Human taxonomy is the classification of the human species within zoological taxonomy. The systematic genus, Homo, is designed to include both anatomically modern humans and extinct varieties of archaic humans. Current humans have been designated as subspecies Homo sapiens sapiens, differentiated from the direct ancestor, Homo sapiens idaltu. Since the introduction of systematic names in the 18th century, knowledge of human evolution has increased drastically, a number of intermediate taxa have been proposed in the 20th to early 21st century; the most accepted taxonomy groups takes the genus Homo as originating between two and three million years ago, divided into at least two species, archaic Homo erectus and modern Homo sapiens, with about a dozen further suggestions for species without universal recognition. The genus Homo is placed in the tribe Hominini alongside Pan; the two genera are estimated to have diverged over an extended time of hybridization spanning 10 to 6 million years ago, with possible admixture as late as 4 million years ago.
A subtribe of uncertain validity, grouping archaic "pre-human" or "para-human" species younger than the Homo-Pan split is Australopithecina. A proposal by Wood and Richmond would introduce Hominina as a subtribe alongside Australopithecina, with Homo the only known genus within Hominina. Alternatively, following Cela-Conde and Ayala, the "pre-human" or "proto-human" genera of Australopithecus, Ardipithecus and Sahelanthropus may be placed on equal footing alongside the genus Homo. An more radical view rejects the division of Pan and Homo as separate genera, which based on the Principle of Priority would imply the re-classification of chimpanzees as Homo paniscus. Prior to the current scientific classification of humans and scientists have made various attempts to classify humans, they offered definitions of schemes for classifying types of humans. Biologists once classified races as subspecies, but today anthropologists reject the concept of race and view humanity as an interrelated genetic continuum.
Taxonomy of the hominins continues to evolve. Human taxonomy on one hand involves the placement of humans within the Taxonomy of the hominids, on the other the division of archaic and modern humans into species and, if applicable, subspecies. Modern zoological taxonomy was developed by Carl Linnaeus during the 1730s to 1750s, he named the human species as Homo sapiens in 1758, as the only member species of the genus Homo, divided into several subspecies corresponding to the great races. The Latin noun homō means "human being"; the systematic name Hominidae for the family of the great apes was introduced by John Edward Gray. Gray supplied Hominini as the name of the tribe including both chimpanzees and humans; the discovery of the first extinct archaic human species from the fossil record dates to the mid 19th century, Homo neanderthalensis, classified in 1864. Since a number of other archaic species have been named, but there is no universal consensus as to their exact number. After the discovery of H. neanderthalensis, which if "archaic" is recognizable as human, late 19th to early 20th century anthropology for a time was occupied with finding the "missing link" between Homo and Pan.
The "Piltdown Man" hoax of 1912 was the presentation of such a transitional species. Since the mid-20th century, knowledge of the development of Hominini has become much more detailed, taxonomical terminology has been altered a number of times to reflect this; the introduction of Australopithecus as a third genus, alongside Homo and Pan, in the Hominini tribe is due to Raymond Dart. Australopithecina as a subtribe containing Australopithecus as well as Paranthropus is a proposal by Gregory & Hellman. More proposed additions to the Australopithecina subtribe include Ardipithecus and Kenyanthropus; the position of Sahelanthropus relative to Australopithecina within Hominini is unclear. Cela-Conde and Ayala propose the recognition of Australopithecus, Ardipithecus and Sahelanthropus as separate genera. Other proposed genera, now considered part of Homo, include: Pithecanthropus, Sinanthropus, Cyphanthropus Africanthropus,Telanthropus, Tchadanthropus; the genus Homo has been taken to originate some two million years ago since the discovery of stone tools in Olduvai Gorge, Tanzania, in the 1960s.
