Anatomy and Physiology – Muscular System

Anatomy and Physiology – Muscular System

INTRODUCTION TO THE MUSCULAR SYSTEM

• The muscular system is composed of specialized cells called muscle fibers.
• Their predominant function is contractibility. Muscles, attached to bones or
• internal organs and blood vessels, are responsible for movement. Nearly all
• movement in the body is the result of muscle contraction. Exceptions to this
• are the action of cilia, the flagellumon sperm cells, and amoeboid movement
• of some white blood cells.
• The integrated action of joints, bones, and skeletal muscles produces
• obvious movements such as walking and running. Skeletal muscles also
• produce more subtle movements that result in various facial expressions, eye
• movements, and respiration.
• In addition to movement, muscle contraction also fulfills some other
• important functions in the body, such as posture, joint stability, and heat
• production. Posture, such as sitting and standing, is maintained as a result of
• muscle contraction. The skeletal muscles are continually making fine
• adjustments that hold the body in stationary positions. The tendons of many
• muscles extend over joints and in this way contribute to joint stability. This
• is particularly evident in the knee and shoulder joints, where muscle tendons
• are a major factor in stabilizing the joint. Heat production, to maintain body
• temperature, is an important by-product of muscle metabolism. Nearly 85

percent of the heat produced in the body is the result of muscle contraction.

Muscular System

• The muscular system is responsible for the movement of the human body.
• Attached to the bones of the skeletal system are about 700 named muscles
• that make up roughly half of a person‘s body weight. Each of these muscles
• is a discrete organ constructed of skeletal muscle tissue, blood vessels,
• tendons, and nerves. Muscle tissue is also found inside of the heart, digestive
• organs, and blood vessels. In these organs, muscles serve to move

substances throughout the body.

Muscle Types

• There are three types of muscle tissue: Visceral, cardiac, and skeletal.
• Visceral Muscle. Visceral muscle is found inside of organs like
• the stomach, intestines, and blood vessels. The weakest of all muscle
• tissues, visceral muscle makes organs contract to move substances through
• the organ. Because visceral muscle is controlled by the unconscious part of
• the brain, it is known as involuntary muscle—it cannot be directly controlled
• by the conscious mind. The term ―smooth muscle‖ is often used to describe
• visceral muscle because it has a very smooth, uniform appearance when
• viewed under a microscope. This smooth appearance starkly contrasts with
• the banded appearance of cardiac and skeletal muscles.
• Cardiac Muscle. Found only in the heart, cardiac muscle is responsible for
• pumping blood throughout the body. Cardiac muscle tissue cannot be
• controlled consciously, so it is an involuntary muscle. While hormones and
• signals from the brain adjust the rate of contraction, cardiac muscle
• stimulates itself to contract. The natural pacemaker of the heart is made of
• cardiac muscle tissue that stimulates other cardiac muscle cells to contract.
• Because of its self-stimulation, cardiac muscle is considered to be auto
• rhythmic or intrinsically controlled.
• The cells of cardiac muscle tissue are striated—that is, they appear to have
• light and dark stripes when viewed under a light microscope. The
• arrangement of protein fibers inside of the cells causes these light and dark
• bands. Striations indicate that a muscle cell is very strong, unlike visceral
• muscles.
• The cells of cardiac muscle are branched X or Y shaped cells tightly
• connected together by special junctions called intercalated disks. Intercalated
• disks are made up of fingerlike projections from two neighboring cells that
• interlock and provide a strong bond between the cells. The branched
• structure and intercalated disks allow the muscle cells to resist high blood
• pressures and the strain of pumping blood throughout a lifetime. These
• features also help to spread electrochemical signals quickly from cell to cell
• so that the heart can beat as a unit.
• Skeletal Muscle. Skeletal muscle is the only voluntary muscle tissue in the
• human body—it is controlled consciously. Every physical action that a
• person consciously performs (e.g. speaking, walking, or writing) requires
• skeletal muscle. The function of skeletal muscle is to contract to move parts
• of the body closer to the bone that the muscle is attached to. Most skeletal
• muscles are attached to two bones across a joint, so the muscle serves to
• move parts of those bones closer to each other.
• Skeletal muscle cells form when many smaller progenitor cells lump
• themselves together to form long, straight, multinucleated fibers. Striated
• just like cardiac muscle, these skeletal muscle fibers are very strong. Skeletal
• muscle derives its name from the fact that these muscles always connect to

the skeleton in at least one place.

