A ligament is the fibrous connective tissue that connects bones to other bones. It is known as articular ligament, articular larua, fibrous ligament, or true ligament. Other ligaments in the body include the: Peritoneal ligament: a fold of peritoneum or other membranes. Fetal remnant ligament: the remnants of a fetal tubular structure. Periodontal ligament: a group of fibers that attach the cementum of teeth to the surrounding alveolar bone. Ligaments are similar to tendons and fasciae; the differences in them are in the connections that they make: ligaments connect one bone to another bone, tendons connect muscle to bone, fasciae connect muscles to other muscles. These are all found in the skeletal system of the human body. Ligaments cannot be regenerated naturally; the study of ligaments is known as desmology. "Ligament" most refers to a band of dense regular connective tissue bundles made of collagenous fibers, with bundles protected by dense irregular connective tissue sheaths. Ligaments connect bones to other bones to form joints.
Some ligaments prevent certain movements altogether. Capsular ligaments are part of the articular capsule, they act as mechanical reinforcements. Extra-capsular ligaments join together in harmony with the other ligaments and provide joint stability. Intra-capsular ligaments, which are much less common provide stability but permit a far larger range of motion. Cruciate ligaments are paired ligaments in the form of a cross. Ligaments are viscoelastic, they strain when under tension and return to their original shape when the tension is removed. However, they cannot retain their original shape when extended past a certain point or for a prolonged period of time; this is one reason why dislocated joints must be set as as possible: if the ligaments lengthen too much the joint will be weakened, becoming prone to future dislocations. Athletes, gymnasts and martial artists perform stretching exercises to lengthen their ligaments, making their joints more supple; the term hypermobility refers to people with more-elastic ligaments, allowing their joints to stretch and contort further.
The consequence of a broken ligament can be instability of the joint. Not all broken ligaments need surgery, but, if surgery is needed to stabilise the joint, the broken ligament can be repaired. Scar tissue may prevent this. If it is not possible to fix the broken ligament, other procedures such as the Brunelli procedure can correct the instability. Instability of a joint can over time lead to wear of the cartilage and to osteoarthritis. One of the most torn ligaments in the body is the anterior cruciate ligament; the ACL is one of the ligaments crucial to knee stability and persons who tear their ACL seek to undergo reconstructive surgery, which can be done through a variety of techniques and materials. One of these techniques is the replacement of the ligament with an artificial material. An artificial ligament is a reinforcing material, used to replace a torn ligament, such as the ACL. Artificial ligaments are a synthetic material composed of a polymer, such as polyacrylonitrile fiber, polypropylene, PET, or polyNaSS poly.
Certain folds of peritoneum are referred to as ligaments. Examples include: The hepatoduodenal ligament, that surrounds the hepatic portal vein and other vessels as they travel from the duodenum to the liver; the broad ligament of the uterus a fold of peritoneum. Certain tubular structures from the fetal period are referred to as ligaments after they close up and turn into cord-like structures: Broström procedure
In vertebrate anatomy, hip refers to either an anatomical region or a joint. The hip region is located lateral and anterior to the gluteal region, inferior to the iliac crest, overlying the greater trochanter of the femur, or "thigh bone". In adults, three of the bones of the pelvis have fused into the hip bone or acetabulum which forms part of the hip region; the hip joint, scientifically referred to as the acetabulofemoral joint, is the joint between the femur and acetabulum of the pelvis and its primary function is to support the weight of the body in both static and dynamic postures. The hip joints have important roles in retaining balance, for maintaining the pelvic inclination angle. Pain of the hip may be the result of numerous causes, including nervous, infectious, trauma-related, genetic; the proximal femur is covered by muscles and, as a consequence, the greater trochanter is the only palpable bony structure in the hip region. The hip joint is a synovial joint formed by the articulation of the rounded head of the femur and the cup-like acetabulum of the pelvis.
