CSCS Chapter 1: Structure and Function of Body Systems PDF

CHAPTER Structure and Function 1 of Body Systems N. Travis Triplett, PhD After completing this chapter, you will be able to describe both the macrostructure and...

CHAPTER Structure and Function 1 of Body Systems N. Travis Triplett, PhD After completing this chapter, you will be able to describe both the macrostructure and microstructure of muscle and bone, describe the sliding-filament theory of muscular contraction, describe the specific morphological and physiological characteristics of different muscle fiber types and predict their relative involvement in different sport events, and describe the anatomical and physiological characteristics of the cardiovascular and respiratory systems. The author would like to acknowledge the significant contributions of Robert T. Harris and Gary R. Hunter to this chapter. 1 2 Essentials of Strength Training and Conditioning Physical exercise and sport performance involve There are approximately 206 bones in the body, effective, purposeful movements of the body. These though the number can vary. This relatively light, strong movements result from the forces developed in muscles, structure provides leverage, support, and protection which move the various body parts by acting through (figure 1.1). The axial skeleton consists of the skull lever systems of the skeleton. These skeletal muscles are (cranium), vertebral column (vertebra C1 through the under the control of the cerebral cortex, which activates coccyx), ribs, and sternum. The appendicular skele- the skeletal muscle cells or fibers through the motor ton includes the shoulder (or pectoral) girdle (left and neurons of the peripheral nervous system. Support for right scapula and clavicle); bones of the arms, wrists, this neuromuscular activity involves continuous delivery and hands (left and right humerus, radius, ulna, carpals, of oxygen and nutrients to working tissues and removal metacarpals, and phalanges); the pelvic girdle (left and of carbon dioxide and metabolic waste by-products from right coxal or innominate bones); and the bones of the working tissues through activities of the cardiovascular legs, ankles, and feet (left and right femur, patella, tibia, and respiratory systems. fibula, tarsals, metatarsals, and phalanges). In order to best apply the available scientific knowl- Junctions of bones are called joints. Fibrous joints edge to the training of athletes and the development of (e.g., sutures of the skull) allow virtually no movement; effective training programs, strength and conditioning cartilaginous joints (e.g., intervertebral disks) allow professionals must have a basic understanding of not limited movement; and synovial joints (e.g., elbow only musculoskeletal function but also those systems and knee) allow considerable movement. Sport and of the body that directly support the work of exercising exercise movements occur mainly about the synovial muscle. Accordingly, this chapter summarizes those joints, whose most important features are low friction aspects of the anatomy and function of the musculo- and large range of motion. Articulating bone ends are skeletal, neuromuscular, cardiovascular, and respiratory covered with smooth hyaline cartilage, and the entire systems that are essential for developing and maintaining joint is enclosed in a capsule filled with synovial fluid. muscular force and power. There are usually additional supporting structures of ligament and cartilage (13). Musculoskeletal System Virtually all joint movement consists of rotation about points or axes. Joints can be categorized by the number The musculoskeletal system of the human body consists of directions about which rotation can occur. Uniaxial of bones, joints, muscles, and tendons configured to joints, such as the elbow, operate as hinges, essentially allow the great variety of movements characteristic of rotating about only one axis. The knee is often referred to human activity. This section describes the various com- as a hinge joint, but its axis of rotation actually changes ponents of the musculoskeletal system, both individually throughout the joint range of motion. Biaxial joints, and in the context of how they function together. such as the ankle and wrist, allow movement about two perpendicular axes. Multiaxial joints, including the Skeleton shoulder and hip ball-and-socket joints, allow movement The muscles of the body do not act directly to exert force about all three perpendicular axes that define space. on the ground or other objects. Instead, they function The vertebral column is made up of vertebral bones by pulling against bones that rotate about joints and separated by flexible disks that allow movement to occur. transmit force to the environment. Muscles can only The vertebrae are grouped into 7 cervical vertebrae in pull, not push; but through the system of bony levers, the neck region; 12 thoracic vertebrae in the middle to muscle pulling forces can be manifested as either pulling upper back; 5 lumbar vertebrae, which make up the lower or pushing forces against external objects. back; 5 sacral vertebrae, which are fused together and What Factors Affect Skeletal Growth in an Adult? There are several things that can positively affect the adult skeleton, and most are a result of muscle use. When the body is subjected to heavy loads (job tasks or resistance training), the bone will increase in density and bone mineral content. If the body performs more explosive movements with impact, similar changes can occur. Some of the higher bone densities have been seen in people who engage in gymnastics or other activities that involve high-strength and high-power movements, some with hard landings (11). Other factors that influence bone adaptations are whether the axial skeleton is loaded and how often this loading occurs (frequency). Since the adaptation period of bone is longer than that of skeletal muscle, it is important to vary the stimulus in terms of frequency, intensity, and type.  