Medicine

23. Musculoskeletal System Anatomy and Physiology

Musculoskeletal System: Anatomy and Physiology

 

 

Objectives

1.     Describe the anatomy and physiology of the bones, muscles, and joints.

2.     Discuss the directional movements of the joints.

3.     Develop questions to be used when completing the focused interview.

4.     Describe the techniques required for assessment of the musculoskeletal system.

5.     Differentiate normal from abnormal findings of the musculoskeletal system.

6.     Describe developmental, cultural, psychosocial, and environmental related variations in assessment and findings.

7.     Discuss focus areas related to overall health of the musculoskeletal system as presented in Healthy People 2010 initiatives.

8.     Apply critical thinking in selected simulations related to physical assessment of the musculoskeletal system.

Overview of the musculoskeletal system

  • The musculoskeletal system provides structure and movement for the body.

  • The skeleton consists of 206 bones.

  • The musculoskeletal system allows the body to stand erect and move, supports and protects organs, produces red blood cells, stores fat and minerals, and generates heat.

Anatomy and physiology review

  • Bones are classified according to their shape and composition.

  • Bone shapes include long, short, flat, and irregular.

  • Bones are composed of osseous tissue arranged in dense, smooth, compact structures, or cancellous spongy structures.

  • Skeletal muscles are composed of fibers arranged in striated bands that attach to bones.

  • A joint is the point where two or more bones meet.

  • Tendons attach muscle to bone, and ligaments attach bone to bone.

  • Joints may be fibrous, cartilaginous, or synovial.

  • The temporomandibular joint permits articulation between the temporal bone and the mandible.

  • The shoulder joint is a ball-and-socket joint in which the head of the humerus articulates with the glenoid capsule of the scapula.

  • The elbow is a hinge joint that allows articulation of the humerus, the radius, and the ulna.

  • The wrist consists of two rows of carpal bones. The distal row articulates with the metacarpals. The scaphoid and lunate bones of the proximal row articulate with the ulna and radius.

  • Each hand has metacarpophalangeal and interphalangeal joints.

  • The hip joint is a ball-and-socket joint composed of the head of the femur as it fits into the acetabulum.

  • The knee is a complex joint consisting of the patella, femur, and tibia.

  • The ankle is a hinge joint that accommodates articulation of the tibia, fibula, and talus.

  • The spine is composed of 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, and the sacrum.

  • Joint movements include flexion, extension, rotation, circumduction, elevation, protrusion, retraction, abduction, adduction, pronation, supination, inversion, eversion, and gliding.

Special considerations

  • When assessing the infant, the nurse should observe spontaneous movements.

  • Fetal positioning may cause musculoskeletal anomalies in the newborn.

  • Newborns usually have flat feet; arches develop during the preschool period.

  • Spina bifida may be indicated if tufts of hair, cysts, or masses appear on the spine.

  • The infant is assessed for congenital hip dislocation.

  • Bone growth is rapid during infancy and steady throughout childhood.

  • Children should be assessed for scoliosis on each healthcare visit.

  • The nurse should determine the physical activities in which the child or adolescent participates.

  • Cartilage in the pelvis is softened in the pregnant female. Lordosis develops to compensate for the enlarging fetus.

  • Aging results in changes in bones, muscles, joints, and connective tissue.

  • Bones change as a result of decreased calcium absorption and reduced osteoblast production.

  • Bone resorption occurs more rapidly than new bone growth in the older adult and results in loss of bone density.

  • Height decreases from 1 to 2 inches between the ages of 20 and 70 as a result of shortening of the vertebral column.

  • Reaction time, speed of movement, agility, and endurance decrease with aging as a result of decrease in muscle size, loss of elasticity of tendons, and increase in the amount of connective tissue in muscles.

  • Joints degenerate, resulting in decreased range of motion and flexibility.

  • Impaired mobility affects the self-esteem of the individual. Conversely, altered body image or self-esteem may promote immobility.

  • Diet influences musculoskeletal health. Protein, calcium, and vitamin D are required for a healthy musculoskeletal system.

  • Bone density is highest among individuals of African descent and lowest among Asians.

  • Activities at work can impact musculoskeletal health. Work-related risks include repetitive motion, lifting, and physical activity.

  • Active lifestyles promote health of the musculoskeletal system.

  • Participation in athletics and sports activities may increase risks for traumatic musculoskeletal injury.

Gathering the data

  • The focused interview includes questions about the function of the musculoskeletal system and the impact of function on activities of daily living.