MUSCLE TYPES

• In the body, there are three types of muscle: skeletal (striated), smooth, and
• cardiac.

Skeletal Muscle

• Skeletal muscle, attached to bones, is responsible for skeletal movements.
• The peripheral portion of the central nervous system (CNS) controls the
• skeletal muscles. Thus, these muscles are under conscious, or voluntary,
• control. The basic unit is the muscle fiber with many nuclei. These muscle
• fibers are striated (having transverse streaks) and each acts independently of

neighboring muscle fibers.

Smooth Muscle

• Smooth muscle, found in the walls of the hollow internal organs such as
• blood vessels, the gastrointestinal tract, bladder, and uterus, is under control
• of the autonomic nervous system. Smooth muscle cannot be controlled
• consciously and thus acts involuntarily. The non-striated (smooth) muscle
• cell is spindle-shaped and has one central nucleus. Smooth muscle contracts

slowly and rhythmically.

Cardiac Muscle

• Cardiac muscle, found in the walls of the heart, is also under control of the
• autonomic nervous system. The cardiac muscle cell has one central nucleus,
• like smooth muscle, but it also is striated, like skeletal muscle. The cardiac
• muscle cell is rectangular in shape. The contraction of cardiac muscle is
• involuntary, strong, and rhythmical.
• Smooth and cardiac muscle will be discussed in detail with respect to their

appropriate systems. This unit mainly covers the skeletal muscular system.

MUSCLE GROUPS

• There are more than 600 muscles in the body, which together account for
• about 40 percent of a person's weight.
• Most skeletal muscles have names that describe some feature of the muscle.
• Often several criteria are combined into one name. Associating the muscle's
• characteristics with its name will help you learn and remember them. The
• following are some terms relating to muscle features that are used in naming
• muscles.
• Size: vastus (huge); maximus (large); longus (long); minimus (small);
• brevis (short).
• Shape: deltoid (triangular); rhomboid (like a rhombus with equal and
• parallel sides); latissimus (wide); teres (round); trapezius (like a
• trapezoid, a four-sided figure with two sides parallel).
• Direction of fibers: rectus (straight); transverse (across); oblique
• (diagonally); orbicularis (circular).
• Location: pectoralis (chest); gluteus (buttock or rump); brachii (arm);
• supra- (above); infra- (below); sub- (under or beneath); lateralis
• (lateral).
• Number of origins: biceps (two heads); triceps (three heads);
• quadriceps (four heads).
• Origin and insertion: sternocleidomastoideus (origin on the sternum
• and clavicle, insertion on the mastoid process); brachioradialis (origin
• on the brachium or arm, insertion on the radius).
• Action: abductor (to abduct a structure); adductor (to adduct a
• structure); flexor (to flex a structure); extensor (to extend a structure);

levator (to lift or elevate a structure); masseter (a chewer).

MUSCLES OF THE HEAD AND NECK

• Humans have well-developed muscles in the face that permit a large variety
• of facial expressions. Because the muscles are used to show surprise,
• disgust, anger, fear, and other emotions, they are an important means of
• nonverbal communication. Muscles of facial expression include frontalis,
• orbicularis oris, laris oculi, buccinator, and zygomaticus.These muscles of
• facial expressions are identified in the illustration below.
• There are four pairs of muscles that are responsible for chewing movements
• or mastication. All of these muscles connect to the mandible and they are
• some of the strongest muscles in the body. Two of the muscles, temporalis
• and masseter, are identified in the illustration above.
• There are numerous muscles associated with the throat, the hyoid bone and
• the vertebral column; only two of the more obvious and superficial neck

muscles are identified in the illustration: sternocleidomastoid and trapezius.