Structure and Function of Body Systems 3 Clavicle Scapula Sternum Humerus Ribs Vertebral column Crest of pelvis (iliac crest) Pelvis Radius Ulna Carpals Metacarpals Femur Patella Tibia Fibula Metatarsals a b FiGure 1.1 (a) Front view and (b) rear view of an adult male human skeleton. E6372/NSCA/fig01.01/508626/alw/r1-pulled make up the rear part of the pelvis; and 3 to 5 coccygeal Fibrous connective tissue, or epimysium, covers the vertebrae, which form a kind of vestigial internal tail body’s more than 430 skeletal muscles. The epimysium extending downward from the pelvis. is contiguous with the tendons at the ends of the muscle (figure 1.3). The tendon is attached to bone periosteum, Skeletal Musculature a specialized connective tissue covering all bones; any contraction of the muscle pulls on the tendon and, in The system of muscles that enables the skeleton to move turn, the bone. Limb muscles have two attachments to is depicted in figure 1.2. The connection point between bone: proximal (closer to the trunk) and distal (farther bones is the joint, and skeletal muscles are attached to from the trunk). The two attachments of trunk muscles bones at each of their ends. Without this arrangement, are termed superior (closer to the head) and inferior movement could not occur. (closer to the feet). Muscle cells, often called muscle fibers, are long Musculoskeletal Macrostructure (sometimes running the entire length of a muscle), cylin- and Microstructure drical cells 50 to 100 µm in diameter (about the diameter Each skeletal muscle is an organ that contains muscle of a human hair). These fibers have many nuclei situated tissue, connective tissue, nerves, and blood vessels. on the periphery of the cell and have a striated appearance  Trapezius Deltoid Infraspinatus Pectoralis major Teres major Biceps brachii Triceps brachii Rectus abdominis Brachialis Latissimus dorsi External oblique Brachioradialis Finger extensors Finger flexors Adductor longus Gluteus maximus Gracilis Semitendinosus Sartorius Biceps femoris Rectus femoris Semimembranosus Vastus lateralis Vastus medialis Gastrocnemius Tibialis anterior Soleus a b FiGure 1.2 (a) Front view and (b) rear view of adult male human skeletal musculature. E6372/NSCA/fig01.02a/508073/alw/r1-pulled E6372/NSCA/fig01.02b/508074/alw/r1-pulled Tendon Muscle belly Epimysium (deep fascia) Fasciculus Endomysium (between fibers) Sarcolemma Sarcoplasm Myofibril Perimysium Myofilaments actin (thin) myosin (thick) Single muscle fiber Nucleus FiGure 1.3 Schematic drawing of a muscle illustrating three types of connective tissue: epimysium (the outer layer), perimysium (surrounding each fasciculus, or group of fibers), and endomysium (surrounding individual fibers). 4 www.ebook3000.com E6372/NSCA/fig01.03/508017/alw/r1-pulled  Structure and Function of Body Systems 5 under low magnification. Under the epimysium the Mitochondrion Opening to T-tubule muscle fibers are grouped in bundles (fasciculi) that may consist of up to 150 fibers, with the bundles sur- rounded by connective tissue called perimysium. Each muscle fiber is surrounded by connective tissue called endomysium, which is encircled by and is contiguous with the fiber’s membrane, or sarcolemma (13). All the connective tissue—epimysium, perimysium, and endomysium—is contiguous with the tendon, so tension developed in a muscle cell is transmitted to the tendon and the bone to which it is attached (see figure 1.3). The junction between a motor neuron (nerve cell) Sarcoplasmic T-tubule reticulum and the muscle fibers it innervates is called the motor end plate, or, more often, the neuromuscular junction Myofibril Sarcolemma (figure 1.4). Each muscle cell has only one neuromuscu- lar junction, although a single motor neuron innervates FiGure 1.5 Sectional view of a muscle fiber. many muscle fibers, sometimes hundreds or even thou- sands. A motor neuron and the muscle fibers it innervates are called a motor unit. All the muscle fibers of a motor of protein filaments, other proteins, stored glycogen and E6372/NSCA/fig01.05/508019/alw/r1-pulled unit contract together when they are stimulated by the fat particles, enzymes, and specialized organelles such motor neuron. as mitochondria and the sarcoplasmic reticulum. The interior structure of a muscle fiber is depicted in Hundreds of myofibrils (each about 1 mm in diam- figure 1.5. The sarcoplasm, which is the cytoplasm of a eter, 1/100 the diameter of a hair) dominate the sarco- muscle fiber, contains contractile components consisting plasm. Myofibrils contain the apparatus that contracts the muscle cell, which consists primarily of two types Dendrites of myofilament: myosin and actin. The myosin fila- ments (thick filaments about 16 nm in diameter, about Nucleus 1/10,000 the diameter of a hair) contain up to 200 myosin molecules. The myosin filament consists of a globular head, a hinge point, and a fibrous tail. The globular heads protrude away from the myosin filament at regular intervals, and a pair of myosin filaments forms a cross- bridge, which interacts with actin. The actin filaments Axon (thin filaments about 6 nm in diameter) consist of two strands arranged in a double helix. Myosin and actin filaments are organized longitudinally in the smallest contractile unit of skeletal muscle, the sarcomere. Sarco- Node of Ranvier meres average about 2.5 mm in length in a relaxed fiber (approximately 4,500 per centimeter of muscle length) Myelin sheath and are repeated the entire length of the muscle fiber (1). Figure 1.6 shows the structure and orientation of the myosin and actin in the sarcomere. Adjacent myosin filaments anchor to each other at the M-bridge in the Neuromuscular center of the sarcomere (the center of the H-zone). Actin junction filaments are aligned at both ends of the sarcomere and are anchored at the Z-line. Z-lines are repeated through Muscle the entire myofibril. Six actin filaments surround each myosin filament, and each actin filament is surrounded by three myosin filaments. It is the arrangement of the myosin and actin fil- aments and the Z-lines of the sarcomeres that gives FiGure 1.4 A motor unit, consisting of a motor neuron skeletal muscle its alternating dark and light pattern, and the muscle fibers it innervates. There are typically which appears as striated under magnification. The dark several hundred muscle fibers in a single motor unit. A-band corresponds with the alignment of the myosin E6372/NSCA/fig01.04/508018/alw/r1-pulled 6 Essentials of Strength Training and Conditioning Actin filament Myosin filament Myofilaments (cross sections) Myofibril M-line I-band A-band I-band A-band Z-line H-zone Z-line M-line Resting state Sarcomere Myosin (thick) filament Head Tail Backbone Tropomyosin Actin (thin) filament Actin Troponin Actin Cross-bridge Myosin Z-line end M-bridge H-zone level FiGure 1.6 Detailed view of the myosin and actin protein filaments in muscle. The arrangement of myosin (thick) and actin (thin) filaments gives skeletal muscle its striated appearance. filaments, whereas the light I-band corresponds with the sarcomere. The I-band also decreases as the Z-lines are E6372/NSCA/fig01.06/508020/alw/r1-pulled areas in two adjacent sarcomeres that contain only actin pulled toward the center of the sarcomere. filaments (13). The Z-line is in the middle of the I-band Parallel to and surrounding each myofibril is an and appears as a thin, dark line running longitudinally intricate system of tubules, called the sarcoplasmic through the I-band. The H-zone is the area in the center reticulum (see figure 1.5), which terminates as vesicles of the sarcomere where only myosin filaments are pres- in the vicinity of the Z-lines. Calcium ions are stored in ent. During muscle contraction, the H-zone decreases as the vesicles. The regulation of calcium controls mus- the actin slides over the myosin toward the