Chapter 1. The Musculoskeletal System

At the completion of this chapter, the reader will be able to:

1. Describe the various types of biological tissue of the musculoskeletal system.

2. Describe the tissue mechanics and structural differences and similarities between muscle, tendons, fascia, and ligaments.

3. Define the various terminologies used to describe joint position, movements, and relationships.

4. Give definitions for commonly used biomechanical terms.

5. Describe the different planes of the body.

6. Describe the different axes of the body and the motions that occur around them.

7. Define the terms osteokinematic motion and arthrokinematic motion.

8. Differentiate between the different types of motion that can occur at the joint surfaces.

9. Describe the basic biomechanics of joint motion in terms of their concave–convex relationships.

10. Define the terms closepacked and openpacked.

Throughout the human body, there are four types of fundamental tissues:

• Epithelial. Covers all internal and external body surfaces and includes structures such as the skin and the inner lining of the blood vessels.
• Connective. Connective tissue (CT), which includes bone, cartilage, tendons, ligaments, and blood tissue, provides structural and metabolic support for other tissues and organs of the body.
• Muscle. Muscles are classified functionally as either voluntary or involuntary, and structurally as either smooth, striated (skeletal), or cardiac. There are approximately 430 skeletal muscles in the body, each of which can be considered anatomically as a separate organ. Of these 430 muscles, about 75 pairs provide the majority of body movements and postures.1
• Nervous. Nervous tissue provides a two-way communication system between the central nervous system (brain and spinal cord) and muscles, sensory organs, and various systems (see Chapter 3).

CT is divided into subtypes according to the matrix that binds the cells. CT proper has a loose flexible matrix, called ground substance. The most common cell within CT proper is the fibroblast. Fibroblasts produce collagen, elastin, and reticulin fibers. Collagen is a group of naturally occurring proteins. The collagens are a family of extracellular matrix (ECM) proteins that play a dominant role in maintaining the structural integrity of various tissues and in providing tensile strength to tissues. The ECM contains proteoglycans, lipids, water, and dissolved electrolytes. Proteoglycans, which are a major component of the ECM, are macromolecules that consist of a protein backbone to which are attached many extended polysaccharide units called glycosaminoglycans (GAGs), of which there are two types: chondroitin sulfate and keratin sulfate.2,3 A simple way to visualize the proteoglycan molecule is to consider a test tube brush, with the stem representing the protein core and the GAGs representing the bristles.4,5

The various characteristics of collagen differ depending on whether it is loose or dense collagen. The anatomic and functional characteristics of loose and dense collagen are summarized in Table 1-1. Collagenous and elastic fibers are sparse and irregularly arranged in loose CT but are tightly packed in dense CT.6 Elastic fibers are composed of a protein called elastin. As its name suggests, elastin provides elastic properties to the tissues in which it is situated.7 Elastin fibers can stretch, but they normally return to their original shape when the tension is released. Thus, the elastic fibers of elastin determine the patterns of distention and recoil in most organs, including the skin and lungs, blood vessels, and CT. Bundles of collagen and elastin combine to form a matrix of CT fascicles. This matrix is organized within the primary collagen bundles as well as between the bundles that surround them.8

The various types of CT as relate to the musculoskeletal system are described as follows:

Fascia

Fascia is viewed as a loose CT that provides support and protection to the joint, and acts as an interconnection between tendons, aponeuroses, ligaments, capsules, nerves, and the intrinsic components of muscle.9,10 This type of CT may be categorized as fibrous or nonfibrous, with the fibrous components consisting mainly of collagen and elastin fibers, and the nonfibrous portion consisting of amorphous ground substance.11

Tendons

The function of tendons is to attach muscle to bone at each end of the muscle, and, when stretched, store elastic energy that contributes to movement.12 In addition, tendons enable the muscle belly to be an optimal distance from the joint upon which it is acting.12 The collagen fibers of tendons (95% of the collagen in tendons is type I, with the remaining 5% being type III and IV) are arranged in a quarter-stagger arrangement, which gives it a characteristic banding pattern and provides high strength and stability.13 A loose CT matrix surrounds the bundles of collagen fibrils. The thickness of each tendon varies and is proportional to the size of the muscle from which it originates. Vascularity within the tendon is relatively sparse and corresponds with the lower metabolic/turnover rate of these tissues. Within the fascicles of tendons, which are held together by loose CT called endotenon, the collagen components are unidirectionally oriented. Endotenon contains blood vessels, lymphatics, and nerves and permits longitudinal movements of individual fascicles when tensile forces are applied to the structure. The CT surrounding groups of fascicles or the entire structure is called the epitenon. Tendons display viscoelastic mechanical properties that confer time-and rate-dependent effects on the tissue. Specifically, tendons are more elastic at lower strain rates and stiffer at higher rates of tensile loading. Tendons deform less than ligaments under an applied load and are able to transmit the load from muscle to bone.8 The resting tendon has a slightly crimped or wave-like appearance. Total tendon strains (percentage deformity) of 1–2% result in the straightening of the crimp pattern of unloaded tendon collagen. Strains of 2–6% are well tolerated by most healthy tendons. However, with a strain higher than 6%, incomplete tears start to occur within the tendon, and complete structural failure typically occurs in the range of 8–10%.14 As with all CT, tendons have a positive adaptive response to repeated physiologic mechanical loading, which results in biologic and mechanical changes. Although tendons withstand strong tensile forces well, they resist shear forces less well, and provide little resistance to compression force. A peritendinous connective-tissue sheath (paratenon) surrounds the entire tendon. This sheath consists of two layers: an inner (visceral) layer and an outer (parietal) layer with occasional connecting bridges (mesotenon). If there is synovial fluid between these two layers, the paratenon is called tenosynovium; if not, it is termed tenovagium.8

A tendon can be divided into three main sections:12

• The bone–tendon junction. At most tendon–bone interfaces, the collagen fibers insert directly into the bone in a gradual transition of material composition. The physical junction of tendon and bone is referred to as an enthesis,15 and is an interface that is vulnerable to acute and chronic injury.16 One role of the enthesis is to absorb and distribute the stress concentration that occurs at the junction over a broader area.
• The tendon midsubstance. Overuse tendon injuries can occur in the midsubstance of the tendon, but not as frequently as at the enthesis.
• The musculotendinous junction (MTJ). The MTJ is the site where the muscle and tendon meet. The MTJ comprises numerous interdigitations between muscle cells and tendon tissue, resembling interlocked fingers. Despite its viscoelastic mechanical characteristics, the MTJ is very vulnerable to tensile failure.17,18

