Chin-up
The chin-up is a strength training exercise. People do this exercise with the intention of strengthening muscles such as the latissimus dorsi and biceps, which extend the shoulder and flex the elbow, respectively, it is a form of pull-up. In the 1970s and 1980s, the term chin-up not only included an overhand/pronated grip, but some authors used it as the default meaning of the term, with an underhand/supinated grip called a "reverse" grip. In the 2010s "chin-up" still includes palms-away lifting; the term "chin-up" is still used to refer to pulling using an overhand-grip. Both pull-ups and chin-ups are among the best exercises for back and overall upper body conditioning. However, they target the muscles a bit differently. Both exercises will work the latissimus dorsi and biceps, but standard chin-ups—with an underhand grip—place more emphasis on the biceps and less on rear deltoid. A chin-up is named by bringing the chin up through space in relation to its position with the bar or other hand grips.
This can be either touching the bar or by bringing the chin over the bar. This exercise is easier for males than females because of the male tendency to have stronger and larger biceps; this is achieved most with vertical forearms that are close to the body. For most, bringing the chin this high is most achieved with a supinated grip. Due to this, the phrase "chin-up" has become associated with pulling with this type of grip; some have delegated the term pull-up to refer to the pronated grip. In spite of this, many refer to pull-ups with a pronated grip as chin-ups, the supine grip is still called a pull-up; some organizations such as the American Council on Exercise have adopted this new terminology, issuing statements such as: "a chin-up differs from a pull-up in that the puller's hands are facing towards him or her in a chin-up, away in a pull-up." Organizations such as the United States Marine Corps, use the term pull-up interchangeably to refer to both the overhand and underhand grips. A chin-up has a variety of different forms.
The movement begins with the arms extended above gripping a hold. It may be fixed, such as a chin-up bar or moving, such as rotating handles; the body is pulled up, with the bar touching the upper chest. A chin-up is considered complete based on a variety of criteria in relation to where the chin should be in respect to the bar, or in respect to the hand grips; the body is lowered until the arms are straight but not in a lockout, the exercise is repeated. Like any pull-up, chin-ups can be performed with a kip, where the legs and back flop around to aid the exercise, or from a dead hang, where the body is kept still. Performing the chin-up can be tricky with a supinated grip, because of the natural tendency to do most of the work with the elbow flexors rather than the shoulder extensors. Initiating the pulling action with scapular depression may help avoid this problem; the exercise is most effective in stretching the working muscles when the body is lowered down to a full extension. A supinated grip involves the biceps more than a pronated grip.
Chin-ups, like most pull-ups, target the latissimus dorsi muscle of the back as a shoulder extensor, scapular downward rotator and scapular depressor, in bringing the spine to the humerus. This is assisted by elbow flexors. Chin-ups, unlike pull-ups highly target the biceps; that is one of the main differences between chin-ups. The lat's functions are assisted, both by shoulder extensors, scapular downward rotators, scapular depressors. Pulling higher with a narrow grip puts the focus on extension rather than adduction of the shoulder. If one leans back at the top of the movement, the focus is shifted somewhat towards scapular retraction and hyperextension; the weight of the legs and pelvis are borne by spinal ligaments and various muscles that flex or extend the spine. If the pelvis is tilted anterior and the legs brought behind, the erector spinae bears more weight. If the pelvis is tilted posterior and the legs brought in front, the rectus abdominis bears more. Sternal chin-ups — this variant employs a fuller range of motion at the top, raising beyond the chin and touching the sternum to the bar.
The elbows are nearly directly below the shoulders this way. This requires adequate scapular depression. If leaning back a sternum-up can be done, not a chin-up, this shifts to requiring scapular retraction. Weighted chin-ups — weight is added dangling from a dipping belt or via weighted belt or vest, ankle weights, medicine ball between the knees, dumbbell between the feet or kettlebells on top of the feet. One-arm chin-ups — one hand grips the bar and the other hand does not assist One-hand chin-ups — One hand grips the bar while the other arm assists by grabbing the forearm of the arm hanging onto the bar; these require far less strength than a one-arm chin-up. Spine chin-ups — in the supine position, the arms are held perpendicular to the body as the grip the bar; this exercise is performed in the horizontal
Skeletal muscle
Skeletal muscle is one of three major muscle types, the others being cardiac muscle and smooth muscle. It is a form of striated muscle tissue, under the voluntary control of the somatic nervous system. Most skeletal muscles are attached to bones by bundles of collagen fibers known as tendons. A skeletal muscle refers to multiple bundles of cells joined together called muscle fibers; the fibers and muscles are surrounded by connective tissue layers called fasciae. Muscle fibers, or muscle cells, are formed from the fusion of developmental myoblasts in a process known as myogenesis. Muscle fibers have more than one nucleus, they have multiple mitochondria to meet energy needs. Muscle fibers are in turn composed of myofibrils; the myofibrils are composed of actin and myosin filaments, repeated in units called sarcomeres, which are the basic functional units of the muscle fiber. The sarcomere is responsible for the striated appearance of skeletal muscle and forms the basic machinery necessary for muscle contraction.