center of the cular contraction. T-tubules, or transverse tubules, run  Structure and Function of Body Systems 7 perpendicular to the sarcoplasmic reticulum and termi- The action of myosin crossbridges pulling on the actin nate in the vicinity of the Z-line between two vesicles. filaments is responsible for the movement of the actin Because the T-tubules run between outlying myofibrils filament. Because only a very small displacement of the and are contiguous with the sarcolemma at the surface of actin filament occurs with each flexion of the myosin the cell, discharge of an action potential (an electrical crossbridge, very rapid, repeated flexions must occur nerve impulse) arrives nearly simultaneously from the in many crossbridges throughout the entire muscle for surface to all depths of the muscle fiber. Calcium is thus measurable movement to occur (13). released throughout the muscle, producing a coordinated resting Phase Under normal resting conditions, little contraction. calcium is present in the myofibril (most of it is stored in the sarcoplasmic reticulum), so very few of the myosin ▶ The discharge of an action potential from a crossbridges are bound to actin. Even with the actin motor nerve signals the release of calcium binding site covered, myosin and actin still interact in a from the sarcoplasmic reticulum into the weak bond, which becomes strong (and muscle tension myofibril, causing tension development in is produced) when the actin binding site is exposed after muscle. release of the stored calcium. excitation–Contraction Coupling Phase Before Sliding-Filament Theory myosin crossbridges can flex, they must first attach to of Muscular Contraction the actin filament. When the sarcoplasmic reticulum is In its simplest form, the sliding-filament theory states stimulated to release calcium ions, the calcium binds that the actin filaments at each end of the sarcomere with troponin, a protein that is situated at regular inter- slide inward on myosin filaments, pulling the Z-lines vals along the actin filament (see figure 1.6) and has a toward the center of the sarcomere and thus shortening high affinity for calcium ions. This causes a shift to occur the muscle fiber (figure 1.7). As actin filaments slide over in another protein molecule, tropomyosin, which runs myosin filaments, both the H-zone and I-band shrink. along the length of the actin filament in the groove of the I-band A-band I-band I-band A-band I-band H-zone Z-line Z-line H-zone Z-line Z-line Myosin Actin a filament filament b A-band E6372/NSCA/fig01.07a/508021/alw/r1-pulledZ-line Z-line E6372/NSCA/fig01.07b/513635/alw/r1-pulled c FiGure 1.7 Contraction of a myofibril. (a) In stretched muscle the I-bands and H-zone are elongated, and there is low force potential due to reduced crossbridge–actin alignment. (b) When muscle contracts (here partially), the I-bands E6372/NSCA/fig01.07c/513636/alw/r1-pulled and H-zone are shortened. Force potential is high due to optimal crossbridge–actin alignment. (c) With contracted muscle, force potential is low because the overlap of actin reduces the potential for crossbridge–actin alignment. 8 Essentials of Strength Training and Conditioning double helix. The myosin crossbridge now attaches much myosin, and resetting of the myosin head position—is more rapidly to the actin filament, allowing force to be repeated over and over again throughout the muscle produced as the actin filaments are pulled toward the fiber. This occurs as long as calcium is available in the center of the sarcomere (1). It is important to understand myofibril, ATP is available to assist in uncoupling the that the amount of force produced by a muscle at any myosin from the actin, and sufficient active myosin instant in time is directly related to the number of myosin ATPase is available for catalyzing the breakdown of ATP. crossbridges bound to actin filaments cross-sectionally relaxation Phase Relaxation occurs when the stim- at that instant in time (1). ulation of the motor nerve stops. Calcium is pumped back into the sarcoplasmic reticulum, which prevents the ▶ The number of crossbridges that are formed link between the actin and myosin filaments. Relaxation between actin and myosin at any instant is brought about by the return of the actin and myosin in time dictates the force production of a filaments to their unbound state. muscle. Contraction Phase The energy for pulling action, Neuromuscular System or power stroke, comes from hydrolysis (breakdown) of adenosine triphosphate (ATP) to adenosine diphos- Muscle fibers are innervated by motor neurons that trans- phate (ADP) and phosphate, a reaction catalyzed by the mit impulses in the form of electrochemical signals from enzyme myosin adenosine triphosphatase (ATPase). the spinal cord to muscle. A motor neuron generally has Another molecule of ATP must replace the ADP on the numerous terminal branches at the end of its axon and myosin crossbridge globular head in order for the head to thus innervates many different muscle fibers. The whole detach from the active actin site and return to its original structure is what determines the muscle fiber type and its position. This allows the contraction process to continue characteristics, function, and involvement in exercise. (if calcium is available to bind to troponin) or relaxation to occur (if calcium is not available). It may be noted Activation of Muscles that calcium plays a role in regulating a large number When a motor neuron fires an impulse or action poten- of events in skeletal muscle besides contraction. These tial, all of the fibers that it serves are simultaneously include glycolytic and oxidative energy metabolism, as activated and develop force. The extent of control of a well as protein synthesis and degradation (10). muscle depends on the number of muscle fibers within each motor unit. Muscles that must function with great ▶ Calcium and ATP are necessary for cross- precision, such as eye muscles, may have motor units bridge cycling with actin and myosin fila- with as few as one muscle fiber per motor neuron. ments. Changes in the number of active motor units in these small muscles can produce the extremely fine gradations recharge Phase Measurable muscle shortening in force that are necessary for precise movements of the transpires only when this sequence of events—binding eyeball. In contrast, the quadriceps muscle group, which of calcium to troponin, coupling of the myosin cross- moves the leg with much less precision, may have sev- bridge with actin, power stroke, dissociation of actin and eral hundred fibers served by one motor neuron. Steps of Muscle Contraction The steps of muscle contraction can be summarized as follows: 1. Initiation of ATP splitting (by myosin ATPase) causes myosin head to be in an “energized” state that allows it to move into a position to be able to form a bond with actin. 2. The release of phosphate from the ATP splitting process then causes the myosin head to change shape and shift. 