  • The focused interview includes general questions, questions related to illness and symptoms, questions according to age, and questions related to the environment.

  • The nurse must consider cultural and language differences when conducting the interview.

  • Physical assessment of the musculoskeletal system includes use of the skills of inspection and palpation.

  • Equipment required for the physical assessment includes an examination gown, a drape, clean nonsterile examination gloves, an examination light, a goniometer, and a tape measure.

  • Demonstration of range of motion increases the client’s comprehension and compliance.

  • The physical assessment is carried out in an orderly manner from head to toe.

  • The two sides of the body are compared for symmetry throughout the assessment.

  • Physical assessment of the joints is conducted as follows: inspection, palpation, range of motion, and assessment of muscle strength.

  • The physical assessment includes assessment of the temporomandibular joints, the shoulders, the elbows, the wrists and hands, the hips, the knees, the ankles and feet, and the spine.

Abnormal findings

  • Abnormal findings of the musculoskeletal system include rheumatic disease, abnormalities of the spine, joint disorders, and trauma-induced disorders.

  • Rheumatic diseases include rheumatoid arthritis, connective tissue diseases, gout, tendinitis, bursitis, and polymyositis.

  • Abnormalities of the spine include kyphosis, lordosis, and scoliosis.

  • Joint disorders include arthritic changes, TMJ syndrome, contractures, carpal tunnel syndrome, synovitis, bunions, hallux valgus, and hammertoes.

  • Traumatic musculoskeletal disorders include sprains, strains, fractures, and dislocations.

Health promotion and client education

  • The nurse should explain the benefits of a healthy diet including calcium and vitamins C and D.

  • Maintaining appropriate weight for height or loss of weight in obesity can decrease risks of joint disease and injury.

  • The nurse should provide information about bone density screening for those at risk for osteoporosis and all adults after age 55.

  • Never smoking or cessation of smoking reduces the risk for osteoporosis.

  • Regular exercise promotes health and function of the musculoskeletal system.

  • Employment of safety measures in the home, work, and social environments reduces the risk of traumatic musculoskeletal injury.

Glossary

abduction   A movement of a limb away from the midline or median plane of the body, along the frontal plane.

acetabulum   A rounded cavity on the right and left lateral sides of the pelvic bone.

adduction   The movement of a limb toward the body midline.

ballottement   A technique used to detect fluid, or to examine or detect floating body structures.

bursae   Small, synovial-fluid-filled sacs that protect ligaments from friction.

calcaneous   Heel bone.

cartilaginous joint   Bones joined by cartilage.

circumduction   The movement in which the limb describes a cone in space: while the distal end of the limb moves in a circle, the joint itself moves only slightly in the joint cavity.

depression    The movement in which the elevated part is moved downward to its original position.

dislocation   A displacement of the bone from its usual anatomical location in the joint.

dorsiflexion   Flexion of the ankle so that the superior aspect of the foot approaches the shin.

elevation   A lifting or moving superiorly along a frontal plane.

eversion   A movement in which the sole of the foot is turned laterally.

extension    A movement that increases the angle between the articulating bones.

fibrous joint   Bones joined by fibrous tissue.

flexion    A bending movement that decreases the angle of the joint and brings the articulating bones closer together.

fracture   A partial or complete break in the continuity of the bone from trauma.

gliding   The simplest type of joint movements. One flat bone surface glides or slips over another similar surface. The bones are merely displaced in relation to one another.

hallux valgus   The great toe is abnormally adducted at the metatarsophalangeal joint.

hyperextension   A bending of a joint beyond 180 degrees.

inversion    A movement in which the sole of the foot is turned medially.

joint   (Articulation) is the point where two or more bones in the body meet.

kyphosis   An exaggerated thoracic dorsal curve that causes asymmetry between the sides of the posterior thorax.

lordosis    An exaggerated lumbar curve that compensates for pregnancy, obesity, or other skeletal changes.

opposition   The movement of touching the thumb to the tips of the other fingers of the same hand.

plantar flexion   Extension of the ankle (pointing the toes).

pronation   Movement of the forearm so that the palm faces posteriorly or inferiorly.

protraction   A nonangular anterior movement in a transverse plane.

retraction   A nonangular posterior movement in a transverse plane.

rotation   The turning movement of a bone around its own long axis.

scoliosis   The spine curves to the right or left, causing an exaggerated thoracic convexity on that side.

sprain   A stretching or tearing of the capsule or ligament of a joint due to forced movement beyond the joint’s normal range.

strain   A partial muscle tear resulting from overstretching or overuse of the muscle.

subluxation    A partial dislocation of the head of the radius which occurs commonly when adults dangle children from their hands or remove their clothing forcibly.

supination   Movement of the forearm so that the palm faces anteriorly or superiorly.

synovial joint   Bones separated by a fluid-filled joint cavity.

tendon   Tough fibrous bands that attach muscle to bone, or muscle to muscle.

tophi   Gout related hard nodules that may appear over the joint.