Ligaments

Skeletal ligaments are fibrous bands of dense CT that connect bones across joints. Ligaments can be named for the bones into which they insert (coracohumeral), their shape (deltoid), or their relationships to each other (cruciate).23 The gross structure of a ligament varies according to location (intra-articular or extra-articular, capsular), and function.24 Ligaments, which appear as dense white bands or cords of CT, are composed primarily of water (approximately 2/3), and of collagen (largely type I collagen (85%), but with small amounts of type III) making up most of the dry weight. The collagen in ligaments has a less unidirectional organization than it does in tendons, but its structural framework still provides stiffness (resistance to deformation—see Chapter 2).25 Small amounts of elastin (1% of the dry weight) are present in ligaments, with the exception of the ligamentum flavum and the nuchal ligament of the spine, which contain more. The cellular organization of ligaments makes them ideal for sustaining tensile load, with many containing functional subunits that are capable of tightening or loosening in different joint positions.26 At the microscopic level, closely spaced collagen fibers (fascicles) are aligned along the long axis of the ligament and are arranged into a series of bundles that are delineated by a cellular layer, the endoligament, and the entire ligament is encased in a neurovascular biocellular layer referred to as the epiligament.23 Ligaments contribute to the stability of joint function by preventing excessive motion,27 acting as guides or checkreins to direct motion, and providing proprioceptive information for joint function through sensory nerve endings (see Chapter 3) and the attachments of the ligament to the joint capsule.28–30 Many ligaments share functions. For example while the anterior cruciate ligament of the knee is considered the primary restraint to anterior translation of the tibia relative to the femur, the collateral ligaments and the posterior capsule of the knee also help in this function.23 The vascular and nerve distribution to ligaments is not homogenous. For example, the middle of the ligament is typically avascular, while the proximal and distal ends enjoy a rich blood supply. Similarly, the insertional ends of the ligaments are more highly innervated than the midsubstance.

Cartilage

Cartilage tissue exists in three forms: hyaline, elastic, and fibrocartilage.

• • Hyaline cartilage, also referred to as articular cartilage, covers the ends of long bones and permits almost frictionless motion to occur between the articular surfaces of a diarthrodial (synovial) joint.31 Articular cartilage is a highly organized viscoelastic material composed of cartilage cells called chondrocytes, water, and an ECM. Articular cartilage, the most abundant cartilage within the body, is devoid of any blood vessels, lymphatics, and nerves.4,5 Most of the bones of the body form first as hyaline cartilage, and later become bone in a process called endochondral ossification. The normal thickness of articular cartilage is determined by the contact pressures across the joint—the higher the peak pressures, the thicker the cartilage.24 Articular cartilage functions to distribute the joint forces over a large contact area, thereby dissipating the forces associated with the load. This distribution of forces allows the articular cartilage to remain healthy and fully functional throughout decades of life. The patellar has the thickest articular cartilage in the body.
• Articular cartilage may be grossly subdivided into four distinct zones with differing cellular morphology, biomechanical composition, collagen orientations, and structural properties, as follows:
  • The superficial zone. The superficial zone, which lies adjacent to the joint cavity, comprises approximately 10–20% of the articular cartilage thickness and functions to protect deeper layers from shear stresses. The collagen fibers within this zone are packed tightly and aligned parallel to the articular surface. This zone is in contact with synovial fluid and is responsible for most of the tensile properties of cartilage.
  • The middle (transitional) zone. In the middle zone, which provides an anatomic and functional bridge between the superficial and deep zones, the collagen fibril orientation is obliquely organized. This zone comprises 40–60% of the total cartilage volume. Functionally, the middle zone is the first line of resistance to compressive forces.
  • The deep or radial layer. The deep layer comprises 30% of the matrix volume. It is characterized by radially aligned collagen fibers that are perpendicular to the surface of the joint, and which have a high proteoglycan content. Functionally the deep zone is responsible for providing the greatest resistance to compressive forces.
  • The tidemark. The tidemark distinguishes the deep zone from the calcified cartilage, the area that prevents the diffusion of nutrients from the bone tissue into the cartilage.
• • Elastic cartilage is a very specialized CT, primarily found in locations such as the outer ear and portions of the larynx.
• • Fibrocartilage functions as a shock absorber in both weight-bearing and non–weight-bearing joints. Its large fiber content, reinforced with numerous collagen fibers, makes it ideal for bearing large stresses in all directions. Fibrocartilage is an avascular, alymphatic, and aneural tissue and derives its nutrition by a double-diffusion system.33 Examples of fibrocartilage include the symphysis pubis, the intervertebral disk, and the menisci of the knee.

Bone

Bone is a highly vascular form of CT, composed of collagen, calcium phosphate, water, amorphous proteins, and cells. It is the most rigid of the CTs (Table 1-2). Despite its rigidity, bone is a dynamic tissue that undergoes constant metabolism and remodeling. The collagen of bone is produced in the same manner as that of ligament and tendon, but by a different cell, the osteoblast.6 At the gross anatomical level, each bone has a distinct morphology comprising both cortical bone and cancellous bone. Cortical bone is found in the outer shell. Cancellous bone is found within the epiphyseal and metaphyseal regions of long bones as well as throughout the interior of short bones.17 Skeletal development occurs in one of two ways:

• Intramembranous ossification. Mesenchymal stem cells within mesenchyme or the medullary cavity of a bone initiate the process of intramembranous ossification. This type of ossification occurs in the cranium and facial bones and, in part, the ribs, clavicle, and mandible.
• Endochondral ossification. The first site of ossification occurs in the primary center of ossification, which is in the middle of the diaphysis (shaft). About the time of birth, a secondary ossification center appears in each epiphysis (end) of long bones. Between the bone formed by the primary and secondary ossification centers, cartilage persists as the epiphyseal (growth) plates between the diaphysis and the epiphysis of a long bone. This type of ossification occurs in the appendicular and axial bones.

The periosteum is formed when the perichondrium, which surrounds the cartilage, becomes the periosteum. Chondrocytes in the primary center of ossification begin to grow (hypertrophy) and begin secreting alkaline phosphatase, an enzyme essential for mineral deposition. Calcification of the matrix follows and apoptosis (a type of cell death involving a programmed sequence of events that eliminates certain cells) of the hypertrophic chondrocytes occurs. This creates cavities within the bone. The exact mechanism of chondrocyte hypertrophy and apoptosis is currently unknown. The hypertrophic chondrocytes (before apoptosis) also secrete a substance called vascular endothelial cell growth factor that induces the sprouting of blood vessels from the perichondrium. Blood vessels forming the periosteal bud invade the cavity left by the chondrocytes and branch in opposite directions along the length of the shaft. The blood vessels carry osteoprogenitor cells and hemopoietic cells inside the cavity, the latter of which later form the bone marrow. Osteoblasts, differentiated from the osteoprogenitor cells that enter the cavity via the periosteal bud, use the calcified matrix as a scaffold and begin to secrete osteoid, which forms the bone trabecula. Osteoclasts, formed from macrophages, break down the spongy bone to form the medullary cavity (bone marrow). The function of bone is to provide support, enhance leverage, protect vital structures, provide attachments for both tendons and ligaments, and store minerals, particularly calcium. Bones also may serve as useful landmarks during the palpation phase of the examination. The strength of a bone is related directly to its density. Of importance to the clinician, is the difference between maturing bone and mature bone. The epiphyseal plate or growth plate of a maturing bone can be divided into four distinct zones:34

• Reserve zone: produces and stores matrix.
• Proliferative zone: produces matrix and is the site for longitudinal bone cell growth.
• Hypertrophic zone: subdivided into the maturation zone, degenerative zone, and zone of provisional calcification. It is within the hypertrophic zone that the matrix is prepared for calcification, and is here that the matrix is ultimately calcified. The hypertrophic zone is the most susceptible of the zones to injury because of the low volume of bone matrix and the high amounts of developing immature cells in this region.35
• Bone metaphysis: the part of the bone that grows during childhood.

The microstructure and composition of skeletal muscle have been studied extensively. The class of tissue labeled skeletal muscle consists of individual muscle cells or fibers that work together to produce the movement of bony levers. A single muscle cell is called a muscle fiber or myofiber. Individual muscle fibers are wrapped in a CT envelope called endomysium. Bundles of myofibers, which form a whole muscle (fasciculus), are encased in the perimysium. The perimysium is continuous with the deep fascia. Groups of fasciculi are surrounded by a connective sheath called the epimysium. Under an electron microscope, it can be seen that each of the myofibers consists of thousands of myofibrils, which extend throughout its length. Myofibrils are composed of sarcomeres arranged in series.36 All skeletal muscles exhibit four characteristics:37

1. Excitability, the ability to respond to stimulation from the nervous system.

2. Elasticity, the ability to change in length or stretch.

3. Extensibility, the ability to shorten and return to normal length.

4. Contractility, the ability to shorten and contract in response to some neural command. The tension developed in skeletal muscle can occur passively (stretch) or actively (contraction). When an activated muscle develops tension, the amount of tension present is constant throughout the length of the muscle, in the tendons, and at the sites of the musculotendinous attachments to bone.1 The tensile force produced by the muscle pulls on the attached bones and creates torque at the joints crossed by the muscle. The magnitude of the tensile force is dependent on a number of factors.

One of the most important roles of CT is to mechanically transmit the forces generated by the skeletal muscle cells to provide movement. Each of the myofibrils contains many fibers called myofilaments, which run parallel to the myofibril axis. The myofilaments are made up of two different proteins: actin (thin myofilaments) and myosin (thick myofilaments) that give skeletal muscle fibers their striated (striped) appearance (Fig. 1-1).36

The striations are produced by alternating dark (A) and light (I) bands that appear to span the width of the muscle fiber. The A bands are composed of myosin filaments, whereas the I bands are composed of actin filaments. The actin filaments of the I band overlap into the A band, giving the edges of the A band a darker appearance than the central region (H band), which contains only myosin. At the center of each I band is a thin, dark Z line. A sarcomere represents the distance between each Z line. Each muscle fiber is limited by a cell membrane called a sarcolemma. The protein dystrophin plays an essential role in the mechanical strength and stability of the sarcolemma.38 Dystrophin is lacking in patients with Duchenne muscular dystrophy.

Three types of contraction are commonly recognized: isometric, concentric, and eccentric (see Chapter 12).39

• Isometric contraction. Isometric contractions do not produce any appreciable change in muscle length.
• Concentric contraction. A concentric contraction produces a shortening of the muscle length. When a muscle contracts concentrically, the distance between the Z lines decreases, the I band and H bands disappear, but the width of the A band remains unchanged.40 This shortening of the sarcomeres is not produced by a shortening of the actin and myosin filaments, but by a sliding of actin filaments over the myosin filaments, which pulls the Z lines together (sliding filament theory).
• Eccentric contraction. An eccentric contraction produces a “lengthening” of the muscle length. In reality, the muscle does not actually lengthen, it merely returns from its shortened position to its normal resting length.

Structures called cross-bridges serve to connect the actin and myosin filaments. The myosin filaments contain two flexible, hinge-like regions, which allow the cross-bridges to attach and detach from the actin filament. During contraction, the cross-bridges attach and undergo power strokes, which provide the contractile force. During relaxation, the cross-bridges detach. This attaching and detaching is asynchronous, so that some are attaching while others are detaching. Thus, at each moment, some of the cross-bridges are pulling, while others are releasing.

The regulation of cross-bridge attachment and detachment is a function of two proteins found in the actin filaments: tropomyosin and troponin (Fig. 1-2). Tropomyosin attaches directly to the actin filament, whereas troponin is attached to the tropomyosin rather than directly to the actin filament.

Each muscle fiber is innervated by a somatic motor neuron. One neuron and the muscle fibers it innervates constitute a motor unit, or functional unit of the muscle. Each motor neuron branches as it enters the muscle to innervate a number of muscle fibers.

The release of a chemical acetylcholine from the axon terminals at the NMJ causes electrical activation of the skeletal muscle fibers. When an action potential propagates into the transverse tubule system (narrow membranous tunnels formed from and continuous with the sarcolemma), the voltage sensors on the transverse tubule membrane signal the release of Ca2+ from the terminal cisternae portion of the SR (a series of interconnected sacs and tubes that surround each myofibril).40 The released Ca2+ then diffuses into the sarcomeres and binds to troponin, displacing the tropomyosin, and allowing the actin to bind with the myosin cross-bridges (Fig. 1-2). Whenever a somatic motor neuron is activated, all of the muscle fibers that it innervates are stimulated and contract with all-or-none twitches. Although the muscle fibers produce all-or-none contractions, muscles are capable of a wide variety of responses, ranging from activities requiring a high level of precision, to activities requiring high tension.

At the end of the contraction (the neural activity and action potentials cease), the SR actively accumulates Ca2+ and muscle relaxation occurs. The return of Ca2+ to the SR involves active transport, requiring the degradation of adenosine triphosphate (ATP) to adenosine diphosphate (ADP)*.40 Because SR function is closely associated with both contraction and relaxation, changes in its ability to release or sequester Ca2+ markedly affect both the time course and magnitude of force output by the muscle fiber.41

*The most readily available energy for skeletal muscle cells is stored in the form of ATP and phosphocreatine (PCr). Through the activity of the enzyme ATPase, ATP promptly releases energy when required by the cell to perform any type of work, whether it is electrical, chemical, or mechanical.

On the basis of their contractile properties, two major types of muscle fiber have been recognized within skeletal muscle: type I (slow-twitch fibers), and type II (fast-twitch fibers) (Table 1-3).42 Slow-twitch fibers are richly endowed with mitochondria and have a high capacity for oxygen uptake. They are, therefore, suitable for activities of long duration or endurance (aerobic), including the maintenance of posture. In contrast, fast-twitch fibers are suited to quick, explosive actions (anaerobic), including such activities as sprinting. A number of muscle subtypes have been recognized based on their qualities and aerobic to anaerobic (see Chapter 15) capabilities.