Connective tissue is present in all muscles as fascia. Enclosing each muscle is a layer of connective tissue known as the epimysium. Muscle fibers are the individual contractile units within a muscle. A single muscle such as the biceps brachii contains many muscle fibers. Another group of cells, the myosatellite cells are found between the basement membrane and the sarcolemma of muscle fibers; these cells are quiescent but can be activated by exercise or pathology to provide additional myonuclei for muscle growth or repair. DevelopmentIndividual muscle fibers are formed during development from the fusion of several undifferentiated immature cells known as myoblasts into long, multi-nucleated cells. Differentiation into this state is completed before birth with the cells continuing to grow in size thereafter. MicroanatomySkeletal muscle exhibits a distinctive banding pattern when viewed under the microscope due to the arrangement of cytoskeletal elements in the cytoplasm of the muscle fibers; the principal cytoplasmic proteins are myosin and actin which are arranged in a repeating unit called a sarcomere.
The interaction of myosin and actin is responsible for muscle contraction. Every single organelle and macromolecule of a muscle fiber is arranged to ensure form meets function; the cell membrane is called the sarcolemma with the cytoplasm known as the sarcoplasm. In the sarcoplasm are the myofibrils; the myofibrils are long protein bundles about 1 micrometer in diameter each containing myofilaments. Pressed against the inside of the sarcolemma are the unusual flattened myonuclei. Between the myofibrils are the mitochondria. While the muscle fiber does not have smooth endoplasmic cisternae, it contains a sarcoplasmic reticulum; the sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the calcium ions needed to cause a muscle contraction. Periodically, it has dilated end sacs known as terminal cisternae; these cross the muscle fiber from one side to the other. In between two terminal cisternae is a tubular infolding called a transverse tubule. T tubules are the pathways for action potentials to signal the sarcoplasmic reticulum to release calcium, causing a muscle contraction.
Together, two terminal cisternae and a transverse tubule form a triad. Muscle architecture refers to the arrangement of muscle fibers relative to the axis of force generation of the muscle; this axis is a hypothetical line from the muscle's origin to insertion. For some longitudinal muscles, such as the biceps brachii, this is a simple concept. For others, such as the rectus femoris or deltoid muscle, it becomes more complicated. While the muscle fibers of a fascicle lie parallel to one another, the fascicles themselves can vary in their relationship to one another and to their tendons; the different fiber arrangements produce broad categories of skeletal muscle architectures including longitudinal, unipennate and multipennate. Because of these different architectures, the tension a muscle can create between its tendons varies by more than its size and fiber-type makeup. Longitudinal architectureThe fascicles of longitudinally arranged, parallel, or fusiform muscles run parallel to the axis of force generation, thus these muscles on a whole function to a single, large muscle fiber.
Variations exist, the different terms are used more specifically. For instance, fusiform refers to a longitudinal architecture with a widened muscle belly, while parallel may refer to a more ribbon-shaped longitudinal architecture. A less common example would be a circular muscle such as the orbicularis oculi, in which the fibers are longitudinally arranged, but create a circle from origin to insertion. Unipennate architectureThe fibers in unipennate muscles are all oriented at the same angle relative to the axis of force generation; this angle reduces the effective force of any individual fiber, as it is pulling off-axis. However, because of this angle, more fibers can be packed into the same muscle volume, increasing the Physiological cross-sectional area; this effect is known as fiber packing, and—in terms of force generation—it more than overcomes the efficiency loss of the off-axis orientation. The trade-off comes in the total excursion. Overall muscle shortening speed is reduced compared to fiber shortening speed, as is the total distance of shortening.
All of these effects scale with pennation angle.