It forms the primary connection between the bones of the lower limb and the axial skeleton of the trunk and pelvis. Both joint surfaces are covered with a strong but lubricated layer called articular hyaline cartilage; the cuplike acetabulum forms at the union of three pelvic bones — the ilium and ischium. The Y-shaped growth plate that separates them, the triradiate cartilage, is fused definitively at ages 14–16, it is a special type of spheroidal or ball and socket joint where the spherical femoral head is contained within the acetabulum and has an average radius of curvature of 2.5 cm. The acetabulum grasps half the femoral ball, a grip augmented by a ring-shaped fibrocartilaginous lip, the acetabular labrum, which extends the joint beyond the equator; the joint space between the femoral head and the superior acetabulum is between 2 and 7 mm. The head of the femur is attached to the shaft by a thin neck region, prone to fracture in the elderly, due to the degenerative effects of osteoporosis.
The acetabulum is oriented inferiorly and anteriorly, while the femoral neck is directed superiorly and anteriorly. The transverse angle of the acetabular inlet can be determined by measuring the angle between a line passing from the superior to the inferior acetabular rim and the horizontal plane; the sagittal angle of the acetabular inlet is an angle between a line passing from the anterior to the posterior acetabular rim and the sagittal plane. It measures 7° at birth and increases to 17° in adults. Wiberg's centre-edge angle is an angle between a vertical line and a line from the centre of the femoral head to the most lateral part of the acetabulum, as seen on an anteroposterior radiograph; the vertical-centre-anterior margin angle is an angle formed from a vertical line and a line from the centre of the femoral head and the anterior edge of the dense shadow of the subchondral bone posterior to the anterior edge of the acetabulum, with the radiograph being taken from the false angle, that is, a lateral view rotated 25 degrees towards becoming frontal.
The articular cartilage angle is an angle formed parallel to the weight bearing dome, that is, the acetabular sourcil or "roof", the horizontal plane, or a line connecting the corner of the triangular cartilage and the lateral acetabular rim. In normal hips in children aged between 11 and 24 months, it has been estimated to be on average 20°, ranging between 18° to 25°, it becomes progressively lower with age. Suggested cutoff values to classify the angle as abnormally increased include:30° up to 4 months of age. 25° up to 2 years of age. The angle between the longitudinal axes of the femoral neck and shaft, called the caput-collum-diaphyseal angle or CCD angle measures 150° in newborn and 126° in adults. An abnormally small angle is known as an abnormally large angle as coxa valga; because changes in shape of the femur affects the knee, coxa valga is combined with genu varum, while coxa vara leads to genu valgum. Changes in CCD angle is the result of changes in the stress patterns applied to the hip joint.
Such changes, caused for example by a dislocation, changes the trabecular patterns inside the bones. Two continuous trabecular systems emerging on auricular surface of the sacroiliac joint meander and criss-cross each other down through the hip bone, the femoral head and shaft. In the hip bone, one system arises on the upper part of auricular surface to converge onto the posterior surface of the greater sciatic notch, from where its trabeculae are reflected to the inferior part of the acetabulum; the other system emerges on the lower part of the auricular surface, converges at the level of the superior gluteal line, is reflected laterally onto the upper part of the acetabulum. In the femur, the first system lines up with a system arising from the lateral part of the femoral shaft to stretch to the inferior portion of the femoral neck and head; the other system lines up with a system in the femur stretching from the medial part of the femoral shaft to the superior part of the femoral head. On the lateral side of the hip joint the fascia lata is strengthened to
Gait is the pattern of movement of the limbs of animals, including humans, during locomotion over a solid substrate. Most animals use a variety of gaits, selecting gait based on speed, the need to maneuver, energetic efficiency. Different animal species may use different gaits due to differences in anatomy that prevent use of certain gaits, or due to evolved innate preferences as a result of habitat differences. While various gaits are given specific names, the complexity of biological systems and interacting with the environment make these distinctions'fuzzy' at best. Gaits are classified according to footfall patterns, but recent studies prefer definitions based on mechanics; the term does not refer to limb-based propulsion through fluid mediums such as water or air, but rather to propulsion across a solid substrate by generating reactive forces against it. Due to the rapidity of animal movement, simple direct observation is sufficient to give any insight into the pattern of limb movement.