MUSCLES OF THE TRUNK

• The muscles of the trunk include those that move the vertebral column, the
• muscles that form the thoracic and abdominal walls, and those that cover the
• pelvic outlet.
• The erector spinae group of muscles on each side of the vertebral column is
• a large muscle mass that extends from the sacrum to the skull. These
• muscles are primarily responsible for extending the vertebral column to
• maintain erect posture. The deep back muscles occupy the space between the
• spinous and transverse processes of adjacent vertebrae.
• The muscles of the thoracic wall are involved primarily in the process of
• breathing. The intercostal muscles are located in spaces between the ribs.
• They contract during forced expiration. External intercostal muscles contract
• to elevate the ribs during the inspiration phase of breathing. The diaphragm
• is a dome-shaped muscle that forms a partition between the thorax and the
• abdomen. It has three openings in it for structures that have to pass from the
• thorax to the abdomen.
• The abdomen, unlike the thorax and pelvis, has no bony reinforcements or
• protection. The wall consists entirely of four muscle pairs, arranged in
• layers, and the fascia that envelops them. The abdominal wall muscles are
• identified in the illustration below.
• The pelvic outlet is formed by two muscular sheets and their associated

fascia.

MUSCLES OF THE UPPER EXTREMITY

• The muscles of the upper extremity include those that attach the scapula to
• the thorax and generally move the scapula, those that attach the humerus to
• the scapula and generally move the arm, and those that are located in the arm
• or forearm that move the forearm, wrist, and hand. The illustration below
• shows some of the muscles of the upper extremity.
• Muscles that move the shoulder and arm include the trapezius and serratus
• anterior. The pectoralis major, latissimus dorsi, deltoid, and rotator cuff
• muscles connect to the humerus and move the arm.
• The muscles that move the forearm are located along the humerus, which
• include the triceps brachii, biceps brachii, brachialis, and brachioradialis.
• The 20 or more muscles that cause most wrist, hand, and finger movements

are located along the forearm.