Latissimus dorsi muscle
The latissimus dorsi is a large, flat muscle on the back that stretches to the sides, behind the arm, is covered by the trapezius on the back near the midline. The word latissimus dorsi comes from Latin and means "broadest of the back", from "latissimus"' and "dorsum"; the pair of muscles are known as "lats" among bodybuilders. The latissimus dorsi is the largest muscle in the upper body; the latissimus dorsi is responsible for extension, transverse extension known as horizontal abduction, flexion from an extended position, internal rotation of the shoulder joint. It has a synergistic role in extension and lateral flexion of the lumbar spine. Due to bypassing the scapulothoracic joints and attaching directly to the spine, the actions the latissimi dorsi have on moving the arms can influence the movement of the scapulae, such as their downward rotation during a pull up; the number of dorsal vertebrae to which it is attached varies from four to eight. A muscular slip, the axillary arch, varying from 7 to 10 cm in length, from 5 to 15 mm in breadth springs from the upper edge of the latissimus dorsi about the middle of the posterior fold of the axilla, crosses the axilla in front of the axillary vessels and nerves, to join the under surface of the tendon of the pectoralis major, the coracobrachialis, or the fascia over the biceps brachii.
This axillary arch crosses the axillary artery, just above the spot selected for the application of a ligature, may mislead a surgeon. It is present in about 7% of the population and may be recognized by the transverse direction of its fibers. Guy et al. extensively described this muscular variant using MRI data and positively correlated its presence with symptoms of neurological impingement. A fibrous slip passes from the upper border of the tendon of the Latissimus dorsi, near its insertion, to the long head of the triceps brachii; this is muscular, is the representative of the dorsoepitrochlearis brachii of apes. This muscular form is sometimes termed the latissimocondyloideus; the latissimus dorsi crosses the inferior angle of the scapula. A study found that, of 100 cadavers dissected: 43% had "a substantial amount" of muscular fibers in the latissimus dorsi originating from the scapula. 36% had few or no muscular fibers, but a "soft fibrous link" between the scapula and the latissimus dorsi 21% had little or no connecting tissue between the two structures.
The lateral margin of the latissimus dorsi is separated below from the obliquus externus abdominis by a small triangular interval, the lumbar triangle of Petit, the base of, formed by the iliac crest, its floor by the obliquus internus abdominis. Another triangle is situated behind the scapula, it is bounded above by the trapezius, below by the latissimus dorsi, laterally by the vertebral border of the scapula. If the scapula is drawn forward by folding the arms across the chest, the trunk bent forward, parts of the sixth and seventh ribs and the interspace between them become subcutaneous and available for auscultation; the space is therefore known as the triangle of auscultation. The latissimus dorsi can be remembered best for insertion as "A Miss Between Two Majors"; as the latissimus dorsi inserts into the floor of the intertubercular groove of the humerus it is surrounded by two major muscles. The teres major inserts medially on the medial lip of the intertubercular groove and the pectoralis major inserts laterally onto the lateral lip.
The latissimus dorsi is innervated by the sixth and eighth cervical nerves through the thoracodorsal nerve. Electromyography suggests that it consists of six groups of muscle fibres that can be independently coordinated by the central nervous system; the latissimus dorsi is responsible for extension, transverse extension known as horizontal abduction, flexion from an extended position, internal rotation of the shoulder joint. It has a synergistic role in extension and lateral flexion of the lumbar spine, assists as a muscle of both forced expiration and an accessory muscle of inspiration. Most latissimus dorsi exercises concurrently recruit the teres major, posterior fibres of the deltoid, long head of the triceps brachii, among numerous other stabilizing muscles. Compound exercises for the'lats' involve elbow flexion and tend to recruit the biceps brachii and brachioradialis for this function. Depending on the line of pull, the trapezius muscles can be recruited as well; the power/size/strength of this muscle can be trained with a variety of different exercises.
Some of these include: Vertical pulling movements such as pull-ups. Horizontal pulling movements such as bent-over row, T-bar row and other rowing exercises. Shoulder extension movements with straight arms such as straight-arm lat pulldowns and Pull-overs. Deadlift. Tight latissimus dorsi has been shown to be a contributor to chronic shoulder pain and chronic back pain; because the latissimus dorsi connects the spine to the humerus, tightness in this muscle can manifest as either sub-optimal glenohumeral joint function which leads to chronic pain or tendinitis in the tendinous fasciae connecting the latissimus dorsi to the thoracic and lumbar spine. The latissimus dorsi is a potential source of muscle for
Muscle tissue
Muscle tissue is a soft tissue that composes muscles in animal bodies, gives rise to muscles' ability to contract. This is opposed to other tissues in muscle such as tendons or perimysium, it is formed during embryonic development through a process known as myogenesis. Muscle tissue varies with location in the body. In mammals the three types are: striated muscle. Smooth and cardiac muscle contracts involuntarily, without conscious intervention; these muscle types may be activated both through interaction of the central nervous system as well as by receiving innervation from peripheral plexus or endocrine activation. Striated or skeletal muscle only contracts voluntarily, upon influence of the central nervous system. Reflexes are a form of nonconscious activation of skeletal muscles, but nonetheless arise through activation of the central nervous system, albeit not engaging cortical structures until after the contraction has occurred; the different muscle types vary in their response to neurotransmitters and endocrine substances such as acetylcholine, adrenaline, nitric oxide and among others depending on muscle type and the exact location of the muscle.
