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Muscle Attachments to Bone

This is an excerpt from Dance Anatomy and Kinesiology 2nd Edition With Web Resource by Karen Clippinger.

Connective tissue is intimately related to muscle tissue in that it provides structural support and serves as points of attachment to the respective bones. As shown in figure 2.4, there are connective tissue coverings of individual muscle fibers (endomysium), bundles of muscle fibers or fascicles (perimysium), and the whole muscle itself (epimysium). The central part of a muscle, which tends to be thicker and in which the contractile cells predominate, is called the muscle belly. Toward the ends of the muscle belly, the muscle cells end; but all these connective tissue coverings continue to attach the muscle to one or more bones: in a direct manner (e.g., trapezius, figure 2.5A), via a cord-like or flat band called a tendon (e.g., biceps brachii, figure 2.5B), or via a sheet-like structure of fibrous tissue called an aponeurosis (e.g., latissimus dorsi, figure 2.5C). Tendons are the most common form of attachment and serve to concentrate the pull of the muscle to a small area on the bone. In essence, these connective tissue attachments allow the tension created by the contractile component of the muscle to be transmitted to the associated bones so that joint movement can occur.

Structure of skeletal muscle and related connective tissue.
Structure of skeletal muscle and related connective tissue.

Attachments of muscles onto bones (A) directly, or indirectly through a (B) tendon or (C) aponeurosis.
Attachments of muscles onto bones (A) directly, or indirectly through a (B) tendon or (C) aponeurosis.

Proximal and Distal Attachments

In most cases this text uses the terms proximal attachment and distal attachment to refer to the specific sites of these connective tissue attachments of muscles onto bone(s) in order to reflect the concept that when a muscle contracts, either end can move, depending on the goal and conditions of the movement. For example, when flexing the elbow as you lift a weight (biceps curl; see table 2.2), it is the distal attachment of the biceps brachii on the forearm that moves, while in a pull-up it is the proximal attachments and segment that move. In many movements of the limbs, open versus closed kinematic chain terminology explained in chapter 1 is used to reflect which attachment is moving. For movements for which kinematic chain terminology does not work well, the text provides clarification by identifying which bone or body segment is moving. The terminology of customary and reverse muscle actions can also be used to describe whether the distal (customary) or proximal (reverse) segment is moving. For example, the thigh moving on the trunk (customary action of hip flexors) is shown in figure 2.6A versus the trunk moving on the thigh (reverse muscle action of hip flexors) in figure 2.6B.

Raising the thigh on the trunk

Raising the trunk on the thigh

Customary and reverse muscle actions of the hip flexors. (A) Raising the thigh on the trunk, (B) raising the trunk on the thigh.
Dancer: Merett Miller as a dancer with Sacramento Ballet

When learning the actions of muscles, it is easier to first picture movements that involve the distal segment moving. Hence, examples of movements given in chapters 3 through 7 in "Description and Functions of Individual Muscles" sections emphasize such distal segment movements. However, examples of the proximal segment moving are often included in these chapters in the "Muscular Analysis of Movements" sections.

Line of Pull of a Muscle

An important tool for predicting the action of a given muscle is to picture its line of pull. You can roughly approximate the line of pull of a muscle by drawing an imaginary double-headed arrow with its base at each attachment and pointing toward the center of the muscle. In instances in which a muscle has a broad attachment, you would draw the arrow to approximately bisect the broad attachment as seen in figure 2.7. Then, picture what movement of the interposed joint would occur if one attachment moved toward the other or if the two attachments moved toward each other. For example, if you imagined the distal attachment of the gluteus medius shown in figure 2.7 moving toward the proximal attachment, hip abduction would be occurring. Readers are encouraged to approach learning muscle actions in this manner rather than by rote memorization to make the learning process easier, to provide a logic self-check procedure, and to promote retention. To facilitate use of this process, summary figures showing the approximate line of pull of key muscles at their respective joints are provided in chapters 3 through 7.

Approximate line of pull of the gluteus medius.
Approximate line of pull of the gluteus medius.