 

 

The musculoskeletal system provides shape and support to the body, allows movement, protects the internal organs, produces red blood cells in the bone marrow (hematopoiesis), and stores calcium and phosphorus in the bones. Although examining this system is usually only a small part of the overall physical assessment, everything we do depends on an intact musculoskeletal system. How extensive an assessment you perform depends largely on each patient’s problems and needs.

Perform a comprehensive musculoskeletal assessment if you detect a musculoskeletal abnormality or uncover a symptom that suggests musculoskeletal involvement. Musculoskeletal problems are common in all age groups. Primary problems may result from congenital, developmental, infectious, neoplastic, traumatic, or degenerative disorders of the system itself. Secondary problems can result from disorders of other body systems.

The goal of a complete musculoskeletal assessment is to detect risk factors, potential problems, or musculoskeletal dysfunction early and then to plan appropriate interventions, including teaching health promotion and disease prevention and implementing treatment measures. By doing so, you can play a significant role in preventing pain and dysfunction in your patients.

 

Anatomy and Physiology Review

Before beginning your assessment, you need to understand how the musculoskeletal system works. It consists of three major components: bones, muscles, and joints. Tendons, ligaments, cartilage, and bursae serve as connecting structures and complete the system. Figure 20.1 illustrates the musculoskeletal system, anterior and posterior views.

 

 

Bones. Composed of osseous tissue, bones are divided into two types: compact bone, which is hard and dense and makes up the shaft and outer layers, and spongy bone, which contains numerous spaces and makes up the ends and centers of the bones. Osteoblasts and osteoclasts are the cells responsible for the continuous process of creating and destroying bone. Osteoblasts form new bone tissue, and osteoclasts break down bone tissue. Bones also contain red marrow,which produces blood cells, and yellow marrow,which is composed mostly of fat.The outer covering of bone, called the periosteum, contains osteoblasts and blood vessels that promote nourishment and formation of new bone tissue.

Bones vary in shape and include long bones  (Fig. 20.2), such as the humerus and femur; short, flat bones, such as the sternum and ribs; and bones with irregular shapes, such as the hips and vertebrae.

 

Muscles. The body is composed of skeletal, smooth, and cardiac muscle.Made up of fasciculi (long muscle fibers) that are arranged in bundles and joined by connective tissue, skeletal muscles attach to bones by way of strong,fibrous cords called  tendons. Ligaments are dense, flexible, strong bands of fibrous connective tissue that tie bones to other bones.

Cartilage is dense connective tissue consisting of fibers embedded in a strong, gel-like substance.Cartilage lacks nerve innervation,blood vessels,and lymph vessels, so it is insensitive to pain and regenerates slowly and minimally after injury. Regeneration occurs primarily at sites where the articular cartilage meets the synovial membrane. Cartilage may be fibrous, hyaline, or elastic.

Fibrous cartilage forms the symphysis pubis and the intervertebral discs.Hyaline cartilage covers the articular bone surfaces (where bones meet at a joint),connects the ribs to the sternum, and is found in the trachea, bronchi, and nasal septum.Elastic cartilage is located in the auditory canal, the external ear, and the epiglottis.

Joints. The joint or articulation is the place where two or more bones meet. Joints provide range of motion (ROM) for the body parts and are classified three ways: by the degree of movement they permit, by the connecting tissues that hold them together, and by the type of motion the structure permits. Figure 20.3 illustrates the fibrous and cartilaginous joints and Figure 20.4 the synovial joints.

A joint is the structure of human body where two or more bones are held together in order to allow various types of movements and moldings in the rigid bony human skeleton. A joint is not exclusively for bones, there can be three different types of joints on the basis of what structures are involved in making it.