STRUCTURE OF SKELETAL MUSCLE

• A whole skeletal muscle is considered an organ of the muscular system.
• Each organ or muscle consists of skeletal muscle tissue, connective tissue,
• nerve tissue, and blood or vascular tissue.
• Skeletal muscles vary considerably in size, shape, and arrangement of fibers.
• They range from extremely tiny strands such as the stapedium muscle of the
• middle ear to large masses such as the muscles of the thigh. Some skeletal
• muscles are broad in shape and some narrow. In some muscles the fibers are
• parallel to the long axis of the muscle; in some they converge to a narrow
• attachment; and in some they are oblique.
• Each skeletal muscle fiber is a single cylindrical muscle cell. An individual
• skeletal muscle may be made up of hundreds, or even thousands, of muscle
• fibers bundled together and wrapped in a connective tissue covering. Each
• muscle is surrounded by a connective tissue sheath called the epimysium.
• Fascia, connective tissue outside the epimysium, surrounds and separates the
• muscles. Portions of the epimysium project inward to divide the muscle into
• compartments. Each compartment contains a bundle of muscle fibers. Each
• bundle of muscle fiber is called a fasciculus and is surrounded by a layer of
• connective tissue called the perimysium. Within the fasciculus, each
• individual muscle cell, called a muscle fiber, is surrounded by connective
• tissue called the endomysium.
• Skeletal muscle cells (fibers), like other body cells, are soft and fragile. The
• connective tissue covering furnish support and protection for the delicate
• cells and allow them to withstand the forces of contraction. The coverings
• also provide pathways for the passage of blood vessels and nerves.
• Commonly, the epimysium, perimysium, and endomysium extend beyond
• the fleshy part of the muscle, the belly or gaster, to form a thick ropelike
• tendon or a broad, flat sheet-like aponeurosis. The tendon and aponeurosis
• form indirect attachments from muscles to the periosteum of bones or to the
• connective tissue of other muscles. Typically a muscle spans a joint and is
• attached to bones by tendons at both ends. One of the bones remains
• relatively fixed or stable while the other end moves as a result of muscle
• contraction.
• Skeletal muscles have an abundant supply of blood vessels and nerves. This
• is directly related to the primary function of skeletal muscle, contraction.
• Before a skeletal muscle fiber can contract, it has to receive an impulse from
• a nerve cell. Generally, an artery and at least one vein accompany each nerve
• that penetrates the epimysium of a skeletal muscle. Branches of the nerve
• and blood vessels follow the connective tissue components of the muscle of
• a nerve cell and with one or more minute blood vessels called capillaries.
• Gross Anatomy of a Skeletal Muscle
• Most skeletal muscles are attached to two bones through tendons. Tendons
• are tough bands of dense regular connective tissue whose strong collagen
• fibers firmly attach muscles to bones. Tendons are under extreme stress
• when muscles pull on them, so they are very strong and are woven into the
• coverings of both muscles and bones.
• Muscles move by shortening their length, pulling on tendons, and moving
• bones closer to each other. One of the bones is pulled towards the other
• bone, which remains stationary. The place on the stationary bone that is
• connected via tendons to the muscle is called the origin. The place on the
• moving bone that is connected to the muscle via tendons is called the
• insertion. The belly of the muscle is the fleshy part of the muscle in between
• the tendons that does the actual contraction.
• Names of Skeletal Muscles
• Skeletal muscles are named based on many different factors, including their
• location, origin and insertion, number of origins, shape, size, direction, and
• function.
• Location. Many muscles derive their names from their anatomical region.
• The rectus abdominis and transverse abdominis, for example, are found in
• theabdominal region. Some muscles, like the tibialis anterior, are named
• after the part of the bone (the anterior portion of the tibia) that they are
• attached to. Other muscles use a hybrid of these two, like the brachioradialis,
• which is named after a region (brachial) and a bone (radius).
• Origin and Insertion. Some muscles are named based upon their connection
• to a stationary bone (origin) and a moving bone (insertion). These muscles
• become very easy to identify once you know the names of the bones that
• they are attached to. Examples of this type of muscle include
• the sternocleidomastoid (connecting thesternum and clavicle to the
• mastoid process of the skull) and the occipitofrontalis (connecting
• the occipital bone to the frontal bone).
• Number of Origins. Some muscles connect to more than one bone or to more
• than one place on a bone, and therefore have more than one origin. A muscle
• with two origins is called a biceps. A muscle with three origins is a triceps
• muscle. Finally, a muscle with four origins is a quadriceps muscle.
• Shape, Size, and Direction. We also classify muscles by their shapes. For
• example, the deltoids have a delta or triangular shape. The serratus muscles
• feature a serrated or saw-like shape. The rhomboid major is a rhombus or
• diamond shape. The size of the muscle can be used to distinguish between
• two muscles found in the same region. The gluteal region contains three
• muscles differentiated by size—the gluteus maximus (large), gluteus medius
• (medium), and gluteus minimus (smallest). Finally, the direction in which
• the muscle fibers run can be used to identify a muscle. In the abdominal
• region, there are several sets of wide, flat muscles. The muscles whose fibers
• run straight up and down are the rectus abdominis, the ones running
• transversely (left to right) are the transverse abdominis, and the ones running
• at an angle are the obliques.
• Function. Muscles are sometimes classified by the type of function that they
• perform. Most of the muscles of the forearms are named based on their
• function because they are located in the same region and have similar shapes
• and sizes. For example, the flexor group of the forearm flexes the wrist and
• the fingers. The supinator is a muscle that supinates the wrist
• over to face palm up. In the leg, there are muscles called adductors whose
• role is to adduct (pull together) the legs.
• Groups Action in Skeletal Muscle
• Skeletal muscles rarely work by themselves to achieve movements in the
• body. More often they work in groups to produce precise movements. The
• muscle that produces any particular movement of the body is known as an
• agonist or prime mover. The agonist always pairs with an antagonist muscle
• that produces the opposite effect on the same bones. For example, the biceps
• brachii muscle flexes the arm at the elbow. As the antagonist for this motion,
• the triceps brachii muscle extends the arm at the elbow. When the triceps is
• extending the arm, the biceps would be considered the antagonist.
• In addition to the agonist/antagonist pairing, other muscles work to support
• the movements of the agonist. Synergists are muscles that help to stabilize a
• movement and reduce extraneous movements. They are usually found in
• regions near the agonist and often connect to the same bones. Because
• skeletal muscles move the insertion closer to the immobile origin, fixator
• muscles assist in movement by holding the origin stable. If you lift
• something heavy with your arms, fixators in the trunk region hold your body