In spite of early attempts to classify gaits based on footprints or the sound of footfalls, it was not until Eadweard Muybridge and Étienne-Jules Marey began taking rapid series of photographs that proper scientific examination of gaits could begin. Milton Hildebrand pioneered the classification of gaits; the movement of each limb was partitioned into a stance phase, where the foot was in contact with the ground, a swing phase, where the foot was lifted and moved forwards. Each limb must complete a cycle in the same length of time, otherwise one limb's relationship to the others can change with time, a steady pattern cannot occur. Thus, any gait can be described in terms of the beginning and end of stance phase of three limbs relative to a cycle of a reference limb the left hindlimb. Gaits are classed as "symmetrical" and "asymmetrical" based on limb movement, it is important to note. In a symmetrical gait, the left and right limbs of a pair alternate, while in an asymmetrical gait, the limbs move together.
Asymmetrical gaits are sometimes termed "leaping gaits", due to the presence of a suspended phase. The key variables for gait are the forelimb-hindlimb phase relationship. Duty factor is the percent of the total cycle which a given foot is on the ground; this value will be the same for forelimbs and hindlimbs unless the animal is moving with a specially trained gait or is accelerating or decelerating. Duty factors over 50 % are considered a "walk". Forelimb-hindlimb phase is the temporal relationship between the limb pairs. If the same-side forelimbs and hindlimbs initiate stance phase at the same time, the phase is 0. If the same-side forelimb contacts the ground half of the cycle than the hindlimb, the phase is 50%. Gait choice can have effects beyond immediate changes in limb movement and speed, notably in terms of ventilation; because they lack a diaphragm and salamanders must expand and contract their body wall in order to force air in and out of their lungs, but these are the same muscles used to laterally undulate the body during locomotion.
Thus, they cannot move and breathe at the same time, a situation called Carrier's constraint, though some, such as monitor lizards, can circumvent this restriction via buccal pumping. In contrast, the spinal flexion of a galloping mammal causes the abdominal viscera to act as a piston and deflating the lungs as the animal's spine flexes and extends, increasing ventilation and allowing greater oxygen exchange. Any given animal uses a restricted set of gaits, different species use different gaits. All animals are capable of symmetrical gaits, while asymmetrical gaits are confined to mammals, who are capable of enough spinal flexion to increase stride length. Lateral sequence gaits during walking and running are most common in mammals, but arboreal mammals such as monkeys, some opossums, kinkajous use diagonal sequence walks for enhanced stability. Diagonal sequence walks and runs are most used by sprawling tetrapods such as salamanders and lizards, due to the lateral oscillations of their bodies during movement.
Bipeds are a unique case, most bipeds will display only three gaits - walking and hopping - during natural locomotion. Other gaits, such as human skipping, are not used without deliberate effort. While gaits can be classified by footfall, new work involving whole-body kinematics and force-plate records has given rise to an alternative classification scheme, based on the mechanics of the movement. In this scheme, movements are divided into running. Walking gaits are all characterized by a'vaulting' movement of the body over the legs described as an inverted pendulum. In running, the kinetic and potential energy fluctuate in-phase, the energy change is passed on to muscles, bones and ligaments acting as springs. Speed governs gait selection, with quadrupedal mammals moving from a walk to a run to a gallop as speed increases; each of these gaits has an optimum speed, at which the minimum calories per meter are consumed, costs increase at slower or faster speeds. Gait transitions occur near the speed where the cost of a fast walk becomes higher than the cost of a slow run.
Unrestrained animals will move at the optimum speed
Anatomical terms of motion
Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints and specific sections of the body; the terminology used describes this motion according to its direction relative to the anatomical position of the joints. Anatomists use a unified set of terms to describe most of the movements, although other, more specialized terms are necessary for describing the uniqueness of the movements such as those of the hands and eyes. In general, motion is classified according to the anatomical plane. Flexion and extension are examples of angular motions, in which two axes of a joint are brought closer together or moved further apart. Rotational motion may occur at other joints, for example the shoulder, are described as internal or external. Other terms, such as elevation and depression, describe movement above or below the horizontal plane. Many anatomical terms derive from Latin terms with the same meaning. Motions are classified after the anatomical planes they occur in, although movement is more than not a combination of different motions occurring in several planes.
