MUSCULOSKELETAL SYSTEM

The skeletal system works as a scaffold that performs tasks of enhancing the body, protecting the soft tissues which make up the body, and facilitating the movement of an organism. Three different skeletal designs are existed to accomplish the functions: hydrostatic Skeleton, exoskeleton, and endoskeleton.

The musculoskeletal system is mainly responsible for giving the body its form, in addition to movement in collaboration with muscles and joints, that support them as levers, and provide them with a place to attach. Other types of functions related to bones characteristics include haematopoiesis, that takes place in bone marrow. It also stores some minerals such as calcium and iron and regulates some hormones. Finally, it assists in the protection of vital organs like viscera, and brain. 

The parts of the axial skeleton include the bones of the skull, hyoid bone of the throat, ossicles of the middle ear, the thoracic cage (ribcage), and the vertebral column; they present the central axis of the body. The axial skeleton is responsible for supporting and protecting the brain, the spinal cord, and the organs in the ventral body cavity. It also acts as a surface to which the muscles responsible for the movement of the head, neck, and trunk are attached, performs respiratory movements, and stabilizes parts of the appendicular skeleton (Fig 2.3).

The Skull 

It consists of 22 bones, and they function to back the structures of the face and to safeguard the brain. These bones are split into two divisions: cranial bones and facial bones (Fig 2.4). The cranial bones form the cranial cavity which surrounds the brain and acts as a surface where the muscles of the head and neck are attached. They are eight bones including the frontal bone, two parietal bones, two temporal bones, occipital bone, sphenoid bone, and the ethmoid bone. The facial bones, however, form the face and the facial cavities for the sense organs (eyes, nose, and mouth), They are 14 including the nasal bones, maxillary bones, zygomatic bones, palatine, vomer, lacrimal bones, the inferior nasal conchae, and the mandible (Fig 2.6). 

The Vertebral Column

It encloses the spinal cord with the purpose of protection and acts as a backup for the head a surface where the ribs and muscles of the back and neck are attached. 26 bones are the components of the adult vertebral column: the 24 vertebrae, the sacrum, and the coccyx bones. The adult vertebrae have 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae. In the adult, five vertebrae form the sacrum, and they are fused into one. Similarly, the coccyx is typically 3–4 vertebrae consolidated into one. Around the age of 70, the consolidation of the sacrum and the coccyx into one vertebra may occur. 

The Thoracic Cage

It represents the bones of the chest. It is composed of sternum, ribs, costal cartilages, and thoracic vertebrae. The thoracic cage surrounds the organs of the thoracic cavity including the heart and lungs with the purpose of protection. In addition, it consolidates the shoulder girdles and upper limbs, and acts as a surface to which the muscles of the back, diaphragm, chest, neck, and shoulders are attached. 

The bones of the upper limbs and the lower limbs represent the components of the appendicular skeleton. Also, the pectoral girdle, or shoulder girdle, that connects the upper limbs to the body, and the pelvic girdle that connects the lower limbs to the body are included in this type of skeletal system (Fig 2.3). 

The Pectoral Girdle

The upper limbs are attached to the axial skeleton through the pectoral girdle, which is composed, in the human body, of the clavicle (or collarbone) in the anterior, and the scapula (or shoulder blades) in the posterior (Fig 2.19). 

The Pelvic Girdle

The lower limbs are attached to the axial skeleton through the pelvic girdle. Because of its function to afford the weight of the body and to allow for the movement, strong ligaments securely connect the pelvic girdle to the axial skeleton. The pelvic girdle also connects the lower limb to the body through deep sockets of robust ligaments. Three pairs of bones: the ilium, ischium, and pubis are fused together to form the adult hip bones.

The Upper Limb 

It consists of three regions: the arm (shoulder to elbow), the forearm (ulna and radius), and the wrist and hand. The longest and the largest bone of the upper limb is the humerus, and it is considered as the only bone of the arm. The place at which two bones are connected together is called articulation. The shoulder is the articulation point of the humerus with the scapula while the elbow is the articulation point with the forearm. The forearm starts from the elbow till the wrist, and it is composed of two bones: the ulna and the radius. The radius lies along the lateral (thumb) side of the forearm and the elbow is considered as its articulation point with the humerus. The ulna lies on the medial side (pinky-finger side) of the forearm, and it exceeds the radius in length (Fig 2.24). The elbow is the articulation point of the ulna with the humerus. The radius and ulna, in turn, articulate with the carpal bones and with each other. It allows, in vertebrates, for a variable degree of turnover of the carpus in regard to the long axis of the limb. The eight bones of the carpus (wrist), the five bones of the metacarpus (palm), and the 14 bones of the phalanges (digits) form the skeletal system of the hand. Three phalanges are the components of each finger, except for the thumb which has only two.