A joint can exist between

1. Two bones (for example the shoulder joint that exists between the scapula and the humerus).

2. A cartilage and a bone (for example the joint that exists between the ribs and the costal cartilages).

3. A cartilage and a cartilage (for example the joint between the 6th and the 7th costal cartilage.

Types of joints

Types of joints

Source: The visual dictionary

Types of joints

Joints occur at numerous places in the body and they differ in structure and function. They are classified as Immovable joints and Movable joints.

Immovable joints:

They are also known as fixed joints. They are those joints in which the relative movements of the bones forming the joint do not occur. In such joints the bones are in actual contact with one another without any type of cartilage in between them for example the joints of the bones of cranium as well as those of the face which fix teeth into jaws.

Movable joints

Movable joints are sometimes called synovial joints. These are the joints in which the bones forming the joints are capable of movements with one another. The opposing surface of the bones of a moveable joint is separated by a piece of cartilage called the articular cartilage. According to the range of movement the movable joints are further divided into two kinds; partially moveable joints and freely moveable joints.

Partially moveable joints: These joints have a very limited range of movement for example the joint between the vertebrae and the joints of the cranium and hip bones.

Freely moveable joints: These joints allow the free movements between the articulating bones. In such joints the articular surfaces are often clothed with cartilage which reduces the friction between the two surfaces of bones making the joint. It is covered by a synovial membrane. This membrane often constitutes a closed sac. The sac contains lubricating synovial fluid which also reduces friction in the joint where it is found. Such freely moveable joints are classified further into three classes according the degree of movement that they allow. These types are Hinge joints, Ball and Socket joints and the Pivot joints.

Hinge joints: This type of joints allows the backward and forward movement in only one plane.

Ball and socket joints: In these types of joints the movement is vast virtually occurring in every plane.

Pivot joints: In these joints rotation is the only possible movement.

 

 

 

 

Bursae. Bursae are small, disc-shaped synovial fluid sacs located at points of friction around joints.They act as cushions, thereby reducing the stress to adjacent structures, and facilitate movement. Two examples of bursae are the prepatellar bursa (in the knee) and the subacromial bursa (in the shoulder).

Interaction with Other Body Systems. The neurological and respiratory systems contribute to maintaining musculoskeletal functioning. A problem in any of these systems may affect the functioning of the musculoskeletal system.

Neurological System. The neurological system is responsible for coordinating the functions of the skeleton and muscles. If your patient has neurological complaints, combine the musculoskeletal and neurological assessments because the spinal cord and nerves originate from the spine and innervate the musculoskeletal structures of the back and the extremities. A dysfunction in the neurological system is often reected as pain, abnormal movement, or paresthesias in the extremities and/or back.The patient’s gait may provide information on muscular weakness or neurological disease.

Back pain is a major source of disability in the United States. A large proportion of the population complains of back pain at one time or another,with the most common complaint being low back pain radiating into the hip and down the leg.This pain is usually of neurological origin and emanates from the sciatic nerve. Pain may also be caused by arthritic disease of the spine or hip or muscle spasm of the lower back. Understanding the anatomy of the back and spinal nerve tracks will help you determine the pain’s origin.

Respiratory System. The respiratory system depends on the thorax, bony structures, and muscles of the chest to protect the lungs and assist with breathing.The accessory muscles,which include the sternocleidomastoid,anterior serrati, scalene, trapezius, intercostal, and rhomboid muscles, come into play when a person is involved in aerobic activities or when the body has intrapulmonary resistance to air movement (e.g., chronic pulmonary lung disease). These accessory muscles enhance ventilation by increasing chest expansion and lung size during inspiration. Intercostal muscles also coordinate rib movement; external intercostal muscles pull the ribs up and out,and internal intercostals pull the ribs down and inward. Contraction of these muscles facilitates air movement into the lungs by decreasing intrathoracic pressure. As these muscles relax, exhalation occurs as the lung recoils. Abdominal muscles can also assist with deep breathing, tachypnea (rapid breathing), exercise, coughing, and sneezing. An intact thoracic cage and normal accessory and abdominal muscles are necessary for respiratory function.A musculoskeletal injury or problem in these areas can result in altered respiratory functioning.

 

Developmental, Cultural, and Ethnic Considerations

Infants and Children. Before birth, a skeleton forms in the fetus; it is rst composed of cartilage and then later ossies into true growing bone. After birth,bone growth continues rapidly during infancy and then steadily during the childhood years. Another growth spurt occurs for both boys and girls during adolescence. Long bones increase in diameter by depositing new bone tissue around the shafts. Lengthening occurs at the epiphyses,which are specialized growth centers (growth plates) located at the ends of long bones.Any injury or infection at these growth plates puts the growing child at risk for bone deformity.Longitudinal growth of the bones continues until closure of the epiphyses,which occurs at age 20.