3. This pulls the actin filament in toward the center of the sarcomere and is referred to as the power stroke; ADP is then released. 4. Once the power stroke has occurred, the myosin head detaches from the actin but only after another ATP binds to the myosin head because the binding process facilitates detachment. 5. The myosin head is now ready to bind to another actin (as described in step 1), and the cycle contin- ues as long as ATP and ATPase are present and calcium is bound to the troponin.  Structure and Function of Body Systems 9 The action potential (electric current) that flows to merge and eventually completely fuse, a condition along a motor neuron is not capable of directly exciting called tetanus (figure 1.8, c and d). This is the maximal muscle fibers. Instead, the motor neuron excites the amount of force the motor unit can develop. muscle fiber(s) that it innervates by chemical transmis- sion. Arrival of the action potential at the nerve terminal Muscle Fiber Types causes release of a neurotransmitter, acetylcholine, Skeletal muscles are composed of fibers that have which diffuses across the neuromuscular junction, markedly different morphological and physiological causing excitation of the sarcolemma. Once a sufficient characteristics. These differences have led to several amount of acetylcholine is released, an action potential is different systems of classification, based on a variety of generated along the sarcolemma, and the fiber contracts. criteria. The most familiar approach is to classify fibers All of the muscle fibers in the motor unit contract and according to twitch time, employing the terms slow- develop force at the same time. There is no evidence twitch and fast-twitch fiber. Because a motor unit is that a motor neuron stimulus causes only some of the composed of muscle fibers that are all of the same type, fibers to contract. Similarly, a stronger action potential it also can be designated using this classification system. cannot produce a stronger contraction. This phenomenon A fast-twitch motor unit is one that develops force and is known as the all-or-none principle of muscle. also relaxes rapidly and thus has a short twitch time. Each action potential traveling down a motor neuron Slow-twitch motor units, in contrast, develop force and results in a short period of activation of the muscle fibers relax slowly and have a long twitch time. within the motor unit. The brief contraction that results Histochemical staining for myosin ATPase content is referred to as a twitch. Activation of the sarcolemma is often used to classify fibers as slow-twitch or fast- results in the release of calcium within the fiber, and twitch. Although the techniques can stain for multiple contraction proceeds as previously described. Force fiber types, the commonly identified fibers are Type I develops if there is resistance to the pulling interaction (slow-twitch), Type IIa (fast-twitch), and Type IIx (fast- of actin and myosin filaments. Although calcium release twitch). Another more specific method is to quantify during a twitch is sufficient to allow optimal activation the amount of myosin heavy chain (MHC) protein; the of actin and myosin, and thereby maximal force of the nomenclature for this is similar to that with the myosin fibers, calcium is removed before force reaches its max- ATPase methodology. imum, and the muscle relaxes (figure 1.8a). If a second The contrast in mechanical characteristics of Type I twitch is elicited from the motor nerve before the fibers and Type II fibers is accompanied by a distinct difference completely relax, force from the two twitches summates, in the ability of the fibers to demand and supply energy and the resulting force is greater than that produced by for contraction and thus to withstand fatigue. Type I a single twitch (figure 1.8b). Decreasing the time inter- fibers are generally efficient and fatigue resistant and val between the twitches results in greater summation have a high capacity for aerobic energy supply, but they of crossbridge binding and force. The stimuli may be have limited potential for rapid force development, as delivered at so high a frequency that the twitches begin characterized by low myosin ATPase activity and low anaerobic power (2, 8). Type II motor units are essentially the opposite, char- acterized as inefficient and fatigable and as having low aerobic power, rapid force development, high myosin d ATPase activity, and high anaerobic power (2, 8). Type IIa and Type IIx fibers differ mainly in their capacity for aerobic–oxidative energy supply. Type IIa fibers, for c example, have greater capacity for aerobic metabolism Force and more capillaries surrounding them than Type IIx b a and therefore show greater resistance to fatigue (3, 7, 9, 12). Based on these differences, it is not surprising that postural muscles, such as the soleus, have a high composition of Type I fibers, whereas large, so-called Frequency locomotor muscles, such as the quadriceps group, have a FiGure 1.8 Twitch, twitch summation, and tetanus of a mixture of both Type I and Type II fibers to enable both E6372/NSCA/fig01.08/508022/alw/r1-pulled motor unit: a = single twitch; b = force resulting from low and high power output activities (such as jogging and summation of two twitches; c = unfused tetanus; d = sprinting, respectively). Refer to table 1.1 for a summary fused tetanus. of the primary characteristics of fiber types. 10 Essentials of Strength Training and Conditioning TABle 1.1 Major Characteristics of Muscle Fiber Types Fiber types Characteristic Type I Type IIa Type IIx Motor neuron size Small Large Large Recruitment threshold Low Intermediate/High High Nerve conduction velocity Slow Fast Fast Contraction speed Slow Fast Fast Relaxation speed Slow Fast Fast Fatigue resistance High Intermediate/Low Low Endurance High Intermediate/Low Low Force production Low Intermediate High Power output Low Intermediate/High High Aerobic enzyme content High Intermediate/Low Low Anaerobic enzyme content Low High High Sarcoplasmic reticulum complexity Low Intermediate/High High Capillary density High Intermediate Low Myoglobin content High Low Low Mitochondrial size, density High Intermediate Low Fiber diameter Small Intermediate Large Color Red White/Red White other means of varying skeletal muscle force involves ▶ Motor units are composed of muscle fibers an increase in force through varying the number of with specific morphological and physio- motor units activated, a process known as recruitment. logical characteristics that determine their In large muscles, such as those in the thigh, motor units functional capacity. are activated at near-tetanic frequency when called on. Increases in force output are achieved through recruit- Motor unit recruitment Patterns ment of additional motor units. The type of motor unit recruited for a given activity Through everyday