The Lower Limb

thigh, the leg, and the foot compose the lower limb in which the bones are represented by the femur (thigh bone), patella (kneecap), tibia and fibula (bones of the leg), tarsals (bones of the ankle), and metatarsals and phalanges (bones of the foot). The bones of the lower limbs bear the resulting forces from the movement, and they support the entire weight of the body. Accordingly, they are stronger and thicker than those of the upper limbs (Fig. 2.31). The femur is the longest, densest, and robustest bone in the body. It along with the pelvis compose the hip joint at the near end. At the distal end, the femur along with tibia, and patella compose the knee joint. The patella, or kneecap, is located anteriorly to the knee joint with a triangular shape. It reduces friction to improve the process of knee extension. The patella is ingrained in the tendon of the femoral extensors (quadriceps). The tibia is the second largest bone in the human body after the femur. It lies directly below the knee. The near end is the articulation point of the tibia with the femur, whereas the distal end is its articulation point with the fibula and the tarsal bones. The fibula presents the lateral part of the ankle joint. It has no articulation point with the femur and it does not responsible for weight. It serves as a surface for muscle attachment. The seven bones of the ankle compose the tarsals. The ankle acts as a carrier for the weight of the body from the tibia and the fibula to the foot. The five bones of the foot form the metatarsals. The 14 bones of the toes form the phalanges. Three phalanges are the components of each toe, except for the big one which has only two.

The main consequences of bone development essentially appear in endochondral and membranous forms. According to the general shape of the bone, the skeletal system is split into the following categories (Fig. 4.1):

Long Bones

They include compacted bones with little marrow such as those in the limbs (humerus, ulna, radius, fibula, femur, metacarpal bones, and phalanges). These bones tend to have a supportive role for weight and movement. Through the process of endochondral ossification, long bones develop when the hyaline cartilage plate is slowly substituted. The two ends of the bones known as the epiphyses are connected via diaphysis, a thick, compact bone which surrounds the marrow cavity. The epiphysis is mainly defined as a spongy bone enveloped by a thin layer of compact bone; the joint is the place where the articular ends meet. The metaphysis is located on the border of the diaphysis and the epiphysis at the neck of the bone, and it is the place of growth during development (Fig.4.2). 

Short Bones

They appear with a more or less cuboid shape when vast spongy bone and marrow are enveloped by a thin external layer of a compact bone. The carpal bones (Fig. 4.3) and tarsal bones (Fig. 4.4) are included in this group.

Flat Bones

They include the thin and curved bones and are composed of an inner layer of spongy bone and two outer layers of compact bone which in turn cover both spongy bone and bone marrow space. They consequently have an essential protective role. Skull bones, sternum, ribs, and scapulae are included in this group. The articular surfaces of this type of bone are enveloped by fibrocartilage (Fig.4.5).

Sesamoid Bones

They are an integral part of tendons, such as the patella bone in the knee. They are located where the tendons cross at the end of long bones. Sesamoid bones play a basic role in protecting the tendons from excess wear and stress by alleviating friction (Fig.4.6).

Irregular Bones

They indicate bones with an unusual shape as their name implies and include the bones of the pelvis, spine, vertebrae, and coxa. They tend to have a protective role towards organs or tissues. It appears when a mass of mostly spongy bone is covered by a thin layer of compact bone. This group is categorized by bone content, and not by shape (Fig 4.5).

Compact and spongy osseous tissue exists in most bones, but its concentration and distribution differ according to the overall function of the bone. Compact and spongy bones almost have the same matrix materials and cells, but they are organized in a different way. 