Skeletal contour changes are also apparent in infants and children. At birth, the spine has a single C-shaped curve. At 3 to 4 months of age, an infant is able to raise his or her head from the prone position, allowing the development of the anterior curve in the cervical neck region. As development progresses and the infant is able to stand and walk, the anterior curve develops in the lumbar region. This occurs between 12 and 18 months of age. A toddler stands with feet wide apart to provide balance as he or she learns to walk.

The school-age child usually stands with the normal adult curvature,which should continue until old age. Throughout childhood, the skeleton continues to grow linearly; muscles and fat are responsible for significant weight increases. Individual muscle fibers continue to grow as the child grows, with a marked growth period noted during adolescence. At this time, muscles are responding to increased growth hormone,adrenal androgens, and testosterone in boys. Muscles vary in growth rate, size, and strength, depending on genetic factors, nutritional status, and amount of exercise.

Common knee deviations in children include genu varum (bowlegs) and genu valgum (knock knees). In a child with genu varum, the knees are approximately 5 cm apart and the medial malleoli touch when the child stands. This variation is common during the first years when the child is beginning to walk,but usually does not persist beyond 2 to 3 years. When genu valgum is present in a child, the knees touch and the medial malleoli are 7.5 cm or more apart when the child is standing. These deviations are considered normal for a child aged 2 to 31⁄2 years and may persist until age 7.

Toddlers often have “potbellies” and  lordosis (accentuated lumbar curve). This posture is normal and helps the child adjust to the change in the center of gravity. It should disappear as the child grows. Spinal deformities in children may be structural,but more commonly are caused by poor posture. Scoliosis (lateral curvature) may become apparent during adolescence,with girls at a higher risk than boys.The spine does not grow straight, and the shoulders and iliac crests are not the same height.Assessing for scoliosis is an important component when working with adolescents.

Additional history questions for parents of infants and children include:

Was there a normal labor and delivery without trauma to the infant? (Trauma increases the risk for fractures of the humerus and clavicle.)

Did the infant need to be resuscitated? (Anoxia may result in hypotonia of muscles.)

Did the infant develop as expected?

Has the child ever had a broken bone or dislocation? If so, how were these injuries treated?

■ Have you noticed any bone deformity, spinal curvature, or unusual shape of the legs, toes, or feet?

Additional history questions for adolescents include:

■ Are you involved in any sports? How often do you play or practice?

■ Do you use protective or special equipment?

■ Do you warm up before participating in an athletic activity?

These questions help assess the safety of the sport for the height and weight of the adolescent and gauge the risk of injuries.

Pregnant Women. Progressive lordosis is the most characteristic change in posture in pregnant women. It compensates for the enlarging fetus by shifting the center of gravity and moving the weight of the enlarging fetus back on the lower extremities.

This shift in balance may create strain on the low back muscles, which may be felt as low back pain during late pregnancy. Anterior cervical flexion during the third trimester, kyphosis (humpback), and slumped shoulders are other postural changes that compensate for lordosis.Upper back changes may put pressure on the ulnar and median nerves, creating aching, numbness, and weakness in the upper extremities of some pregnant women. Increased mobility in the sacroiliac, sacrococcygeal,and symphysis pubis joints in the pelvis occurs in preparation for delivery,which also contributes to the characteristic waddling gait.

Older Adults. With aging, bone density decreases because loss of bone tissue occurs more rapidly than formation of new bone tissue. This results in osteoporosis, with women at a higher risk than men and whites affected more often than African Americans.

Kyphosis with a backward head tilt to compensate and slight flexion of the hips and knees are postural changes that occur with aging. Decreased height is the most noticeable postural change. It occurs as a result of shortening of the vertebral column secondary to the loss of water content and thinning of the intervertebral discs.Both men and women can expect a decrease in height beginning in their 40s and continuing until age 60. After this age, height decreases occur secondary to osteoporotic changes in the height of individual vertebrae. This collapse of vertebrae results in a shortening of the trunk and comparatively long extremities. This change in the spine, kyphosis and loss of height due to collapse and wedging of the vertebral column, is often referred to as dowager’s hump.

Contour changes also occur as a result of the distribution of subcutaneous fat. Most men and women gain weight in their fourth and fifth decades, losing fat in the face and depositing it in the abdomen and hips. This distribution pattern continues into later years, but fat continues to decrease in the periphery, especially the forearms.