Motions can be split into categories relating to the nature of the joints involved: Gliding motions occur between flat surfaces, such as in the intervertebral discs or between the carpal and metacarpal bones of the hand. Angular motions occur over synovial joints and causes them to either increase or decrease angles between bones. Rotational motions move a structure in a rotational motion along a longitudinal axis, such as turning the head to look to either side. Apart from this motions can be divided into: Linear motions, which move in a line between two points. Rectilinear motion is motion in a straight line between two points, whereas curvilinear motion is motion following a curved path. Angular motions occur when an object is around another object decreasing the angle; the different parts of the object do not move the same distance. Examples include a movement of the knee, where the lower leg changes angle compared to the femur, or movements of the ankle; the study of movement is known as kinesiology.
A categoric list of movements of the human body and the muscles involved can be found at list of movements of the human body. The prefix hyper- is sometimes added to describe movement beyond the normal limits, such as in hypermobility, hyperflexion or hyperextension; the range of motion describes the total range of motion. For example, if a part of the body such as a joint is overstretched or "bent backwards" because of exaggerated extension motion it can be described as hyperextended. Hyperextension increases the stress on the ligaments of a joint, is not always because of a voluntary movement, it may be other causes of trauma. It may be used in surgery, such as in temporarily dislocating joints for surgical procedures; these are general terms. Most terms have a clear opposite, so are treated in pairs. Flexion and extension describe movements; these terms come from the Latin words with the same meaning. Flexion describes a bending movement that decreases the angle between a segment and its proximal segment.
For example, bending the elbow, or clenching a hand into a fist, are examples of flexion. When sitting down, the knees are flexed; when a joint can move forward and backward, such as the neck and trunk, flexion refers to movement in the anterior direction. When the chin is against the chest, the head is flexed, the trunk is flexed when a person leans forward. Flexion of the shoulder or hip refers to movement of the leg forward. Extension is the opposite of flexion, describing a straightening movement that increases the angle between body parts. For example, when standing up, the knees are extended; when a joint can move forward and backward, such as the neck and trunk, extension refers to movement in the posterior direction. Extension of the hip or shoulder moves the leg backward. Abduction is the motion of a structure away from the midline while adduction refer to motion towards the center of the body; the centre of the body is defined as the midsagittal plane. These terms come from Latin words with similar meanings, ab- being the Latin prefix indicating "away," ad- indicating "toward," and ducere meaning "to draw or pull".
Abduction refers to a motion that pulls a part away from the midline of the body. In the case of fingers and toes, it refers to spreading the digits apart, away from the centerline of the hand or foot. Abduction of the wrist is called radial deviation. For example, raising the arms up, such as when tightrope-walking, is an example of abduction at the shoulder; when the legs are splayed at the hip, such as when doing a star jump or doing a split, the legs are abducted at the hip. Adduction refers to a motion that pulls a structure or part toward the midline of the body, or towards the midline of a limb. In the case of fingers and toes, it refers to bringing the digits together, towards the centerline of the hand or foot. Adduction of the wrist is called ulnar deviation. Dropping the arms to the sides, bringing the knees together, are examples of adduction. Ulnar deviation is the hand moving towards the ulnar styloid. Radial deviation is the hand moving towards the radial styloid; the terms elevation and depression refer to movement below the horizontal.
They derive from the Latin terms with similar meaningsElevation refers to movement in a superior direction. For example
Orthopedic surgery or orthopedics spelled orthopaedics, is the branch of surgery concerned with conditions involving the musculoskeletal system. Orthopedic surgeons use both surgical and nonsurgical means to treat musculoskeletal trauma, spine diseases, sports injuries, degenerative diseases, infections and congenital disorders. Nicholas Andry coined the word in French as orthopédie, derived from the Ancient Greek words ὀρθός orthos and παιδίον paidion, published Orthopedie in 1741; the word was assimilated into English as orthopædics. Though, as the name implies, the discipline was developed with attention to children, the correction of spinal and bone deformities in all stages of life became the cornerstone of orthopedic practice; as with many words derived with the "æ" ligature, simplification to either "ae" or just "e" is common in North America. In the US, the majority of college and residency programs, the American Academy of Orthopaedic Surgeons, still use the spelling with the digraph ae, though hospitals use the shortened form.
