Biomechanics of Skeletal Muscle - 거의 알지만 언젠가 꼭 full text 번역해야

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Biomechanics of Skeletal Muscle.pdf

Skeletal muscle is a fascinating biological tissue able to transform chemical energy to mechanical energy. The focus of this chapter is on the mechanical behavior of skeletal muscle as it contributes to function and dysfunction of the musculoskeletal system. Although a basic understanding of the energy transformation from chemical to mechanical energy is essential to a full understanding of the behavior of muscle, it is beyond the scope of this book. The reader is urged to consult other sources for a discussion of the chemical and physiological interactions that produce and affect a muscle contraction [41,52,86].

Skeletal muscle has three basic performance parameters that describe its function:

1. Movement production

2.  Force production

3.
Endurance

The production of movement and force is the mechanical outcome of skeletal muscle contraction. The factors that influence these parameters are the focus of this chapter. A brief description of the morphology of muscles and the physiological processes that produce contraction needed to understand these mechanical parameters are also presented here. Specifically the purposes of this chapter are to

# Review briefly the structure of muscle and the mechanism of skeletal muscle contraction

# Examine the factors that influence a muscle’s ability to produce a motion

# Examine the factors that influence a muscle’s ability to produce force

# Consider how muscle architecture is specialized to optimize a muscle’s ability to produce force or joint motion

#
Demonstrate how an understanding of these factors can be used clinically to optimize a person’s performance

#
Discuss the adaptations that muscle undergoes with prolonged changes in length and activity

STRUCTURE OF SKELETAL MUSCLE 

The functional unit that produces motion at a joint consists of two discrete units, the muscle belly and the tendon that

binds the muscle belly to the bone. The structure of the muscle belly itself is presented in the current chapter. The structure and mechanical properties of the tendon, composed of connective tissue, are presented in Chapter 6. The muscle   belly consists of the muscle cells, or fibers, that produce the contraction and the connective tissue encasing the muscle

fibers. Each is discussed below.

Structure of an Individual Muscle Fiber

 A skeletal muscle fiber is a long cylindrical, multinucleated cell that is filled with smaller units of filaments (Fig. 4.1 ). These  filamentous structures are roughly aligned parallel to the muscle fiber itself. The largest of the filaments is the myofibril, composed of subunits called sarcomeres  that are arranged end to end the length of the myofibril. Each sarcomere also contains filaments, known as myofilaments.  There are two types of myofilaments within each sarcomere. The thicker myofilaments are composed of myosin protein molecules, and the thinner myofilaments are composed of molecules of the protein actin. Sliding of the actin myofilament on the myosin chain is the basic mechanism of muscle contraction.

THE SLIDING FILAMENT THEORY OF MUSCLE CONTRACTION

 The sarcomere, containing the contractile proteins actin and myosin, is the basic functional unit of muscle. Contraction of a whole muscle is actually the sum of singular contraction events occurring within the individual sarcomeres. Therefore, it is necessary to understand the organization of the sarcomere. The thinner actin chains are more abundant than the myosin myofilaments in a sarcomere. The actin myofilaments are anchored at both ends of the sarcomere at the Z-line and project into the interior of the sarcomere where they surround a thicker myosin myofilament (Fig. 4.2 ). This arrangement of myosin myofilaments surrounded by actin myofilaments is repeated throughout the sarcomere, filling its interior and giving the muscle fiber its characteristic striations. The amount of these contractile proteins within the cells is strongly related to a muscle’s contractile force [6,7,27].

Contraction results from the formation of cross-bridges between the myosin and actin myofilaments, causing the actin chains to “slide” on the myosin chain (Fig. 4.3 ). The tension of the contraction depends upon the number of cross-bridges formed between the actin and myosin myofilaments. The number of cross-bridges formed depends not only on the abundance of the actin and myosin molecules, but also on the frequency of the stimulus to form cross-bridges. Contraction is initiated by an electrical stimulus from the associated motor neuron causing depolarization of the muscle

fiber. When the fiber is depolarized, calcium is released into the cell and binds with the regulating protein troponin. The combination of calcium with troponin acts as a trigger, causing actin to bind with myosin, beginning the contraction.

Cessation of the nerve’s stimulus causes a reduction in calcium levels within the muscle fiber, inhibiting the cross bridges between actin and myosin. The muscle relaxes [86]. If  frequency, new cross-bridges are formed before prior interactions are completely severed, causing a fusion of succeeding contractions. Ultimately a sustained, or tetanic, contraction is produced. Modulation of the frequency and magnitude of the initial stimulus has an effect on the force of

contraction of a whole muscle and is discussed later in this chapter.

The Connective Tissue System within the Muscle Belly

 The muscle belly consists of the muscle cells, or fibers, and the connective tissue that binds the cells together (Fig. 4.4 ). The outermost layer of connective tissue that surrounds the entire muscle belly is known as the epimysium.  The muscle belly is divided into smaller bundles or fascicles by additional connective tissue known as perimysium . Finally individual fibers within these larger sheaths are surrounded by more connective tissue, the endomysium.  Thus the entire muscle belly is invested in a large network of connective tissue that then is bound to the connective tissue tendons at either end of the muscle. The amount of connective tissue within a muscle and the size of the connecting tendons vary widely from muscle to  muscle. The amount of connective tissue found within an individual muscle influences the mechanical properties of that muscle and helps explain the varied mechanical responses of individual muscles. The contribution of the connective tissue to a muscle’s behavior is discussed later in this chapter.

