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Chapter: Anatomy and Physiology for health Professionals: Support and Movement: Muscular System

Skeletal muscles usually function in groups, through the nervous device stimulating the desired muscles to execute the plan function.

You are watching: Muscles that help maintain upright posture are fixators.


Afterstudying this chapter, readers should be maybe to:

1. Distinguishbetween the origin and insertion that a skeleton muscle.

2. Differentiatebetween flexion and extension.

3. Differentiatebetween parallel, convergent, pennate, and circular muscles.

4. Whichtype of lever system is most usual in the bones muscles?

5. Identifythe action of the rhomboid major, subclavius, and also trapezius muscles.

6. Explainthe movements of the deltoid, pectoralis major, and also teres major.

7. Namethe muscles of the abdominal wall and explain the action of the rectusabdominis.

8. Explainthe muscles that flex and also extend the thigh.

9. Describethe quadriceps femoris group and also the function of the muscles it contains.

10. Describethe duty of the gastrocnemius.


Skeletal muscles normally functionin groups, with the nervous system stimulating the wanted muscles come performthe intended function. A muscle that con-tracts to carry out most the a desiredmovement is referred to as a prime mover or agonist. A good example is the pectoralis significant muscle, i beg your pardon is a element moverof arm flexion. Various other muscles, known as synergists, occupational with a element mover to make its action an ext effective by adding asmall amount of added force. For example, when you bend her forearm, theago-nist muscles space the biceps and also the synergists room the triceps. In musclesthat cross numerous joints, contrac-tion causes movement at every the spannedjoints unless other muscles act together stabilizers of the joints. Other flex-orsmay reason some undesirable movements in a joint, yet synergists avoid thisand allow the total force that the prime mover to occur in the desireddirections. Part synergists, recognized as fixators,may also assist one agonist by preventing one more joint from relocating to stabilizethe origin of the agonist. Fixator muscle run-ning indigenous the axial skeleton tothe scapula reason the scapula to be immobilized. Only desired movements canthen happen at the shoulder joint. The muscle that aid maintain uprightposture are fixators.

Other muscles act as antagonists come primemov-ers. They reason movement in the opposite direction. In the earlier example,the triceps room the antagonists come the biceps. Smooth body activity depends onantagonists be sure while prime movers contract. Muscles might work the contrary toeach various other or with each other to manage various movements. Antagonists and theirprime movers are situated on the opposite political parties of joints across which theyfunction. An instance is the ­pectoralis major, i beg your pardon acts together an antagonist tothe latissimus dorsi, the element mover the extends the arm. The is necessary tounderstand the antagonists deserve to actually additionally be element movers together aslatissimus dorsi when it acts as the element mover of arm extension.

Origins andInsertions

One finish of a skeleton muscleusually is fastened to a fairly immovable part (origin) in ~ a movable joint. The other finish connects come a movable component (insertion) on theother side of the joint. As contraction occurs, the insertion is pulled towardthe origin. There may be much more than one origin or insertion such together in thebiceps brachii muscle that the arm. As soon as this muscle contracts, the insertionbeing pulled toward its origin causes the forearm come flex at the elbow (FIGURE 10-1). Musclecon-traction produces specific actionsor movements.


The head of a muscle is the partclosest to its origin­. The term flexiondescribes a decrease in the edge of a joint, for example, a activity of thefore-arm that causes it to bend at the elbow. The term expansion describes rise in the edge of a joint, for ­example, a motion of theforearm that straightens the elbow.

Arrangementof skeleton Muscles

Skeletal muscles, according tothe plan of your fascicles, are divided into four distinct types:parallel muscles, convergent muscles, pennate mus-cles, and also circular muscles.The pennate muscles space subdivided right into unipennate, bipennate, and multipennate­ muscles.


Most skeletal muscles areclassified as parallel muscles, in i beg your pardon the fascicles room parallel come the long axes. Part areflat muscular bands with broad attachments dubbed aponeuroses at every end, vice versa, othersare thick and cylindrical having actually tendons in ~ one or both ends. An instance of aflat parallel muscle is the sartorius muscle, located in the thigh. Once theyare thick and cylindrical, they have actually a spindle shape with a main body. An example of a parallel muscle isthe biceps brachii, which has actually aspindle shape and also an increased body. Sometimes, such spindle-shaped muscles arereferred come asfusiform­ muscles.


In a convergentmuscle, the muscle fascicles extendover a large area, converging top top a solitary attachment site. The muscle may pullon a tendon, aponeurosis, or a slender tape of collagen fibers. This band isknown together a raphe. An instance of a convergent muscle is the pectoralis major, i m sorry is triangular or fan-shaped.


In a pennatemuscle, the fascicles produce a com-monangle through the tendon and also muscle fibers traction at an angle. This method pennatemuscles carry out not relocate tendons as far as parallel muscles do, yet they have moretension because they have much more muscle fibers and myofibrils. The fascicles andmuscle fibers space short and obliquely attached come the central tendon funningthe entire length that the muscle. An instance of a unipennate muscle is the extensor digitorum the the leg, i beg your pardon hasits fascicles inserted into just one next of the tendon. One instance of abipennate muscle is the rectus femoris inthe thigh, which has fascicles placed intothe tendon native opposite sides. It each other the shape of a feather. Onemultipennate muscle is the del-toid ofthe shoulder, which appears as plenty of featherlocated side by side, through each that them inserted into one big tendon.


In a circularmuscle or sphincter, the fascicles are arranged about an opened in aconcentric pattern. Muscle contractions reason a to decrease in the diameter that theopening, such together the orbicularis orismuscle the the mouth or the orbicularisoculi muscle that the eye.

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Arrangementof Fascicles

The selection of motion and also strengthof a muscle are based upon the setup of that is fascicles. Skeletal musclefibers have the right to shorten to about 70% of your resting size when theycontract. Therefore, longer and much more parallel muscle fibers, in compar-ison toa muscle’s lengthy axis, way the more the muscle is able come shorten. Parallelfascicle arrange-ments market the most capacity to shorten, yet are no as strongas other types of fascicle arrangements. The strength of a muscle is based on thenumber of muscle fibers in it. An ext muscle fibers offer an ext strength. Forexample, bipennate and multipennate muscle have much more fibers and also are verystrong while only shortening slightly.