upright and immobile so that you maintain your balance while lifting.

Skeletal Muscle Histology

• Skeletal muscle fibers differ dramatically from other tissues of the body due
• to their highly specialized functions. Many of the organelles that make up
• muscle fibers are unique to this type of cell.
• The sarcolemma is the cell membrane of muscle fibers. The sarcolemma acts
• as a conductor for electrochemical signals that stimulate muscle cells.
• Connected to the sarcolemma are transverse tubules (T-tubules) that help
• carry these electrochemical signals into the middle of the muscle fiber. The
• sarcoplasmic reticulum serves as a storage facility for calcium ions (Ca2+)
• that are vital to muscle contraction. Mitochondria, the ―power houses‖ of the
• cell, are abundant in muscle cells to break down sugars and provide energy
• in the form of ATP to active muscles. Most of the muscle fiber‘s structure is
• made up of myofibrils, which are the contractile structures of the cell.
• Myofibrils are made up of many proteins fibers arranged into repeating
• subunits called sarcomeres. The sarcomere is the functional unit of muscle
• fibers. (See Macronutrients for more information about the roles of sugars

and proteins.)

Sarcomere Structure

• Sarcomeres are made of two types of protein fibers: thick filaments and thin
• filaments.
• Thick filaments. Thick filaments are made of many bonded units of the
• protein myosin. Myosin is the protein that causes muscles to contract.
• Thin filaments. Thin filaments are made of three proteins:
• Actin. Actin forms a helical structure that makes up the bulk of the thin
• filament mass. Actin contains myosin-binding sites that allow myosin to
• connect to and move actin during muscle contraction.
• Tropomyosin. Tropomyosin is a long protein fiber that wraps around actin
• and covers the myosin binding sites on actin.
• Troponin. Bound very tightly to tropomyosin, troponin moves tropomyosin
• away from myosin binding sites during muscle contraction.
• Function of Muscle Tissue
• The main function of the muscular system is movement. Muscles are the
• only tissue in the body that has the ability to contract and therefore move the
• other parts of the body.
• Related to the function of movement is the muscular system‘s second
• function: the maintenance of posture and body position. Muscles often
• contract to hold the body still or in a particular position rather than to cause
• movement. The muscles responsible for the body‘s posture have the greatest
• endurance of all muscles in the body—they hold up the body throughout the
• day without becoming tired.
• Another function related to movement is the movement of substances inside
• the body. The cardiac and visceral muscles are primarily responsible for
• transporting substances like blood or food from one part of the body to
• another.
• The final function of muscle tissue is the generation of body heat. As a result
• of the high metabolic rate of contracting muscle, our muscular system
• produces a great deal of waste heat. Many small muscle contractions within
• the body produce our natural body heat. When we exert ourselves more than
• normal, the extra muscle contractions lead to a rise in body temperature and
• eventually to sweating.
• Skeletal Muscles as Levers
• Skeletal muscles work together with bones and joints to form lever systems.
• The muscle acts as the effort force; the joint acts as the fulcrum; the bone
• that the muscle moves acts as the lever; and the object being moved acts as
• the load.
• There are three classes of levers, but the vast majority of the levers in the
• body are third class levers. A third class lever is a system in which the
• fulcrum is at the end of the lever and the effort is between the fulcrum and
• the load at the other end of the lever. The third class levers in the body serve
• to increase the distance moved by the load compared to the distance that the
• muscle contracts.
• The tradeoff for this increase in distance is that the force required to move
• the load must be greater than the mass of the load. For example, the biceps
• brachia of the arm pulls on the radius of the forearm, causing flexion at
• the elbow joint in a third class lever system. A very slight change in the
• length of the biceps causes a much larger movement of the forearm and
• hand, but the force applied by the biceps must be higher than the load moved

by the muscle.