experiences, we are quite aware that a is determined by its physiological characteristics (table given muscle can vary its level of force output according 1.2). For an activity such as distance running, slow- to the level required by a particular task. This ability twitch motor units are engaged to take advantage of to vary or gradate force is essential for performance of their remarkable efficiency, endurance capacity, and smooth, coordinated patterns of movement. Muscular resistance to fatigue. If additional force is needed, as in force can be graded in two ways. One is through varia- a sprint at the end of a race, the fast-twitch motor units tion in the frequency at which motor units are activated. are called into play to increase the pace; unfortunately, If a motor unit is activated once, the twitch that arises exercise at such intensity cannot be maintained very does not produce a great deal of force. However, if the long. If the activity requires near-maximal performance, frequency of activation is increased so that the forces of as in a power clean, most of the motor units are called the twitches begin to overlap or summate, the resulting into play, with fast-twitch units making the more signif- force developed by the motor unit is much greater. This icant contribution to the effort. Complete activation of method of varying force output is especially important the available motor neuron pool is probably not possible in small muscles, such as those of the hand. Even at in untrained people (4, 5, 6). Although the large fast- low forces, most of the motor units in these muscles twitch units may be recruited if the effort is substantial, are activated, albeit at a low frequency. Force output of under most circumstances it is probably not possible to the whole muscle is intensified through increase in the activate them at a high enough frequency for maximal frequency of firing of the individual motor units. The force to be realized.  Structure and Function of Body Systems 11 TABle 1.2 relative involvement of Muscle ▶ Proprioceptors are specialized sensory Fiber Types in Sport events receptors that provide the central nervous Event Type I Type II system with information needed to maintain muscle tone and perform complex coordi- 100 m sprint Low High nated movements. 800 m run High High Marathon High Low Muscle Spindles Olympic weightlifting Low High Muscle spindles are proprioceptors that consist of Soccer, lacrosse, hockey High High several modified muscle fibers enclosed in a sheath of connective tissue (figure 1.9). These modified fibers, American football wide receiver Low High called intrafusal fibers, run parallel to the normal, or American football lineman Low High extrafusal, fibers. Muscle spindles provide informa- Basketball, team handball Low High tion concerning muscle length and the rate of change in length. When the muscle lengthens, spindles are Volleyball Low High stretched. This deformation activates the sensory neuron Baseball or softball pitcher Low High of the spindle, which sends an impulse to the spinal cord, Boxing High High where it synapses (connects) with motor neurons. This results in the activation of motor neurons that innervate Wrestling High High the same muscle. Spindles thus indicate the degree to 50 m swim Low High which the muscle must be activated in order to overcome Field events Low High a given resistance. As a load increases, the muscle is stretched to a greater extent, and engagement of muscle Cross-country skiing, biathlon High Low spindles results in greater activation of the muscle. Tennis High High Muscles that perform precise movements have many Downhill or slalom skiing High High spindles per unit of mass to help ensure exact control of their contractile activity. A simple example of muscle Speed skating High High spindle activity is the knee jerk reflex. Tapping on the Track cycling Low High tendon of the knee extensor muscle group below the Distance cycling High Low patella stretches the muscle spindle fibers. This causes activation of extrafusal muscle fibers in the same muscle. Rowing High High Sensory neuron ▶ The force output of a muscle can be varied Intrafusal fiber through change in the frequency of activa- tion of individual motor units or change in the number of activated motor units. Motor Proprioception neuron Muscle Proprioceptors are specialized sensory receptors spindle located within joints, muscles, and tendons. Because these receptors are sensitive to pressure and tension, they relay information concerning muscle dynamics to the conscious and subconscious parts of the central nervous Extrafusal fiber system. The brain is thus provided with information con- cerning kinesthetic sense, or conscious appreciation of the position of body parts with respect to gravity. Most FiGure 1.9 Muscle spindle. When a muscle is stretched, of this proprioceptive information, however, is processed deformation of the muscle spindle activates the sensory at subconscious levels so we do not have to dedicate neuron, which sends an impulse to the spinal cord, conscious activity toward tasks such as maintaining where it synapses with a motor neuron, causing the E6372/NSCA/fig01.09/508026/alw/r1-pulled posture or position of body parts. muscle to contract. 12 Essentials of Strength Training and Conditioning How Can Athletes improve Force Production? Incorporate phases of training that use heavier loads in order to optimize neural recruitment. Increase the cross-sectional area of muscles involved in the desired activity. Perform multimuscle, multijoint exercises that can be done with more explosive actions to optimize fast-twitch muscle recruitment. A knee jerk occurs as these fibers actively shorten. This, The GTOs’ inhibitory process is thought to provide a in turn, shortens the intrafusal fibers and causes their mechanism that protects against the development of discharge to cease. excessive tension. The effect of GTOs is therefore min- imal at low forces; but when an extremely heavy load is Golgi Tendon Organs placed on the muscle, reflexive inhibition mediated by Golgi tendon organs (GTOs) are proprioceptors the GTOs causes the muscle to relax. The ability of the located in tendons near the myotendinous junction and motor cortex to override this inhibition may be one of are in series, that is, attached end to end, with extrafusal the fundamental adaptations to heavy resistance training. muscle fibers (figure 1.10). Golgi tendon organs are activated when the tendon attached to an active muscle is stretched. As tension in the muscle increases, discharge Cardiovascular System of the GTOs increases. The sensory neuron of the GTO The primary roles of the cardiovascular system are to synapses with an inhibitory interneuron in the spinal transport nutrients and remove waste and by-products cord, which in turn synapses with and inhibits a motor while assisting with maintaining the environment for neuron that serves the