Learning Objectives

Compare and also contrast agonist and antagonist musclesDescribe just how fascicles space arranged within a bones muscleExplain the significant events of a skeletal muscle contraction in ~ a muscle in generating force

To relocate the skeleton, the tension developed by the contraction of the fibers in many skeletal muscles is transferred to the tendons. The tendons are strong bands of dense, continual connective tissue that affix muscles come bones. The bone connection is why this muscle organization is called skeletal muscle.

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Interactions the Skeletal muscle in the Body

To traction on a bone, the is, to adjust the edge at its synovial joint, which basically moves the skeleton, a skeleton muscle must likewise be attached come a fixed part of the skeleton. The moveable finish of the muscle the attaches come the bone being pulled is dubbed the muscle’s insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. Throughout forearm flexion—bending the elbow—the brachioradialis assists the brachialis.

Figure 1. Prime Movers and also Synergists. The biceps brachii flex the lower arm. The brachoradialis, in the forearm, and brachialis, situated deep come the biceps in the top arm, are both synergists that aid in this motion.

Although a variety of muscles might be connected in an action, the primary muscle connected is called the prime mover, or agonist. Come lift a cup, a muscle referred to as the biceps brachii is in reality the element mover; however, because it deserve to be assisted by the brachialis, the brachialis is dubbed a synergist in this action (Figure 1). A synergist can also be a fixator that stabilizes the bone that is the attachment because that the prime mover’s origin.

A muscle with the opposite activity of the element mover is called an antagonist. Antagonists beat two necessary roles in muscle function:

They keep body or body position, such together holding the eight out or stand erectThey control rapid movement, as in shadow boxing there is no landing a punch or the ability to inspect the motion of a limb

For example, to prolong the knee, a team of 4 muscles referred to as the quadriceps femoris in the anterior compartment the the thigh are activated (and would certainly be called the agonists that knee extension). However, come flex the knee joint, an the opposite or antagonistic collection of muscles referred to as the hamstrings is activated.

As you can see, this terms would also be reversed for the the opposite action. If you take into consideration the first action as the knee bending, the hamstrings would be dubbed the agonists and also the quadriceps femoris would certainly then be referred to as the antagonists. See Table 1 for a list of part agonists and also antagonists.

Table 1. Agonist and Antagonist skeleton Muscle PairsAgonistAntagonistMovement
Biceps brachii: in the anterior compartment of the armTriceps brachii: in the posterior compartment of the armThe biceps brachii flexes the forearm, conversely, the triceps brachii expand it.
Hamstrings: team of three muscles in the posterior compartment that the thighQuadriceps femoris: team of four muscles in the anterior compartment that the thighThe hamstrings flex the leg, whereas the quadriceps femoris expand it.
Flexor digitorum superficialis and flexor digitorum profundus: in the anterior compartment the the forearmExtensor digitorum: in the posterior compartment that the forearmThe flexor digitorum superficialis and also flexor digitorum profundus flex the fingers and also the hand in ~ the wrist, vice versa, the extensor digitorum extends the fingers and also the hand at the wrist.

There are also skeletal muscles that carry out not pull versus the skeleton because that movements. For example, there room the muscle that develop facial expressions. The insertions and also origins of face muscles room in the skin, so that certain individual muscles contract to type a laugh or frown, form sounds or words, and also raise the eyebrows. There additionally are skeletal muscle in the tongue, and the outside urinary and anal sphincters that permit for voluntarily regulation the urination and defecation, respectively. In addition, the diaphragm contracts and also relaxes to change the volume of the pleural cavities but it does not relocate the skeleton to perform this.

Everyday Connections: Exercise and Stretching

When exercising, the is important to first warm increase the muscles. Extending pulls on the muscle fibers and it additionally results in an boosted blood circulation to the muscles being worked. Without a suitable warm-up, the is possible that you might either damages some that the muscle fibers or pull a tendon. A traction tendon, nevertheless of location, results in pain, swelling, and diminished function; if it is moderate come severe, the injury could immobilize friend for prolonged period.

Recall the discussion around muscles cross joints to develop movement. Many of the joints girlfriend use throughout exercise are synovial joints, which have actually synovial liquid in the joint an are between 2 bones. Exercise and also stretching may likewise have a useful effect on synovial joints. Synovial liquid is a thin, but viscous film through the consistency of egg whites. Once you very first get up and also start moving, her joints feeling stiff because that a number of reasons. After suitable stretching and also warm-up, the synovial fluid may end up being less viscous, permitting for better joint function.

Patterns the Fascicle Organization

Skeletal muscle is enclosed in connective organization scaffolding at 3 levels. Every muscle fiber (cell) is extended by endomysium and also the entire muscle is covered by epimysium. Once a team of muscle fibers is “bundled” together a unit in ~ the totality muscle by an additional covering that a connective tissue called perimysium, that bundled team of muscle fibers is dubbed a fascicle. Fascicle arrangement by perimysia is associated to the force generated through a muscle; it additionally affects the range of movement of the muscle. Based upon the patterns of fascicle arrangement, skeleton muscles have the right to be classified in numerous ways. What follows are the most typical fascicle arrangements.