The loss of subcutaneous fat in certain parts of the body contributes to marked bony prominences. The tips of the vertebrae, ribs, and iliac crests may become very noticeable and the body hollows (e.g., cheeks and axillae) become deeper. There is loss of muscle mass,a decrease in muscle size, and some muscle atrophy,which contributes to generalized weakness.These muscle changes can cause decreased agility, an abnormal gait with uneven rhythm and short steps, and a wide base of support.Other complicating factors include fear of falling, osteoporosis, painful arthritic joints,poor vision, and peripheral neuropathy.

Lifestyle can affect the musculoskeletal changes that occur with aging. A sedentary lifestyle leads to decreased muscle mass and strength and increased muscle atrophy. Decreased speed and strength, resistance to fatigue, reaction time, and coordination are changes that can be prevented by physical exercise. Physical activity also prevents or delays bone loss in aging adults.

Additional history questions for the aging adult should elicit information about loss of function, self-care deficits,or safety risks that may occur as a result of aging, injury, or illness.Ask the following questions:

■ Have you broken any bones recently? If so, how?

■ Have you noticed any weakness over the past months?

■ Have you had any increase in stumbling or falls?

■ Do you use mobility aids like a cane or walker to help you get around?

People of Different Cultures /Ethnic Groups. When assessing the musculoskeletal system, you need to consider your patient’s ethnicity because it can affect musculoskeletal anthropomorphic and physical characteristics.

A basic primer on bones, muscles, tendons, ligaments, nerves, and cartilages.

The skeletal system includes the bones of the skeleton and the cartilages, ligaments, and other connective tissue that stabilize or connect the bones. In addition to supporting the weight of the body, bones work together with muscles to maintain body position and to produce controlled, precise movements. Without the skeleton to pull against, contracting muscle fibers could not make us sit, stand, walk, or run.

Bones

There are 206 bones in the adult body. The bones of the body perform five main functions.

·                                 Provide support for the body — The skeletal system provides structural support for the entire body. Individual bones or groups of bones provide a framework for the attachment of soft tissues and organs.

·                                 Store minerals and lipids — Calcium is the most abundant mineral in the body. (Ninety-nine percent of the body's calcium is found in the skeleton.) The calcium salts of bone are a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in body fluids. The bones of the skeleton also store energy reserves as lipids in areas filled with yellow marrow.

·                                 Produce blood cells — Red blood cells, white blood cells, and other blood elements are produced in the red marrow, which fills the internal cavities of many bones.

·                                 Protect body organs — Many soft tissues and organs are surrounded by skeletal elements. For example, the rib cage protects the heart and lungs, the skull protects the brain, the vertebrae protect the spinal cord, and the pelvis protects the delicate reproductive organs.

·                                 Provide leverage and movement — Many bones function as levers that can change the magnitude and direction of the forces generated by muscles.

Bone structure

Each bone in the skeleton contains two forms of tissue: compact (dense) bone that is relatively solid and spongy (cancellous) bone that forms an open network of struts and plates. Compact bone is found on the external surface of the bone. Spongy bone is located inside the bone. The proportion of compact and spongy bone varies with the shape of the bone. Compact bone is thickest where stresses arrive from a limited range of directions. Spongy bone is located where bones are not heavily stressed or where stresses arrive from many directions. Spongy bone is much lighter than compact bone, which helps reduce the weight of the skeleton and makes it easier for muscles to move the bones.

Bone development and growth

The growth of the skeleton determines the size and proportions of the body. Bones begin to form in a mother's womb about six weeks after fertilization, and portions of the skeleton do not stop growing until about the age of 25. Most bones originate as hyaline cartilage. The cartilage is gradually converted to bone through a process called ossification. Bone growth begins at the center of the cartilage. As bones enlarge, bone growth activity shifts to the ends of the bones (an area commonly called the growth plate), which results in an increase in bone length.

Bone growth "factoids"

·                                 Twenty percent of the adult skeleton is replaced each year.

·                                 Moderate amounts of physical activity and weight-bearing activities are essential to stimulate bone maintenance and to maintain adequate bone strength.

Other elements of the musculoskeletal system

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·                                 Joints — These are where two bones interconnect. Each joint reflects a compromise between stability and range of motion. For example, the bones of the skull are very stable but with little motion, whereas the shoulder joint allows for a full range of motion but is a relatively unstable joint.