Homo habilis would be the first "human" species by definition, its type specimen being the OH 7 fossils. However, the discovery of more fossils of this type has opened up the debate on the delineation of H. habilis from Australopithecus. The LD 350-1 jawbone fossil discovered in 2013, dated to 2.8 Mya, has been argued as being transitional between the two. It is disputed whether H. habilis was the first hominin to use stone tools, as Australopithecus garhi, dated to c. 2.5 Mya, has been found along with stone tool implements. Fossil KNM-ER 1470 is now seen as either a third early species of Homo at about 2 million years ago, or alternatively as transitional between Australopithecus and Homo. Wood and Richmond proposed that Gray's tribe Hominini be designated as comprising all species after the chimpan
The skull is a bony structure that forms the head in vertebrates. It provides a protective cavity for the brain; the skull is composed of two parts: the mandible. In the human, these two parts are the neurocranium and the viscerocranium or facial skeleton that includes the mandible as its largest bone; the skull forms the anterior most portion of the skeleton and is a product of cephalisation—housing the brain, several sensory structures such as the eyes, ears and mouth. In humans these sensory structures are part of the facial skeleton. Functions of the skull include protection of the brain, fixing the distance between the eyes to allow stereoscopic vision, fixing the position of the ears to enable sound localisation of the direction and distance of sounds. In some animals such as horned ungulates, the skull has a defensive function by providing the mount for the horns; the English word "skull" is derived from Old Norse "skulle", while the Latin word cranium comes from the Greek root κρανίον.
The skull is made up of a number of fused flat bones, contains many foramina, fossae and several cavities or sinuses. In zoology there are openings in the skull called fenestrae. For details and the constituent bones, see Neurocranium and Facial skeleton The human skull is the bony structure that forms the head in the human skeleton, it forms a cavity for the brain. Like the skulls of other vertebrates, it protects the brain from injury; the skull consists of two parts, of different embryological origin—the neurocranium and the facial skeleton. The neurocranium forms the protective cranial cavity that surrounds and houses the brain and brainstem; the upper areas of the cranial bones form the calvaria. The membranous viscerocranium includes the mandible; the facial skeleton is formed by the bones supporting the face Except for the mandible, all of the bones of the skull are joined together by sutures—synarthrodial joints formed by bony ossification, with Sharpey's fibres permitting some flexibility.
Sometimes there can be extra bone pieces within the suture known as sutural bones. Most these are found in the course of the lambdoid suture; the human skull is considered to consist of twenty-two bones—eight cranial bones and fourteen facial skeleton bones. In the neurocranium these are the occipital bone, two temporal bones, two parietal bones, the sphenoid and frontal bones; the bones of the facial skeleton are the vomer, two inferior nasal conchae, two nasal bones, two maxilla, the mandible, two palatine bones, two zygomatic bones, two lacrimal bones. Some sources count the maxilla as having two bones; some of these bones—the occipital, frontal, in the neurocranium, the nasal and vomer, in the facial skeleton are flat bones. The skull contains sinuses, air-filled cavities known as paranasal sinuses, numerous foramina; the sinuses are lined with respiratory epithelium. Their known functions are the lessening of the weight of the skull, the aiding of resonance to the voice and the warming and moistening of the air drawn into the nasal cavity.
The foramina are openings in the skull. The largest of these is the foramen magnum that allows the passage of the spinal cord as well as nerves and blood vessels; the many processes of the skull include the zygomatic processes. The skull is a complex structure; the skull roof bones, comprising the bones of the facial skeleton and the sides and roof of the neurocranium, are dermal bones formed by intramembranous ossification, though the temporal bones are formed by endochondral ossification. The endocranium, the bones supporting the brain are formed by endochondral ossification, thus frontal and parietal bones are purely membranous. The geometry of the skull base and its fossae, the anterior and posterior cranial fossae changes rapidly; the anterior cranial fossa changes during the first trimester of pregnancy and skull defects can develop during this time. At birth, the human skull is made up of 44 separate bony elements. During development, many of these bony elements fuse together into solid bone.