Elsewhere, usage is not uniform: in Canada, both spellings are acceptable. Many developments in orthopedic surgery have resulted from experiences during wartime. On the battlefields of the Middle Ages the injured were treated with bandages soaked in horses' blood which dried to form a stiff, but unsanitary, splint; the term orthopedics meant the correcting of musculoskeletal deformities in children. Nicolas Andry, a professor of medicine at the University of Paris coined the term in the first textbook written on the subject in 1741, he advocated the use of exercise and splinting to treat deformities in children. His book was directed towards parents, while some topics would be familiar to orthopedists today, it included'excessive sweating of the palms' and freckles. Jean-André Venel established the first orthopedic institute in 1780, the first hospital dedicated to the treatment of children's skeletal deformities, he developed the club-foot shoe for children born with foot deformities and various methods to treat curvature of the spine.
Advances made in surgical technique during the 18th century, such as John Hunter's research on tendon healing and Percival Pott's work on spinal deformity increased the range of new methods available for effective treatment. Antonius Mathijsen, a Dutch military surgeon, invented the plaster of Paris cast in 1851. However, up until the 1890s, orthopedics was still a study limited to the correction of deformity in children. One of the first surgical procedures developed was percutaneous tenotomy; this involved cutting a tendon the Achilles tendon, to help treat deformities alongside bracing and exercises. In the late 1800s and first decades of the 1900s, there was significant controversy about whether orthopedics should include surgical procedures at all. Examples of people who aided the development of modern orthopedic surgery were Hugh Owen Thomas, a surgeon from Wales, his nephew, Robert Jones. Thomas became interested in orthopedics and bone-setting at a young age and, after establishing his own practice, went on to expand the field into general treatment of fracture and other musculoskeletal problems.
He advocated enforced rest as the best remedy for fractures and tuberculosis and created the so-called'Thomas Splint', to stabilize a fractured femur and prevent infection. He is responsible for numerous other medical innovations that all carry his name:'Thomas's collar' to treat tuberculosis of the cervical spine,'Thomas's manoeuvre', an orthopedic investigation for fracture of the hip joint, Thomas test, a method of detecting hip deformity by having the patient lying flat in bed,'Thomas's wrench' for reducing fractures, as well as an osteoclast to break and reset bones. Thomas's work was not appreciated in his own lifetime, it was only during the First World War that his techniques came to be used for injured soldiers on the battlefield. His nephew, Sir Robert Jones, had made great advances in orthopedics in his position as Surgeon-Superintendent for the construction of the Manchester Ship Canal in 1888, he was responsible for the injured among the 20,000 workers, he organized the first comprehensive accident service in the world, dividing the 36 mile site into 3 sections, establishing a hospital and a string of first aid posts in each section.
He had the medical personnel trained in fracture management. He managed 3,000 cases and performed 300 operations in his own hospital; this position enabled him to improve the standard of fracture management. Physicians from around the world came to Jones’ clinic to learn his techniques. Along with Alfred Tubby, Jones founded the British Orthopaedic Society in 1894. During the First World War, Jones served as a Territorial Army surgeon, he observed that treatment of fractures both at the front and in hospitals at home was inadequate, his efforts led to the introduction of military orthopedic hospitals. He was appointed Inspector of Military Orthopaedics, with responsibility over 30,000 beds; the hospital in Ducane Road, Hammersmith became the model for both British and American military orthopedic hospitals. His advocacy of the use of Thomas splint for the initial treatment of femoral fractures reduced mortality of compound fractures of the femur from 87% to less than 8% in the period from 1916 to 1918.