·                                 Tendons — These attach muscle to bone.

·                                 Ligaments — These attach bone to bone.

·                                 Skeletal muscles — These muscles contract to pull on tendons and move the bones of the skeleton. In addition to producing skeletal movement, muscles also maintain posture and body position, support soft tissues, guard entrances and exits to the digestive and urinary tracts, and maintain body temperature.

·                                 Nerves — Nerves control the contraction of skeletal muscles, interpret sensory information, and coordinate the activities of the body's organ systems.

·                                 Cartilage — This is a type of connective tissue. It is a firm gel-like substance. The body contains three major types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage.

·                                 Hyaline cartilage is the most common type of cartilage. This type of cartilage provides stiff but somewhat flexible support. Examples in adults include the tips of ribs (where they meet the sternum) and part of the nasal septum. Another example is articular cartilage, which is cartilage that covers the ends of bones within a joint. The surfaces of articular cartilage are slick and smooth, which reduces friction during joint movement.

·                                 Elastic cartilage provides support but can tolerate distortion without damage and return to its original shape. The external flap of the ear is one place where elastic cartilage can be found.

·                                 Fibrocartilage resists compression, prevents bone-to-bone contact, and limits relative movement. Fibrocartilage can be found within the knee joint, between the pubic bones of the pelvis, and between the spinal vertebrae.

Cartilage heals poorly, and damaged fibrocartilage in joints such as the knee can interfere with normal movements. The knee contains both hyaline cartilage and fibrocartilage. The hyaline cartilage covers bony surfaces and fibrocartilage pads in the joint prevent contact between bones during movement. Injuries to the joints can produce tears in the fibrocartilage pads, and the tears do not heal. Eventually, joint mobility is severely reduced.

Musculoskeletal system terms

Looking at and analysing the anatomy and physiology of the musculoskeletal system can be challenging as there are many new terms to learn and understand.  For this reason some of the basic terms you are likely to encounter in your role as a personal trainer are explained in the following table.

 

Term

Definition

Example

Anterior

The front

The mouth is on the anterior part of the head

Posterior

The back

The spine is on the posterior part of the trunk

Superior

Toward the head

The head is superior to the stomach

Inferior

Toward the feet

The knee is inferior to the hip

Lateral

Toward the side/outside

The ears are on the lateral part of the head

Medial

Toward the midline/inside

The big toe is medial to the little toe

Proximal

Nearer the trunk

The thighs are proximal to the toes

Distal

Further from the trunk

The toes are distal to the thighs

Prone

Face down

Lying on stomach about to do a push up

Supine

Face up

Lying on back about to do a sit up/crunch

Superficial

Nearer to the surface

The skin is more superficial than the stomach

Deep

Further from the surface

The stomach is more deep than the skin

  

Another term which is important to understand is the ‘anatomical position’.

http://www.ptdirect.com/images/personal-training-anatomical-positionThe anatomical position refers to a position of the body that is commonly used when analysing it.  The anatomical position is achieved by following the points below, an example of this can also be seen in the adjacent image.

1.                     Stand upright with head facing forward

2.                     Arms hanging by sides

3.                     Palms of hands facing forward

4.                     Feet slightly separated

5.                     Toes pointing forward

 

Using the anatomical position allows everyone who works with the body to start from the same reference point so their descriptions in practice and research make sense to a wider audience.

Knowing the anatomical position also helps you understand movement descriptions and work effectively with other professionals such as doctors and physiotherapists etc.

The spinal cord can be divided to several regions:

cervical (C1-C6)

cervicothoracic (C7-T2)

thoracolumbar (T3-L3)

lumbosacral (L3-S2)

sacral (S3 onwards)

Nerves originating from the spinal cord and the segmental spinal nerves innervate the limbs. 

The forelimb nerves include the suprascapular (C5-6), the musculocutaneous (C5-7), the ulna/median (Originates from the brachial plexus, which is formed from C5-T1) and the radial (C5-T1).

The hindlimb nerves include the obturator (L2-4), the femoral (L2-4) and the sciatic (L4-S3). The sciatic nerve branches to the tibial nerve and the peroneal nerve. 

Structure and Function

The spinal cord is constructed of the marginal layer which has axons and white matter, the mantle which contains cell bodies and grey matter and the spinal canal. This canal conducts sensory information from the peripheral nervous system (both somatic and autonomic) to the brain, conducts motor information from the brain to various effectors and acts as a minor reflex center. 

