The Muscular System Together, almost all of the 700 voluntarily controlled muscles of your body compose the muscular system The function of most muscles is to produce movements of body parts

Muscles Create Pulling Forces The essence of muscle function is that muscles create pulling forces. When a muscle contracts, it attempts to pull in toward its center. This results in a pulling force being placed on its attachments.

Muscles Create Pulling Forces This pulling force is equal on both of its attachments. A muscle then pulls on one or the other of its attachments This is guided by the nervous system

Origin and Insertion When a skeletal muscle contracts, it pulls one of the articulating bones toward the other The two articulating bones usually do not move equally in response to the contraction One bone will remain stationary, or near its original position either due to stabilization, or because its structure makes it less moveable

Origin and Insertion The attachment to the more stationary bone is commonly referred to as the origin The attachment to the muscle’s other tendon to the more moveable bone is commonly referred to as the insertion

Lever Systems In producing movement, bones act as levers and the joints function as the fulcrums of these levers A lever is a rigid structure that can move around a fixed point called a fulcrum

Lever Systems A lever is acted on at 2 different points by 2 different forces: the effort (E) which causes the movement and the load (L) or resistance which opposes the movement The effort is the force exerted by the muscular contraction The load is typically the weight of the body part or resistance that the moving part is trying to overcome Motion occurs when the effort applied to the bone at its insertion exceeds the load

Lever Systems The relative distance between the fulcrum and the load and the point at which the effort is applied determine whether a given lever operates at a mechanical advantage or a mechanical disadvantage

Lever Systems Levers are categorized into 3 types according to the position of the fulcrum, the effort and the load: First class levers Second class levers Third class levers

Lever Systems First Class Levers the fulcrum is between the effort and the load Ex. See-saw

Lever Systems Second Class Levers The load is between the fulcrum and the effort Ex. Wheelbarrow They always produce a mechanical advantage because the load is always closer to the fulcrum than the effort This type of lever produces the most force

Lever Systems Third Class Levers The effort is between the fulcrum and the load Ex. a pair of forceps The most common levers in the body Always produce a mechanical disadvantage because the effort is always closer to the fulcrum than the load

Skeletal Muscle Architecture The skeletal muscle fibers (cells) within a muscle are arranged in bundles known as fascicles Within a fascicle, all muscle fibers are parallel to one another

Skeletal Muscle Architecture The fascicles, however, may form 1 of 5 patterns Parallel Fusiform Circular Triangular Pennate Fascicular arrangement affects a muscle’s power and range of motion

Skeletal Muscle Architecture Arrangement of Fascicles Circular Fascicles arranged in concentric rings (e.g., orbicularis oris) Triangular (Convergent) Fascicles converge toward a single tendon insertion (e.g., pectoralis major)

Skeletal Muscle Architecture Arrangement of Fascicles Parallel Fascicles parallel to the long axis of a straplike muscle (e.g., sartorius) Fusiform Spindle-shaped muscles with parallel fibers (e.g., biceps brachii) Pennate Short fascicles attach obliquely to a central tendon running the length of the muscle (e.g., rectus femoris)

Figure 10.1 (a) (b) (e) (d) (g) (f) (c) Circular (orbicularis oris) (b) Convergent (pectoralis major) (c) Parallel (sartorius) (d) Unipennate (extensor digitorum longus) (f) Fusiform (biceps brachii) (g) Multipennate (deltoid) (e) Bipennate (rectus femoris)

Skeletal Muscle Architecture As a muscle fiber contracts, it shortens to about 70% of its resting length, therefore, the longer the fibers in a muscle, the greater the range of motion it can produce

Skeletal Muscle Architecture The power of a muscle depends not on length, but on its total cross sectional area, because a short muscle can contract as forcefully as a long one. So… the more fibers per unit of cross- sectional area a muscle has, the more power it can produce