The bones of the roof of the skull are separated by regions of dense connective tissue called fontanelles. There are six fontanelles: one anterior, one posterior, two sphenoid, two mastoid. At birth these regions are fibrous and moveable, necessary for birth and growth; this growth can put a large amount of tension on the "obstetrical hinge", where the squamous and lateral parts of the occipital bone meet. A possible complication of this tension is rupture of the great cerebral vein; as growth and ossification progress, the connective tissue of the fontanelles is invaded and replaced by bone creating sutures. The five sutures are the two squamous sutures, one coronal, one lambdoid, one sagittal suture; the posterior fontanelle closes by eight weeks, but the anterior fontanel can remain open up to eighteen months. The anterior fontanelle is located at the junction of the parietal bones. Careful observation will show that you can count a baby's heart
Meat is animal flesh, eaten as food. Humans have killed animals for meat since prehistoric times; the advent of civilization allowed the domestication of animals such as chickens, rabbits and cattle. This led to their use in meat production on an industrial scale with the aid of slaughterhouses. Meat is composed of water and fat, it is edible raw, but is eaten after it has been cooked and seasoned or processed in a variety of ways. Unprocessed meat will spoil or rot within hours or days as a result of infection with and decomposition by bacteria and fungi. Meat is important in economy and culture though its mass production and consumption has been determined to pose risks for human health and the environment. Many religions have rules about which meat may not be eaten. Vegetarians may abstain from eating meat because of concerns about the ethics of eating meat, environmental effects of meat production or nutritional effects of consumption; the word meat comes from the Old English word mete. The term is related to mad in Danish, mat in Swedish and Norwegian, matur in Icelandic and Faroese, which mean'food'.
The word mete exists in Old Frisian to denote important food, differentiating it from swiets and dierfied. Most meat refers to skeletal muscle and associated fat and other tissues, but it may describe other edible tissues such as offal. Meat is sometimes used in a more restrictive sense to mean the flesh of mammalian species raised and prepared for human consumption, to the exclusion of fish, other seafood, poultry, or other animals. In the context of food, meat can refer to "the edible part of something as distinguished from its covering", for example, coconut meat. Paleontological evidence suggests that meat constituted a substantial proportion of the diet of the earliest humans. Early hunter-gatherers depended on the organized hunting of large animals such as bison and deer; the domestication of animals, of which we have evidence dating back to the end of the last glacial period, allowed the systematic production of meat and the breeding of animals with a view to improving meat production.
Animals that are now principal sources of meat were domesticated in conjunction with the development of early civilizations: Sheep, originating from western Asia, were domesticated with the help of dogs prior to the establishment of settled agriculture as early as the 8th millennium BCE. Several breeds of sheep were established in ancient Mesopotamia and Egypt by 3500–3000 BCE. Today, more than 200 sheep-breeds exist. Cattle were domesticated in Mesopotamia after settled agriculture was established about 5000 BCE, several breeds were established by 2500 BCE. Modern domesticated cattle fall into the groups Bos taurus and Bos taurus indicus, both descended from the now-extinct aurochs; the breeding of beef cattle, cattle optimized for meat production as opposed to animals best suited for work or dairy purposes, began in the middle of the 18th century. Domestic pigs, which are descended from wild boars, are known to have existed about 2500 BCE in modern-day Hungary and in Troy. Pork sausages and hams were of great commercial importance in Greco-Roman times.
Pigs continue to be bred intensively as they are being optimized to produce meat best suited for specific meat products. Other animals have been raised or hunted for their flesh; the type of meat consumed varies much between different cultures, changes over time, depending on factors such as tradition and the availability of the animals. The amount and kind of meat consumed varies by income, both between countries and within a given country. Horses are eaten in France, Italy and Japan, among other countries. Horses and other large mammals such as reindeer were hunted during the late Paleolithic in western Europe. Dogs are consumed in South Korea and Vietnam. Dogs are occasionally eaten in the Arctic regions. Dog meat has been consumed in various parts of the world, such as Hawaii, Japan and Mexico. Cats are consumed in Southern China and sometimes in Northern Italy. Guinea pigs are raised for their flesh in the Andes. Whales and dolphins are hunted for their flesh, in Japan, Siberia, the Faroe Islands, Iceland, Saint Vincent and the Grenadines and by two small communities in Indonesia.