The use of intramedullary rods to treat fractures of the femur and tibi
Heredity is the passing on of traits from parents to their offspring, either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic information of their parents. Through heredity, variations between individuals can accumulate and cause species to evolve by natural selection; the study of heredity in biology is genetics. In humans, eye color is an example of an inherited characteristic: an individual might inherit the "brown-eye trait" from one of the parents. Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype; the complete set of observable traits of the structure and behavior of an organism is called its phenotype. These traits arise from the interaction of its genotype with the environment; as a result, many aspects of an organism's phenotype are not inherited. For example, suntanned skin comes from the interaction between a person's sunlight. However, some people tan more than others, due to differences in their genotype: a striking example is people with the inherited trait of albinism, who do not tan at all and are sensitive to sunburn.
Heritable traits are known to be passed from one generation to the next via DNA, a molecule that encodes genetic information. DNA is a long polymer; the sequence of bases along a particular DNA molecule specifies the genetic information: this is comparable to a sequence of letters spelling out a passage of text. Before a cell divides through mitosis, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. A portion of a DNA molecule that specifies a single functional unit is called a gene. Within cells, the long strands of DNA form condensed structures called chromosomes. Organisms inherit genetic material from their parents in the form of homologous chromosomes, containing a unique combination of DNA sequences that code for genes; the specific location of a DNA sequence within a chromosome is known as a locus. If the DNA sequence at a particular locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations.
If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism. However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by multiple interacting genes within and among organisms. Developmental biologists suggest that complex interactions in genetic networks and communication among cells can lead to heritable variations that may underlie some of the mechanics in developmental plasticity and canalization. Recent findings have confirmed important examples of heritable changes that cannot be explained by direct agency of the DNA molecule; these phenomena are classed as epigenetic inheritance systems that are causally or independently evolving over genes. Research into modes and mechanisms of epigenetic inheritance is still in its scientific infancy, this area of research has attracted much recent activity as it broadens the scope of heritability and evolutionary biology in general.
DNA methylation marking chromatin, self-sustaining metabolic loops, gene silencing by RNA interference, the three dimensional conformation of proteins are areas where epigenetic inheritance systems have been discovered at the organismic level. Heritability may occur at larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment; this generates a legacy of effect that modifies and feeds back into the selection regime of subsequent generations. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors. Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits, group heritability, symbiogenesis; these examples of heritability that operate above the gene are covered broadly under the title of multilevel or hierarchical selection, a subject of intense debate in the history of evolutionary science.
When Charles Darwin proposed his theory of evolution in 1859, one of its major problems was the lack of an underlying mechanism for heredity. Darwin believed in the inheritance of acquired traits. Blending inheritance would lead to uniformity across populations in only a few generations and would remove variation from a population on which natural selection could act; this led to Darwin adopting some Lamarckian ideas in editions of On the Origin of Species and his biological works. Darwin's primary approach to heredity was to outline how it appeared to work rather than suggesting mechanisms. Darwin's initial model of heredity was adopted by, heavily modified by, his cousin Francis Galton, who laid the framework for the biometric school of heredity. Galton found no evidence to support the aspects of Darwin's pangenesis model, which relied on acquired traits; the inheritance of acquired traits was shown to have little basis in the 1880s when August Weismann cut the tails off many generations of mice and found that their offspring continued to develop tails.
Scientists in Antiquity had a variety of ideas about heredity: Theophrastus proposed that male flowers caused f
A tendon or sinew is a tough band of fibrous connective tissue that connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments. Ligaments join one bone to bone, while tendons connect muscle to bone for a proper functioning of the body. Histologically, tendons consist of dense regular connective tissue fascicles encased in dense irregular connective tissue sheaths. Normal healthy tendons are composed of parallel arrays of collagen fibers packed together, they are anchored to bone by Sharpey's fibres. The dry mass of normal tendons, which makes up about 30% of their total mass, is composed of about 86% collagen, 2% elastin, 1–5% proteoglycans, 0.2% inorganic components such as copper and calcium. The collagen portion is made up of 97–98% type I collagen, with small amounts of other types of collagen; these include type II collagen in the cartilaginous zones, type III collagen in the reticulin fibres of the vascular walls, type IX collagen, type IV collagen in the basement membranes of the capillaries, type V collagen in the vascular walls, type X collagen in the mineralized fibrocartilage near the interface with the bone.