Motor Units

• Nerve cells called motor neurons control the skeletal muscles. Each motor
• neuron controls several muscle cells in a group known as a motor unit.
• When a motor neuron receives a signal from the brain, it stimulates all of the
• muscles cells in its motor unit at the same time.
• The size of motor units varies throughout the body, depending on the
• function of a muscle. Muscles that perform fine movements—like those of
• the eyes or fingers—have very few muscle fibers in each motor unit to
• improve the precision of the brain‘s control over these structures. Muscles
• that need a lot of strength to perform their function—like leg or arm
• muscles—have many muscle cells in each motor unit. One of the ways that
• the body can control the strength of each muscle is
• many motor units to activate for a given function. This explains why the
• same muscles that are used to pick up a pencil are also used to pick up a

bowling ball.

Contraction Cycle

• Muscles contract when stimulated by signals from their motor neurons.
• Motor neurons contact muscle cells at a point called the Neuromuscular
• Junction (NMJ). Motor neurons release neurotransmitter chemicals at the
• NMJ that bond to a special part of the sarcolemma known as the motor end
• plate. The motor end plate contains many ion channels that open in response
• to neurotransmitters and allow positive ions to enter the muscle fiber. The
• positive ions form an electrochemical gradient to form inside of the cell,
• which spreads throughout the sarcolemma and the T-tubules by opening
• even more ion channels.
• When the positive ions reach the sarcoplasmic reticulum, Ca2+ ions are
• released and allowed to flow into the myofibrils. Ca2+ ions bind to troponin,
• which causes the troponin molecule to change shape and move nearby
• molecules of tropomyosin. Tropomyosin is moved away from myosin
• binding sites on actin molecules, allowing actin and myosin to bind together.
• ATP molecules power myosin proteins in the thick filaments to bend and
• pull on actin molecules in the thin filaments. Myosin proteins act like oars
• on a boat, pulling the thin filaments closer to the center of a sarcomere. As
• the thin filaments are pulled together, the sarcomere shortens and contracts.
• Myofibrils of muscle fibers are made of many sarcomeres in a row, so that
• when all of the sarcomeres contract, the muscle cells shortens with a great
• force relative to its size.
• Muscles continue contraction as long as they are stimulated by a
• neurotransmitter. When a motor neuron stops the release of the
• neurotransmitter, the process of contraction reverses itself. Calcium returns
• to the sarcoplasmic reticulum; troponin and tropomyosin return to their
• resting positions; and actin and myosin are prevented from binding.
• Sarcomeres return to their elongated resting state once the force of myosin
• pulling on actin has stopped.
• Types of Muscle Contraction
• The strength of a muscle‘s contraction can be controlled by two factors: the
• number of motor units involved in contraction and the amount of stimulus
• from the nervous system. A single nerve impulse of a motor neuron will
• cause a motor unit to contract briefly before relaxing. This small contraction
• is known as a twitch contraction. If the motor neuron provides several
• signals within a short period of time, the strength and duration of the muscle
• contraction increases. This phenomenon is known as temporal summation. If
• the motor neuron provides many nerve impulses in rapid succession, the
• muscle may enter the state of tetanus, or complete and lasting contraction. A
• muscle will remain in tetanus until the nerve signal rate slows or until the
• muscle becomes too fatigued to maintain the tetanus.
• Not all muscle contractions produce movement. Isometric contractions are
• light contractions that increase the tension in the muscle without exerting
• enough force to move a body part. When people tense their bodies due to
• stress, they are performing an isometric contraction. Holding an object still
• and maintaining posture are also the result of isometric contractions. A
• contraction that does produce movement is an isotonic contraction. Isotonic
• contractions are required to develop muscle mass through weight lifting.