Sub-categorization of muscle tissue is possible, depending on among other things the content of myoglobin, myosin ATPase etc. Muscle cells are elongated cells ranging from several millimetres to about 10 centimetres in length and from 10 to 100 micrometres in width; these cells are joined together in tissues that may be either striated or smooth, depending on the presence or absence of organized repeated arrangements of myofibrillar contractile proteins called myofilaments. Striated muscle is further classified as either cardiac muscle. Striated muscle is subject to conscious control, while smooth muscle is not. Thus, muscle tissue can be described as being one of three different types: Skeletal muscle, striated in structure and under voluntary control, is anchored by tendons to bone and is used to effect skeletal movement such as locomotion and to maintain posture. An average adult male is made up of 42% of skeletal muscle and an average adult female is made up of 36%, it has striations unlike smooth muscle.
Smooth muscle, neither striated in structure nor under voluntary control, is found within the walls of organs and structures such as the esophagus, intestines, uterus, bladder, blood vessels, the arrector pili in the skin. In vertebrates, there is a third muscle tissue recognized: Cardiac muscle, found only in the heart, is a striated muscle similar in structure to skeletal muscle but not subject to voluntary control. Cardiac and skeletal muscles are "striated" in that they contain sarcomeres and are packed into regular arrangements of bundles. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles. Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or near-permanent contractions. Skeletal muscle is further divided into several subtypes: Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color.
It can sustain aerobic activity. Type I muscle fiber are sometimes broken down into Type I and Type Ic categories, as a result of recent research. Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speed:Type IIa, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red when deoxygenated. Type IIx, less dense in mitochondria and myoglobin; this is the fastest muscle type in humans. It can contract more and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful. N. B. in some books and articles this muscle in humans was, called type IIB. Type IIb, anaerobic, glycolytic, "white" muscle, less dense in mitochondria and myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of their flesh. Smooth muscle is an involuntary non-striated muscle, it is divided into two subgroups: the multiunit smooth muscle.
Within single-unit cells, the whole bundle or sheet contracts as a syncytium. Multiunit smooth muscle tissues innervate individual cells. Smooth muscle is found within the walls of blood vessels such as in the tunica media layer of large and small arteries and veins. Smooth muscle is found in lymphatic vessels, the urinary bladder, uterus and female reproductive tracts, gastrointestinal tract, respiratory tract, arrector pili of skin, the ciliary muscle, iris of the eye; the structure and function is the same in smooth muscle cells in different organs, but the
Eye
Eyes are organs of the visual system. They provide organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs and arthropods; the simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, sufficient for the entrainment of circadian rhythms.
From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment and to the pretectal area to control the pupillary light reflex. Complex eyes can distinguish colours; the visual fields of many organisms predators, involve large areas of binocular vision to improve depth perception. In other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses, which have monocular vision; the first proto-eyes evolved among animals 600 million years ago about the time of the Cambrian explosion. The last common ancestor of animals possessed the biochemical toolkit necessary for vision, more advanced eyes have evolved in 96% of animal species in six of the ~35 main phyla. In most vertebrates and some molluscs, the eye works by allowing light to enter and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye; the cone cells and the rod cells in the retina detect and convert light into neural signals for vision.
The visual signals are transmitted to the brain via the optic nerve. Such eyes are roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and an iris; the eyes of most cephalopods, fish and snakes have fixed lens shapes, focusing vision is achieved by telescoping the lens—similar to how a camera focuses. Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye; each sensor has its own photosensitive cell. Some eyes have up to 28,000 such sensors, which are arranged hexagonally, which can give a full 360° field of vision. Compound eyes are sensitive to motion; some arthropods, including many Strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing different, high-resolution images.