• Type A: possess good aerobic and anaerobic characteristics
• Type B: possess fair aerobic and poor anaerobic characteristics
• Type AB: possess aerobic and anaerobic characteristics somewhere between types A and B
• Type C: somewhat rare in humans

Based on the above, six fiber types can be distinguished: I, IC, IIA, IIB, IIAB, and IIC.43 The type II (fast-twitch) fibers are separated based on mitochondria content into those that have a high complement of mitochondria (type IIA) and those that are mitochondria poor (type IIB). Type IIC fibers exhibit structural features of both red and white fibers and thus have fast contraction times and good fatigue resistance. Within this list, there appears to be a consistent order of recruitment based on fiber size: type I fibers first, followed by IC, IIC, IIA, IIAB, and, finally, IIB fibers.43 The smaller type I fibers are the easiest to stimulate as they have the smaller motor units.

Theory dictates that a muscle with a large percentage of the total cross-sectional area occupied by slow-twitch type I fibers should be more fatigue resistant than one in which the fast-twitch type II fibers predominate.

Different activities place differing demands on a muscle (Table 1-4).44 In humans, most limb muscles contain a relatively equal distribution of each muscle fiber type, whereas the back and trunk demonstrate a predominance of slow-twitch fibers. Although it would seem possible that physical training may cause fibers to convert from slow twitch to fast twitch or the reverse, this has not been shown to be the case.45 However, fiber conversion from type IIB to type IIAB and IIA, and vice versa, has been found to occur with training.46

The effectiveness of a muscle to produce movement depends on a number of factors. These include the location and orientation of the muscle attachment relative to the joint, the limitations or laxity present in the musculotendinous unit, the type of contraction, the point of application, and the actions of other muscles that cross the joint.1

Muscles serve a variety of roles depending on the required movement:

• Prime agonist. A muscle that is directly responsible for producing movement.
• Synergist (supporter). Performs a cooperative muscle function in relation to the agonist. Synergists can function as stabilizers or neutralizers.
  • Stabilizers (fixators). Muscles that contract statically to steady or support some part of the body against the pull of the contracting muscles, against the pull of gravity, or against the effect of momentum and recoil in certain vigorous movements.
  • Neutralizers. Muscles that act to prevent an undesired action of one of the movers.
• Antagonist. A muscle that has an effect opposite to that of the agonist.

As previously mentioned, depending on the type of muscular contraction, the length of a muscle can remain the same (isometric), shorten (concentric), or “lengthen” (eccentric). The rate of muscle length change substantially affects the force that a muscle can develop during contraction.

• Concentric contractions. As the speed of a concentric contraction increases, the force it is capable of producing decreases.50,51 The slower speed of contraction is thought to produce greater forces than can be produced by increasing the number of cross-bridges formed. This relationship can be viewed as a continuum, with the optimum velocity for the muscle somewhere between the slowest and fastest rates. At very slow speeds, the force that a muscle can resist or overcome rises rapidly up to 50% greater than the maximum isometric contraction.50,51
• Eccentric contractions. The following changes in force production occur during an eccentric contraction:
  • Rapid eccentric contractions generate more force than do slow ones (slower eccentric contractions).
  • During slow eccentric muscle actions, the work produced approximates that of an isometric contraction.50,51

The force and speed of a muscle contraction are based on the requirement of an activity and are dependent on the ability of the central nervous system to control the recruitment of motor units.1 The motor units of slow-twitch fibers generally have lower thresholds and are relatively easier to activate than those of the fast-twitch motor units. Consequently, the slow-twitch fibers are the first to be recruited, even when the resulting limb movement is rapid.57

As the force requirement, speed requirement, or duration of an activity increases, motor units with higher thresholds are recruited. 58

Although each muscle contains the contractile machinery to produce the forces for movement, it is the tendon that transmits these forces to the bones in order to achieve movement or stability of the body in space.8 The angle of insertion the tendon makes with a bone determines the line of pull, whereas the tension generated by a muscle is a function of its angle of insertion. A muscle generates the greatest amount of torque when its line of pull is oriented at a 90-degree angle to the bone, and it is attached anatomically as far from the joint center as possible.1

Just as there are optimal speeds of length change, and optimal muscle lengths, there are optimal insertion angles for each of the muscles. The angle of insertion of a muscle, and therefore its line of pull, can change during dynamic movements.56 The angle of pennation is the angle created between the fiber direction and the line of pull. When the fibers of a muscle lie parallel to the long axis of the muscle, there is no angle of pennation. The number of fibers within a fixed volume of muscle increases with the angle of pennation.56 Although maximum tension can be improved with pennation, the range of shortening of the muscle is reduced. Muscle fibers can contract to about 60% of their resting length. Since the muscle fibers in pennate muscles are shorter than the nonpennate equivalent, the amount of contraction is similarly reduced. Muscles that need to have large changes in length without the need for very high tension, such as the sartorius muscle, do not have pennate muscle fibers.56 In contrast, pennate muscle fibers are found in those muscles in which the emphasis is on a high capacity for tension generation rather than range of motion (e.g., gluteus maximus).

Although the respiratory muscles share some mechanical similarities with skeletal muscles, they are distinct from other skeletal muscles in several aspects as follows63,64:

• Whereas skeletal muscles of the limbs overcome inertial loads, the respiratory muscles overcome primarily elastic and resistive loads.
• The respiratory muscles are under both voluntary and involuntary control.
• The respiratory muscles are similar to the heart muscles, in that they have to contract rhythmically and generate the required forces for ventilation throughout the entire life span of the individual. The respiratory muscles, however, do not contain pacemaker cells and are under the control of mechanical and chemical stimuli, requiring neural input from higher centers to initiate and coordinate contraction.
• The resting length of the respiratory muscles is a relationship between the inward recoil forces of the lung and the outward recoil forces of the chest wall. Changes in the balance of recoil forces will result in changes in the resting length of the respiratory muscles. Thus, simple and everyday life occurrences such as changes in posture may alter the operational length and the contractile strength of the respiratory muscles.65 If uncompensated, these length changes can lead to decreases in the output of the muscles, and hence, a reduction in the ability to generate volume changes.65 The skeletal muscles of the limbs, on the other hand, are not constrained to operate at a particular resting length.

Arthrology is the study of the classification, structure, and function of joints. A joint represents the junction between two or more bones. Joints are regions where bones are capped and surrounded by CTs that hold the bones together and determine the type and degree of movement between them.66 An understanding of the anatomy and biomechanics of the various joints is required to be able to assess and treat a patient thoroughly. When classified according to movement potential, joints may be classified as synarthrosis or diarthrosis (synovial).