• Muscle tone is a natural condition in which a skeletal muscle stays partially
• contracted at all times. Muscle tone provides a slight tension on the muscle
• to prevent damage to the muscle and joints from sudden movements, and
• also helps to maintain the body‘s posture. All muscles maintain some
• amount of muscle tone at all times, unless the muscle has been disconnected
• from the central nervous system due to nerve damage.
• Functional Types of Skeletal Muscle Fibers
• Skeletal muscle fibers can be divided into two types based on how they
• produce and use energy: Type I and Type II.
• Type I fibers are very slow and deliberate in their contractions. They are
• very resistant to fatigue because they use aerobic respiration to produce
• energy from sugar. We find Type I fibers in muscles throughout the body for
• stamina and posture. Near the spine and neck regions, very high
• concentrations of Type I fibers hold the body up throughout the day.
• Type II fibers are broken down into two subgroups: Type II A and Type II
• B.
• Type II A fibers are faster and stronger than Type I fibers, but do not have as
• much endurance. Type II A fibers are found throughout the body, but
• especially in the legs where they work to support your body throughout a
• long day of walking and standing.
• Type II B fibers are even faster and stronger than Type II A, but have even
• less endurance. Type II B fibers are also much lighter in color than Type I
• and Type II A due to their lack of myoglobin, an oxygen-storing pigment.
• We find Type II B fibers throughout the body, but particularly in the upper
• body where they give speed and strength to the arms and chest at the
• expense of stamina.
• Muscle Metabolism and Fatigue
• Muscles get their energy from different sources depending on the situation
• that the muscle is working in. Muscles use aerobic respiration when we call
• on them to produce a low to moderate level of force. Aerobic respiration
• requires oxygen to produce about 36-38 ATP molecules from a molecule of
• glucose. Aerobic respiration is very efficient, and can continue as long as a
• muscle receives adequate amounts of oxygen and glucose to keep
• contracting. When we use muscles to produce a high level of force, they
• become so tightly contracted that oxygen carrying blood cannot enter the
• muscle. This condition causes the muscle to create energy using lactic acid
• fermentation, a form of anaerobic respiration. Anaerobic respiration is much
• less efficient than aerobic respiration—only 2 ATP are produced for each
• molecule of glucose. Muscles quickly tire as they burn through their energy
• reserves under anaerobic respiration.
• To keep muscles working for a longer period of time, muscle fibers contain
• several important energy molecules. Myoglobin, a red pigment found in
• muscles, contains iron and stores oxygen in a manner similar to hemoglobin
• in the blood. The oxygen from myoglobin allows muscles to continue
• aerobic respiration in the absence of oxygen. Another chemical that helps to
• keep muscles working is cretin phosphate. Muscles use energy in the form of
• ATP, converting ATP to ADP to release its energy. Cretin phosphate

donates its phosphate group to ADP to turn it back into ATP in order to provide extra energy to the muscle. Finally, muscle fibers contain energystoring glycogen, a large macromolecule made of many linked glucoses.

• Active muscles break glucoses off of glycogen molecules to provide an
• internal fuel supply.
• When muscles run out of energy during either aerobic or anaerobic
• respiration, the muscle quickly tires and loses its ability to contract. This
• condition is known as muscle fatigue. A fatigued muscle contains very little
• or no oxygen, glucose or ATP, but instead has many waste products from
• respiration, like lactic acid and ADP. The body must take in extra oxygen
• after exertion to replace the oxygen that was stored in myoglobin in the
• muscle fiber as well as to power the aerobic respiration that will rebuild the
• energy supplies inside of the cell. Oxygen debt (or recovery oxygen uptake)
• is the name for the extra oxygen that the body must take in to restore the
• muscle cells to their resting state. This explains why you feel out of breath
• for a few minutes after a strenuous activity—your body is trying to restore

itself to its normal state.