• The striations in skeletal muscles are due to a regular arrangement of actin and myosin filaments. (e-adventure.net)
  • The proteins are arranged in a specific pattern, with actin filaments in the light bands (I-bands) and myosin filaments in the dark bands (A-bands). (e-adventure.net)
  • When a muscle contracts, the actin filaments slide over the myosin filaments. (e-adventure.net)
  • Muscle contraction occurs by a sliding filament mechanism whereby actin filaments slide inward among the myosin filaments. (lookformedical.com)
  • M-lines - run through the center of the sarcomere and anchor the thick myosin filaments in their center. (wikilectures.eu)
  • I-band (isotropic) - part of the sarcomere where actin filaments do not overlap with myosin filaments. (wikilectures.eu)
  • A-band (anisotropic) - the darker part of the sarcomere where myosin filaments are located (including the section where mysoin overlaps with actin). (wikilectures.eu)
  • H-zone - lighter part of the sarcomere where only myosin filaments are found. (wikilectures.eu)
  • It consists in the sliding of heavy myosin filaments along actin filaments. (wikilectures.eu)
  • They contain the actin and myosin filaments and are responsible for the contraction and relaxation of muscle. (golifescience.com)
  • When a muscle contracts, the actin and myosin filaments slide past each other, resulting in the shortening of the muscle. (golifescience.com)
  • A new study from researchers at the University of Arizona and Ohio State University took a different approach to the problem: They performed direct tests of both human and chimp muscle fibers, and used computer models to determine how strong chimps really are. (discovermagazine.com)
  • Direct tests of muscle fibers actually found many similarities between our cells and theirs. (discovermagazine.com)
  • In terms of static strength and contraction speed, human muscle fibers kept pace with chimp muscles. (discovermagazine.com)
  • Where we differ, however, is in the type of fibers that together make up our skeletal muscles. (discovermagazine.com)
  • Chimp muscle fibers are longer than ours too, and the net result is that, pound-for-pound, their muscles are more powerful. (discovermagazine.com)
  • The Homo genus began developing larger brains that sucked energy away from muscles and selected for more economical muscle fibers. (discovermagazine.com)
  • In skeletal muscles, acetylcholine is released by nerve terminals at the motor end plate-the point of synaptic communication between motor neurons and muscle fibers. (jove.com)
  • Skeletal muscles are made of long, cylindrical cells called muscle fibers. (e-adventure.net)
  • The striations found in skeletal muscle tissue are due to the arrangement of proteins in the muscle fibers. (e-adventure.net)
  • The central protein in the muscle fibers is called myosin. (e-adventure.net)
  • According to this theory, muscle fibers contract because the myosin and actin filaments slide past each other. (e-adventure.net)
  • Compared to skeletal muscle, smooth muscle fibers are much shorter. (mokshayogaamazonica.com)
  • Within this muscle and protective tissue there may be thousands of muscle fibers (also known as muscle cells) that are bundled together and wrapped in another connective tissue covering called the endomysium . (mokshayogaamazonica.com)
  • Smooth muscle fibers are bound together into sheets or bundles by reticular fibers and frequently elastic nets are also abundant. (lookformedical.com)
  • Skeletal muscle fibers contain a set of contractile proteins actin and myosin , which allow the muscle to contract by sliding against each other. (wikilectures.eu)
  • The structures are connected by ligaments , which are referred to as the epimysium (the layer surrounding the entire muscle), the perimysium (the layer surrounding the fiber bundles) and the endomysium (enclosing the individual muscle fibers). (wikilectures.eu)
  • A muscle is a bundle of many cells called fibers . (howstuffworks.com)
  • You can think of muscle fibers as long cylinders , and compared to other cells in your body, muscle fibers are quite big. (howstuffworks.com)
  • Each fascicle is made up of individual muscle fibers, which are long, cylindrical cells. (golifescience.com)
  • These muscle fibers are surrounded by a layer of connective tissue called the perimysium. (golifescience.com)
  • Myofibrils are smaller structures within muscle fibers that are made up of protein filaments called actin and myosin. (golifescience.com)
  • Skeletal muscle is made up of long, cylindrical muscle fibers that are surrounded by connective tissue called the perimysium. (golifescience.com)