Modern agriculture employs a number of techniques, such as progeny testing, to speed artificial selection by breeding animals to acquire the qualities desired by meat producers. For instance, in the wake of well-publicised health concerns associated with saturated fats in the 1980s, the fat content of United Kingdom beef and lamb fell from 20–26 percent to 4–8 percent within a few decades, due to both selective breeding for leanness and changed methods of butchery. Methods of genetic engineering aimed at improving the meat production qualities of animals are now becoming available. Though it is a old industry, meat production continues to be shaped by the evolving demands of customers; the trend towards selling meat in pre-packaged cuts has increased the demand for larger breeds of cattle, which are better suited to producing such cuts. More animals not exploited for their meat are now being farmed the more agile and mobile species, whose muscles tend to be developed better than those of cattle, sheep or pigs.
Examples are the various antelope species, the zebra, water buffalo and camel, as well as non-
The plantaris is one of the superficial muscles of the superficial posterior compartment of the leg, one of the fascial compartments of the leg. It is composed of a long thin tendon. While not as thick as the achilles tendon, the plantaris tendon is the longest tendon in the human body. Not including the tendon, the plantaris muscle is 5–10 cm long and is absent in 8-12% of the population, it is one of the plantar flexors in the posterior compartment of the leg, along with the gastrocnemius and soleus muscles. The plantaris is considered an unimportant muscle and acts with the gastrocnemius; the plantaris muscle arises from the inferior part of the lateral supracondylar ridge of the femur at a position superior to the origin of the lateral head of gastrocnemius. It passes posterior to the knee joint in an inferomedial direction and becomes tendinous distally to insert into the Achilles tendon, it separately inserts into the medial side of the calcaneus. The plantaris muscle is innervated by the tibial nerve a branch of the sciatic nerve in the sacral plexus.
Signaling for contraction begins in the frontal lobe of the brain with the pre-central gyrus. Upper motor neurons are stimulated and send a signal through the internal capsule and down the corticospinal tract. Decussation of the lateral corticospinal tract occurs in the medullary pyramids the fibers continue down the contralateral side of the spinal cord. Upper motor neurons synapse with lower motor neurons at the anterior horn of the spinal cord in the sacral plexus; the lower motor neuron fibers continue down the sciatic nerve and diverge into the tibial and common fibular nerves. The tibial nerve runs medially at the knee joint; when the tibial nerve receives an action potential, the plantaris muscle contracts, providing weak plantar flexion of the foot and weak flexion of the knee. The muscle may arise from the oblique popliteal ligament. Interdigitations with the lateral head of the gastrocnemius and a fibrous extension of the muscle to the patella are not unusual; the plantaris acts to flex the knee joint.
The plantaris muscle may provide proprioceptive feedback information to the central nervous system regarding the position of the foot. The unusually high density of proprioceptive receptor end organs supports this notion, its motor function is so minimal that its long tendon can be harvested for reconstruction elsewhere with little functional deficit. Mistaken for a nerve by new medical students, the muscle was useful to other primates for grasping with their feet. A common injury, attributed to the plantaris muscle is a condition called tennis leg. Although pain in the calf can be attributed to a rupture of the plantaris muscle, recent ultrasound research has shown that tennis leg more arises from tears in the musculotendinous junction of the medial gastrocnemius. In one clinical study, 94 out of 141 patients diagnosed with tennis leg were found with a partial rupture of the gastrocnemius muscle while rupture of the plantaris tendon was only seen in 2 patients. Injury may occur from running, jumping, or pushing off one leg as in sports such as tennis and soccer which require quick foot movement in a certain direction.