Possessing detailed hyperspectral colour vision, the Mantis shrimp has been reported to have the world's most complex colour vision system. Trilobites, which are now extinct, had unique compound eyes, they used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods; the number of lenses in such an eye varied, however: some trilobites had only one, some had thousands of lenses in one eye. In contrast to compound eyes, simple eyes are those. For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision; some insect larvae, like caterpillars, have a different type of simple eye which provides only a rough image, but can possess resolving powers of 4 degrees of arc, be polarization sensitive and capable of increasing its absolute sensitivity at night by a factor of 1,000 or more. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot "see" in the normal sense.
They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can no more; this enables snails to keep out of direct sunlight. In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents—in this way the bearers can spot hot springs and avoid being boiled alive. There are ten different eye layouts—indeed every technological method of capturing an optical image used by human beings, with the exceptions of zoom and Fresnel lenses, occur in nature. Eye types can be categorised into "simple eyes", with one concave photoreceptive surface, "compound eyes", which comprise a number of individual lenses laid out on a convex surface. Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for any behaviour or environment; the only limitations specific to eye types are that of resolution—the physics of compound eyes prevents them from achieving a resolution better than 1°.
Superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to
Biceps
The biceps biceps brachii, is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join to form a single muscle belly, attached to the upper forearm. While the biceps crosses both the shoulder and elbow joints, its main function is at the elbow where it flexes the forearm and supinates the forearm. Both these movements are used when opening a bottle with a corkscrew: first biceps unscrews the cork it pulls the cork out; the biceps is one of three muscles in the anterior compartment of the upper arm, along with the brachialis muscle and the coracobrachialis muscle, with which the biceps shares a nerve supply. The biceps muscle has two heads, the short head and the long head, distinguished according to their origin at the coracoid process and supraglenoid tubercle of the scapula, respectively. From its origin on the glenoid, the long head remains tendinous as it passes through the shoulder joint and through the intertubercular groove of the humerus.
Extending from its origin on the coracoid, the tendon of the short head runs adjacent to the tendon of the coracobrachialis as the conjoint tendon. Unlike the other muscles in the anterior compartment of the arm, the biceps muscle crosses two joints, the shoulder joint and the elbow joint. Both heads of the biceps join in the middle upper arm to form a single muscle mass near the insertion of the deltoid to form a common muscle belly, although several anatomic studies have demonstrated that the muscle bellies remain distinct structures without confluent fibers; as the muscle extends distally, the two heads rotate 90 degrees externally before inserting onto the radial tuberosity. The short head inserts distally on the tuberosity while the long head inserts proximally closer to the apex of the tuberosity; the bicipital aponeurosis called the lacertus fibrosus, is a thick fascial band that organizes close to the musculotendinous junction of the biceps and radiates over and inserts onto the ulnar part of the antebrachial fascia.
The tendon that attaches to the radial tuberosity is or surrounded by a bursa, the bicipitoradial bursa, which ensures frictionless motion between the biceps tendon and the proximal radius during pronation and supination of the forearm. Two muscles lie underneath the biceps brachii; these are the coracobrachialis muscle, which like the biceps attaches to the coracoid process of the scapula, the brachialis muscle which connects to the ulna and along the mid-shaft of the humerus. Besides those, the brachioradialis muscle is adjacent to the biceps and inserts on the radius bone, though more distally. Traditionally described as a two-headed muscle, biceps brachii is one of the most variable muscles of the human body and has a third head arising from the humerus in 10% of cases —most originating near the insertion of the coracobrachialis and joining the short head—but four and seven supernumerary heads have been reported in rare cases. One study found a higher than expected number of female cadavers with a third head of biceps brachii, equal incidence between sides of the body, uniform innervation by musculocutaneous nerve.
The distal biceps tendons are separated in 40% and bifurcated in 25% of cases. The biceps shares its nerve supply with the other two muscles of the anterior compartment; the muscles are supplied by the musculocutaneous nerve. Fibers of the fifth and seventh cervical nerves make up the components of the musculocutaneous nerve which supply the biceps; the biceps works across three joints. The most important of these functions is to flex the elbow. Besides, the long head of biceps prevents the upward displacement of the head of the humerus. In more detail, the actions are, by joint: Proximal radioulnar joint – Contrary to popular belief, the biceps brachii is not the most powerful flexor of the forearm, a role which belongs to the deeper brachialis muscle; the biceps brachii functions as a powerful supinator of the forearm. This action, aided by the supinator muscle, requires the elbow to be at least flexed. If the elbow, or humeroulnar joint, is extended, supination is primarily carried out by the supinator muscle.