same muscle. The result is a all the body’s functions. The cardiovascular system reduction in tension within the muscle and tendon. Thus, plays key roles in the regulation of the body’s acid–base whereas spindles facilitate activation of the muscle, system, fluids, and temperature, as well as a variety of neural input from GTOs inhibits muscle activation. other physiological functions. This section describes the anatomy and physiology of the heart and the blood Inhibitory interneuron vessels. Heart Tendon The heart is a muscular organ composed of two inter- connected but separate pumps; the right side of the heart pumps blood through the lungs, and the left side pumps Muscle blood through the rest of the body. Each pump has two chambers: an atrium and a ventricle (figure 1.11). The right and left atria deliver blood into the right and left Motor ventricles. The right and left ventricles supply the main neuron force for moving blood through the pulmonary and peripheral circulations, respectively (13). Valves Sensory neuron The tricuspid valve and mitral valve (bicuspid valve) (collectively called atrioventricular [AV] valves) pre- vent the flow of blood from the ventricles back into the atria during ventricular contraction (systole). The aortic Golgi tendon organ valve and pulmonary valve (collectively, the semilunar valves) prevent backflow from the aorta and pulmonary FiGure 1.10 Golgi tendon organ (GTO). When an arteries into the ventricles during ventricular relaxation extremely heavy load is placed on the muscle, discharge of the GTOE6372/NSCA/fig01.10/508027/alw/r1-pulled occurs. The sensory neuron of the GTO (diastole). Each valve opens and closes passively; that activates an inhibitory interneuron in the spinal cord, is, each closes when a backward pressure gradient pushes which in turn synapses with and inhibits a motor neuron blood back against it, opening when a forward pressure serving the same muscle. gradient forces blood in the forward direction (13).  Structure and Function of Body Systems 13 Head and upper extremity Aorta Superior vena cava Pulmonary artery To right lung To left lung Pulmonary veins Aortic valve From left lung From right lung Pulmonary valve Left atrium Right atrium Tricuspid valve Mitral valve Left ventricle Right ventricle Inferior vena cava Trunk and lower extremity FiGure 1.11 Structure of the human heart and the course of blood flow through its chambers. Conduction System The fibers of the node are contiguous with the muscle fibers of the atrium, with the result that each electrical A specialized electrical conduction system (figure 1.12) impulse that begins in the SA node normally spreads controls the mechanical contraction of theE6372/NSCA/fig01.11/508028/alw/r1-pulled heart. The immediately into the atria. The conductive system is conduction system is composed of organized so that the impulse does not travel into the ven- the sinoatrial (SA) node—the intrinsic pace- tricles too rapidly, allowing time for the atria to contract maker—where rhythmic electrical impulses are and empty blood into the ventricles before ventricular normally initiated; contraction begins. It is primarily the AV node and its associated conductive fibers that delay each impulse the internodal pathways that conduct the impulse entering into the ventricles. The AV node is located in from the SA node to the atrioventricular node; the posterior septal wall of the right atrium (13). the atrioventricular (AV) node, where the The left and right bundle branches lead from the AV impulse is delayed slightly before passing into bundle into the ventricles. Except for their initial portion, the ventricles; where they penetrate the AV barrier, these conduction the atrioventricular (AV) bundle, which con- fibers have functional characteristics quite opposite ducts the impulse to the ventricles; and those of the AV nodal fibers. They are large and transmit the left bundle branch and right bundle branch, impulses at a much higher velocity than the AV nodal which further divide into the Purkinje fibers and fibers. Because these fibers give way to the Purkinje conduct impulses to all parts of the ventricles. fibers, which more completely penetrate the ventricles, the impulse travels quickly throughout the entire ven- The SA node is a small area of specialized muscle tricular system and causes both ventricles to contract at tissue located in the upper lateral wall of the right atrium. approximately the same time (13). 14 Essentials of Strength Training and Conditioning 2 R 1 P T P Millivolts SA node 0 Internodal Q pathways S AV node 1 Left bundle branch 2 Purkinje fibers Right bundle FiGure 1.13 Normal electrocardiogram. branch E6372/NSCA/fig01.13/508031/alw/r2-pulled FiGure 1.12 The electrical conduction system of the S-wave), and a T-wave. The P-wave and the QRS heart. complex are recordings of electrical depolarization, that is, the electrical stimulus that leads to mechanical The SA node normally controls heart rhythmicity contraction. Depolarization is the reversal of the mem- becauseE6372/NSCA/fig01.12/508029/alw/r1-pulled its discharge rate is considerably greater (60-80 brane electrical potential, whereby the normally negative times per minute) than that of either the AV node (40-60 potential inside the membrane becomes slightly positive times per minute) or the ventricular fibers (15-40 times and the outside becomes slightly negative. The P-wave per minute). Each time the SA node discharges, its is generated by the changes in the electrical potential of impulse is conducted into the AV node and the ventricu- cardiac muscle cells that depolarize the atria and result lar fibers, discharging their excitable membranes. Thus, in atrial contraction. The QRS complex is generated by these potentially self-excitatory tissues are discharged the electrical potential that depolarizes the ventricles before self-excitation can actually occur. and results in ventricular contraction. In contrast, the The inherent rhythmicity and conduction properties T-wave is caused by the electrical potential generated as of the myocardium (heart muscle) are influenced by the ventricles recover from the state of depolarization; the cardiovascular center of the medulla, which trans- this process, called repolarization, occurs in ventricu- mits signals to the heart through the sympathetic and lar muscle shortly after depolarization. Although atrial parasympathetic nervous systems, both of which are repolarization occurs as well, its wave formation usually components of the autonomic nervous system. The atria occurs during the time of ventricular depolarization and are supplied with a large number of both sympathetic and is thus masked by the QRS complex (13). parasympathetic neurons, whereas the ventricles receive sympathetic fibers almost exclusively. Stimulation of the Blood Vessels sympathetic nerves accelerates depolarization of the SA The central and peripheral circulation form a single node (the chronotropic effect), which causes the heart to closed-circuit system with two components: an arterial beat faster. Stimulation of the parasympathetic nervous system, which carries blood away from the heart, and system slows the rate of SA node discharge, which slows a venous system, which returns blood toward the heart the heart rate. The resting heart rate normally ranges (figure 1.14). The blood vessels of each system are from 60 to 100 beats/min; fewer than 60 beats/min is identified here. called bradycardia, and more than 100 beats/min is called tachycardia. Arteries The function of arteries is to rapidly transport blood Electrocardiogram pumped from the heart. Because blood pumped from The electrical activity of the heart can be recorded at the heart is under relatively high pressure, arteries have the surface of the body; a graphic representation of strong, muscular walls. Small branches of arteries called this activity is called an electrocardiogram (ECG). arterioles act as control vessels through which blood A normal ECG, seen in figure 1.13, is composed of a enters the capillaries. Arterioles play a major role in the P-wave, a QRS complex (the QRS complex is often regulation of blood flow to the capillaries. Arterioles three separate waves: a Q-wave, an R-wave, and an have strong, muscular walls that are capable of closing  Structure and Function of Body Systems 15 Pulmonary to a great degree and thereby act as a reservoir for blood, circulation: 9% either in small or in large amounts (13). In addition, some veins, such as those in the legs, contain one-way valves that help maintain venous return by preventing retrograde blood flow. ▶ The cardiovascular system transports nutri- ents and removes waste products while helping to maintain the environment for all Heart: 7% the body’s functions. The blood transports oxygen from the lungs to the tissues for use in cellular metabolism; and it transports carbon dioxide, the most abundant by-prod- Arteries: 13% uct of metabolism, from the tissues to the lungs, where it is removed from the body. Arterioles and capillaries: 7% Blood Two paramount functions of blood are the transport of oxygen from the lungs to the tissues for use in cellular Veins, venules, and metabolism and the removal of carbon dioxide, the most venous sinuses: 64% abundant by-product of metabolism, from the tissues FiGure 1.14 The arterial (right) and venous (left) com- to the lungs. The transport of oxygen is accomplished ponents ofE6372/NSCA/fig01.14/508032/alw/r1-pulled the circulatory system. The percent values by hemoglobin, the iron–protein molecule carried by indicate the distribution of blood volume throughout the red blood cells. Hemoglobin also has an additional the circulatory system at rest. important role as an acid–base buffer, a regulator of hydrogen ion concentration, which is crucial to the rates of chemical reactions in cells. Red blood cells, the the arteriole completely or allowing it to be dilated many major component of blood, have other functions as well. times their size, thus vastly altering blood flow to the For instance, they contain a large quantity of carbonic capillaries in response to the needs of the tissues (13). anhydrase, which catalyzes the reaction between carbon dioxide and water to facilitate carbon dioxide removal. Capillaries The function of capillaries is to facilitate exchange of oxygen, fluid, nutrients, electrolytes, hormones, and respiratory System other substances between the blood and the interstitial The primary function of the respiratory system is the fluid in the various tissues of the body. The capillary basic exchange of oxygen and carbon dioxide. The anat- walls are very thin and are permeable to these, but not omy of the human respiratory system is shown in figure all, substances (13). 1.15. As air passes through the nose, the nasal cavities perform three distinct functions: warming, humidifying, Veins and purifying the air (13). Air is distributed to the lungs Venules collect blood from the capillaries and gradually by way of the trachea, bronchi, and bronchioles. The converge into the progressively larger veins, which trachea is called the first-generation respiratory passage, transport blood back to the heart. Because the pressure and the right and left main bronchi are the second-gen- in the venous system is very low, venous walls are thin, eration passages; each division thereafter is an additional although muscular. This allows them to constrict or dilate generation (bronchioles). There are approximately 23 What is the Skeletal Muscle Pump? The skeletal muscle pump is the assistance that contracting muscles provide to the circulatory system. The muscle pump works with the venous system, which contains the one-way valves for blood return to the heart. The contracting muscle compresses the veins, but since the blood can flow only in the direction of the valves, it is returned to the heart. This mechanism is one of the reasons that individuals are told to keep moving around after exercise to avoid blood pooling in the lower extremities. On the flip side, it is important to periodically squeeze muscles during prolonged sitting to facilitate blood return to the heart. 16 Essentials of Strength Training and Conditioning Conchae Pharynx Epiglottis Glottis Larynx, vocal cords Esophagus Trachea Pulmonary Left main artery bronchus Right main bronchus Pulmonary vein Alveoli Bronchiole FiGure 1.15 Gross anatomy of the human respiratory system. E6372/Baechle/fig 01.15/508033/JanT/R1 generations before the air finally reaches the alveoli, push the abdomen upward against the bottom of the where gases are exchanged in respiration (13). diaphragm (13). The second method for expanding the lungs is to raise ▶ The primary function of the respiratory the rib cage. Because the chest cavity is small and the system is the basic exchange of oxygen and ribs are slanted downward while in the resting position, carbon dioxide. elevating the rib cage allows the ribs to project almost directly forward so that the sternum can move forward and away from the spine. The muscles that elevate the exchange of Air rib cage are called muscles of inspiration and include The amount and movement of air and expired gases in the external intercostals, the sternocleidomastoids, and out of the lungs are controlled by expansion and the anterior serrati, and the scaleni. The muscles that recoil of the lungs. The lungs do not actively expand and depress the chest are muscles of expiration and include recoil themselves but rather are acted upon to do so in the abdominal muscles (rectus abdominis, external and two ways: by downward and upward movement of the internal obliques, and transversus abdominis) and the diaphragm to lengthen and shorten the chest cavity and internal intercostals (13). by elevation and depression of the ribs to increase and Pleural pressure is the pressure in the narrow space decrease