Isolated plantaris muscle strains are rare and ruptures occur in concurrence with injury to other muscles in the posterior compartment of the lower leg. Symptoms of a plantaris muscle rupture may include an audible popping sound in the area during physical activity, pain in the posterior side of the lower leg, persistent soreness, it may be painful when trying to flex the ankle. Compartmental syndrome Anatomy photo:15:st-0412 at the SUNY Downstate Medical Center PTCentral
Muscle hypertrophy involves an increase in size of skeletal muscle through a growth in size of its component cells. Two factors contribute to hypertrophy: sarcoplasmic hypertrophy, which focuses more on increased muscle glycogen storage. A range of stimuli can increase the volume of muscle cells; these changes occur as an adaptive response that serves to increase the ability to generate force or resist fatigue in anaerobic conditions. Strength training, or resistance exercise, brings about neural and muscular adaptations which increase the capacity of an athlete to exert force through voluntary muscular contraction. After an initial period, in which neuro-muscular adaptation dominates, a process of muscular hypertrophy is observed whereby the size of muscle tissue increases; this increase in size is due to growth from adding sarcomeres as well as an increase in non-contractile elements like sarcoplasmic fluid. The precise mechanisms which induce muscular hypertrophy are not understood, with accepted hypotheses regarding some combination of mechanical tension, metabolic fatigue, muscular damage as relevant factors.
Progressive overload, a strategy of progressively increasing resistance or repetitions over successive bouts of exercise in order to maintain a high level of effort, is one fundamental principle of training associated with muscular hypertrophy. Across the research literature, a wide variety of resistance exercise training modalities have all been shown to elicit similar hypertrophic responses in muscle tissue. Muscular hypertrophy plays an important role in competitive bodybuilding as well as strength sports like powerlifting and Olympic weightlifting; the best approach to achieve muscle growth remains controversial. Muscular hypertrophy can be increased through strength training and other short-duration, high-intensity anaerobic exercises. Lower-intensity, longer-duration aerobic exercise does not result in effective tissue hypertrophy. During a workout, increased blood flow to metabolically active areas causes muscles to temporarily increase in size known as being "pumped up" or getting "a pump".
About two hours after a workout and for seven to eleven days, muscles swell due to an inflammation response as tissue damage is repaired. Longer-term hypertrophy occurs due to more permanent changes in muscle structure. Biological factors and training variables can affect muscle hypertrophy. Individual differences in genetics account for a substantial portion of the variance in existing muscle mass. A classical twin study design estimates that about 52% of the variance in lean body mass is estimated to be heritable and that about 45% of the variance in muscle fiber proportion is genetic as well. During puberty in males, hypertrophy occurs at an increased rate. Natural hypertrophy stops at full growth in the late teens; as testosterone is one of the body's major growth hormones, on average, males find hypertrophy much easier to achieve than females and on average, have about 60% more muscle mass than women. Taking additional testosterone, as in anabolic steroids, will increase results, it is considered a performance-enhancing drug, the use of which can cause competitors to be suspended or banned from competitions.
Testosterone is a medically regulated substance in most countries, making it illegal to possess without a medical prescription. Anabolic steroid use can cause testicular atrophy, cardiac arrest, gynecomastia. A positive energy balance, when more calories are consumed rather than burned, is required for anabolism and therefore muscle hypertrophy. An increased requirement for protein branch chained amino acids, is required for elevated protein synthesis, seen in athletes training for muscle hypertrophy. Training variables, in the context of strength training, such as frequency and total volume directly affect the increase of muscle hypertrophy. A gradual increase in all of these training variables will yield the muscular hypertrophy; the message filters down to alter the pattern of gene expression. The additional contractile proteins appear to be incorporated into existing myofibrils. There appears to be some limit to how large a myofibril can become: at some point, they split; these events appear to occur within each muscle fiber.