Synarthrosis

There are three major types of synarthroses, based on the type of tissue uniting the bone surfaces66:

• Fibrous joints, which are joined by dense fibrous CT. Three types exist:
  • Suture (e.g., suture of the skull).
  • Gomphosis (e.g., tooth and mandible or maxilla articulation).
  • Syndesmosis (e.g., tibiofibular or radioulnar joints).
• Cartilaginous joints, originally referred to as amphiarthrosis joints, are stable joints that allow for minimal or little movement. These joints exist in humans in one of two ways: synchondrosis (e.g., manubriosternal joints) and symphysis (e.g., symphysis pubis)

Diarthrosis

This type of joint generally unites long bones and permits free bone movement and great mobility. These joints are characterized by a fibroelastic joint capsule, which is filled with a lubricating substance called synovial fluid. Consequently these joints are often referred to as synovial joints (see later). Examples include but are not limited to the hip, knee and shoulder, and elbow joints. Synovial joints are further classified based on complexity:

• Simple: a single pair of articular surfaces one male, or convex, surface and one female, or concave, surface.
• Compound: a single joint capsule containing more than a single pair of mating articulating surfaces.
• Complex: contain an intra-articular inclusion within the joint class such as a meniscus or disk that increases the number of joint surfaces.

Synovial joints have five distinguishing characteristics: joint cavity, articular cartilage, synovial fluid, synovial membrane, and a fibrous capsule. All synovial joints of the body are provided with an array of corpuscular (mechanoreceptors) and noncorpuscular (nociceptors) receptor endings imbedded in articular, muscular, and cutaneous structures with varying characteristic behaviors and distributions depending on the articular tissue (see Chapter 3). Synovial joints can be broadly classified according to structure or analogy (Fig. 1-3) into the following categories67:

• Spheroid. As the name suggests, a spheroid joint is a freely moving joint in which a sphere on the head of one bone fits into a rounded cavity in the other bone. Spheroid (ball and socket) joints allow motions in three planes (see later). Examples of a spheroid joint surface include the heads of the femur and humerus.
• Trochoid. The trochoid joint is characterized by a pivot-like process turning within a ring, or a ring on a pivot, the ring being formed partly of bone, partly of ligament. Trochoid joints permit only rotation. Examples of a trochoid joint include the humeroradial joint and the atlantoaxial joint.
• Condyloid. This type of joint is characterized by an ovoid articular surface, or condyle. One bone may articulate with another by one surface or by two, but never more than two. If two distinct surfaces are present, the joint is called condylar, or bicondylar. The elliptical cavity of the joint is designed in such a manner as to permit the motions of flexion, extension, adduction, abduction, and circumduction, but no axial rotation. The wrist-joint is an example of this form of articulation.
• Ginglymoid. A ginglymoid joint is a hinge joint. It is characterized by a spool-like surface and a concave surface. An example of a ginglymoid joint is the humeroulnar joint.
• Ellipsoid. Ellipsoid joints are similar to spheroid joints in that they allow the same type of movement albeit to a lesser magnitude. The ellipsoid joint allows movement in two planes (flexion, extension; abduction, adduction) and is biaxial. Examples of this joint can be found at the radiocarpal articulation at the wrist and the metacarpophalangeal articulation with the phalanges.
• Planar. As its name suggests, a planar joint is characterized by flat surfaces that slide over each other. Movement at this type of joint does not occur about an axis and is termed nonaxial. Examples of a planar joint include the intermetatarsal joints and some intercarpal joints.

In reality, no joint surface is planar or resembles a true geometric form. Instead, joint surfaces are either convex in all directions, or concave in all directions; that is they resemble either the outer or inner surface of a piece of eggshell.68 Because the curve of an eggshell varies from point to point, these articular surfaces are called ovoid. The other major type of articular surface is the sellar joint.68 Sellar joints are characterized by a convex surface in one cross-sectional plane, and a concave surface in the plane perpendicular to it (Fig. 1-3). Examples of a sellar joint include the interphalangeal joints, the carpometacarpal joint of the thumb, the humeroulnar joint, and the calcaneocuboid joints.

Physiological Range of Motion

The amount of available physiological joint motion is based on a number of factors, including

• the integrity of the joint surfaces and the amount of joint motion;
• the mobility and pliability of the soft tissues that surround a joint;
• the degree of soft-tissue approximation that occurs;
• the amount of scarring that is present69—interstitial scarring or fibrosis can occur in and around the joint capsules, within the muscles, and within the ligaments as a result of previous trauma;
• age—joint motion tends to decrease with increasing age;
• gender—in general, females have more joint motion than males.

Synovial Fluid

Articular cartilage is subject to a great variation of loading conditions, and joint lubrication through synovial fluid is necessary to minimize frictional resistance between the weight-bearing surfaces. Fortunately, synovial joints are blessed with a very superior lubricating system, which permits a remarkably frictionless interaction at the joint surfaces. A lubricated cartilaginous interface has a coefficient of friction* of 0.002.70 By way of comparison, ice on ice has a higher coefficient of friction (0.03).70 The composition of synovial fluid is nearly the same as blood plasma, but with a decreased total protein content and a higher concentration of hyaluronan.71

*Coefficient of friction is a ratio of the force needed to make a body glide across a surface compared with the weight or force holding the two surfaces in contact.

Indeed, synovial fluid is essentially a dialysate of plasma to which hyaluronan has been added.73 Hyaluronan is a GAG that is continually synthesized and released into synovial fluid by specialized synoviocytes.73,74 The mechanical properties of synovial fluid permit it to act as both a cushion and a lubricant to the joint. Diseases such as osteoarthritis, affect the thixotropic properties (thixotropy is the property of various gels becoming fluid when disturbed, as by shaking) of synovial fluid, resulting in reduced lubrication and subsequent wear of the articular cartilage and joint surfaces.75,76 It is well established that damaged articular cartilage in adults has a very limited potential for healing (see Chapter 2), because it possesses neither a blood supply nor lymphatic drainage.77

Bursae

Closely associated with some synovial joints are flattened, saclike structures called bursae that are lined with a synovial membrane and filled with synovial fluid. The bursa produces small amounts of fluid, allowing for smooth and almost frictionless motion between contiguous muscles, tendons, bones, ligaments, and skin.78–80 A tendon sheath is a modified bursa. A bursa can be a source of pain if it becomes inflamed or infected.

When describing movements, it is necessary to have a starting position as the reference position. This starting position is referred to as the anatomic reference position. The anatomic reference position for the human body is described as the erect standing position with the feet just slightly separated and the arms hanging by the side, the elbows straight, and the palms of the hand facing forward (Fig. 1-4).

Directional Terms

Directional terms are used to describe the relationship of body parts or the location of an external object with respect to the body.81 The following are commonly used directional terms:

• Superior or cranial. Closer to the head.
• Inferior or caudal. Closer to the feet.
• Anterior or ventral. Toward the front of the body.
• Posterior or dorsal. Toward the back of the body.
• Medial. Toward the midline of the body.
• Lateral. Away from the midline of the body.
• Proximal. Closer to the trunk.
• Distal. Away from the trunk.
• Superficial. Toward the surface of the body.
• Deep. Away from the surface of the body in the direction of the inside of the body.

In general, there are two types of motions: translation, which occurs in either a straight or curved line, and rotation, which involves a circular motion around a pivot point. Movements of the body segments occur in three dimensions along imaginary planes and around various axes of the body.

Planes of the Body

There are three traditional planes of the body corresponding to the three dimensions of space: sagittal, frontal, and transverse81 (Fig. 1-5).

• Sagittal. The sagittal plane, also known as the anterior-posterior or median plane, divides the body vertically into left and right halves of equal size.
• Frontal. The frontal plane, also known as the lateral or coronal plane, divides the body equally into front and back halves.
• Transverse. The transverse plane, also known as the horizontal plane, divides the body equally into top and bottom halves.

Because each of these planes bisects the body, it follows that each plane must pass through the center of gravity.* If the movement described occurs in a plane that passes through the center of gravity, that movement is deemed to have occurred in a cardinal plane. An arc of motion represents the total number of degrees traced between the two extreme positions of movement in a specific plane of motion.82 If a joint has more than one plane of motion, each type of motion is referred to as a unit of motion. For example, the wrist has two units of motion: flexion–extension (anterior–posterior plane) and ulnar–radial deviation (lateral plane).82

*The center of gravity may be defined as the point at which the three planes of the body intersect each other. The line of gravity is defined as the vertical line at which the two vertical planes intersect each other.

Few movements involved with functional activities occur in the cardinal planes. Instead, most movements occur in an infinite number of vertical and horizontal planes parallel to the cardinal planes (see discussion that follows).

Axes of the Body

Three reference axes are used to describe human motion: frontal, sagittal, and longitudinal (Fig. 1-6). The axis around which the movement takes place is always perpendicular to the plane in which it occurs.

• Frontal. The frontal axis, also known as the transverse axis, is perpendicular to the sagittal plane.
• Sagittal. The sagittal axis is perpendicular to the frontal plane.
• Longitudinal. The longitudinal axis, also known as the vertical axis, is perpendicular to the transverse plane.

Most movements occur in planes and around axes that are somewhere in between the traditional planes and axes. However, nominal identification of every plane and axis of movement is impractical. The structure of the joint determines the possible axes of motion that are available. For example, a hinge joint has only a frontal–horizontal axis. Condyloid (ovoid) joints have both a frontal–horizontal and a sagittal–horizontal axis. Ball-and-socket joints have frontal, sagittal–horizontal, and vertical axes. The axis of rotation remains stationary only if the convex member of a joint is a perfect sphere and articulates with a perfect reciprocally shaped concave member. The planes and axes for the more common planar movements (Fig. 1-7) are as follows:

• Flexion, extension, hyperextension, dorsiflexion, and plantar flexion occur in the sagittal plane around a frontal–horizontal axis.
• Abduction and adduction, side flexion of the trunk, elevation and depression of the shoulder girdle, radial and ulnar deviation of the wrist, and eversion and inversion of the foot occur in the frontal plane around a sagittal–horizontal axis.
• Rotation of the head, neck, and trunk; internal rotation and external rotation of the arm or leg; horizontal adduction and abduction of the arm or thigh; and pronation and supination of the forearm occur in the transverse plane around the longitudinal axis.
• Arm circling and trunk circling are examples of circumduction. Circumduction involves an orderly sequence of circular movements that occur in the sagittal, frontal, and intermediate oblique planes, so that the segment as a whole incorporates a combination of flexion, extension, abduction, and adduction. Circumduction movements can occur at biaxial and triaxial joints. Examples of these joints include the tibiofemoral, radiohumeral, hip, glenohumeral, and the spinal joints.

Both the configuration of a joint and the line of pull of the muscle acting at a joint determine the motion that occurs at a joint:

• A muscle whose line of pull is lateral to the joint is a potential abductor.
• A muscle whose line of pull is medial to the joint is a potential adductor.
• A muscle whose line of pull is anterior to a joint has the potential to extend or flex the joint. At the knee, an anterior line of pull may cause the knee to extend, whereas at the elbow joint, an anterior line of pull may cause flexion of the elbow.
• A muscle whose line of pull is posterior to the joint has the potential to extend or flex a joint (refer to preceding example).

The number of independent modes of motion at a joint is called the degrees of freedom (DOF). A joint can have up to 3 degrees of angular freedom, corresponding to the three dimensions of space.83 If a joint can swing in one direction or can only spin, it is said to have 1 DOF.84–87 The proximal interphalangeal joint is an example of a joint with 1 DOF. If a joint can spin and swing in one way only or it can swing in two completely distinct ways, but not spin, it is said to have 2 DOF.84–87 The tibiofemoral joint, temporomandibular joint, proximal and distal radioulnar joints, subtalar joint, and talocalcaneal joint are examples of joints with 2 DOF. If the bone can spin and also swing in two distinct directions then it is said to have 3 DOF.84–87 Ball-and-socket joints such as the shoulder and hip have 3 DOF.

Because of the arrangement of the articulating surfaces—the surrounding ligaments and joint capsules—most motions around a joint do not occur in straight planes or along straight lines. Instead, the bones at any joint move through space in curved paths. This can best be illustrated using Codman's paradox.

1. Stand with your arms by your side, palms facing inward, thumbs extended. Notice that the thumb is pointing forward.

2. Flex one arm to 90 degrees at the shoulder so that the thumb is pointing up.

3. From this position, horizontally extend your arm so that the thumb remains pointing up but your arm is in a position of 90 degrees of glenohumeral abduction.

4. From this position, without rotating your arm, return the arm to your side and note that your thumb is now pointing away from your thigh.

Referring to the start position, and using the thumb as the reference, it can be seen that the arm has undergone an external rotation of 90 degrees. But where and when did the rotation take place? Undoubtedly, it occurred during the three separate, straight-plane motions or swings that etched a triangle in space. What you have just witnessed is an example of a conjunct rotation—a rotation that occurs as a result of joint surface shapes—and the effect of inert tissues rather than contractile tissues. Conjunct rotations can only occur in joints that can rotate internally or externally. Although not always apparent, most joints can so rotate. Consider the motions of elbow flexion and extension. While fully flexing and extending your elbow a number of times, watch the pisiform bone and forearm. If you watch carefully you should notice that the pisiform and the forearm move in a direction of supination during flexion, and pronation during extension of the elbow. The pronation and supination motions are examples of conjunct rotations.

Most habitual movements, or those movements that occur most frequently at a joint, involve a conjunct rotation. However, the conjunct rotations are not always under volitional control. In fact, the conjunct rotation is only under volitional control in joints with 3 DOF (glenohumeral and hip joints). In joints with fewer than 3 DOF (hinge joints, such as the tibiofemoral and ulnohumeral joints), the conjunct rotation occurs as part of the movement but is not under voluntary control. The implications become important when attempting to restore motion at these joints: the mobilizing techniques must take into consideration both the relative shapes of the articulating surfaces as well as the conjunct rotation that is associated with a particular motion (see Chapter 10).

Kinematics is the study of motion. Kinetics is the term applied to the forces acting on the body (see Chapter 2). In studying joint kinematics, two major types of motion are involved: (1) osteokinematic and (2) arthrokinematic.

Osteokinematic Motion

Osteokinematic motion occurs when any object forms the radius of an imaginary circle about a fixed point. The axis of rotation for osteokinematic motions is oriented perpendicular to the plane in which the rotation occurs.81 The distance traveled by the motion may be a small arc or a complete circle and is measured as an angle, in degrees. All human body segment motions involve osteokinematic motions. Examples of osteokinematic motion include abduction or adduction of the arm, flexion of the hip or knee, and side bending of the trunk.

Moment Arm

To understand the concept of a moment arm, an understanding of the anatomy and movement (kinematics) of the joint of interest is necessary. Although muscles produce linear forces, motions at joints are all rotary. For example, some joints can be considered to rotate about a fixed point. A good example of such a joint is the elbow. At the elbow joint, where the humerus and ulna articulate, the resulting rotation occurs primarily about a fixed point, referred to as the center of rotation (COR). In the case of the elbow joint, this COR is relatively constant throughout the joint range of motion. However, in other joints (for example the knee) the COR moves in space as the knee joint flexes and extends because the articulating surfaces are not perfect circles. In the case of the knee, it is not appropriate to discuss a single COR—rather we must speak of a COR corresponding to a particular joint angle, or, using the terminology of joint kinematics, we must speak of the instant center of rotation (ICR), that is, the COR at any “instant” in time or space. Thus, the moment arm is defined as the perpendicular distance from the line of force application to the axis of rotation.

Arthrokinematic Motion

The motions occurring at the joint surfaces are termed arthrokinematic movements. At each synovial articulation, the articulating surface of each bone moves relative to the shape of the other articulating surface. A normal joint has an available range of active, or physiologic, motion, which is limited by a physiologic barrier as tension develops within the surrounding tissues, such as the joint capsule, ligaments, and CT. At the physiologic barrier, there is an additional amount of passive, or accessory, range of motion. The small motion, which is available at the joint surfaces, is referred to as accessory motion, or joint-play motion. This motion can only occur when resistance to active motion is applied, or when the patient's muscles are completely relaxed.88

Beyond the available passive range of motion, the anatomic barrier is found. This barrier cannot be exceeded without disruption to the integrity of the joint.

Both the physiologic (osteokinematic) and accessory (arthrokinematic) motions occur simultaneously during movement and are directly proportional to each other, with a small increment of accessory motion resulting in a larger increment of osteokinematic motion. Normal arthrokinematic motions must occur for full-range physiologic motion to take place. Mennell89,90 introduced the concept that full, painless, active range of motion is not possible without these motions and that a restriction of arthrokinematic motion results in a decrease in osteokinematic motion. Three fundamental types of movement exist between joint surfaces (Fig. 1-8):91

• Roll. A roll occurs when the points of contact on each joint surface are constantly changing (see Fig. 1-8). This type of movement is analogous to a tire on a car as the car rolls forward. The term rock is often used to describe small rolling motions.
• Slide. A slide is a pure translation. It occurs if only one point on the moving surface makes contact with varying points on the opposing surface (see Fig. 1-8). This type of movement is analogous to a car tire skidding when the brakes are applied suddenly on a wet road. This type of motion also is referred to as translatory or accessory motion. Although the roll of a joint always occurs in the same direction as the swing of a bone, the direction of the slide is determined by the shape of the articulating surface (Fig. 1-9). This rule is often referred to as the concave–convex rule: If the joint surface is convex relative to the other surface, the slide occurs in the opposite direction to the osteokinematic motion (see Fig. 1-9). If, on the other hand, the joint surface is concave, the slide occurs in the same direction as the osteokinematic motion. The clinical significance of the concave–convex rule is described in Chapter 10.
• Spin. A spin is defined as any movement in which the bone moves but the mechanical axis remains stationary. A spin involves a rotation of one surface on an opposing surface around a longitudinal axis (see Fig. 1-8). This type of motion is analogous to the pirouette performed in ballet. Spin motions in the body include internal and external rotation of the glenohumeral joint when the humerus is abducted to 90 degrees; and at the radial head during forearm pronation and supination.

Most anatomic joints demonstrate composite motions involving a roll, slide, and spin.

Osteokinematic and arthrokinematic motions are directly proportional to each other, and one cannot occur completely without the other. It therefore follows that if a joint is not functioning correctly, one or both of these motions may be at fault. When examining a patient with movement impairment, it is critical that the clinician determine whether the osteokinematic motion or the arthrokinematic motion is restricted so that the intervention can be made as specific as possible.

In the extremities, osteokinematic motion is controlled by the amount of flexibility of the surrounding soft tissues of the joint, where flexibility is defined as the amount of internal resistance to motion. In contrast, the arthrokinematic motion is controlled by the integrity of the joint surfaces and the supporting tissues of the joint. This characteristic can be noted clinically in a chronic rupture of the anterior cruciate ligament of the knee. Upon examination of that knee, the arthrokinematic motion (joint slide or glide) is found to be increased, illustrated by a positive Lachman's test, but the range of motion of the knee, its osteokinematic motion, is not affected (see Chapter 20).

In the spine, the osteokinematic motion is controlled by both the flexibility of the surrounding soft tissues and by the integrity of the joint surfaces and the supporting tissues of the joint. This characteristic can be noted clinically when examining the craniovertebral joint, where a restriction in the arthrokinematic motion (joint slide or glide) can be caused by either a joint restriction or an adaptively shortened suboccipital muscle (see Chapter 23).

The examination of these motions and their clinical implications are described in Chapter 4.

Biomechanical levers can be defined as rotations of a rigid surface about an axis. For simplicity sake, levers are usually described using a straight bar which is the lever, and the fulcrum, which is the point on which the bar is resting. The effort force attempts to cause movement of the load. That part of the lever between the fulcrum and the load is load arm. There are three types of levers:

• First-class: occurs when two forces are applied on either side of an axis and the fulcrum lies between the effort and the load (Fig. 1-10), like a seesaw. Examples in the human body include the contraction of the triceps at the elbow joint, or tipping of the head forwards and backwards.
• Second-class: occurs when the load (resistance) is applied between the fulcrum and the point where the effort is exerted (Fig. 1-10). This has the advantage of magnifying the effects of the effort so that it takes less force to move the resistance. Examples of 2nd class levers in everyday life include, the nutcracker, and the wheelbarrow—with the wheel acting as the fulcrum. Examples of second class levers in the human body include weight bearing plantarflexion (rising up on the toes) (Fig. 1-10). Another would be an isolated contraction of the brachioradialis to flex the elbow, which could not occur without the other elbow flexors being paralyzed.
• Third class: occurs when the load is located at the end of the lever (Fig. 1-10) and the effort lies between the fulcrum and the load (resistance), like a drawbridge or a crane. The effort is exerted between the load and the fulcrum. The effort expended is greater than the load, but the load is moved a greater distance. Most movable joints in the human body function as third-class levers—flexion at the elbow.