  • Microscopically, the discs appear as folded, dark-staining lines that run perpendicular to the direction of cardiac muscle fibers. (humanbiomedia.org)
  • Thus, we investigated the force output and work produced by single skinned fibers of rat soleus muscles during and after ramp contractions at a constant velocity. (frontiersin.org)
  • Skeletal muscle consists of fibers (cells). (medscape.com)
  • To move an object, referred to as load, the sarcomeres in the muscle fibers of the skeletal muscle must shorten. (achievingthedream.org)
  • The actual group of muscle fibers in a muscle innervated by a single motor neuron is called a motor unit . (achievingthedream.org)
  • A small motor unit is an arrangement where a single motor neuron supplies a small number of muscle fibers in a muscle. (achievingthedream.org)
  • Muscle fibers can broadly be divided into 3 muscles types: skeletal, cardiac, and smooth muscle. (medscape.com)
  • Likewise, the number of muscle fibers within each of these, as well as the shape of muscles (depending on their function), can also vary greatly. (medscape.com)
  • What remains constant, however, is that the muscle fibers are aligned in the same direction so that individual muscle fibers can work in concert. (medscape.com)
  • When the muscles fibers and the associated tendon are arranged along the same axis, the muscle is termed a parallel muscle. (medscape.com)
  • Bipennate muscles have muscles fibers inserting at an angle on both sides of a central tendon. (medscape.com)
  • Each muscle fascicle represents a group of muscle fibers bound together by a layer of connective tissue termed the perimysium. (medscape.com)
  • Only 60% of the axons in the nerve to a given muscle are motor to the muscle fibers that make up the bulk of the muscle. (medscape.com)
  • The nerve supply branches within the muscle belly, forming a plexus from which groups of axons emerge to supply the muscle fibers. (medscape.com)
  • The axons supply single motor endplates placed about halfway along the muscle fibers. (medscape.com)
  • A motor unit comprises a motor neuron in the spinal cord or brainstem together with the squad of muscle fibers it innervates. (medscape.com)
  • In large muscles (eg, the flexors of the hip or knee), each motor unit contains 1200 or more muscle fibers. (medscape.com)
  • In small muscles (eg, the intrinsic muscles of the hand), each unit contains 12 or fewer muscle fibers. (medscape.com)
  • Muscle fibers are long and cylindrical in shape. (medscape.com)
  • Isometric contractions involve sarcomere shortening and increasing muscle tension, but do not move a load, as the force produced cannot overcome the resistance provided by the load. (achievingthedream.org)
  • For example, if one attempts to lift a hand weight that is too heavy, there will be sarcomere activation and shortening to a point, and ever-increasing muscle tension, but no change in the angle of the elbow joint. (achievingthedream.org)
  • It is at the level of the myofiber where the basic contractile element of muscle, the sarcomere, is found. (medscape.com)
  • Calcium-dependent activation of the thin filament mediated by the troponin-tropomyosin complex is key in the regulation of actin-myosin based muscle contraction. (bvsalud.org)
  • Skeletal muscle tissue and cardiac muscle tissue are the two type of muscle tissues found in humans. (e-adventure.net)
  • Each skeletal muscle is enveloped by a protective tissue sheath called the epimysium . (mokshayogaamazonica.com)
  • A process leading to shortening and/or development of tension in muscle tissue. (lookformedical.com)
  • The nonstriated involuntary muscle tissue of blood vessels. (lookformedical.com)
  • This transmission of impulses makes cardiac muscle tissue similar to nerve tissue, although cardiac muscle cells are notably connected to each other by intercalated discs . (wikidoc.org)
  • Intercalated discs support synchronized contraction of cardiac tissue. (wikidoc.org)
  • Skeletal muscle is composed of muscle tissue of mesenchymal origin, containing specialized cell types . (wikilectures.eu)
  • In this post, we'll delve into the structure and function of muscle tissue, including the different types of muscle and how they contribute to the overall functioning of the body. (golifescience.com)
  • Maintaining healthy skeletal muscle tissue is important for overall health and well-being. (golifescience.com)
  • Muscle tissue also helps maintain posture and produce heat. (medscape.com)
  • Protein aids in the regeneration of muscle tissue that has been damaged throughout the activity. (exercisedaily.com)
  • Gross inspection of a skeletal muscle reveals collections of muscle fascicles surrounded by a layer connective tissue termed the epimysium. (medscape.com)
