humoral length, without depending upon external regulation to make alterations. The physiological importance of the mechanism lies mainly in maintaining left and right ventricular output equality.

Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. The presence of sarcomeres manifests as a series of bands visible along the muscle fibers, which is responsible for the striated appearance observed in microscopic images of this tissue. There are two types of striated muscle:

Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.

<span class="mw-page-title-main">Muscular hydrostat</span> Body part type that consists mainly of muscles with no skeletal support

A muscular hydrostat is a biological structure found in animals. It is used to manipulate items or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid in a separate compartment, as in a hydrostatic skeleton.

In biomechanics, Hill's muscle model refers to the 3-element model consisting of a contractile element (CE) in series with a lightly-damped elastic spring element (SE) and in parallel with lightly-damped elastic parallel element (PE). Within this model, the estimated force-velocity relation for the CE element is usually modeled by what is commonly called Hill's equation, which was based on careful experiments involving tetanized muscle contraction where various muscle loads and associated velocities were measured. They were derived by the famous physiologist Archibald Vivian Hill, who by 1938 when he introduced this model and equation had already won the Nobel Prize for Physiology. He continued to publish in this area through 1970. There are many forms of the basic "Hill-based" or "Hill-type" models, with hundreds of publications having used this model structure for experimental and simulation studies. Most major musculoskeletal simulation packages make use of this model.

A pennate or pinnate muscle is a type of skeletal muscle with fascicles that attach obliquely to its tendon. This type of muscle generally allows higher force production but a smaller range of motion. When a muscle contracts and shortens, the pennation angle increases.

Muscle is a soft tissue, one of the four basic types of animal tissue. Muscle tissue gives skeletal muscles the ability to contract. Muscle is formed during embryonic development, in a process known as myogenesis. Muscle tissue contains special contractile proteins called actin and myosin which interact to cause movement. Among many other muscle proteins present are two regulatory proteins, troponin and tropomyosin.

Undulatory locomotion is the type of motion characterized by wave-like movement patterns that act to propel an animal forward. Examples of this type of gait include crawling in snakes, or swimming in the lamprey. Although this is typically the type of gait utilized by limbless animals, some creatures with limbs, such as the salamander, forgo use of their legs in certain environments and exhibit undulatory locomotion. In robotics this movement strategy is studied in order to create novel robotic devices capable of traversing a variety of environments.

Role of skin in locomotion describes how the integumentary system is involved in locomotion. Typically the integumentary system can be thought of as skin, however the integumentary system also includes the segmented exoskeleton in arthropods and feathers of birds. The primary role of the integumentary system is to provide protection for the body. However, the structure of the skin has evolved to aid animals in their different modes of locomotion. Soft bodied animals such as starfish rely on the arrangement of the fibers in their tube feet for movement. Eels, snakes, and fish use their skin like an external tendon to generate the propulsive forces need for undulatory locomotion. Vertebrates that fly, glide, and parachute also have a characteristic fiber arrangements of their flight membranes that allows for the skin to maintain its structural integrity during the stress and strain experienced during flight.

<span class="mw-page-title-main">Physiological cross-sectional area</span> Area perpendicular to fiber direction

In muscle physiology, physiological cross-sectional area (PCSA) is the area of the cross section of a muscle perpendicular to its fibers, generally at its largest point. It is typically used to describe the contraction properties of pennate muscles. It is not the same as the anatomical cross-sectional area (ACSA), which is the area of the crossection of a muscle perpendicular to its longitudinal axis. In a non-pennate muscle the fibers are parallel to the longitudinal axis, and therefore PCSA and ACSA coincide.

In vitro muscle testing is a method used to characterize properties of living muscle tissue after removing it from an organism, which allows more extensive and precise quantification of its properties than in vivo testing. In vitro muscle testing has provided the bulk of scientific knowledge of muscle structure and physiology, and how both relate to organismal performance. Stem cell research relies on in vitro muscle testing to establish sole muscle cell function and its individual behavior apart from muscle cells in the presence of nonmuscle cells seen in in vitro studies.

A key component in lateral force transmission in skeletal muscle is the extracellular matrix (ECM). Skeletal muscle is a complex biological material that is composed of muscle fibers and an ECM consisting of the epimysium, perimysium, and endomysium. It can be described as a collagen fiber-reinforced composite. The ECM has at least three functions: (1) to provide a framework binding muscle fibers together and ensure their proper alignment, (2) to transmit the forces, either from active muscle contraction or ones passively imposed on it, and (3) providing lubricated surfaces between muscle fibers and bundles enabling the muscle to change shape. The mechanical properties of skeletal muscle depend on both the properties of muscle fibers and the ECM, and the interaction between the two. Contractile forces are transmitted laterally within intramuscular connective tissue to the epimysium and then to the tendon. Due to the nature of skeletal muscle, direct measurements are not possible, but many indirect studies and analyses have shown that the ECM is an important part of force transmission during muscle contraction.

<span class="mw-page-title-main">Architectural gear ratio</span> Ratio between muscle-shortening velocity and fiber-shortening velocity

Architectural gear ratio, also called anatomical gear ratio (AGR) is a feature of pennate muscle defined by the ratio between the longitudinal strain of the muscle and muscle fiber strain. It is sometimes also defined as the ratio between muscle-shortening velocity and fiber-shortening velocity.

Anatomical terminology is used to uniquely describe aspects of skeletal muscle, cardiac muscle, and smooth muscle such as their actions, structure, size, and location.

References

1 2 3 4 5 6 Lieber, Richard L., Friden, Jan (November 2002). "Functional and clinical significance of skeletal muscle architecture" (PDF). Muscle & Nerve. 23 (11): 1647–1666. doi:10.1002/1097-4598(200011)23:11<1647::aid-mus1>3.3.co;2-d. PMID 11054744 . Retrieved November 17, 2012.{{cite journal}}: CS1 maint: multiple names: authors list (link)
1 2 3 4 Narici, Marco (April 1999). "Human skeletal muscle architecture studied in vivo by non-invasive imaging techniques: functional significance and applications" (PDF). Journal of Electromyography and Kinesiology. 9 (2): 97–103. doi:10.1016/s1050-6411(98)00041-8. PMID 10098710 . Retrieved November 20, 2012.
1 2 3 4 5 6 7 8 9 10 Liem, Karel F., Bemis, William E., Walker, Warren F. Jr., Grande, Lance (2001). Function anatomy of the vertebrates: an evolutionary perspective. Emily Barosse. ISBN 0-03-022369-5.{{cite book}}: CS1 maint: multiple names: authors list (link)
1 2 Brooks, Darrell MD (2012). "Functional microvascular muscle transplantation" . Retrieved November 20, 2012.
↑ Erickson, Donna, Baer, Thomas, and Harris, Katherine S. "The role of the strap muscles in pith lowering" (PDF). Haskins Laboratory: Status Report: 275–284. Retrieved November 20, 2012.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Moore, Keith L. (2018). Clinically oriented anatomy (Eighth ed.). Philadelphia: Wolters Kluwer. pp. 30–32. ISBN 9781496347213.
↑ Dean, Mason N., Azizi, Emanuel, Sumers, Adam, P. (2007). "Uniform strain in broad muscles: active and passive effect of the twisted tendon of the spotted ratfish Hydrolagus colliei". Journal of Experimental Biology. 210 (19): 3395–3406. doi: 10.1242/jeb.007062 . PMID 17872993 . Retrieved November 20, 2012.{{cite journal}}: CS1 maint: multiple names: authors list (link)
1 2 3 Alexander, R. M. & A. Vernon. (1975). "The dimensions of knee and ankle muscles and the forces they exert". J Hum Mov Stud. 1: 115–123.
1 2 "The Musculature". 2004. Retrieved November 29, 2012.
1 2 Kier, William M. (1985). "Tongues, tentacles and trunks: the biomechanics of movement in muscular-hydrostats". Zoological Journal of the Linnean Society. 83 (4): 307–324. doi:10.1111/j.1096-3642.1985.tb01178.x.
↑ National Skeletal Muscle Research center (January 2006). "Muscle Physiology" . Retrieved November 29, 2012.
1 2 Azizi, Emanuel, Brainerd, Elizabeth L. and Roberts, Thomas J. (2008). "Variable gearing in pennate muscles". PNAS. 105 (5): 1745–1750. Bibcode:2008PNAS..105.1745A. doi: 10.1073/pnas.0709212105 . PMC 2234215 . PMID 18230734.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Enright, K; Morton, J; Iga, J; Drust, B (2015). "The effect of concurrent training organisation in youth elite soccer players" (PDF). European Journal of Applied Physiology. 115 (11): 2367–81. doi:10.1007/s00421-015-3218-5. PMID 26188880. S2CID 14667961.