the back-to-front diameter of the chest cavity between the lung pleura and the chest wall pleura (mem- (13). Normal, quiet breathing is accomplished almost branes enveloping the lungs and lining the chest walls). entirely by movement of the diaphragm. During inspi- This pressure is normally slightly negative. Because the ration, contraction of the diaphragm creates a negative lung is an elastic structure, during normal inspiration the pressure (vacuum) in the chest cavity, and air is drawn expansion of the chest cage is able to pull on the surface into the lungs. During expiration, the diaphragm simply of the lungs and creates a more negative pressure, thus relaxes; the elastic recoil of the lungs, chest wall, and enhancing inspiration. During expiration, the events are abdominal structures compresses the lungs, and air is essentially reversed (13). expelled. During heavy breathing, the elastic forces Alveolar pressure is the pressure inside the alveoli alone are not powerful enough to provide the necessary when the glottis is open and no air is flowing into or out respiratory response. The extra required force is achieved of the lungs. In fact, in this instance the pressure in all mainly by contraction of the abdominal muscles, which parts of the respiratory tree is the same all the way to the  Structure and Function of Body Systems 17 How important is it to Train the Muscles of respiration? Regular exercise in general is beneficial for maintaining respiratory muscle function. Both endurance exercise, which involves repetitive contraction of breathing muscles, and resistance exercise, which taxes the diaphragm and abdominal muscles especially because of their use for stabilization and for increasing intra-abdominal pressure (Valsalva maneuver) during exertion, can result in some muscle training adaptations. This can help to preserve some of the pulmonary function with aging. However, it is generally not necessary to specifically train the muscles of respiration except following surgery or during prolonged bed rest when the normal breathing patterns are compromised. alveoli and is equal to the atmospheric pressure. To cause low concentration. The rates of diffusion of the two inward flow of air during inspiration, the pressure in the gases depend on their concentrations in the capillaries alveoli must fall to a value slightly below atmospheric and alveoli and the partial pressure of each gas (13). pressure. During expiration, alveolar pressure must rise At rest, the partial pressure of oxygen in the alveoli above atmospheric pressure (13). is about 60 mmHg greater than that in the pulmonary During normal respiration at rest, only 3% to 5% of capillaries. Thus, oxygen diffuses into the pulmonary the total energy expended by the body is required for capillary blood. Similarly, carbon dioxide diffuses in the pulmonary ventilation. During very heavy exercise, opposite direction. This process of gas exchange is so however, the amount of energy required can increase to rapid as to be thought of as instantaneous (13). as much as 8% to 15% of total body energy expenditure, especially if the person has any degree of increased airway resistance, as occurs with exercise-induced Conclusion asthma. Precautions, including physician evaluation of Knowledge of musculoskeletal, neuromuscular, car- the athlete, are often recommended, depending on the diovascular, and respiratory anatomy and physiology potential level of impairment. is important for the strength and conditioning pro- fessional to have in order to understand the scientific exchange of respiratory Gases basis for conditioning. This includes knowledge of the With ventilation, oxygen diffuses from the alveoli into function of the macrostructure and microstructure of the pulmonary blood, and carbon dioxide diffuses from the skeleton and muscle fibers, muscle fiber types, and the blood into the alveoli. The process of diffusion is a interactions between tendon and muscle and between the simple random motion of molecules moving in opposite motor unit and its activation, as well as the interactions directions through the alveolar capillary membrane. The of the heart, vascular system, lungs, and respiratory energy for diffusion is provided by the kinetic motion of system. This information is necessary for developing the molecules themselves. Net diffusion of the gas occurs training strategies that will meet the specific needs of from the region of high concentration to the region of the athlete. Key TerMS A-band atrium endomysium acetylcholine axial skeleton epimysium actin biaxial joints extrafusal fibers action potential bone periosteum fasciculi all-or-none principle bradycardia fast-twitch fiber alveolar pressure bronchi fibrous joints alveoli bronchiole Golgi tendon organ (GTO) aortic valve capillary hemoglobin appendicular skeleton cartilaginous joints hyaline cartilage arterial system crossbridge H-zone arteriole depolarization I-band artery diastole inferior atrioventricular (AV) bundle diffusion intrafusal fibers atrioventricular (AV) node distal left bundle branch atrioventricular (AV) valves electrocardiogram (ECG) mitral valve  motor neuron P-wave tendon motor unit QRS complex tetanus multiaxial joints red blood cell trachea muscle fiber repolarization tricuspid valve muscle spindle right bundle branch tropomyosin myocardium sarcolemma troponin myofibril sarcomere T-tubule myofilament sarcoplasm T-wave myosin sarcoplasmic reticulum twitch neuromuscular junction semilunar valves Type I fiber parasympathetic nervous system sinoatrial (SA) node Type IIa fiber perimysium sliding-filament theory Type IIx fiber pleura slow-twitch fiber uniaxial joints pleural pressure superior vein power stroke sympathetic nervous system venous system proprioceptor synovial fluid ventricle proximal synovial joints venule pulmonary valve systole vertebral column Purkinje fibers tachycardia Z-line STudy QueSTioNS 1. Which of the following substances regulates muscle actions? a. potassium b. calcium c. troponin d. tropomyosin 2. Which of the following substances acts at the neuromuscular junction to excite the muscle fibers of a motor unit? a. acetylcholine b. ATP c. creatine phosphate d. serotonin 3. When throwing a baseball, an athlete’s arm is rapidly stretched just before throwing the ball. Which of the following structures detects and responds to that stretch by reflexively increasing muscle activity? a. Golgi tendon organ b. muscle spindle c. extrafusal muscle d. Pacinian corpuscle 4. From which of the following is the heart’s electrical impulse normally initiated? a. AV node b. SA node c. the brain d. the sympathetic nervous system 5. Which of the following occurs during the QRS complex of a typical ECG? I. depolarization of the atrium II. repolarization of the atrium III. repolarization of the ventricle IV. depolarization of the ventricle a. I and III only b. II and IV only c. I, II, and III only d. II, III, and IV only 18