Parallel muscles have fascicles that space arranged in the exact same direction together the lengthy axis that the muscle (Figure 2). The majority of skeletal muscles in the body have this form of organization. Part parallel muscles are level sheets that broaden at the ends to make large attachments. Various other parallel muscles space rotund through tendons at one or both ends. Muscles the seem to it is in plump have actually a huge mass the tissue located in the middle of the muscle, between the insertion and also the origin, which is known as the central body. A more common name for this muscle is belly. When a muscle contracts, the contractile yarn shorten it to an even larger bulge. Because that example, extend and then flex her biceps brachii muscle; the large, center section is the ship (Figure 3). When a parallel muscle has actually a central, big belly the is spindle-shaped, an interpretation it tapers as it extends to its origin and insertion, it occasionally is called fusiform.

Figure 2. Muscle Shapes and Fiber Alignment. The skeletal muscles of the body typically come in seven various general shapes.

Figure 3. Biceps Brachii Muscle Contraction. The big mass at the facility of a muscle is called the belly. Tendons arise from both ends of the belly and connect the muscle come the bones, allowing the skeleton come move. The tendons that the bicep affix to the upper arm and also the forearm. (credit: Victoria Garcia)

Circular muscles are also called sphincters (see Figure 2). As soon as they relax, the sphincters’ concentrically arranged bundles of muscle fibers boost the size of the opening, and also when castle contract, the dimension of the opening shrinks come the allude of closure. The orbicularis oris muscle is a circular muscle that goes roughly the mouth. Once it contracts, the dental opening becomes smaller, as once puckering the lips for whistling. Another example is the orbicularis oculi, among which surrounds each eye. Consider, for example, the names of the two orbicularis muscle (orbicularis oris and also oribicularis oculi), where component of the first name the both muscles is the same. The first part that orbicularis, orb (orb = “circular”), is a recommendation to a ring or one structure; the may likewise make one think of orbit, such as the moon’s path around the earth. The word oris (oris = “oral”) describes the oral cavity, or the mouth. The word oculi (ocular = “eye”) refers to the eye.

There are various other muscles transparent the body called by their form or location. The deltoid is a large, triangular-shaped muscle the covers the shoulder. That is so-named because the Greek letter delta looks like a triangle. The rectus abdomis (rector = “straight”) is the right muscle in the anterior wall surface of the abdomen, when the rectus femoris is the right muscle in the anterior compartment that the thigh.

When a muscle has actually a widespread development over a sizable area, but then the fascicles come to a single, typical attachment point, the muscle is called convergent. The attachment point for a convergent muscle might be a tendon, an aponeurosis (a flat, broad tendon), or a raphe (a an extremely slender tendon). The big muscle ~ above the chest, the pectoralis major, is an instance of a convergent muscle due to the fact that it converges ~ above the greater tubercle the the humerus via a tendon. The temporalis muscle of the cranium is another.

Pennate muscles (penna = “feathers”) blend into a tendon that runs v the main region that the muscle because that its totality length, somewhat choose the quill that a feather with the muscle arranged comparable to the feathers. Due to this design, the muscle yarn in a pennate muscle have the right to only pull at one angle, and as a result, contracting pennate muscles perform not move their tendons an extremely far. However, since a pennate muscle typically can hold much more muscle fibers in ~ it, that can develop relatively an ext tension because that its size. There are three subtypes that pennate muscles.

In a unipennate muscle, the fascicles are located on one next of the tendon. The extensor digitorum of the forearm is an instance of a unipennate muscle. A bipennate muscle has fascicles top top both political parties of the tendon. In some pennate muscles, the muscle yarn wrap around the tendon, sometimes creating individual fascicles in the process. This setup is referred to as multipennate. A typical example is the deltoid muscle of the shoulder, i beg your pardon covers the shoulder however has a single tendon that inserts on the deltoid tuberosity the the humerus.

Because the fascicles, a section of a multipennate muscle like the deltoid have the right to be engendered by the nervous mechanism to change the direction of the pull. For example, as soon as the deltoid muscle contracts, the eight abducts (moves far from midline in the sagittal plane), yet when just the anterior fascicle is stimulated, the arm will abduct and flex (move anteriorly in ~ the shoulder joint).

The bar System that Muscle and Bone Interactions

Skeletal muscles do not work by themselves. Muscles are arranged in pairs based upon their functions. For muscles attached to the skeleton of the skeleton, the connection determines the force, speed, and variety of movement. These characteristics depend on each other and also can define the basic organization of the muscular and skeletal systems.

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The skeleton and also muscles act with each other to move the body. Have you ever before used the back of a hammer to remove a pond from wood? The take care of acts together a lever and the head the the hammer acts as a fulcrum, the fixed suggest that the pressure is used to when you pull back or press down top top the handle. The effort used to this mechanism is the pulling or pushing on the take care of to eliminate the nail, i beg your pardon is the load, or “resistance” to the motion of the handle in the system. Our musculoskeletal system works in a similar manner, v bones gift stiff levers and the articular endings of the bones—encased in synovial joints—acting together fulcrums. The fill would be an item being lifted or any type of resistance to a activity (your head is a load when you room lifting it), and also the effort, or used force, comes from contracting skeleton muscle.

Self-Check Questions

Take the quiz listed below to check your understanding of the interaction of bones Muscles: