Basic Concepts of the Musculoskeletal System PDF

Chapter 3 Basic Concepts of the Musculoskeletal System The major learning concepts in this chapter relate to } equipment used in anatomy learning and research; } mechanical and physiological functions of the skeletal system; } components of bone and the different types of bone cells; } structure...

Chapter 3 Basic Concepts of the Musculoskeletal System The major learning concepts in this chapter relate to } equipment used in anatomy learning and research; } mechanical and physiological functions of the skeletal system; } components of bone and the different types of bone cells; } structure of long bones and the mechanical properties of bone tissue; } the functions, shapes, and organisation of whole bones; } classifying joints according to their structure or function; } characteristic features of synovial joints; } protection, lubrication, and wear of synovial joints; } the joint as the functional unit of the musculoskeletal system; } structural features of muscles and their main actions; } distinguishing properties of skeletal muscle; } the biological and mechanical bases of muscular contraction; } the electrical and mechanical responses of muscle to stimulation; and } the major factors that determine strength and range of joint motion. 31 32 | Biophysical Foundations of Human Movement T he purpose of this chapter is to introduce key column, are other bones that form the framework concepts related to the structure and function of the human body, much like the beams and studs of the skeletal system, the system of joints of a house or the chassis of a car. These analogies (the articular system), and the muscular system and are imperfect, however, because they imply that the to describe the tools for measuring these systems. skeletal system has only mechanical functions when, as we discuss later, the physiological functions of TOOLS FOR bone are equally important. MEASUREMENT Mechanical Functions Language is very important for basic gross anatomy of the Skeletal System because of the descriptions required to explain the The most obvious mechanical function of the skeletal position, relative size, and relationships of each system is providing support for weight bearing. The anatomical feature. Learning anatomy from a text- skeletal system also provides protection of internal book can be very difficult but becomes easier with organs (e.g., protection of the brain by some bones the aid of atlases that have artistic illustrations or of the skull and of the heart and lungs by the ribs). photographs of the various structures. The major bones of the limbs provide rigid links The concepts introduced in this chapter are between joints; these bones also provide sites for presented as general descriptions, but the reader muscle attachment. These functions of the skeleton, should realise that many of these concepts have in providing linkages and sites for muscle attach- been verified experimentally using quantitative ment, facilitate human movement and form the basis methods. Bone density in living humans can be of the mechanical models of the human body. determined using radiological and other more direct techniques. Bone structure can be visualised under Physiological Functions a microscope; however, because bone is a hard of the Skeletal System tissue, special preparation techniques are required. Bone is a living, dynamic tissue. The framework for a Chemical analyses can be performed to determine house is often made of timber that was once a living the composition of bone. Movement relies on the tree. Similarly, the bones in an anatomy laboratory integrity of joints, and goniometers are examples of are changed from their condition in the living body. instruments used to measure ranges of joint motion. Thus, in addition to its mechanical functions, the Muscles produce forces that have an external effect, living skeletal system has important physiological and these effects can be captured by different types functions. When subjected to large forces over a long of dynamometers that measure muscle strength. The period, the framework of the house may start to fail, electrical signal generated just before a muscle con- just as the metal in a car may start to fatigue or rust. traction can also be detected, recorded, and analysed But living bone has the advantage that it may heal using electronic equipment and computers. Muscles when broken and even carry out maintenance to change shape when they contract; when a muscle is prevent failure. Some bone researchers believe that artificially stimulated, the amount of deformation the stimuli to bone adaptation include microcracks can be measured. that form during increased levels of physical activ- ity. (Adaptation in response to physical activity is SKELETAL SYSTEM explored further in chapter 6.) Bone tissue is also involved in the storage of The skeletal system is exquisitely structured to fulfill essential minerals such as calcium and phospho- both its mechanical and physiological functions. We rous. Additionally, bone marrow produces blood first examine those functions and then show how cells and is part of the body’s immune system. both the composition and architectural structures of the skeletal system support them. Composition of Bone Functions of the In a car, the metal in the engine tends to be thick and rigid because it must resist deformation, but Skeletal System the body panels are more cosmetic and help absorb Humans have an endoskeleton and are vertebrate the energy of impact during a collision. As with animals. Attached to our backbone, or vertebral the metal in a car, the composition of bone in the  Basic Concepts of the Musculoskeletal System | 33 skeleton is not uniform. This makes sense because one-third organic material. This is the proportion each of the constituents of bone must accommodate of calcium salts to collagen in healthy adult bone. different physiological and mechanical functions. Almost all the body’s calcium is stored in bone and is released from there as required by other tissues, Mechanical Properties Provided such as skeletal muscle, which requires calcium to by Components of Bone contract. As already noted, collagen fibres compose most of It has been argued that both stiffness and flexibility the organic component of dry bone. Bone cells are are important properties of bone. In a car the axles another organic component of bone. In the matrix must be stiff to resist deformation as they trans- of bone, the basic cells are called osteocytes. Also fer the forces from the engine to the wheels, but associated with bone are bone-forming cells, called springs in the suspension deform while the shock osteoblasts. Depending on local environmental absorbers dampen the motion. Engineers can use conditions, osteocytes can become osteoblasts and different materials to suit particular purposes; they vice versa so that adaptive responses can occur choose steel, aluminum, plastic, or other alternatives when there are mechanical stimuli. There are also depending on the purpose of the part. To produce bone-eroding cells, called osteoclasts. Remodelling the optimal mechanical properties, materials engi- of bone, which is a continual process in adults, neers develop alloys that consist of different types involves organised erosion and deposition of bone of metal bonded together. tissue by the various cells. A full cycle of remodel- Is bone like a homogeneous metal, such as copper ling takes about 3 mo. or iron, or does it consist of different components? The answer is the latter; consequently, bone is described as a composite material. One useful anal- Types of Bone ogy for bone is fibreglass, which consists of glass Metallurgists and engineers have at their disposal fibres cured in an epoxy resin. The final product, different building materials, whereas the two types fibreglass, has mechanical properties that are supe- of bone in the human skeleton are made of the same rior to those of its individual components. material. Compact and spongy bone differ mainly With regard to the mechanical properties of bone, in their porosity, although compact bone is more comparison with other materials is useful. Bone is organised than spongy bone. similar to wood from the oak tree in its properties Spongy bone (also termed cancellous or trabecu- of strength and stiffness. Bone is about as flexible as lar bone) is a lattice meshwork of bony rods (tra- fibreglass, although weaker, but is stronger and more beculae; figure 3.1a). This meshwork arrangement flexible than ordinary glass. Ceramics in general are makes spongy bone much more springy than com- about one-third to one-half as strong in compres- pact bone. In spongy bone, each osteocyte is close sion as bone. The blocks of older car engines were to a nutrient supply because the bony tissue is sur- manufactured from cast iron, which is similar to rounded by blood vessels and associated material. bone in its tensile strength, but bone is three times Compact bone (also called cortical or dense lighter (that is, less dense) and much more flexible. bone) is much more solid than spongy bone. When Bone is clearly a remarkable physical material. bone is remodelled to become compact, bone cells About one quarter of the mass of bone in the may be too far from their nutrient supply because living body is water. Adult bone, after removal from the blood vessels are surrounded by bony tissue. the body and drying, is about two-thirds inorganic If compact bone were not organised in a specific crystals consisting mainly of calcium and phospho- way, this lack of nutritive supply would result in rous. The main mechanical components of bone the death of each cell. To overcome this problem, are collagen (most of the organic component) and compact bone contains a basic structural unit calcium salts (most of the inorganic component). (Haversian system or osteon) that is repeated many The collagen provides toughness and flexibility, so it times. Haversian canals, longitudinally arranged in contributes to the tensile strength; bone’s hardness the shafts of long bones, carry blood vessels and are and rigidity are attributable mainly to its calcium surrounded by layers of bone (lamellae). In each salts, which also contribute principally to the com- lamella, osteocytes are contained in lacunae. There pressive strength. According to one calculation, the is a limit to the number of lamellae so that bone cells optimal mineralisation of bone, in terms of strength are not too far from their supply of nutrition. Figure and flexibility properties, is two-thirds mineral to 3.1b illustrates the microscopic organisation of this 34 | Biophysical Foundations of Human Movement Articular cartilage compact bone in contrast with the organisation of spongy bone (figure 3.1a). Most of the calcium in bone is in the compact bone, but the calcium in the spongy bone is more easily released into the blood when required. Architecture of Bone In engineering, an important concept is the rela- Cancellous tionship between strength and mass. Elite athletes (trabecular) bone have a good power-to-weight relationship, which is the same idea. The analysis of bone architecture allows us to conclude that the bones in our body are very efficient in that they are able to withstand large forces and at the same time are relatively light. Periosteum Bone Shape and Organisation Compact bone When different functional requirements exist, there are variations in the arrangements of the Endosteum two types of bone as well as differently shaped bones. Bone shape and predominant function have Medullary a specific structure–function relationship. The long cavity bones of the limbs have many attached muscles and a act as rigid links between major joints (figure 3.2a). The flat bones of the skull help protect the brain. Their structure, consisting of two layers of compact bone with spongy bone Lamellae in between (figure 3.2b), Abernethy/E5730/Fig. 3.1a/439889/Pulled/R1 Circumferential is similar to that of some lamellae protective helmets. The Haversian canal ribs protect vital organs in the chest such as the heart. Periosteum The short bones of the hindfoot have a thin layer of cortical bone around the outside and are filled with spongy bone (figure 3.2c). These weight-bearing Osteones bones resist the large com- pressive forces generated Osteocytes during running and help cushion ground reaction b forces. All the bones illus- Figure 3.1 (a) The trabeculae of the spongy bone and (b) the architecture trated in figure 3.2 con- of compact bone in the shaft of a long bone, shown in longitudinal and sist of both compact and Abernethy/E5730/Fig. 3.1a/439890/Pulled/R1 spongy bone in differing transverse sections. Adapted, by permission, from J. Watkins, 1999, Structure and function of the musculoskeletal system proportions and structural (Champaign, IL: Human Kinetics), 112-113. arrangements.  Basic Concepts of the Musculoskeletal System | 35 Long bone easily like a springboard. Also compare the Flat bone of skull deformation of high- and low-density foam mattresses under the weight of a person. Architecture of Long Bones b The bones of the lower limb are often under compression during standing, whereas the bones of the upper limb may be subjected to tension when one is holding a load in the hand or performing a giant swing on the Short bone high bar (see figure 3.4). During forward bending of the trunk, there are shear forces Navicular Talus between adjacent vertebrae (see figure 3.4). Medial cuneiform During walking, the long bones of the lower Calcaneum limb may be acted on by bending and tor- sional (longitudinal twisting) forces. The compressive forces on the ends of the femur a c in the thigh are not aligned in the same straight line, causing bending, which also Figure 3.2 Examples of bones with different shapes to occurs because of the bend of the shaft of serve different functions. Muscles are attached to most the femur. Twisting about the long axis of the bones to facilitate movement. (a) Long bones of the foot immediately after heel strike in jogging, limbs, (b) flat bones 3.2 Abernethy/E5730/Fig. of a,b,c/439891, the skull, and (c) short 439892, bones of 439893/Pulled/R1 commonly called hindfoot pronation, pro- the hindfoot. duces torsion of the main leg bone, the tibia. These patterns of loading require optimal Note the arrangements of compact and spongy bone illustrated in the different parts of a vertebra Spinous process (figure 3.3). A vertebra is classified as an irregular bone, but is more like a composite bone consisting Lamina of three main components. The vertebral body is the main weight-bearing part of the vertebra and is con- structed like a short bone; in section one can see the thin layer of compact bone surrounding a mass of spongy bone. The lamina of the vertebral, or neural, arch protects the spinal cord and is constructed like a flat bone; in section it looks like one of the ribs that help protect the heart and lungs. The spinous and Transverse process transverse processes have areas of attachment for ligaments and muscles, providing leverage for these structures, and they are constructed like long bones. Vertebral Amount, density, and distribution of bone mate- body rial have major effects on the mechanical properties of a whole bone. Compare the resistance of a long plank of wood to bending in the vertical direction Figure 3.3 A vertebra, classified as an when it is supported at both ends and placed on irregular bone, has three main components: its narrow side, where it does not deform, with the the weight-bearing vertebral body, the flat resistance when it is on its wide side, where it bends lamina, and the long Abernethy/E5730/Fig. processes. 3.3/439894/Pulled/R1 36 | Biophysical Foundations of Human Movement Compression Tension Torsion Shear Bending Tibia Radius Tibia Femur Vertebra Figure 3.4 Examples of the different types of forces that may act within and between bones. Major forces that the long bones of the limbs, particularly the lower limbs, must withstand are compression along their long axes, bending, and twisting. The bones of the upper limb may be subjected to tension. Abernethy/E5730/Fig. 3.4/439895/Pulled/R1 arrangement of the bone material, but what is the Articular cartilage most efficient architecture? The shafts of long bones are hollow, giving them mechanical advantages over Subchondral bone a solid rod of the same mass. If a pipe and a solid rod contain the same amount of material, the pipe is stronger in bending than the rod, so a hollow shaft is more efficient in terms of the relationship between strength of the whole bone and its weight. A hollow shaft also resists twisting better than a solid rod. It appears from mechanical analyses that the shapes of our bones are close to optimal for the activities that most humans perform. During movement, especially when landing from a jump, forces are transferred from one bone to the Tire next via the joints. A large contact area between the bones results in less pressure on the ends of the Rim bones, so expanded ends are advantageous. Much of the material forming the expanded ends is spongy bone that absorbs energy during impact. Everybody knows the difference between jumping on a trampo- line and jumping off the trampoline onto a concrete floor. Compact bone is like concrete because it does not deform much but is very strong, whereas spongy bone is more like a trampoline that cushions the force of landing as it deforms. In fact, spongy bone Spokes is 10 to15 times more flexible than compact bone. At each end of a typical long bone, a thin outer layer of compact bone protects the overlying cartilage during Figure 3.5 The expanded Abernethy/E5730/Fig. end of a long bone 3.5/439896/Pulled/R2-alw impact and transfers the forces to the underlying is compared with a bicycle wheel consisting of spongy bone (figure 3.5). The deformable articular spokes, rim, and tire.  Basic Concepts of the Musculoskeletal System | 37 cartilage acts like an air-filled bicycle tire, which joints allow a relatively large amount of movement; is supported by a light but solid rim, like the thin consider, for example, the ranges of motion that are layer of compact subchondral bone. The trabeculae possible at both the shoulder and elbow joints. of the expanded ends line up along the major lines of force to perform a function similar to that of the Char acteristic Features spokes of a wheel, which help support and maintain the shape of the rim and cushion the forces produced of Synovial Joints by bumps in the road. Recall that the composition A number of characteristic features are associated of compact bone and spongy bone is the same; just with a typical synovial joint (figure 3.6). On the the organisation is different. ends of the bones forming the joint there is articu- lar cartilage, which consists of collagen fibres in ARTICULAR SYSTEM a liquid matrix. Articular cartilage has a water component of about 80% and has been likened to a The system of the joints between the bones is called sponge from which water can be squeezed. When the articular system because the union of bones is not being deformed, it absorbs water because of an articulation. Joints are important because they the dissolved chemicals that it contains. Articular allow movement, but they must remain stable during cartilage forms a relatively smooth bearing surface movement. The study of joints is called arthrology. and acts, through its capacity to deform, to cushion forces, like a tire on a bicycle wheel (figure 3.5) Classification of Joints A joint capsule forms part of the boundary of We all know that joints allow cars and bicycles to the joint. The joint capsule contains a high propor- move. These are relatively simple joints compared tion of collagen fibres and provides some intrinsic with the synovial joints found in the human body; in fact, engineers are still attempting to explain the functions of biological joints. The design of artificial joints for replacement of human joints such as hips and knees is progressing rapidly, but replacement joints still do not work as well as the natural joints. To give some idea of the complexity of natural Joint cavity joints, we can contrast them with the joints in a containing synovial Synovial car. The bearing surfaces in the artificial joints of fluid membrane a car are usually smooth, hard metal, whereas the articular cartilage is less smooth but quite deform- able. Artificial joint surfaces are very regular, usu- ally part of a circle or sphere, whereas human joint surfaces are ovoid (egg shaped), resulting in much more complex movement patterns. To remove debris in the joints of a car, the car must be taken to the local garage for an oil change. Any effects of wear result in rapid deterioration of artificial joints; how- ever, in a synovial joint, cell debris can be removed continuously. The materials between the bones forming the joint may differ, providing the basis for the struc- Joint capsule tural classification of joints. All anatomical joints may be described as fibrous, cartilaginous, or syno- Articular vial. Most of the major joints we are familiar with, cartilage such as the shoulder, elbow, knee, and ankle, are examples of synovial joints, so this section focuses exclusively on these joints. The major function of joints is to allow move- Figure 3.6 Representation of a typical synovial ment but, at the same time, remain stable. Synovial joint indicating its characteristic features. Abernethy/E5730/Fig. 3.6/439897/Pulled/R1 38 | Biophysical Foundations of Human Movement stability to the joint as well as some resistance to structural and functional criteria on which each motion. Forming the inner layer of the joint capsule system is based. is the synovial membrane, which has a number of Anatomically, each synovial joint can be clas- important functions. It produces the fluid in the sified on the basis of the approximate geometric joint and removes the cell debris that results from form of its articulating surfaces. For example, the wear and tear in the joint. hip looks like a ball in a socket (figure 3.7a), much A major characteristic of a synovial joint is the like the joint between a car and a trailer, so it is cavity bounded by the articular cartilage and the called a ball-and-socket joint. Synovial joints can synovial membrane of the joint capsule. In this joint also be classified according to the gross movements cavity is a small amount of synovial fluid that con- permitted. For example, because the ankle joint tains constituents of blood, substances secreted by allows movement basically in only one plane, it is the synovial membrane, and some products result- called a hinge joint (figure 3.7b). The ankle could ing from abrasive wear within the joint. also be classified as a uniaxial joint because it allows The synovial fluid has three important func- movement about only one principal axis. The joints tions: lubrication, protection, and nutrition. The forming the knuckles of the fingers are classified as viscosity of the fluid can change according to local biaxial because they allow motion about two prin- environmental conditions, so it may be thick and cipal axes (figure 3.7c). Note that the fingers can be protective, like grease, or thin and lubricating, like moved in two directions at right angles—backward oil. Because articular cartilage does not receive an and forward and from side to side. adequate blood supply, nutrients are supplied via The complexity of organisation of the joint struc- the synovial fluid in contact with the cartilage. tures is another criterion for classification. If a joint Pressure during movement helps squeeze fluid out consists of two bones and one pair of articulating from the cartilage. When the pressure is removed, surfaces, it can be classified as a simple joint; an the liquid can seep back into the cartilage. Physical example is the knuckles of the fingers (figure 3.7c). If activity thus promotes the nutritional function of there is more than one pair of articulating surfaces, synovial fluid. such as in the elbow joint capsule (where there are To help maintain the integrity of the synovial three pairs), the joint is classified as compound joint, associated ligaments attach from bone to (figure 3.7d). Sometimes synovial joints contain bone and cross the joint. Ligaments consist of intra-articular structures, such as a cartilaginous collagen fibres, also a constituent of bone. In liga- disk or meniscus; these joints can be classified as ments, the collagen forms about 90% of the struc- complex (figure 3.7e). ture, and the fibres tend to run parallel to each other. The ligaments help the joint capsule provide stability and they function to guide the joint’s movements. In doing so, they provide some resistance to joint motion. Ligaments are basically passive structures that resist tensile forces, which tend to separate the bones b forming the joint. Classification a of Synovial Joints One can use the classical anatomical view to classify synovial joints on a descriptive basis or use the view of engineers, who e are interested in movement between joint c d surfaces in contact, to produce a system to explain joint lubrication and wear. It Figure 3.7 Representations of some types of synovial joints: (a) is not our aim in this section to provide spheroidal (ball-and-socket) joint, (b) hinge-like joint, (c) simple the details of the alternative classifica- joint, (d) compound joint, and (e) complex joint containing an tion systems; instead, we summarise the intra-articular disk. Abernethy/E5730/Fig. 3.7a,b,c,d,e/439898, 439899, 439900, 439901, 439902/Pulled/R1  Basic Concepts of the Musculoskeletal System | 39 R anges of Movement Allowed by Synovial Joints Movements that occur at synovial joints have tra- ditionally been described in terms of the major planes of the body (figure 3.8). The sagittal plane Spin is a vertical plane dividing the body into left and right parts. The coronal (frontal) plane is a verti- cal plane dividing the body into front (anterior) and back (posterior) parts. The transverse plane is a horizontal plane dividing the body into top (superior) and bottom (inferior) parts. Anatomi- cally, movements that occur in the sagittal plane have been termed flexion (when the angle between Slide the limb segments decreases) and extension (when the angle between the limb segments increases; figure 3.8). These gross descriptions of directions of movements of body segments can be contrasted with the more mechanical approach in which the terminology relates to the movement that occurs between the articular surfaces in contact. Roll Transverse plane Figure 3.9 The motions of spin, slide, and roll between articular surfaces in a synovial joint. Abernethy/E5730/Fig. 3.9/439904/Pulled/R1 Using the engineering approach, we can describe the relative motion between articular surfaces as a Extension Flexion spin, a slide, or a roll (figure 3.9). The movements can be likened to the spinning of a top, the skidding of a tire on the road, and the normal rolling of a tire on the road, respectively. These movements have an effect on the frictional resistance. For example, resistance is much less when the wheels of a car are Flexion Extension rolling freely than when the brakes are applied and the tires slide along the road. Often combinations of all movements occur during joint motion, such as during flexion and extension at the knee. Joint Protection, Coronal plane Sagittal plane Lubrication, and Wear Figure 3.8 The major planes of the human body The structures in contact in a synovial joint are with respect to the anatomical position. The move- ments of flexion and extension of the elbow and the articular cartilages on the ends of each bone. Abernethy/E5730/Fig. 3.8/439903/Pulled/R1 The cartilage is smooth and deformable, so it can knee are illustrated. cushion forces applied to its surface. The subchon- Reprinted, by permission, from J.H. Wilmore and D.L. Costill, 2004, Physiology of sport and exercise, 3rd ed. (Champaign, IL: Human dral bone, which is the thin layer of compact bone Kinetics), 679. under the cartilage, provides a solid base and helps 40 | Biophysical Foundations of Human Movement protect the cartilage from damage. This thin layer of } Muscle forces are transmitted to bony attach- compact bone sits on a more deformable network of ments via tendons; thus, it is the muscle– bony rods forming the spongy bone (figure 3.5). This tendon unit that is of major significance. organisational structure is similar to that of a bicycle } Nerves are also associated with joints. As wheel with spokes, where the spokes maintain the discussed in chapter 15, motor nerves in the shape of the rim but also help cushion forces from central nervous system provide some control the ground. (A solid bicycle wheel would be too stiff over the muscles producing actions at a joint. for general purposes.) Sensory nerves provide feedback about joint The synovial fluid acts as a lubricant so that position and movement from a variety of sen- friction between articular cartilages in a synovial sors located in the joint capsule and ligaments joint is less than between, for example, two blocks as well as in the muscles and tendons. of smooth ice. Synovial joints have a number of important characteristics that differentiate them Injury to any intrinsic joint structure or to the from artificial joints. Movement at a synovial joint associated structures will result in functional can best be described as oscillatory (backward impairment of the entire joint and adjacent body and forward), but in most load-bearing joints in segments. Functional impairment of one joint often machines the angular motion occurs in only one results in a chain reaction that affects adjacent direction (around and around). The load-bearing joints. For example, an ankle or knee injury will surface in a synovial joint is deformable cartilage, affect the whole lower limb and therefore adversely which is both elastic and porous, and the synovial affect performance during walking or running. fluid has particular chemical properties that allow Because of altered function of the injured joint, and it to act as a lubricant. This arrangement is much possibly because of pain associated with the injury, more complicated than that of most joints in cars, the other joints must attempt to compensate, usually in which the bearing surfaces are very hard. resulting in a limping gait. The Joint as the MUSCULAR SYSTEM Functional Unit of the The muscular system comprises the main effector structures for human movement. It is an important Musculoskeletal System part of the musculoskeletal system because it pro- Human movement studies and functional anatomy duces joint motion (see part II on biomechanics), but emphasise movement; therefore, the joint is the it is also an important part of the neuromuscular focus of functional musculoskeletal anatomy. The system in that it is controlled by the central nervous characteristic features of a synovial joint have been system (see part IV on motor control). listed (see figure 3.6), but each joint has associated with it a number of other structures. These include Structure of the the following. Muscular System } On either side of the joint are bones that act as We can identify major muscles that lie just beneath levers and aid force cushioning, as explained the skin, such as the pecs, lats, and deltoid and earlier (see figure 3.5). biceps muscles. Understanding the mechanisms } Skeletal muscles have a role in movement of action of these muscles relies on knowledge of because they cross joints and thus initiate and muscular tissue at a number of levels of organisa- control movement. The forces they produce tion, including its microscopic structure. across the joint also stabilise it. Thus, muscles Association of Muscles are secondary stabilisers, in addition to the associated ligaments. If you contract all the With Other Structures muscle groups about a joint simultaneously so As with bones and joints, the structure of muscles is that no movement occurs, the joint becomes related to their function. Muscles cross joints, and much more stable. The joint is also stiffer, as the major skeletal muscles of the trunk and limbs are you would realise if somebody else attempted attached to bones at both ends. The muscle–tendon to move it. unit therefore consists of a chain of structures, typi- cally bone–tendon–muscle–tendon–bone.  Basic Concepts of the Musculoskeletal System | 41 The attachment sites of muscle and tendon are Muscle fibres are oriented in the direction of pull important because they determine the action of each of the whole muscle (e.g., spindle) or at an angle to muscle. Whenever a muscle shortens, it tends to this direction (e.g., unipennate). The hip and thigh pull the two attachment sites closer together, so the region contains many examples of these shapes. relationship between direction of pull of the muscle Semitendinosus (one of the hamstring muscles in and the axis of rotation of the joint determines the the back of the thigh) has a spindle shape, whereas resulting joint action. This type of analysis is the another hamstring muscle (semimembranosus) is a basis used to define the actions listed in tables in all unipennate muscle. Rectus femoris (one of the four basic texts on gross anatomy. Functional anatomists, muscles forming the quadriceps muscle group at the starting with Duchenne in the late 19th century, front of the thigh) is a bipennate muscle. Under- have used other techniques to attempt to unravel neath the large gluteus maximus in the buttock is the complexity of muscle contraction when different the fan-shaped gluteus medius. muscles act simultaneously or when joint position Skeletal muscle cells are elongated and contain is changed from the normal anatomical position many nuclei; hence, muscle cells are called fibres. illustrated in figure 3.8. The alternating thick and thin filaments produce characteristic striations that one can clearly see Structur al Features of Muscles when viewing muscle tissue through an electron So far in this discussion, we have used the term microscope. This appearance is produced by tens muscle in relation to only one type of muscle—skel- of thousands of repeating units in series that form etal muscle. There are, however, other types of muscle each fibre. These repeating units, called sarcomeres, tissue. Smooth muscle is found in the walls of the are visible in a photograph taken by an electron digestive system and certain blood vessels. Cardiac microscope. The sarcomere is the structural and muscle forms the major part of the walls of the heart. functional unit of muscle. You can gain an idea of In this and subsequent functional anatomy chapters, its structure by looking at figure 3.11. The function we restrict our discussion to skeletal muscle. of the components of the sarcomere is discussed Not all muscles look the same. The typical in “Muscle Contractions” later in this chapter. The muscle has a belly and tapers toward the tendon connective tissue element of the muscle consists of that attaches it to bone at each end, but there are thin sheets surrounding each fibre, each bundle of other shapes. Muscles attach to bone either directly fibres, and the whole muscle itself. or via a tendon. Examples of muscles with different The structural and functional unit of the neu- architectural arrangements are shown in figure 3.10. romuscular system is a motor unit consisting of a Spindle Bipennate Fan Unipennate Figure 3.10 Examples of muscles with different shapes and different arrangements of fibres. Abernethy/E5730/Fig. 3.10/439905/Pulled/R1  Periosteum Tendon Myofilaments Myosin filament Actin filament Striations Myofibril Sarcolemma Sarcoplasm Nucleus Fascia Muscle fiber Skeletal muscle Epimysium Perimysium Endomysium Thin (actin) filament Thick (myosin) filament Cross-bridge Sarcomere Z-line H-zone Z-line Thin (actin) filament Thick (myosin) filament I-band A-band I-band Figure 3.11 A typical muscle comprises bundles (fascicles) of muscle fibres. Each muscle fibre, in turn, Abernethy/E5730/Fig. contains many myofibrils that are made 3.11/440145/Pulled/R1 of repeating series of sarcomeres. In the A band at the middle of each sarcomere are myosin filaments. Overlapping slightly with each end of every myosin filament are actin filaments. The actin filaments stretch from their attachment to the Z disk at either end of the sarcomere toward the H zone in the middle of the sarcomere. Adapted, by permission, from R.S. Behnke, 1999, Kinetic anatomy (Champaign, IL: Human Kinetics), 13. 42  Basic Concepts of the Musculoskeletal System | 43 Muscle as a tissue has three main properties: 1. its excitability in response to nerve stimula- tion, Ventral horn 2. its contractility in response to the stimula- motor unit tion, and 3. its conductivity, which allows the electri- cal signal produced by the muscle fibres in response to neural stimulation to travel along those fibres. A whole muscle has two additional properties that arise primarily from its structure and the mechanical characteristics of the connective tissue within it: 1. extensibility, and 2. elasticity. Because muscle fibre tissue responds to neural Motor unit activation by producing a force, it is regarded as an active tissue, in contrast to the passive connective tissue component, which can only resist applied forces. Skeletal muscle Muscle Contr actions fibre Muscle fibres operate by producing a force that tends to shorten the muscle. Knowledge of the microstruc- ture of muscle tissue is necessary for understanding this process. In the lengthened position there is little overlap between the longitudinal thick and thin protein Figure 3.12 Illustration of a motor unit in a limb muscle where the nerve fibre originates in the filaments contained in each sarcomere. The muscle Abernethy/E5730/Fig. spinal cord. 3.12/439906/Pulled/R1 shortens (contracts) by increasing that overlap. Thus, researchers proposed the sliding filament hypothesis of muscle contraction, which states that the shortening of the sarcomere results from nerve and the muscle fibres that it controls (figure the thick and thin filaments sliding toward one 3.12). Muscles over which we have fine control, such another. Further research led to the cross-bridge as the small muscles in the hand, have relatively hypothesis, which states that muscles shorten few muscle fibres per motor unit, whereas the large when cross-bridges are formed as the thick myosin muscles of the lower limb, over which we have only filaments attach themselves to the thin actin fila- relatively coarse control, have many more muscle ments connected to the Z disk on either end of the fibres per motor unit. sarcomere. These bridges are the means by which the myosin filaments pull the actin filaments toward Distinguishing Properties themselves, shortening the muscles. Both hypoth- of Muscles eses, which are generally accepted, indicate that The macro- and microstructure of muscle give it five there are both upper and lower limits to sarcomere properties that are crucial to its function. (and hence muscle) length. 44 | Biophysical Foundations of Human Movement Activation by a nerve supplying a muscle prompts 4. The calcium ions expose active sites on the both an electrical response and a mechanical actin (thin) myofilaments, to which the myosin response. The electrical response spreads over the filaments immediately attach. surface of each muscle fibre, causing a series of steps 5. By means of these attachments (cross-bridges), to occur. Here we highlight only some of the major each myosin filament pulls the actin filaments steps in a process known as excitation–contraction that overlap with it at either end toward its coupling. centre, producing cross-bridge cycling, which 1. In the neuromuscular (nerve–muscle) junc- is the mechanical response to the electrical tion, a chemical is released from the end of signal (figure 3.13). the nerve fibre, which causes a rapid change The whole process takes a couple milliseconds. in voltage in the muscle. Although almost all the calcium in the body is stored 2. The electrical signal travels over the surface in bones, the 1% that is unbound in skeletal muscle of, and along, the muscle fibres. is essential for muscle contraction. 3. The electrical signal causes the release of Cross-bridge cycling involves four steps (figure calcium ions into the cytoplasm (intracellular 3.13): (1) attachment of the myosin to the active site fluid) of the muscle fibre. This is an impor- of the actin filament (when the active site is exposed tant part of the process because it is the link in the presence of calcium ions), (2) pivoting of between excitation and contraction. the myosin head to pull the actin toward it (power Troponin Actin Tropomyosin a Myosin filament Active binding site b Next active binding site c Figure 3.13 The cross-bridge cycle. The thick filament binds to the active site on the thin filament, pulls the thin filament toward it, and then detaches, ready to repeat the process. Reprinted, by permission, from J.H. Wilmore and D.L. Costill, 1999, Physiology of sport and exercise, 3rd ed. (Champaign, IL: Human Kinetics), 42. Abernethy/E5730/Fig. 3.13/439907/Pulled/R1  Basic Concepts of the Musculoskeletal System | 45 stroke), (3) release of the myosin head from the muscle–tendon unit does not change, so this type actin, and (4) reactivation of the myosin (return to of action is important for the stabilisation of joints. the straight-head configuration of myosin). Muscles may therefore act concentrically to produce Electrical activity of a muscle can be detected movement, eccentrically to control movement, or using electrodes in a technique called electromy- isometrically to maintain posture and enhance ography (EMG), which involves detecting the small joint stability. Joint stability is also a byproduct electrical signal produced by muscle slightly before of the dynamic types of action. In figure 3.14, the it contracts and then using appropriate hardware and elbow-flexor muscles act concentrically to move the software to record and analyse the signal. A basic load up against the resistance of gravity and then tool in functional anatomy, EMG has been used to act eccentrically to control the downward move- determine muscle roles in movement situations, to ment. The descriptions of joint action that often provide biological feedback to a person attempting appear in tables in anatomy books are based on the to improve motor performance, and to investigate assumption that the joint movement is produced by the effects of strength training. a concentric muscle action. As a muscle shortens it also thickens, which can be observed on the skin overlying superficial muscles. Explaining Joint Actions This mechanical response of muscle can be detected The joint actions caused by muscles would be rela- using a variety of techniques, collectively termed tively simple to predict and explain if each muscle mechanomyography (MMG). In MMG, a muscle is crossed only one joint; that is, if all muscles were artificially suprastimulated and the muscle response is detected and analysed. The size of the maximum muscle deformation and the time it takes to reach this maximal deformation indicate some of the physi- Movement up ological functions of the muscle. The effects of muscle fatigue and injury can be quantified using MMG. Mechanics of Muscular Action The force produced by muscle activation places tension on the attached tendon or bone to produce joint motion. As we shall see, muscles do not always shorten when they produce force. Types of Contr actions When muscles contract they can perform three main actions. It is the muscular system that provides the power for the human body to perform work. Muscles No movement cross joints so that their contractions produce move- ment, and whenever a muscle contracts it tends to shorten and pull the two bony attachments closer together. This action of a muscle is termed concen- tric, in contrast to an eccentric action that occurs when a muscle is activated but is lengthening. In this situation other forces (such as external loads) Movement down prevent the muscle from shortening. During eccen- tric actions, muscles are controlling the movement produced by the other forces. (For example, when lowering a weight, a muscle is controlling the move- ment produced by gravity working on the weight.) Figure 3.14 The three major types of skeletal muscle action are isometric, concentric, and When a muscle relaxes it is not producing any eccentric. Isometric actions occur in static situa- force and is therefore not controlling the movement Abernethy/E5730/Fig. tions 3.14/439908/Pulled/R1 (no movement), whereas concentric (muscle produced by gravity. During an isometric action, shortening) and eccentric (muscle lengthening) the muscle is activated but the overall length of the actions occur during dynamic tasks. 46 | Biophysical Foundations of Human Movement monoarticular. However, in addition to monoarticu- unsupported limb as far back as possible at the hip lar muscles, there are muscles that cross two joints (extend the hip). Now try to bend (flex) the knee (biarticular), such as the hamstring muscle group as far as possible (figure 3.15a). You will notice that at the back of the thigh, and others that cross more the range of knee flexion is less than normal when than two joints (polyarticular), such as the muscles the hip is extended. (You can verify this by trying in the forearm that bend the fingers. A basic rule for the same knee movement with the hip flexed.) Now predicting joint movement is that when a muscle is lie on your back and bend (flex) one hip so that the activated it shortens and tends to produce all the thigh is vertical, and then attempt to straighten the joint actions of which it is capable. If these actions knee (figure 3.15b). Most people are unable to do do not occur, it must be that external forces are pres- this. If you can straighten your knee in this posi- ent or that other muscles are preventing some of the tion, flex the hip more and try again. These two actions. In these situations even a purely mechanical examples principally indicate the lower and upper analysis of the muscular system is quite complex. length limits of the hamstring muscle group that In a whole muscle, the shortest length is deter- crosses both the hip and knee joints. mined by how much the actin and myosin filaments To demonstrate the lower limit of the muscles in each sarcomere of the muscle tissue can overlap, that bend the fingers, bend (flex) the wrist so the but the upper limit is determined mainly by the palm of the hand is brought as close as possible to extensibility of the muscle’s connective tissue com- the forearm and then try to make a fist. Notice this ponent. When joints are moved through their full is a very weak grip compared with your strongest range, most muscles in the human body normally power grip. Notice the position of the wrist when the operate well within their available length range. In grip is strongest. The muscles that provide most of some cases, however, the limits are reached. Biar- the strength for bending the fingers have their bellies ticular and polyarticular muscles may be limited in the forearm and tendons that cross a number of because their lengths are determined by motion at joints, including the wrist and others in the hand. a series of joints. When the wrist is flexed, the finger-flexing muscles Try these movements to demonstrate both the are already shortened; they have almost run out upper and lower length limits of muscles crossing of the capacity to shorten any further to produce more than one joint. Stand on one foot and bend the finger flexion. a b Figure 3.15 Exercises used to demonstrate the (a) shortened and (b) lengthened positions of the ham- string muscle group. Abernethy/E5730/Fig. 3.15a, b/439909, 439910/Pulled/R1  Basic Concepts of the Musculoskeletal System | 47 THE JOINT-STABILISING ROLE OF MUSCLE–TENDON UNITS Bioengineers are fascinated with the human body produce the movements at the joints, control the because many are involved in the design of replace- movements, or both. ment parts, such as artificial hips and knees. A What is the primary mechanism that helps pre- human synovial joint is analogous to a tent that relies vent our ligaments from tearing and our joints from on the interaction between the following structures dislocating? It is the stabilising influence of the for its stability: The bones with cartilaginous ends contraction of surrounding musculature. In terms are like the poles of a tent that resist compressive of injury prevention, the time required to damage a forces, so the mechanical properties of bone and car- ligament is less than the time of a simple muscular tilage have been measured using specially designed reflex response to muscle stretch. This means that equipment. A tent that relied solely on the poles the stabilising muscles must be activated before would be very unstable; it would simply blow over the imposition of external deforming forces or they in windy conditions. For extra stability, guy ropes cannot prevent joint dislocation and ligament tears. are attached to the poles, and these ropes must be When you step on the edge of a hole that you have flexible (or attached to springs) to perform their not seen, you are likely to sustain a sprained ankle functions optimally. because you have relied on the ligaments for stability Ligaments are flexible structures that stabilise and, unaided by surrounding musculature, the liga- joints. Their tension varies through the range of ments are usually not up to the job in that situation. joint motion because of the complex shapes of joint If you see the same hole ahead of time, however, surfaces, so joint stability is dependent on joint posi- the surrounding muscles are activated before you tion. Joints must not be completely stable because step; the muscle–tendon units add stability and you they must also allow movement. Stabilising features are much less likely to sprain your ankle joint. The of joints, which tents do not share, are the muscle– activation patterns of muscles during movement are tendon units that are under neural control and that a major area of research. Limitations to R ange Determinants of Strength of Joint Motion What is muscular strength? Mechanically, strength When a joint reaches the end of its range of motion, is determined by both the force generated by mus- this limitation has several possible causes in addi- cular contraction and the leverage of the muscle at tion to the intrinsic features of the joint illustrated the joint. This concept, which is termed moment in figure 3.6. When joint motion occurs, the body of force, is examined in detail in chapter 7. Muscle segments bend such that the tissues on one side are force is proportional to the cross-sectional area of compressed while those on the opposite side are the muscle. stretched. The most obvious limiting factor is there- fore the tension in the joint capsule and its associated ligaments on the side of the joint where stretching is SUMMARY occurring. In addition, and most commonly, stretch The musculoskeletal system consists of bones, of the associated muscles and tendons restricts the joints, and muscles. Bone tissue is both hard and range of joint motion. Some of this resistance is also tough. Compact bone is particularly strong, whereas provided through stretching of the skin. Sometimes spongy bone is better for shock absorption. Indi- apposition of soft tissues also restricts movement vidual bones have a variety of shapes; each shape and, rarely, apposition of bony parts forming the joint is particularly suited for performing one of the restricts movement. This bony contact would occur functions of bone. only at the extreme end of range and is potentially Joint mobility and stability tend to be compet- injurious in dynamic situations. ing requirements, so the structure of each joint is a 48 | Biophysical Foundations of Human Movement compromise. The many types of joints are classified stimulation can be detected and used as a tool for on the basis of their structure, the gross movements measuring maximum muscle performance. Joint they allow, or the motions that occur between the motion may be limited by the length of muscle– articular surfaces in contact. tendon units, which may be increased by flexibility Skeletal muscle tissue looks striated in a longi- exercises. Strength is determined by muscle cross- tudinal section under a microscope. A muscle fibre sectional area (related to the number of fibres in consists of up to tens of thousands of sarcomeres parallel) and leverage of the muscle that produces in series (joined end to end). Connective tissue, joint motion. an important component of whole muscles, pro- vides some of the properties of the muscle–tendon FURTHER READING unit. The sliding filament hypothesis of muscle Basmajian, J.V., & de Luca, C.J. (1985). Muscles alive: Their contraction describes what happens when muscle functions revealed by electromyography (5th ed.). Balti- changes length and is based on electron micro- more: Williams & Wilkins. graphs of muscle at different lengths. According Jenkins, D.B. (2009). Hollinshead’s functional anatomy of the to the cross-bridge hypothesis, cross-bridges on limbs and back (9th ed.). St. Louis: Saunders/Elsevier. the thick myofilaments pull the thin myofilaments Levangie, P.K., & Norkin, C.C. (2011). Joint structure and toward them after excitation from the nerve that function: A comprehensive analysis (5th ed.). Philadelphia: supplies the muscle. Length changes resulting from F.A. Davis. neural activation are dependent on the net effect of Martini, F., Timmons, M.J., & Tallitsch, R.B. (2012). Human a number of forces. The muscle–tendon unit may anatomy (7th ed.). Boston: Pearson Benjamin Cummings. act isometrically to stabilise joints, concentrically Nordin, M., & Frankel, V.H. (2001). Basic biomechanics of the to produce movement, and eccentrically to control musculoskeletal system (3rd ed.). Philadelphia: Lippincott movement. The electrical response of muscle to Williams & Wilkins. neural stimulation can be detected and used as a Oatis, C.A. (2009). Kinesiology: The mechanics and pathome- tool in the study of normal muscle contraction. chanics of human movement (2nd ed.). Baltimore: Lippin- The mechanical response of muscle to artificial cott Williams & Wilkins.