Sensory Pathways

http://en.wikivet.net/images/thumb/0/09/Spinal_cord_tracts_-_English.png/200px-Spinal_cord_tracts_-_English.png

Spinal cord tracts

he spinal cord contains a number of sensory (ascending) pathways or tracts contained within the white matter. These pathways allow sensory information such as pain, touch, temperature or kinaesthesia (conscious proprioception) to be passed through the spinal cord and on to higher levels of the brain. 

Vasculature

It is important to note that there is no direct vasculature to the spinal cord but instead there are a number of choroid plexuses that act as an exchanger between the vasculature of the spinal cord/brain and the fluid surrounding these structures. This distinction is referred to as the "blood-brain barrier". 

The vasculature of the spinal cord has a close relationship with the
 cerebrospinal fluid (CSF) within the subarachnoid space. This CSF effectively forms a water jacket that buoys up the spinal cord and protects it from external influences. Therefore it is extremely important that the CSF has the appropriate properties in order to undertake this role. The vasculature of the spinal cord therefore has to provide the appropriate level of oxygen, pressure, pH and nutrients to maintain homeostasis of the spinal cord. As the CSF also performs this role within the skull, the vasculature of the brain has an important relationship with every aspect of the ventricles and subarachnoid space within the central nervous system. 

Arterial Supply

The spinal cord is supplied by three main arteries that run along its length; the Ventral Spinal Artery, and paired Dorsolateral Spinal Arteries. The ventral spinal artery is the largest and follows the ventral fissure of the spinal cord. The dorsolateral arteries run close to the groove from which the dorsal nerve roots arise. Together with these three main arteries, the spinal cord is also supplied by branches from regional arteries including branches in the cervical, intercostal, lumbar and sacral regions. These regional arteries enter the spine at the intervertebral foramina, often accompanying the roots of spinal nerves. These regional arteries also form plexuses into which the three main longitudinal arteries run. The number and type of arteries that enter the spine from regional branches varies with species and also between individuals. 
The
 ventral spinal artery supplies the main "core" of the spinal cord, i.e. the grey matter. It also partially supplies the white matter via the ventral fissure, although the majority of the white matter is supplied by radial branches of the dorsolateral arteries. There are also a number of anastamoses between both sets of arteries.

Venous Supply

Along the length of the spinal cord runs the vertebral venous plexus which drains the blood from the vertebrae and surrounding musculature. This venous plexus gives rise to veins that then leave the vertebrae via the intervertebral foramina and then go on to join the major venous channels of the neck and the trunk; namely the vertebral, cranial caval, azygous and caudal caval veins. 

The venous plexus consists of paired channels within the epidural space that lie in a
 ventral position to the spinal cord. Each side of the pair is connected to its opposing plexus around the vertebrae resulting in a ladder-type pattern of venous vessels. The connections between each side are via the intervertebral foramina and these vessels are in close proximity to the spinal nerves. 

The veins around the plexuses have no valves and can theoretically pass blood in either direction. The vessels are able to adjust their size/pressure to compensate for intrathoracic pressure. This intermittency of flow causes an increased risk of septic or neoplastic disease within the vertebral column. Where blood is impeded or where flow may become temporarily held stagnant, this may allow tumor seeds or micro-organisms to settle within tributaries.

Spinal Cord Blood Supply Considerations

When giving an epidural or a subarachnoid puncture, the above described anatomy results in there being an increased risk of haemorrhage. The risk of haemorrhage is greatest at the atlanto-occipital space where there are particularly large tributaries.

Lymphatics

There are no lymphatic vessels or nodes within the spinal cord or other central nervous tissue.

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....

Muscular System Anatomy

Muscle Types
There are three types of muscle tissue: Visceral, cardiac, and skeletal.

1.                             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.
 

2.                             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 thebrain 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 autorhythmic 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.
 

3.                             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.

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. Thesupinator is a muscle that supinates the wrist by rolling it 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:


1.                                                     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. 

2.                                                     Tropomyosin. Tropomyosin is a long protein fiber that wraps around actin and covers the myosin binding sites on actin.

3.                                                     Troponin. Bound very tightly to tropomyosin, troponin moves tropomyosin away from myosin binding sites during muscle contraction.

Muscular System Physiology

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 by determining how 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.

1.                             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.
 

2.                             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 creatine phosphate. Muscles use energy in the form of ATP, converting ATP to ADP to release its energy. Creatine 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 energy-storing 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.