FACTORS THAT INFLUENCE A MUSCLE’S ABILITY TO PRODUCE A MOTION

An essential function of muscle is to produce joint movement. The passive range of motion (ROM) available at a joint

depends on the shape of the articular surfaces as well as on the surrounding soft tissues. However the joint’s active ROM depends on a muscle’s ability to pull the limb through a joint’s available ROM. Under normal conditions, active ROM is approximately equal to a joint’s passive ROM. However there is a wide variation in the amount of passive motion available at joints throughout the body. The knee joint is capable of flexing through an arc of approximately 140 , but the metacarpophalangeal (MCP) joint of the thumb usually is capable of no more than about 90  of flexion. Joints that exhibit large ROMs require muscles capable of moving the joint through the entire range. However such muscles are unnecessary at joints with smaller excursions. Thus muscles exhibit structural specializations that influence the magnitude of the excursion that is produced by a contraction. These specializations are 

• The length of the fibers composing the muscle

• The length of the muscle’s moment arm.

How each of these characteristics affects active motion of a joint is discussed below.

Effect of Fiber Length on Joint Excursion

Fiber length has a significant influence on the magnitude of the joint motion that results from a muscle contraction. The fundamental behavior of muscle is shortening, and it is this shortening that produces joint motion. The myofilaments in each sarcomere are 1 to 2 μm long; the myosin myofilaments are longer than the actin myofilaments [125,149].

Thus sarcomeres in humans are a few micrometers in length, varying from approximately 1.25 to 4.5 μm with muscle

contraction and stretch [90–92,143]. Each sarcomere can shorten to approximately the length of its myosin molecules.

Because the sarcomeres are arranged in series in a myofibril, the amount of shortening that a myofibril and, ultimately, a muscle fiber can produce is the sum of the shortening in all of the sarcomeres. Thus the total shortening of a muscle fiber depends upon the number of sarcomeres arranged in series within each myofibril. The more sarcomeres in a fiber, the longer the fiber is and the more it is able to shorten (Fig. 4.5). The amount a muscle fiber can shorten is proportional to its length [15,89,155]. A fiber can shorten roughly 50 to 60% of its length [44,155], although  there is some evidence that fibers exhibit varied shortening capabilities [15].

The absolute amount of shortening a fiber undergoes is a function of its fiber length. Similarly, the amount a whole

muscle can shorten is dictated by the length of its constituent fibers. An individual whole muscle is composed mostly of fibers of similar lengths [15]. However there is a wide variation in fiber lengths found in the human body, ranging from a few centimeters to approximately half a meter [86,146]. The length of the fibers within a muscle is a function of the architecture of that muscle rather than of the muscle’s total length. The following describes how fiber length and muscle architecture are related.

ARCHITECTURE OF SKELETAL MUSCLE

 Although all skeletal muscle is composed of muscle fibers, the arrangement of those fibers can vary significantly among muscles. This fiber arrangement has marked effects on a muscle’s ability to produce movement and to generate force. Fiber arrangements have different names but fall into two major categories, parallel  and pennate  [42] (Fig. 4.6 ). In general, the fibers within a parallel fiber muscle are approximately parallel to the length of the whole muscle. These muscles can be classified as either fusiform  or strap  muscles. Fusiform muscles have tendons at both ends of the muscle so that the muscle fibers taper to insert into the tendons. Strap muscles have less prominent tendons, and therefore their

fibers taper less at both ends of the whole muscle. Parallel

fiber muscles are composed of relatively long fibers, although

these fibers still are shorter than the whole muscle. Even the

sartorius muscle, a classic strap muscle, contains fibers that

are only about 90% of its total length.

In contrast, a pennate muscle has one or more tendons

that extend most of the length of the whole muscle.

Fibers run obliquely to insert into these tendons. Pennate

muscles fall into subcategories according to the number of

tendons penetrating the muscle. There are unipennate,

bipennate,  and multipennate  muscles. A comparison of

two muscles of similar total length, one with parallel fibers

and the other with a pennate arrangement, helps to illustrate

the effect of fiber arrangement on fiber length (Fig. 4.7 ). The

muscle with parallel fibers has longer fibers than those found

in the pennate muscle. Because the amount of shortening

that a muscle can undergo depends on the length of its fibers,

the muscle with parallel fibers is able to shorten more than

the pennate muscle. If fiber length alone affected joint excursion,

the muscle with parallel fibers would produce a larger

joint excursion than the muscle composed of pennate fibers  [90]. However, a muscle’s ability to move a limb through an

excursion also depends on the length of the muscle’s moment

arm. Its effect is described below.

Effect of Muscle Moment Arms on Joint Excursion

 Chapter 1 defines the moment arm of a muscle as the perpendicular

distance between the muscle and the point of

rotation. This moment arm depends on the location of the

muscle’s attachment on the bone and on the angle between

the line of pull of the muscle and the limb to which the muscle

attaches. This angle is known as the angle of application

 (Fig. 4.8 ). The location of an individual muscle’s attachment

on the bone is relatively constant across the population.

Therefore, the distance along the bone between the muscle’s

attachment and the center of rotation of the joint can be

estimated roughly by anyone with a knowledge of anatomy

and can be measured precisely as well [57,81,95,151]. This