In some cases with very broad or multiple proximal or distal attachments, it may be necessary to use multiple arrows, as different portions of the muscle may have different actions due to different lines of pull relative to axes of the joint. In such instances, the muscle is often divided into portions such as the anterior, middle, and posterior deltoid as seen in figures 2.15 and 2.16.

It is also important to realize that the line of pull of a muscle may change its relation to the axis of rotation of a given joint in various ranges of motion, causing the muscle to change its action. For purposes of simplicity, this text primarily emphasizes the classic actions of muscles from anatomical position, but interested readers are encouraged to deepen their understanding of muscles by reading journal articles and texts that address the detailed actions of muscles in different ranges of motion and with different tasks. Some of this work is based on continuing Basmajian's approach (Basmajian and DeLuca, 1985) of studying the electrical activity (electromyography; see Tests and Measurements 2.1) of muscles in different movements.

Mechanical Model of Muscle

The connective tissue associated with muscles not only is vital for attaching muscles to bone but also influences the behavior of muscle as represented in the three-component mechanical model originally developed by A.V. Hill in 1938 and expanded upon by other investigators in more recent years (Oatis, 2009; Winters, 1990). In this model (figure 2.8), the ability of muscle to contract via the actin and myosin myofilaments previously described (sliding filament theory) is represented by the contractile component (CC) or active component of muscle. In contrast, the components that do not require active contraction, also termed the passive components, are represented in this model by two elastic components - the parallel elastic component and the series elastic component. The connective tissue coverings of muscle previously described contribute to the parallel elastic component (PEC), a component that surrounds or lies parallel to the contractile proteins. The muscle tendon contributes to the series elastic component (SEC), a component that lies in line with or in series with the contractile proteins. Various other structures also contribute to these elastic components, including structural proteins within and between muscle fibers that help give form to muscle but are not capable of contraction (Hunter and Brown, 2010).

Three-component mechanical model of muscle.
Three-component mechanical model of muscle.

When stretch is applied to the muscle - tendon complex, it elongates both the parallel and elastic components, and the muscle exhibits viscoelastic properties. The elastic response can be modeled as a spring (figure 2.9): Energy stored in the elastic component of muscle with application of a stretch can be recovered when the stretch is released (recoverable deformation), just as a spring will quickly recoil to its resting position when the tension is removed. These elastic components give rise to muscle's property of elasticity. Viscous or plastic properties are usually modeled by a hydraulic cylinder or dash pot, as shown in figure 2.9, and reflect putty-like behavior, in which the elongation produced by a force remains after the force is removed (permanent deformation). Together, the elastic and viscous properties of connective tissue are termed viscoelastic, and it is this viscoelastic response that gives rise to muscle's property of extensibility. The average muscle fiber can be stretched 1.5 times its resting length (Hamilton and Luttgens, 2002).

Viscoelastic properties of connective tissue.
Viscoelastic properties of connective tissue.

The study of the viscoelastic characteristic of muscle has been instrumental in developing cur
rent recommendations for effective muscle conditioning programs. For example, with stretching exercises, the goal is to achieve flexibility that persists over time, so the emphasis should be on maximizing plastic elongation (Taylor et al., 1990). This can be achieved by the use of a slow, lower-force, longer-duration stretch applied to warmed muscles. More specifically, two to four repetitions of a 10- to 30-second stretch are currently recommended (American College of Sports Medicine, 2011).

Conversely, to emphasize greater force production of a muscle, the goal is to maximize elastic elongation. Application of a rapid, higher-force stretch, immediately preceding a shortening (concentric) contraction of the same muscle, termed the stretch - shortening cycle, has been shown to markedly enhance force production (Horita et al., 2002). Factors in addition to the recoil effect offered by the elastic components of the muscle - tendon complex, including those that relate to neural control of the movement and muscle activation (Bobbert and Casius, 2005; Hirayama at al., 2012; Malisoux et al., 2006), likely also contribute to this enhanced force production. The stretch - shortening cycle is used in walking and running and acts to reduce the energy cost of these locomotor movements (Thelen et al., 2005). Examples of the use of the stretch - shortening cycle in dance-specific movements include the use of a quick demi-plié before a jump or the small counterrotation of the trunk preceding multiple turns.



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