  • A myofibril (set of actin and myosin myofilaments) is divided into regular sections, so-called sarcomeres , which are the basic functional unit. (wikilectures.eu)
  • The myofilaments in myocardial contractile cells have the same composition and arrangement as found in skeletal muscle cells. (humanbiomedia.org)
  • Humans tend to favor a type of muscle fiber called MHC I that allows for greater endurance and reduced energy consumption, but at the cost of raw strength. (discovermagazine.com)
  • The signal continues down a nerve to the specific muscle fiber such as those found in the biceps, where the action potentials terminate at the motor end plate. (jove.com)
  • There, the motor neuron establishes synaptic contact with the muscle fiber and triggers the release of the neuro transmitter acetylcholine, which diffuses across the synaptic cleft and binds to receptors. (jove.com)
  • This prevents transmission of electrical signals between the motor neuron and muscle fiber and impairs skeletal muscle contraction. (jove.com)
  • As a result of this, the entire muscle fiber shortens or contracts, which enables movement. (e-adventure.net)
  • The nuclei in the muscle fiber are concentrated under the cytoplasmatic membrane, which is why skeletal muscle can be easily distinguished from other types under the microscope. (wikilectures.eu)
  • Neural control regulates concentric, eccentric and isometric contractions, muscle fiber recruitment, and muscle tone. (achievingthedream.org)
  • As you have learned, every skeletal muscle fiber must be innervated by the axon terminal of a motor neuron in order to contract. (achievingthedream.org)
  • Each muscle fiber is innervated by only one motor neuron. (achievingthedream.org)
  • This article mainly focuses on the end organ of this complex interaction, the muscle fiber (myofiber). (medscape.com)
  • Thanks to the alternation of actin-myosin complexes, transverse striations are visible microscopically. (wikilectures.eu)
  • Unlike skeletal muscles, smooth muscles present in the walls of internal organs are innervated by the autonomic nervous system and undergo involuntary contractions. (jove.com)
  • Alongside myosin, there's another protein called actin which is a thin, stringy protein. (e-adventure.net)
  • The striated appearance of each cardiac muscle cell is due to the same protein arrangement as in skeletal muscle cells. (e-adventure.net)
  • Thin muscle filaments are made of actin protein and have a twisting shape. (mokshayogaamazonica.com)
  • The protein constituents of muscle, the major ones being ACTINS and MYOSINS. (lookformedical.com)
  • Actin and myosin are protein filaments that are found within the sarcomeres of muscle. (golifescience.com)
  • The sarcomeres contain protein filaments called actin and myosin, which are responsible for muscle contraction. (golifescience.com)
  • This study investigated the effect of (-)-Epigallocatechin-3-gallate (EGCG) non-covalent/covalent grafting onto myofibrillar protein (MP) by high-intensity ultrasound (HIU) on its water-solubility and filament forming behavior. (bvsalud.org)
  • Smooth muscle is not under conscious control and is stimulated by the autonomic nervous system. (golifescience.com)
  • Voluntary muscles , such as those attached to our skeleton, work when we consciously tell them to. (mokshayogaamazonica.com)
  • A subtype of striated muscle, attached by TENDONS to the SKELETON. (lookformedical.com)
  • Skeletal muscles attach to the skeleton and come in pairs -- one muscle to move the bone in one direction and another to move it back the other way. (howstuffworks.com)
  • Skeletal muscle is attached to the bones of the skeleton and is responsible for movement of the limbs and other parts of the body. (golifescience.com)
  • The interplay between these filaments allowed striated muscles to maximize contractile force and efficiency. (e-adventure.net)
  • There are two major types of cardiac muscle cells: myocardial contractile cells and myocardial conducting cells. (humanbiomedia.org)
  • They can do a short, single contraction ( twitch ) or a long, sustained contraction ( tetanus ). (howstuffworks.com)
  • a) Mention one significant difference between each of the following : [5] (i) Growth and Development (ii) Muscle twitch and Tetanus (iii) Heartwood and Sapwood (iv) Leghaemoglobin and Haemoglobin (v) Collateral vascular bundle and Concentric vascular bundle. (icsesolutions.com)
  • Tetanus is the continued state of contraction of a muscle fibre stimulated by many nerve impulses or electric shocks. (icsesolutions.com)
  • Muscles are tissues with elastic properties, able to contract and then relax after an exciting stimulus. (wikilectures.eu)