When a machine puts out more force than is put in, the machine is said to have mechanical advantage (MA). The MA of the musculoskeletal lever is defined as the ratio of the internal moment arm to the external moment arm. Depending on the location of the axis of rotation, the first-class lever can have an MA equal to, less than, or greater than 183 Second-class levers always have an MA greater than 1. Third-class levers always have an MA less than 1. The majority of muscles throughout the musculoskeletal system function with an MA of much less than 1. Therefore, the muscles and underlying joints must ‘pay the price’ by generating and dispersing relative large forces, respectively, even for seemingly low-load activities.83

When a body moves, it will do so in accordance with its kinematics, which in the human body takes place through arthrokinematic and osteokinematic movements. The expression kinematic chain is used in rehabilitation to describe the function or activity of an extremity or trunk in terms of a series of linked chains. A kinematic chain refers to a series of articulated segmented links, such as the connected pelvis, thigh, leg, and foot of the lower extremity.83 According to kinematic chain theory, each of the joint segments of the body involved in a particular movement constitutes a link along the kinematic chain. Because each motion of a joint is often a function of other joint motions, the efficiency of an activity can be dependent on how well these chain-links work together.92

Two types of kinematic chain systems are recognized: closed kinematic chain (CKC) systems, and the open kinematic chain (OKC) system (Table 1-5).93

Closed Kinematic Chain

A variety of definitions for a CKC activity have been proposed:

1. Palmitier94 defined an activity as closed if both ends of the kinetic chain are connected to an immovable framework, thus preventing translation of either the proximal, or distal joint center, and creating a situation whereby movement at one joint produces a predictable movement at all other joints.

2. Gray95 considered a closed-chain activity to involve fixation of the distal segment so that joint motion takes place in multiple planes, and the limb is supporting weight.

3. Dillman96 described the characteristics of closed-chain activities to include relatively small joint movements, low joint accelerations, greater joint compressive forces, greater joint congruity, decreased shear, stimulation of joint proprioception, and enhanced dynamic stabilization through muscle coactivation.97

4. Kibler97 defines a closed-chain activity as a sequential combination of joint motions that have the following characteristics:

•     a. The distal segment of the kinetic chain meets considerable resistance.
•     b. The movement of the individual joints, and translation of their instant centers of rotation, occurs in a predictable manner that is secondary to the distribution of forces from each end of the chain.

Examples of closed kinematic chain exercises (CKCEs) involving the lower extremities include the squat and the leg press. The activities of walking, running, jumping, climbing, and rising from the floor all incorporate closed kinetic chain components. An example of a CKCE for the upper extremities is the push-up, or when using the arms to rise out of a chair.

Open Kinematic Chain

It is generally accepted that the difference between OKC and CKC activities is determined by the movement of the end segment. The traditional definition for an open-chain activity included all activities that involved the end segment of an extremity moving freely through space, resulting in isolated movement of a joint.

Examples of an open-chain activity include lifting a drinking glass and kicking a soccer ball. Open kinematic chain exercises (OKCEs) involving the lower extremity include the seated knee extension and prone knee flexion. Upper extremity examples of OKCE include the biceps curl and the military press.

Many activities, such as swimming and cycling, traditionally viewed as OKC activities, include a load on the end segment; yet the end segment is not “fixed” and restricted from movement. This ambiguity of definitions for CKC and OKC activities has allowed some activities to be classified in opposing categories.96 Thus, there has been a growing need for clarification of OKC and CKC terminology, especially when related to functional activities.

The work of Dillman and colleagues96 and then Lephart and Henry98 has attempted to address the confusion. Dillman and colleagues96 proposed three classifications of activity because of the gray area between CKC and OKC activity. These classifications were based on the boundary condition, either moveable or fixed, and the presence or absence of a load on the end segment. An activity with a fixed boundary and no load does not exist, resulting in three classifications:

1. Moveable no load. These activities involve a moveable end with no load and closely resemble the extreme open-chain activity. An example of this type of activity is hitting a ball with a tennis racket.

2. Moveable external load.These activities involve a movable end with an external load and include a combination of open- and closed-chain actions, because they are characterized by cocontractions of the muscles around the joints. An example of this type of activity is the military press.

3. Fixed external load. These activities involve a fixed end with an external load, and closely resemble the extreme closed-chain activity. An example of this type of activity is the push-up.

Lephart and Henry suggested that a further definition could be made by analyzing the following characteristics of an activity:

• Direction of force.
• Magnitude of load.
• Muscle action.
• Joint motion.
• Neuromuscular function.

Under Lephart and Henry's classification, activities could be subdivided into four groups:

1. Activities that involve a fixed boundary with an external and axial load. An example of this type of activity is the use of a slide board.

2. Activities that involve a movable boundary with an external and axial load. An example of this type of activity is the bench press.

3. Activities that involve a movable boundary with an external and rotary load. An example of this type of activity is a resisted proprioceptive neuromuscular facilitation (PNF) motion pattern (see Chapter 10).

4. Activities that involve a moveable boundary with no load. An example of this type of activity is position training

Although both the Dillman and Lephart and Henry models appear to be describing the same concept, the Lephart and Henry model is distinct in that it incorporates diagonal or rotary components to the movements. These diagonal and rotary movements feature in the vast majority of functional activities.

Joint movements usually are accompanied by a relative compression (approximation) or distraction (separation) of the opposing joint surfaces. These relative compressions or distractions affect the level of congruity of the opposing surfaces. The position of maximum congruity of the opposing joint surfaces is termed the close-packed position of the joint. The position of least congruity is termed the open-packed position. Thus, movements toward the close-packed position of a joint involve an element of compression, whereas movements out of this position involve an element of distraction.

Close-Packed Position

The close-packed position of a joint is the joint position that results in:

• Maximal tautness of the major ligaments.
• Maximal surface congruity.
• Minimal joint volume.
• Maximal stability of the joint.

Once the close-packed position is achieved, no further motion in that direction is possible. This is the often-cited reason why most fractures and dislocations occur when an external force is applied to a joint that is in its close-packed position. In addition, many of the traumatic injuries of the upper extremities result from falling on a shoulder, elbow or wrist, which are in their close-packed position. This type of injury, a fall on an outstretched hand is often referred to as a FOOSH injury. The close-packed positions for the various joints are depicted in Table 1-6.

Open-Packed Position

In essence, any position of the joint, other than the close-packed position, could be considered as an open-packed position. The open-packed position, also referred to as the loose-packed position of a joint, is the joint position that results in:

• Slackening of the major ligaments of the joint.
• Minimal surface congruity.
• Minimal joint surface contact.
• Maximal joint volume.
• Minimal stability of the joint.

The open-packed position permits maximal distraction of the joint surfaces. Because the open-packed position causes the brunt of any external force to be borne by the joint capsule or surrounding ligaments, most capsular or ligamentous sprains occur when a joint is in its open-packed position. The open-packed positions for the various joints are depicted in Table 1-7.