Skeletal Muscle Architecture Muscles can be named because of: Location—bone or body region associated with the muscle Shape—e.g., deltoid muscle (deltoid = triangle) Relative size—e.g., maximus (largest), minimus (smallest), longus (long) Direction of fibers or fascicles—e.g., rectus (fibers run straight), transversus, and oblique (fibers run at angles to an imaginary defined axis) Number of origins—e.g., biceps (2 origins) and triceps (3 origins) Location of attachments—named according to point of origin or insertion Action—e.g., flexor or extensor, muscles that flex or extend, respectively

“Contraction” When it comes to the study of muscle function, the operative word is contract because that is what muscles do. Doesn’t necessarily shorten So what is a muscle contraction? When a muscle attempts to shorten

Muscle Contraction For a muscle to shorten, it must move one or both of its attachments. Therefore the resistance to shortening is usually the weight of the body parts to which the muscle is attached.

Brachialis Contraction For the brachialis to contract and shorten, it must move the forearm toward the arm and/or move the arm toward the forearm.

Brachialis Contraction However, even if the brachialis contracts with insufficient strength to shorten, it is important to understand that it is still exerting a pulling force on its attachments. This pulling force can play an important role in musculoskeletal function. It is the pulling force of a muscle that defines it as contracting—not whether it is shortening or not.

Reverse Actions When the proximal attachment moves toward the distal one instead of the distal one moving toward the proximal one, it is called a reverse action.

Concentric Contraction A shortening contraction of a muscle is called a concentric contraction. The word “concentric” literally means “with center.” In other words, when a concentric contraction occurs, the muscle moves toward its center.

Concentric Contraction Agonist When a muscle contracts and generates sufficient force to move one or both of its attachments, it is the mover of the joint action that is occurring and called the mover or agonist. By definition, when a mover muscle contracts, it contracts concentrically.

Eccentric Contractions If a muscle contracts, but the resistance force is greater than the muscle’s contraction force, not only will the muscle not succeed in shortening, the muscle’s attachment will actually be pulled farther away from the center of the muscle. This will result in a lengthening of the muscle as it contracts.

Eccentric Contractions Antagonist Because the muscle that eccentrically contracts opposes the joint action movement that is occurring, it is called the antagonist. By definition, when an antagonist muscle contracts, it contracts eccentrically.

Contracted Versus Relaxed By definition, when a mover muscle (Agonist) contracts and shortens, it contracts concentrically. When an antagonist muscle contracts and lengthens, it contracts eccentrically. A muscle can be relaxed as it shortens or lengthens

Skeletal Muscle Function 1. Prime movers (Agonist) Provide the major force for producing a specific movement 2. Antagonists Oppose or reverse a particular movement

Skeletal Muscle Function 3. Synergists Add force to a movement Reduce undesirable or unnecessary movement 4. Fixators Synergists that immobilize a bone or muscle’s origin

Isometric Contractions An isometric contraction occurs when the force of the muscle’s contraction is equal to the resistance force. Because the two forces are equal, the muscle is neither able to win and shorten nor lose and lengthen; instead, it stays the same length as it contracts. This is defined as an isometric contraction.

Isometric Contractions Deltoid isometrically contracting

Isometric Contractions Stabilization muscle If the muscle stays the same length, its attachments do not move. Function: fixes, or in other words stabilizes, its bony attachment.

Roles of Muscles Muscles can Contract concentrically and shorten as movers, Contract eccentrically and lengthen as antagonists, Or contract isometrically and stay the same length as stabilizers.

Naming a Muscle’s Attachments Origin/Insertion The classic method to name a muscle’s attachments is to describe one attachment as the origin and the other as the insertion. Although the exact definitions have varied, the origin is usually defined as the more fixed attachment and the insertion as the more mobile attachment.

Naming a Muscle’s Attachments Problems with origin/insertion terminology In recent years, use of the terms origin and insertion has been decreasing in favor. Why? It tends to promote the idea that the proximal attachment is always fixed. This limits a true understanding of how the muscular system works. It adds to the stress of students who are first learning muscles by adding an unneeded designation to the attachments.