The biceps is a powerful supinator of the forearm due to the distal attachment of the muscle at the radial tuberosity, on the opposite side of the bone from the supinator muscle. When flexed, the biceps pulls the radius back into its neutral supinated position in concert with the supinator muscle. Elbow – The biceps brachii functions as an important flexor of the forearm when the forearm is supinated. Functionally, this action is performed when lifting an object, such as a bag of groceries or when performing a biceps curl; when the forearm is in pronation, the brachialis and supinator function to flex the forearm, with minimal contribution from the biceps brachii. It is important to note that regardless of forearm position, the force exerted by the biceps brachii remains the same; that is, the biceps can only exert so much force, as forearm position changes, other muscles must compensate. Shoulder – Several weaker functions occur at the glenohumeral, or shoulder, joint; the biceps brachii weakly assists in forward flexion of the shoulder joint.
It may con
Muscle contraction
Muscle contraction is the activation of tension-generating sites within muscle fibers. In physiology, muscle contraction does not mean muscle shortening because muscle tension can be produced without changes in muscle length such as holding a heavy book or a dumbbell at the same position; the termination of muscle contraction is followed by muscle relaxation, a return of the muscle fibers to their low tension-generating state. Muscle contractions can be described based on two variables: tension. A muscle contraction is described as isometric if the muscle tension changes but the muscle length remains the same. In contrast, a muscle contraction is isotonic if muscle tension remains the same throughout the contraction. If the muscle length shortens, the contraction is concentric. In natural movements that underlie locomotor activity, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time-varying manner. Therefore, neither length nor tension is to remain the same in muscles that contract during locomotor activity.
In vertebrates, skeletal muscle contractions are neurogenic as they require synaptic input from motor neurons to produce muscle contractions. A single motor neuron is able to innervate multiple muscle fibers, thereby causing the fibers to contract at the same time. Once innervated, the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction, explained by the sliding filament theory; the contraction produced can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials. In skeletal muscles, muscle tension is at its greatest when the muscle is stretched to an intermediate length as described by the length-tension relationship. Unlike skeletal muscle, the contractions of smooth and cardiac muscles are myogenic, although they can be modulated by stimuli from the autonomic nervous system; the mechanisms of contraction in these muscle tissues are similar to those in skeletal muscle tissues. Muscle contractions can be described based on two variables: length.
Force itself can be differentiated as either load. Muscle tension is the force exerted by the muscle on an object whereas a load is the force exerted by an object on the muscle; when muscle tension changes without any corresponding changes in muscle length, the muscle contraction is described as isometric. If the muscle length changes while muscle tension remains the same the muscle contraction is isotonic. In an isotonic contraction, the muscle length can either shorten to produce a concentric contraction or lengthen to produce an eccentric contraction. In natural movements that underlie locomotor activity, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time-varying manner. Therefore, neither length nor tension is to remain constant when the muscle is active during locomotor activity. An isometric contraction of a muscle generates tension without changing length. An example can be found when the muscles of the forearm grip an object. In isotonic contraction, the tension in the muscle remains constant despite a change in muscle length.
This occurs. In concentric contraction, muscle tension is sufficient to overcome the load, the muscle shortens as it contracts; this occurs. During a concentric contraction, a muscle is stimulated to contract according to the sliding filament theory; this occurs throughout the length of the muscle, generating a force at the origin and insertion, causing the muscle to shorten and changing the angle of the joint. In relation to the elbow, a concentric contraction of the biceps would cause the arm to bend at the elbow as the hand moved from the leg to the shoulder. A concentric contraction of the triceps would change the angle of the joint in the opposite direction, straightening the arm and moving the hand towards the leg. In eccentric contraction, the tension generated while isometric is insufficient to overcome the external load on the muscle and the muscle fibers lengthen as they contract. Rather than working to pull a joint in the direction of the muscle contraction, the muscle acts to decelerate the joint at the end of a movement or otherwise control the repositioning of a load.
This can occur voluntarily. Over the short-term, strength training involving both eccentric and concentric contractions appear to increase muscular strength more than training with concentric contractions alone. However, exercise-induced muscle damage is greater during lengthening contractions. During an eccentric contraction of the biceps muscle, the elbow starts the movement while bent and straightens as the hand moves away from the shoulder. During an eccentric contraction of the triceps muscle, the elbow starts the movement straight and bends as the hand moves towards the shoulder. Desmin and other z-line proteins are involved in eccentric contractions, but their mechanism is poorly understood in comparison to crossbridge cycling in concentric contractions. Though the muscle is doing a negative amount of mechanical work, (work is being d