That is, hypertrophy results from the growth of each muscle cell, rather than an increase in the number of cells. Skeletal muscle cells are however unique in the body in that they can contain multiple nuclei, the number of nuclei can increase. Cortisol decreases amino acid uptake by muscle tissue, inhibits protein synthesis; the short-term increase in protein synthesis that occurs subsequent to resistance training returns to normal after 28 hours in adequately fed male youths. Another study determined that muscle protein synthesis was elevated 72 hours following training. A small study performed on young and elderly found that ingestion of 340 grams of lean beef did not increase muscle protein synthesis any more than ingestion of 113 grams of lean beef. In both groups, muscle protein synthesis increased by 50%; the study concluded that more than
A primate is a eutherian mammal constituting the taxonomic order Primates. Primates arose 85–55 million years ago from small terrestrial mammals, which adapted to living in the trees of tropical forests: many primate characteristics represent adaptations to life in this challenging environment, including large brains, visual acuity, color vision, altered shoulder girdle, dexterous hands. Primates range in size from Madame Berthe's mouse lemur, which weighs 30 g, to the eastern gorilla, weighing over 200 kg. There are 190 -- 448 species of living primates, depending on. New primate species continue to be discovered: over 25 species were described in the first decade of the 2000s, eleven since 2010. Primates are divided into two distinct suborders; the first is the strepsirrhines - lemurs and lorisids. The second is haplorhines - the "dry-nosed" primates - tarsier and ape clades, the last of these including humans. Simians are monkeys and apes, cladistically including: the catarrhines consisting of the Old World monkeys and apes.
Forty million years ago, simians from Africa migrated to South America by drifting on debris, gave rise to the New World monkeys. Twenty five million years ago the remaining Old World simians split into Old World monkeys. Common names for the simians are the baboons, macaques and great apes. Primates have large brains compared to other mammals, as well as an increased reliance on visual acuity at the expense of the sense of smell, the dominant sensory system in most mammals; these features are more developed in monkeys and apes, noticeably less so in lorises and lemurs. Some primates are trichromats, with three independent channels for conveying color information. Except for apes, primates have tails. Most primates have opposable thumbs. Many species are sexually dimorphic. Primates have slower rates of development than other sized mammals, reach maturity and have longer lifespans. Depending on the species, adults may live in solitude, in mated pairs, or in groups of up to hundreds of members; some primates, including gorillas and baboons, are terrestrial rather than arboreal, but all species have adaptations for climbing trees.
Arboreal locomotion techniques used include leaping from tree to tree and swinging between branches of trees. Primates are among the most social of animals, forming pairs or family groups, uni-male harems, multi-male/multi-female groups. Non-human primates have at four types of social systems, many defined by the amount of movement by adolescent females between groups. Most primate species remain at least arboreal: the exceptions are some great apes and humans, who left the trees for the ground and now inhabit every continent. Close interactions between humans and non-human primates can create opportunities for the transmission of zoonotic diseases virus diseases, including herpes, ebola and hepatitis. Thousands of non-human primates are used in research around the world because of their psychological and physiological similarity to humans. About 60% of primate species are threatened with extinction. Common threats include deforestation, forest fragmentation, monkey drives, primate hunting for use in medicines, as pets, for food.
Large-scale tropical forest clearing for agriculture most threatens primates. The English name "primates" is derived from Old French or French primat, from a noun use of Latin primat-, from primus; the name was given by Carl Linnaeus. The relationships among the different groups of primates were not understood until recently, so the used terms are somewhat confused. For example, "ape" has been used either as an alternative for "monkey" or for any tailless human-like primate. Sir Wilfrid Le Gros Clark was one of the primatologists who developed the idea of trends in primate evolution and the methodology of arranging the living members of an order into an "ascending series" leading to humans. Used names for groups of primates such as "prosimians", "monkeys", "lesser apes", "great apes" reflect this methodology. According to our current understanding of the evolutionary history of the primates, several of these groups are paraphyletic: a paraphyletic group is one which does not include all the descendants of the group's common ancestor.
In contrast with Clark's methodology, modern classifications identify only those groupings that are monophyletic. The cladogram below shows one possible classification sequence of the living primates: groups that use common names are shown on the right. All groups with scientific names are monophyletic, the sequence of scientific classification reflects the evolution
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