Compact Bone 

It is the stronger of the two types of osseous tissue. It is dense and heavyweight with an ability to bear pressing forces. It is directly connected to the periosteum forming the outer cortex of all bones (Fig 4.8). Through taking a microscopic look, the very tiny component or the microscopic structural unit of the compact bone appears as a group of concentric circles that are highly organized looking like tree trunks. Each group or each unit is called an osteon or a Haversian system. Each circle inside this unit is known as a lamella (plural = lamellae), and it is composed of collagen and calcified matrix. The collagen fibers make the osteons able to resist twisting forces in different directions through running at perpendicular angles to each other. The central canal or what is called Haversian canal is running down the center of each osteon. It contains lymphatic vessels, blood vessels and nerves which are split into small branches at right angles through what is called Volkmann’s canals, to reach its destination in the periosteum and endosteum. Each central canal has a lining of the endosteum giving the osteons a chance to be reconstructed and remodelled over time. The lacunae which are existed at the borders of adjacent lamellae besiege their osteocytes and the canaliculi of all lacunae are connected together and ultimately to the central canal. In this way and in spite of the immune calcified matrix, nutrients are transferred to the osteocytes and wastes are removed from them.

Spongy Bone

As you move, in long bones, from the outer layer of cortical compact bone to the inner medullary cavity, the bone transfers to spongy bone. Spongy bone (also called cancellous bone), like compact bone, is made of osteocytes besieged in lacunae, but in contrary to compact bone which appears in concentric circles, the lacunae take the form of a lattice-like network of matrix spikes called trabeculae (singular = trabecula) which are enveloped by the endosteum that can facilitate the process of remodeling them. Each trabecula appears along lines of stress alleviating the compressive forces and directing them to the more solid compact bone. Spongy bone is lightweight and flexible, and it is known with its ability to accommodate the different needs of human body. It tends to have supportive task for having open spaces (Fig 4.8). In addition, red bone marrow appears in some spaces of spongy bones, and it is protected by the trabeculae, where hematopoiesis occurs.

The superficial features of bones differ according to their location and function in the body. Articulations, Holes and Projections are the three general categories of bone features. An articulation is the meeting point of two bone surfaces, and the articulation is called joint. These surfaces work together in harmony and tend to adjust to one another. In other words, one is rounded, and the other is cupped to facilitate the function of the articulation. A hole is a slit or an opening through which blood vessels and nerves can enter the bone. The size and shape of a hole can mirror, with other markings, the size of the vessels and nerves that break through the bone at these points. A projection is a region of a bone that protrudes above the surface of the bone. It is the area where tendons and ligaments are attached to the surface. The size and shape of a projection are signs of the forces exerted through the attachment to the bone.  Bone features are crucial to anatomists, physicians, and surgeons because they give anatomic indications regarding the surrounding environment including the structures.  

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The joint refers to the point at which two or more bones meet together. It is also called an articulation. Joints function as a facilitator for movement, such as the movement of limbs, and a provider for stability, such as the stability in the bones of the skull. Joints can be classified in two ways the 1st way is based on their function while the 2nd is based on their structure. 

According to the structure, joints are split into fibrous, bony, cartilaginous, and synovial joints. This classification relies on the material composing the joint and the existence or nonexistence of a cavity in the joint.

Fibrous Joints

The bones of fibrous joints are characterized by the absence of cavity and accordingly they do not move, or they are able to move slightly. These bones are connected together by fibrous connective tissue. Sutures, syndesmoses, and gomphoses represent the three types of fibrous joints. Sutures are joints that are existed only in the skull. The short fibers of connective tissue represent the components of fibrous joints and they connect the skull bones firmly in place. Syndesmoses are joints in which the bones are hold by fibers of connective tissue that are longer than those that connect the sutures and so they are characterized by more movement than sutures. The joint of the tibia and fibula in the ankle is an example of a syndesmosis. Gomphoses are observed between teeth and their sockets, and they indicate how a tooth and its socket comfort together. It happens. The periodontal ligament is the connective tissue that links the tooth to the socket (Fig 5.1).

Cartilaginous Joints

The bones of cartilaginous joints are connected by cartilage. Synchondroses and symphyses represent the two types of cartilaginous joints. The bones, in a synchondrosis, are hold by hyaline cartilage. The epiphyseal plates of growing bones in children are the places where synchondroses are existed. In symphyses, whereas the bones are connected by fibrocartilage, the ends of the bones are covered by hyaline cartilage. The places between vertebrae are the sites where symphyses are found. Cartilaginous joints are characterized by little movement. (Fig 5.2).

Synovial Joints

Synovial joints are the only joints which are characterized by the existence of a space or a cavity between the adjoining bones. This space is called as the synovial cavity, and it is full of synovial fluid which acts as a lubricator for the joint, alleviating friction between the bones and giving a chance for greater movement. The bones of Synovial joints are connected by the ligaments of an articular capsule. These capsule envelopes the entire bone and it consists of connective tissue that facilitates movement of the joint. On the other hand, an articular cartilage, called a hyaline cartilage, covers the ends of the bones. Examples of synovial joints are shoulders, elbows, and knees. Among the three structural joint types, synovial joints can perform the greatest movement (Fig 5.3).

Based on the shape and structure, six different categories are further included in synovial joints as follows: planar, hinge, pivot, condyloid, saddle, or ball-and-socket joints. The shape of the joint affects the type of movement permitted by the joint.

Planar Joints

The bones of planar joints have surfaces with articulating features that can be slightly curved or flat. It can be called gliding joints for its ability to perform gliding movements. However, rotation is not involved in the movements of planar joints; it is due to the range of motion which is limited. The tarsal bones of the foot and the carpal bones in the hand are examples of Planar joints (Fig 5.10). 

Hinge Joints

In hinge joints, one bone is observed moving while the other is fixed and stable, like the hinge of a door. In other words, the end of one bone is somewhat rounded and it fits into the end of another bone which is somewhat hollow. A hinge joint is found in the elbow. Also, the knee is sometimes categorized as a modified hinge joint (Fig 5.10). 

Pivot Joints

Pivot joints appear through a ring formed by the end of one bone inside which the rounded end of another bone is inserted. This rounded end moves around its axis allowing for rotational movement. The joint of the first and second vertebrae of the neck is an example of a pivot joint; the head can move back and forth. Also, the joint of the wrist is another example of pivot joint; the palm of the hand can be turned up and down (Fig 5.10). 

Condyloid Joints

In condyloid joints, the end of one bone with an oval shape is embedded into the end of another bone with a similarly oval-shaped hollow. An ellipsoidal joint is another name of condyloid joints. Through this structure, the angular movement is allowed along two axes. Condyloid joints are found in the joints of the wrist and fingers, which can move both side to side as well as up and down (Fig 5.10). 

Saddle Joints 

Saddle joints consist of the ends of two bones with a saddle that has concave and convex parts fitting into each other. The angular movement is allowed in this type of joints but with a greater range of motion that that in condyloid joints which give a chance for the same movement. The thumb joint is an example of a saddle joint (Fig 5.10). 

Ball-and-Socket Joints

A rounded, ball-like end of one bone and a cuplike socket of another bone are the main components of ball-and-socket joints. In this structure, the range of motion is the greatest and hence, all movement types in all directions are permitted. The shoulder and hip joints are examples of ball-and-socket joints (Fig 5.10). 

Different types of movements are observed in Synovial Joints because of the wide range of motion. Gliding, angular, rotational, or special movement are four different types of movement produced by synovial Joints (Fig. 5.4).

Gliding Movement

When the surfaces of relatively flat bones move past each other, gliding movements are produced with very little rotational or angular movement of the bones. The joints that produce gliding movements are those of the carpal and tarsal bones (Fig. 5.5).

Angular Movement

When a change in the angle between the bones of a joint occurs, angular movements are produced. Different types of angular movements are observed including flexion, extension, hyperextension, abduction, adduction, and circumduction. Decreasing the angle between the bones can produce flexion or bending (Fig. 5.6). Moving the wrist to stir the hand toward the forearm and moving the forearm upward at the elbow are instances of flexion. In contrary to flexion, extension is to increase the angle between the bones of a joint as in straightening or unbending a limb after flexion (Fig. 5.6).  Hyperextension refers to further extension of a regular anatomical position. As in bending the wrist to move the hand away from the forearm and moving the neck back to look upward. Abduction is an angular movement produced through moving a bone away from the midline of the body (Fig. 5.7).. Moving the arms or legs laterally to lift them straight out to the side is an example of abduction. Adduction occurs when a bone moves toward the midline of the body as in the movement of the limbs inward after abduction. Circumduction is the movement of a limb in a circular motion, as in moving the arm in a circular motion (Fig. 5.7).

Rotational Movement

When a bone rotates around its longitudinal axis, rotational movement occurs. Medial rotation is produced in case rotation happens toward the midline of the body while lateral rotation is seen in case rotation happens away from the midline of the body. An example of rotation is found in the movement of the head and legs from side to side (Fig. 5.8).

Special Movements

Special movements refer to the movements which cannot be categorized as gliding, angular, or rotational. Moving the soles of the feet inward, toward the midline of the body is called Inversion (Fig. 5.9). On the other hand, moving the soles of the foot outward, away from the midline of the body refers to Eversion (Fig. 5.9). Moving a bone anteriorly in the horizontal plane is Protraction. After protection, the movement of a joint back into a position is called retraction. The movement of a bone upward is known as elevation, as in shrugging to leave the scapulae. When the scapulae return to the normal position, it is called depression which is a movement downward of a bone. Bending the ankle for the toes to be lifted toward the knee is called dorsiflexion (Fig. 5.9).When standing on the toes and the heel is lifted, the ankle is bending, and it is called plantarflexion (Fig. 5.9). Moving the radius and ulna bones of the forearm for the palm to face forward is known as supination. In contrary when the palm faces backward it is called pronation. 

Tendons are cords of dense connective tissues consisting of parallel bundles of collagenous fibers, this allows tendons to tolerate high tensile strength and have resistance against longitudinal force. On each side of ends of muscles, there are tendons attaching muscles proximally at point of origin on the periosteum of bones, and distally at point of insertion. Tendons are flexible enough to transfer the mechanical force to the bones, acting as mechanical bridge. 

Similar to tendons, ligaments are connective tissue, consisting of strong regular collagen fibers. They exist mostly in joints, gathering bones and stabilizing joints by holding two ends of bones. Ligaments are also found in other organs like the womb, which stabilize the womb in pelvis and transmit neurovascular bundles. 

Capsular ligaments 

Articulation ligaments help in stabilizing and maintaining integrity of joints. A local thickening of the articular capsule is called capsular ligament. An example of the capsular ligament is the iliofemoral ligament of the hip joint.

Intracapsular ligaments 

Intracapsular ligaments are located inside the fibrous capsule of joints. It allows a wider range of movement by connecting surfaces of joints. Examples include anterior and posterior cruciate ligament of the knee joint.

Extracapsular ligaments 

Extracapsular ligaments are located outside the fibrous capsule of joints. They could be located proximal to joints like medial collateral ligament of the ankle joint, or distal like vertebral ligaments. Their main function is maintaining stability of structures composing joints and preventing dislocations. 

Cartilage is a type of connective tissue composed of dense network of collagen fibers embedded in gelatinous ground substance full of proteoglycan and elastin fibers, that gives cartilage a more flexible characteristics than bones. Specialized cartilage cells are called chondrocytes, diffused throughout the cartilage structure. In specific, the musculoskeletal system contains type of cartilage called articular cartilage, which is a highly specified tissue lines the articulating surfaces of bones in joints. Articular cartilage is not perfused with vessels, lymphatics and it is not innervated. Mainly, it provides a smooth, lubricated surface for friction between bones, and facilitates the transmission of loads. Note: Articular cartilage is not capable of full recovery when an injury happens. 

Hyaline cartilage 

It has a glossy blue-white appearance because it consists of type II collagen fibers. It is mostly found at the end of bones in moveable joints as articular cartilage. It is also found in 

the nose, larynx, trachea and ribs.

Elastic cartilage 

Elastic cartilage has a yellowish appearance, and it is more compliant cartilage due to the presence of elastic fibers in addition to collagen fibers. It forms the external ear, the auditory tube of the middle ear, and the epiglottis.

Fibrocartilage 

It is stronger and tougher cartilage consisting of collagen fibers type I. It is found in intervertebral disks, pubic and at the insertions of ligaments and tendons.

Bursa (plural, bursas or bursae) are lubricated sac filled with fluid, found between bones and muscles and tendons around joints. It is lined by a synovial membrane. It is of great importance in reducing pressure applied on joints and lowering resistance against movement. Bursae are mostly found near large joints in the extremities, a common example is the bursae of the knee joint, which is the suprapatellar bursa, found superior to the patella, between the femur and the tendon of the quadriceps femoris muscle. These structures reduce friction between knee joint components in order to do flexion and extension movements of the joint smoothly.

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Muscles form a major tissue type, composed of fibers enveloped with fibrous tissue called fascia. Muscle fibers vary in shape, size, and mode of action to adapt with different movements need to be achieved. A human body contains more than 600 muscles, consisting of around half of the body weight of the average human. Different types of muscle tissue incorporate to achieve many body movements. 

Muscles are scattered all over the body attached to bones, skin, or other muscles. This attachment is done either by tendons, which are tough cord like structure, or aponeuroses, that consists of thin sheaths containing less vessels and nerves than tendons. The palm of the hand, ventral abdominal and the dorsal lumbar regions, are some body regions with aponeuroses. In order to accomplish balance movements, most muscles movements have contrary movements from other muscles. The biceps and triceps muscles are an example of two contrary movements: biceps are responsible for arm flexion, while triceps are responsible for arm extension. 

Skeletal Muscles

Skeletal muscles are considered voluntary muscles that let human bodies move and give them posture. They help in performing every day activities, in addition, they help maintain skeleton stability. There are a wide range of sizes and shapes and arrangement of fibers of skeletal muscles, to meet all body movements demands. Stapedius muscle, one of the intratympanic muscles, is considered as the smallest muscle in the body; while largest muscle in body is the gluteus maximus. Skeletal muscles are characterized by alternating between light and dark bands, which give them a striated appearance. They are attached to bones by tendons across a connecting joint. 

Smooth Muscles

Smooth muscles are abundant in viscera organs such as stomach, intestine, bladder and blood vessels. They are unstriated involuntary muscles. They are built of thin layers or sheets that form walls of internal organs. 

Cardiac Muscles

Unlike skeletal muscles and smooth muscles, although cardiac muscle is involuntary, it is unstriated muscle. Myocardium contains cardiac muscle fibers and functions as a pumper for blood via blood vessels to perfuse body organs. Myocardium forms a great portion of the heart wall in addition to other cardiac components. 

A connective tissue sheath called epimysium surrounds the whole muscle. The muscle consists of many bundles, each bundle consists of hundreds to thousands of muscle fibers. Each fiber contains many myofibrils, and myofibrils in turns contain myofilaments. When myofibrils bundled together, they form a consistency of repeated thick and thin filaments and appear in striated form. Skeletal muscle fibers are classified into two types: 

Type 1

They are rich in capillaries and organelles responsible for energy production (mitochondria). They use aerobic respiration. Due to the presence of mitochondria and myoglobin they appear dark. 

Type 2-A

They use mainly aerobic respiration, but can also use anaerobic respiration, thus it is called intermediate fibers. They have fewer mitochondria making them light.

Type 2-B

These fibers also appear white due to containing even less mitochondria than type 2A. They use anaerobic glycolysis to produce energy. They store amounts of glycogen and produce short bursts of movement. 

Cardiac muscle has unique characteristics. The heart is capable of having a its own regular rhythm due to specialized cardia muscles called pacemakers. They generate pulses from certain points in heart, these pulses are conducted through organized wavelike pattern to other cardiac muscle fibers causing contraction of the heart. Pacemakers send signals constantly; they may fasten or slow heartbeats due to physiological or medical conditions. Smooth muscle fibers have an oval shape and are much shorter than skeletal ones. 

Skeletal muscles have two ends: proximal end is the origin point, it is least moveable and mostly fleshy; distal end is the insertion point, and it is most moveable and contain mostly fibrous tissue. 

Types of Attachments

Many structures participate in binding muscles to other components: Tendons usually bind muscles to bones, they consist of fibrous cord-like connective tissue, they help in producing movement; Aponeurosis is a thin sheath of connective tissue lies on the surface of some muscles; Raphe is a seamlike union between two tendinous ends of a flat muscle such as mylohyoid raphe. 

The arrangement of muscle bundles specifies direction of muscles. When bundles are arranged in the same directions of muscle axis, it would be parallel; these muscles have more range of movement and are less powerful. When parallel muscles have large central region with tapered ends it would be a fusiform, like biceps muscle. Circular muscles called sphincters, they increase and tighten the size of opening or orifice. A convergent or triangle muscle is the muscle which has a widespread expansion to one attachment. Pennate muscles have feather like shape which fibers are oblique to line, they could be unipennate, bipennate and multipennate. Pennate muscles are more powerful and have less range of movement. 

Usually, many muscles involve achieving one action. However, one muscle is responsible principally for the movement and it is called prime mover or Agonist; for example, flexion movement of elbow joint and forearm is done by Biceps Brachii as a prime mover. A muscle that assists the prime mover is called synergist, such as Brachialis that assists Biceps Brachii in flexion of the forearm. A muscle that acts with an opposite movement of the prime mover is called antagonist, it holds limb position and controls rapid movement; an example is Triceps Brachii as an antagonist for flexion movement that produces extension of the elbow joint and forearm. Fixator muscles are responsible for preserving prime mover origin to act efficiently, an example is Deltoid muscle for Biceps prime mover muscle.

Muscle fibers require stimuli to produce contraction such as nerve impulses, or they contract spontaneously by pacemaker like cardiac muscles. These actions initiate a depolarization process, which involves shifting in electrical charges around the plasma membrane of muscle. After that a sequence of actions results in contraction of muscle fibers. Eventually, energy is produced, and movement is achieved. When stimuli input resolves, the muscle relaxes. 

Fast and Slow Twitch

ATP is a molecule that produces energy in the human body as a result of many types of metabolism processes. Muscle fibers depend on ATP molecules to act, but muscle fibers differ in the speed they contract according to how quickly it acts on ATP. Thus, skeletal muscle fibers are classified to Fast twitch and slow twitch fibers. 

St fibers: are type 1 fibers. They have the most durability effect. Because St fibers support aerobic metabolism, they tire at a slower rate. Thus, they are mostly responsible for stabilization and postural control. They’re also used in endurance activities, such as running, cycling, or swimming. 

Ft fibers: are types 2-A and 2-B fibers. FT fibers break down ATP twice as fast as ST fibers. Ft fibers depend partially on aerobic metabolism, and some are highly dependent on anaerobic metabolism. As a result, they are quicker to fatigue but are more capable of producing a greater and quicker force. Ft fibers achieve activities include sprinting and weightlifting. 

Skeletal muscles comprise of a mix between these two types. The ratio of each one differs greatly between individuals. For example. Endurance athletes often have more ST fibers, while athletes like sprinters or powerlifters often have more FT fibers.

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Fractures occur for different reasons including the application of significant force the occurrence of repetitive trauma over a long time, the weak nature of a bone itself such as with osteoporosis, or structural problems as in Paget's disease. Vertebral fractures associated with high-energy trauma and cancer, wrist fractures and hip fractures associated with osteoporosis as well as fractures of long bones are among common fractures. The type and location of fractures can determine the degree of pain and seriousness. Complications of serious fractures may include flail chest, fat embolism or compartment syndromes.

Fractures are classified by types and nature whether they are open or closed. Closed fracture is when the bone does not tear through the skin, and still being invisible to the naked eye. Open fracture is when the bone tears through the skin making an obvious cut (Fig 6.2). It might rebound into the skin, but the skin tear still remains. Displaced fracture is when the bone deviates from its original alignment with the adjacent bones non-displaced fracture is when the bone does not deviate from its original aligned position (Fig 6.4). See table 6.1 for the common types of fractures. 

Bones and bone tissues are common sites for cancer. "primary" cancers are those that arise in bone, and they are rare. "Secondary" cancers are the metastases that develop within bone, with the most common locations in lung, breast, prostate, and kidney. There are two types of secondary cancers that can affect bone: the first type can destroy bone and it is called a "lytic" cancer while the second can create bone and it is referred to as a "sclerotic" cancer. Also, there are cancers of the bone marrow which arise inside the bone with bad effect on bone tissue; instances are multiple myeloma and leukemia. Bones may also be sensitive to cancers in different parts of the body which may emit parathyroid hormone or parathyroid hormone-related peptide. This results in the increasing process of bone reabsorption that can lead to bone fractures (Fig 6.5).  Cancers cause the weakness and distortion of bone tissues which become more prone to fractures. Accordingly, compression of the spinal cord, devastation of the marrow with the possibility of immunosuppression, bruising and bleeding may occur causing bone pain. In case the cancer is metastatic, other symptoms based on the location of the original cancer might be observed. 

When there is reduced bone mineral density (Fig 6.10). Osteoporosis occurs, a disease increasing the likelihood of fractures. It is defined in women by the World Health Organization (WHO) as a bone mineral density of 2.5 standard deviations below peak bone mass, relative to the age and sex-matched average. Dual energy X-ray absorptiometry (DEXA) is used to measure this density with the term "established osteoporosis" including the presence of a fragility fracture. Women after menopause are more prone to Osteoporosis and it is called "postmenopausal osteoporosis". However, it may develop in men and premenopausal women as a result of smoking and certain medications, specifically glucocorticoids or with the existence of particular hormonal disorders and other chronic diseases No symptoms appear for Osteoporosis until a fracture occurs. Consequently, DEXA scans are often resorted to in people with one or more risk factors, who are at risk of fracture and have developed osteoporosis. Advice to stop smoking, exercise regularly, have a healthy diet and decrease alcohol consumption are included in Osteoporosis treatment. Calcium and trace mineral supplements may also be advised, as vitamin D. Bisphosphonates, Strontium ranelate, and hormone replacement therapy are included in the medications used to treat Osteoporosis. 