Collagen fibres coalesce into macroaggregates. After secretion from the cell, the cleaved by procollagen N- and C-proteinases, the tropocollagen molecules spontaneously assemble into insoluble fibrils. A collagen molecule is about 300 nm long and 1–2 nm wide, the diameter of the fibrils that are formed can range from 50–500 nm. In tendons, the fibrils assemble further to form fascicles, which are about 10 mm in length with a diameter of 50–300 μm, into a tendon fibre with a diameter of 100–500 μm. Fascicles are bound by the endotendineum, a delicate loose connective tissue containing thin collagen fibrils. and elastic fibres. Groups of fascicles are bounded by the epitenon. Filling the interstitia within the fascia where the tendon is located is the paratenon a fatty areolar tissue; the collagen in tendons are held together with proteoglycan components including decorin and, in compressed regions of tendon, which are capable of binding to the collagen fibrils at specific locations. The proteoglycans are interwoven with the collagen fibrils – their glycosaminoglycan side chains have multiple interactions with the surface of the fibrils – showing that the proteoglycans are important structurally in the interconnection of the fibrils.
The major GAG components of the tendon are dermatan sulfate and chondroitin sulfate, which associate with collagen and are involved in the fibril assembly process during tendon development. Dermatan sulfate is thought to be responsible for forming associations between fibrils, while chondroitin sulfate is thought to be more involved with occupying volume between the fibrils to keep them separated and help withstand deformation; the dermatan sulfate side chains of decorin aggregate in solution, this behavior can assist with the assembly of the collagen fibrils. When decorin molecules are bound to a collagen fibril, their dermatan sulfate chains may extend and associate with other dermatan sulfate chains on decorin, bound to separate fibrils, therefore creating interfibrillar bridges and causing parallel alignment of the fibrils; the tenocytes produce the collagen molecules, which aggregate end-to-end and side-to-side to produce collagen fibrils. Fibril bundles are organized to form fibres with the elongated tenocytes packed between them.
There is a three-dimensional network of cell processes associated with collagen in the tendon. The cells communicate with each other through gap junctions, this signalling gives them the ability to detect and respond to mechanical loading. Blood vessels may be visualized within the endotendon running parallel to collagen fibres, with occasional branching transverse anastomoses; the internal tendon bulk is thought to contain no nerve fibres, but the epitenon and paratenon contain nerve endings, while Golgi tendon organs are present at the junction between tendon and muscle. Tendon length varies from person to person. Tendon length is, in practice, the deciding factor regarding potential muscle size. For example, all other relevant biological factors being equal, a man with a shorter tendons and a longer biceps muscle will have greater potential for muscle mass than a man with a longer tendon and a shorter muscle. Successful bodybuilders will have shorter tendons. Conversely, in sports requiring athletes to excel in actions such as running or jumping, it is beneficial to have longer than average Achilles tendon and a shorter calf muscle.
Tendon length is determined by genetic predisposition, has not been shown to either increase or decrease in response to environment, unlike muscles, which can be shortened by trauma, use imbalances and a lack of recovery and stretching. Traditionally, tendons have been considered to be a mechanism by which muscles connect to bone as well as muscles itself, functioning to transmit forces; this connection allows tendons to passively modulate forces during locomotion, providing additional stability with no active work. However, over the past two decades, much research focused on the elastic properties of some tendons and their ability to function as springs. Not all tendons are required to perform the same functional role, with some predominantly positioning limbs, such as the fingers when writing and others acting as springs to make locomotion more efficient. Energy storing tendons can recover energy at high efficiency. For example, during a human stride, the Achilles tendon stretches as the ankle joint dorsiflexes.
During the last portion of the stride, as the foot plantar-flexes (pointing the