  • This improved muscle capability is related to various mechanisms, including pre-activation, stretch-reflex responses and elastic recoil from serial elastic tissues. (frontiersin.org)
  • It is a twitch muscle only and contracts involuntarily . (howstuffworks.com)
  • A muscle twitch is the single isolated contraction of the muscle libre by a single nerve impulse or by a single electric shock of adequate strength, followed by immediate relaxation. (icsesolutions.com)
  • During contraction, the I-band and H-zone shorten, the A-band remains the same. (wikilectures.eu)
  • 2 . Fish Oil - Fish oil contains Omega 3 fatty acids and will reduce inflammation in the joints as well as shorten the length of your muscle pain. (dealpain.org)
  • Unlike skeletal muscle , which contracts in response to nerve stimulation, and like single unit smooth muscle, cardiac muscle is myogenic , meaning that it is self-excitable stimulating contraction without a requisite electrical impulse coming from the central nervous system. (wikidoc.org)
  • if two cardiac muscle cells are in contact, whichever one contracts first will stimulate the other to contract, and so on. (wikidoc.org)
  • Skeletal muscle has a complex structure that is essential to how it contracts. (howstuffworks.com)
  • Unlike smooth muscle, cardiac muscle has limited stretching ability but it can contract with the strength of a skeletal muscle. (mokshayogaamazonica.com)
  • Smooth muscles are not striated, and their appearance differs significantly from skeletal and cardiac muscles. (e-adventure.net)
  • skeletal , smooth , and cardiac . (mokshayogaamazonica.com)
  • Smooth muscle is found throughout the internal organs of the body, including the digestive tract, blood vessels, airways, and the female uterus. (mokshayogaamazonica.com)
  • Smooth muscle has the ability to stretch and maintain tension for long periods of time. (mokshayogaamazonica.com)
  • Because the contractions of smooth muscle are not under our conscious control, it is considered an involuntary muscle. (mokshayogaamazonica.com)
  • Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle . (wikidoc.org)
  • Muscles belong to the locomotor system and are divided into several classes - striated muscle, smooth muscle, cardiac myocardium. (wikilectures.eu)
  • It can stretch in a limited way, like smooth muscle, and contract with the force of a skeletal muscle. (howstuffworks.com)
  • Smooth muscle is found in the walls of organs and structures such as the digestive tract, blood vessels, and the uterus. (golifescience.com)
  • Smooth muscle is made up of small, spindle-shaped cells that are arranged in sheets or layers. (golifescience.com)
  • Electrical stimuli: Applying electrical stimuli between cardiac and smooth muscle cells causes the muscles to contract. (medscape.com)
  • Hormonal stimuli: Oxytocin stimulates smooth muscle in the uterus to contract during labor . (medscape.com)
  • Through contraction, muscle provides motion of the body (skeletal muscle), motion of blood (cardiac muscle), and motion of hollow organs such as the uterus, esophagus, stomach, intestines, and bladder (smooth muscle). (medscape.com)
  • The sarcoplasmic reticulum (smooth endoplasmic reticulum) stores calcium, which is released into the sarcoplasm during muscle contraction. (medscape.com)
  • In two sessions, 25 healthy participants performed isometric reference (ISO), shortening hold (SHO) and SSC contractions on an isokinetic dynamometer. (nature.com)
  • In stretch-hold experiments, the force or torque during active stretch but also in the isometric hold phase after active stretch is enhanced compared to a fixed-end reference contraction. (nature.com)
  • The steady-state force or torque is decreased after shortening-hold (SHO) experiments compared to an isometric hold phase of a fixed-end contraction at the same muscle length and activation level. (nature.com)
  • However, muscle tension also is generated when the muscle is contracting against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions and isometric contractions. (achievingthedream.org)
  • An isometric contraction occurs as the muscle produces tension without changing the angle of a skeletal joint. (achievingthedream.org)
  • In everyday living, isometric contractions are active in maintaining posture and maintaining bone and joint stability. (achievingthedream.org)
  • Most actions of the body are the result of a combination of isotonic and isometric contractions working together to produce a wide range of outcomes (Figure 10.13). (achievingthedream.org)
  • During isometric contractions, muscle length does not change because the load exceeds the tension the muscle can generate. (achievingthedream.org)