When the joint has an unordinary separation in bones, joint dislocation also namely luxation occurs, and it comes as a result of sudden shock or trauma as a fall or force. Muscles, nerves, tendons, and ligaments are badly affected by joint dislocation which can happen in any minor joint (toes, fingers, etc.) or major joint (shoulder, knees, etc.). Shoulder dislocation is the most common. Subluxation refers to a partial dislocation (Fig 6.6). A skilled manipulation to bring the bones back to their normal is considered as a treatment for joint dislocation. It is called closed reduction and it should be conducted by trained medical professionals only, because of the sensitivity of soft tissues, nerves and vascular structures around the dislocation that can damage by the reduction. Even after treatment, the likelihood of another dislocation in the future is still existed as the ligaments fixing the bones in the correct position can be damaged or loosened, making it easier for the joint to be dislocated once again. Some congenital conditions, such as Ehlers-Danlos Syndrome or hypermobility syndrome increase the probability of individuals’ exposure to dislocations. Intense pain, difficulty in moving joint, bruising, redness or deformity of joint area, joint instability, and reduced muscle strength are symptoms of any type of dislocation.

When bursae are over irritated due to chronic overuse, trauma or infection, they become inflamed. Inflammation of bursae is called bursitis (burs/o, itis: inflammation). Well-known conditions of bursitis are "housemaid's knee", "soldier's heel", or "tennis elbow", thus the most commonly affected joints are the shoulder, elbow, knee and hip. Common symptoms include tenderness, pain, redness, or swelling of the affected area, difficulty in movement, and fever if infectious pathogens are present. Treatment consists of many aspects, medications such as NSAIDs, conservative therapy such as physiotherapy and rest, and as second line,  an injection of a corticosteroid into the joint may be required.

Sprain is a common injury most happened most in ankle. Fractures or dislocations do not exist in this injury. A stretching or torn event happens secondary to trauma in joint. Symptoms include swelling, pain, instability of joint, and loss of the ability of move or function. Treatments include applying ice, getting enough rest, and using compression bandages.

It is a chronic condition in which an individual suffers from pain and tenderness all over the musculoskeletal system. There is no obvious cause for fibromyalgia, it is suggested that genetic mutations may play a role in it. Risk factors include age, sex, chronic illnesses, stress and traumas. People who are above 40, females, affected with lupus, rheumatoid arthritis, or ankylosing spondylitis are more susceptible to suffer from fibromyalgia. There is no cure, but symptomatic treatment can help some patients.

Muscle cramps or muscle spasms are sudden painful contractions of one or more muscles. They could range from mild to severe, causing intensive pain and preventing the use of muscle. Sometimes muscle cramps could be idiopathic, without obvious reason; some causes include doing exercise in hot environment, dehydration, muscle stain or inappropriate use of muscle, even medications and some medical conditions like calcium deficiency could cause muscle cramps. Some risk factors may increase the probability of suffering from muscle cramps such as age, pregnancy, or having diabetes, or nerve, liver, or thyroid disorders. 

MD is a group of more than 30 genetic progressive diseases that involve a permanent weakness in muscles. All types of MD are based on mutations in the genetic code for dystrophin, which is an important element of sarcolemma stability. Symptoms vary in onset from infancy through to adulthood. The most common form of muscular dystrophy is Duchenne’s muscular dystrophy (DMD). AS other muscle dystrophies, it happens due to abnormality in dystrophin gene, that located on chromosome X. Thus, it affects males predominantly. One main point in (DMD), is once a child’s first walk, symptoms of DMD mostly appear. By age 10 to 12 years these children lose their ability to ambulate. DMD is terminal disease, which death occurs in the late teens or early adulthood. The most likely cause of death is cardiopulmonary compromise. Thus, patients need to get respiratory and cardiac assessment regularly, and may be put on ventilator. 

It is a very common injury that involves overstretching of a muscle and/or tendon. Some cases may be severe such a tear could happen. It must be differentiated from fractures or dislocations of joints. Symptoms include weakness and muscle spasms in addition to pain. Initial treatment consists of rest, ice, compression, and administer nonsteroidal anti-inflammatory drug therapy. Physical therapy could also be essential. 

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