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[one-1] 1 2 3 4 5 6 Lieber, Richard L., Friden, Jan (November 2002). "Functional and clinical significance of skeletal muscle architecture" (PDF). Muscle & Nerve. 23 (11): 1647–1666. doi:10.1002/1097-4598(200011)23:11<1647::aid-mus1>3.3.co;2-d. PMID 11054744 . Retrieved November 17, 2012.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[two-2] 1 2 3 4 Narici, Marco (April 1999). "Human skeletal muscle architecture studied in vivo by non-invasive imaging techniques: functional significance and applications" (PDF). Journal of Electromyography and Kinesiology. 9 (2): 97–103. doi:10.1016/s1050-6411(98)00041-8. PMID 10098710 . Retrieved November 20, 2012.

[three-3] 1 2 3 4 5 6 7 8 9 10 Liem, Karel F., Bemis, William E., Walker, Warren F. Jr., Grande, Lance (2001). Function anatomy of the vertebrates: an evolutionary perspective. Emily Barosse. ISBN 0-03-022369-5.{{cite book}}: CS1 maint: multiple names: authors list (link)

[four-4] 1 2 Brooks, Darrell MD (2012). "Functional microvascular muscle transplantation" . Retrieved November 20, 2012.

[five-5] Erickson, Donna, Baer, Thomas, and Harris, Katherine S. "The role of the strap muscles in pith lowering" (PDF). Haskins Laboratory: Status Report: 275–284. Retrieved November 20, 2012.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[Moore1-6] Moore, Keith L. (2018). Clinically oriented anatomy (Eighth ed.). Philadelphia: Wolters Kluwer. pp. 30–32. ISBN 9781496347213.

[six-7] Dean, Mason N., Azizi, Emanuel, Sumers, Adam, P. (2007). "Uniform strain in broad muscles: active and passive effect of the twisted tendon of the spotted ratfish Hydrolagus colliei". Journal of Experimental Biology. 210 (19): 3395–3406. doi: 10.1242/jeb.007062 . PMID 17872993 . Retrieved November 20, 2012.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[seven-8] 1 2 3 Alexander, R. M. & A. Vernon. (1975). "The dimensions of knee and ankle muscles and the forces they exert". J Hum Mov Stud. 1: 115–123.

[nine-9] 1 2 "The Musculature". 2004. Retrieved November 29, 2012.

[ten-10] 1 2 Kier, William M. (1985). "Tongues, tentacles and trunks: the biomechanics of movement in muscular-hydrostats". Zoological Journal of the Linnean Society. 83 (4): 307–324. doi:10.1111/j.1096-3642.1985.tb01178.x.

[eight-11] National Skeletal Muscle Research center (January 2006). "Muscle Physiology" . Retrieved November 29, 2012.

[eleven-12] 1 2 Azizi, Emanuel, Brainerd, Elizabeth L. and Roberts, Thomas J. (2008). "Variable gearing in pennate muscles". PNAS. 105 (5): 1745–1750. Bibcode:2008PNAS..105.1745A. doi: 10.1073/pnas.0709212105 . PMC 2234215 . PMID 18230734.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[13] Enright, K; Morton, J; Iga, J; Drust, B (2015). "The effect of concurrent training organisation in youth elite soccer players" (PDF). European Journal of Applied Physiology. 115 (11): 2367–81. doi:10.1007/s00421-015-3218-5. PMID 26188880. S2CID 14667961.

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