  • The main functions of the muscular system, which contains more than 600 individual muscles, is to provide movement for the body, provide heat, and maintain body posture. (mokshayogaamazonica.com)
  • Therefore, the muscular system is closely integrated with the skeletal system and the nervous system. (mokshayogaamazonica.com)
  • Most of the skeletal muscular system is arranged into groups of agonists and antagonist muscles that work in concert to provide efficient and controlled motion. (medscape.com)
  • This release of calcium initiates actin-myosin crossbridge activity and the observation of the muscle shortening and contracting. (jove.com)
  • In contrast to skeletal muscle , cardiac muscle cannot contract in the absence of extracellular calcium ions as well as extracellular potassium ions. (wikidoc.org)
  • The in vitro motility assay provides a quantitative and precise method to study the calcium sensitivity of the reconstituted myosin-thin filament motile system. (bvsalud.org)
  • Here we present a simple and robust protocol to perform calcium-dependent motility of ß-cardiac myosin and regulated thin filaments. (bvsalud.org)
  • The arrangement of myosin and actin filaments in parallel produces an efficient and powerful contraction mechanism. (e-adventure.net)
  • Binding of acetylcholine to its receptors on the sarcolemma allows entry of sodium ions into the cell and triggers an action potential in the muscle cell. (jove.com)
  • The striped appearance of skeletal muscles appears due to the arrangement of myosin and actin filaments. (e-adventure.net)
  • A: While skeletal and cardiac muscles both have stripes, the arrangement of filaments in the two muscle types is slightly different. (e-adventure.net)
  • These two factors likely explain much of the human-chimp strength gap, but other factors such as joint mechanics and muscle-tendon interactions play a role as well, with chimp arms better suited for the hanging and grabbing movements that they do every day. (discovermagazine.com)
  • Neutralizing muscles - canceling unwanted movements of the main and auxiliary muscles. (wikilectures.eu)
  • Therefore, the physiological relevance of rFE might be particularly important for movements at greater muscle-tendon unit lengths. (nature.com)
  • Communicating junctions allow the passage of ions between the muscle cells and help synchronize contraction movements. (humanbiomedia.org)
  • Higher demand for ATP from the anaerobic pathways place on the participating muscles during strength training with complex, multijoint weightlifting exercises or by performing a weightlifting circuit that alternates between upper- and lower-body movements. (exercisedaily.com)
  • The largest muscle, the gluteus maximus in the buttocks, enables humans to run upright quickly. (mokshayogaamazonica.com)
  • Residual force depression (rFD) following active muscle shortening and residual force enhancement (rFE) following active muscle stretch ( Abbott and Aubert, 1952 ) are fundamentally accepted mechanical properties of skeletal muscle ( Rassier, 2017 ). (frontiersin.org)
  • This can increase to 10% under moderately hypoxic conditions, but under more severe hypoxic conditions, not enough energy can be liberated by lactate production to sustain ventricular contractions. (wikidoc.org)
  • It is possible to regenerate glycogen in muscles from lactate. (exercisedaily.com)
  • The T tubules are only found at the Z lines, whereas in skeletal muscle, they are found at the junction of the A and I bands. (humanbiomedia.org)
  • Therefore, there is one-half as many T tubules in cardiac muscle as in a skeletal muscle. (humanbiomedia.org)
  • This summary discusses the basic anatomy of skeletal muscle, key features of skeletal muscle histology and physiology, and important presentations of muscular disease. (medscape.com)
  • In gross anatomy, the nerves to skeletal muscles are branches of mixed peripheral nerves. (medscape.com)
  • These muscles usually contract voluntarily , meaning that you think about contracting them and your nervous system tells them to do so. (howstuffworks.com)
  • All of these muscle activities are under the exquisite control of the nervous system. (achievingthedream.org)
  • A crucial aspect of nervous system control of skeletal muscles is the role of motor units. (achievingthedream.org)
  • For example, when you think about moving your arm using your biceps muscle, your brain sends a signal down a nerve cell telling your biceps muscle to contract. (howstuffworks.com)
  • The amount of force that the muscle creates varies -- the muscle can contract a little or a lot depending on the signal that the nerve sends. (howstuffworks.com)
  • Types of Muscle Contractions During isotonic contractions, muscle length changes to move a load. (achievingthedream.org)