Naming a Muscle’s Attachments There is alternative and simpler terminology to name attachments For these reasons, naming a muscle’s attachments by simply describing their location is gaining favor.

Learning Muscles Three major aspects must be learned: The attachments of the muscle The actions of the muscle The nerve innervation to the muscle

Learning Muscle Attachments Generally speaking, the attachments of a muscle must be memorized. However, times exist when clues are given about the attachments of a muscle by the muscle’s name. Ex. Coracobrachialis

Learning Muscle Actions Unlike muscle attachments, muscle actions do not have to be memorized. Instead, by understanding the simple concept that a muscle pulls at its attachments to move a body part, the action or actions of a muscle can be reasoned out.

Five-Step Approach to Learning Muscles Step 1 Look at the name of the muscle to see if it gives you any “free information” that saves you from having to memorize attachments or actions of the muscle.

Five-Step Approach to Learning Muscles Step 2 Learn the general location of the muscle well enough to be able to visualize the muscle on your body.

Five-Step Approach to Learning Muscles Step 3 Use this general knowledge of the muscle’s location (step 2) to figure out the actions of the muscle. Look at the direction of the muscle fibers relative to the joint that it crosses By doing this, you can see the line of pull of the muscle relative to the joint. The best approach to see the line of pull is to ask the following three questions: What joint does the muscle cross? Where does the muscle cross the joint? How does the muscle cross the joint?

Five-Step Approach to Learning Muscles Question 1: What joint does the muscle cross? The following rule applies: If a muscle crosses a joint, it can have an action at that joint. Question 2: Where does the muscle cross the joint? Anteriorly, posteriorly, medially, or laterally? The following general rules apply: Anteriorly = usually flexes a body part at that joint Posteriorly = usually extends a body part at that joint Laterally = usually abducts or laterally flexes a body part at that joint Medially = usually adducts a body part at that joint. Question 3: How does the muscle cross the joint? Whether it crosses the joint with its fibers running vertically or horizontally

Five-Step Approach to Learning Muscles With a muscle that has a horizontal direction to its fibers, another factor must be considered when looking at how this muscle crosses the joint. Muscles that run horizontally (in the transverse plane) and wrap around the bone before attaching to it create a rotation action when they contract and pull on the attachment.

Five-Step Approach to Learning Muscles Step 4 Go back and learn (memorize, if necessary) the specific attachments of the muscle.

Five-Step Approach to Learning Muscles Step 5 Look at the relationship of this muscle to other muscles (and other soft tissue structures) of the body. Ex. Is this muscle superficial or deep? What other muscles (and other soft tissue structures) are located near this muscle?

Functional Group Approach to Learning Muscles Once the five-step approach to learning muscles has been used a few times and learned, it is extremely helpful to begin to transition to the functional group approach to learning muscles. What is a functional group of muscles? A group of muscles that all share the same function, in other words, have a common joint action.

Functional Group Approach to Learning Muscles Elbow joint flexors functional group For example, if the biceps brachii has been studied and it is seen that it crosses the elbow joint anteriorly and flexes it, then it is easier to see and learn that the brachialis also flexes the elbow joint because it also crosses it anteriorly. In fact, all muscles that cross the elbow joint anteriorly belong to the functional group of elbow joint flexors.

Functional Group Approach to Learning Muscles Elbow joint extensors functional group Similarly, all muscles that cross the elbow joint posteriorly belong to the functional group of elbow joint extensors.

Shoulder Joint Functional Groups Applying the functional group approach to the shoulder joint, it is seen that: All muscles that cross it anteriorly with a vertical fiber direction (or at least a vertical component to their fiber direction) flex it. All muscles that cross it posteriorly with a vertical fiber direction extend it.

Shoulder Joint Functional Groups (…cont’d.) All muscles that cross it laterally, abduct it; and all muscles that cross it medially adduct it. Functional groups of medial and lateral rotators are not as segregated location-wise, but with closer inspection, it is seen that all medial rotators of the shoulder joint wrap in the same direction, and all lateral rotators wrap in the other direction.