  • 0.05) less depressed joint torque in the SSC at the longest muscle-tendon unit length compared to the corresponding SHO condition, without any differences in knee kinematics and fascicle behavior. (nature.com)
  • This combination of eccentric and concentric contractions is the most important type of daily muscle action and plays a significant role in natural locomotion such as walking, running or jumping. (frontiersin.org)
  • A key feature of SSCs is the increase in muscular force and work during the concentric phase of a SSC by more than 50% compared with concentric muscle actions without prior stretch (SSC-effect). (frontiersin.org)
  • i ) partial detachment of XBs during the eccentric phase of a SSC, ( ii ) significantly enhanced forces and mechanical work during the concentric phase of SSCs compared with shortening contractions with and without XB-inhibition, and ( iii ) no residual force depression after SSCs. (frontiersin.org)
  • On the contrary, SSCs - eccentric muscle action immediately followed by concentric muscle action - play a significant role in natural locomotion. (frontiersin.org)
  • There are two types of isotonic contractions: concentric and eccentric. (achievingthedream.org)
  • A concentric contraction involves the muscle shortening to move a load. (achievingthedream.org)
  • Muscles that control the heart or digestive system are called involuntary muscles because they work on their own without conscious effort on our part. (mokshayogaamazonica.com)
  • Skeletal muscle is under conscious control and can be voluntarily contracted or relaxed. (golifescience.com)
  • An example of this is the biceps brachii muscle contracting when a hand weight is brought upward with increasing muscle tension. (achievingthedream.org)
  • Cylindrical muscles typically have greater mass at the center of the muscle, leading to a central body or muscle belly (eg, biceps brachii). (medscape.com)
  • Contraction remains until the stimulation continues. (icsesolutions.com)
  • Motor points have been identified for all major muscle groups for the purpose of functional electrical stimulation by physical therapists, in order to increase muscle power. (medscape.com)
  • Cardiac muscle cells are short and fat, each containing a single, central nucleus. (e-adventure.net)
  • The cells in cardiac muscles form a regular, repeating pattern, which gives the muscle its striated appearance. (e-adventure.net)
  • These connections between cells allow the cardiac muscles to contract and relax in a coordinated manner. (e-adventure.net)
  • Large, multinucleate single cells, either cylindrical or prismatic in shape, that form the basic unit of SKELETAL MUSCLE. (lookformedical.com)
  • Coordinated contractions of cardiac muscle cells during systole propel blood out of the atria and ventricles to the blood vessels of the systemic and pulmonary circulatory systems . (wikidoc.org)
  • Specialized pacemaker cells in the sinoatrial node normally determine the overall rate of contractions, with an average resting pulse of 72 beats per minute. (wikidoc.org)
  • The cardiac muscle cells would still fire in the absence of a functioning SA node pacemaker, albeit in a chaotic and ineffective manner. (wikidoc.org)
  • Skeletal muscle cells are grouped into primary bundles (fasciculi), secondary bundles and finally bundles of higher orders. (wikilectures.eu)
  • Muscle cells have a complicated and well-organized structure, with each part doing a certain job. (golifescience.com)
  • Their function is similar in many respects to neurons, although they are specialized muscle cells. (humanbiomedia.org)
  • Anchoring junctions strongly bind the ends of adjacent muscle cells together and resist the forces exerted by contraction. (humanbiomedia.org)
  • Skeletal muscle is actually formed by the fusion of hundreds of embryonic cells. (medscape.com)
  • Cardiac muscle is a type of involuntary striated muscle found in the walls of the myocardium . (wikidoc.org)
  • Cardiac muscle is found only in your heart, and its big features are endurance and consistency . (howstuffworks.com)
  • No significant difference of joint torque was found in the steady-state for all SSC-magnitudes compared to the corresponding SHO contractions in session 1. (nature.com)
  • A muscle may be used over and over for and tighten over time, but the muscle may not go into spasm until a simple movement occurs. (dealpain.org)
  • An eccentric contraction occurs as the muscle tension diminishes and the muscle lengthens. (achievingthedream.org)
  • However, holding your head in an upright position occurs not because the muscles cannot move the head, but because the goal is to remain stationary and not produce movement. (achievingthedream.org)