By now you will be familiar with the basic structure and organization of muscle (in particular, skeletal muscle). In this chapter we want to expand upon your knowledge of muscle by looking at how it actually works and look more specifically at muscular attachments as well as some basic muscle nomenclature. In chapter nine we looked at the three types of human muscle, namely cardiac, smooth and skeletal muscle. In this chapter we will expand upon our knowledge base of the three types of muscle but will focus predominantly on the properties of skeletal muscle. Skeletal muscle is arguably the most versatile of human muscle due to the number, size and shapes of skeletal muscle. It is also the largest and most powerful. However, even though skeletal, cardiac, and smooth muscle are structurally and mechanically different, they all basically operate the same way once a stimulus has been provided. Therefore, all muscles have similar actions even though their responsibilities may differ. In this next section we will take a look at the basic responsibilities and actions of muscle. The most basic function of muscle can be described as a contraction. However, this contraction can come in many forms allowing muscle to perform a great variety of movements both precise and powerful.
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Human muscle can generate large amounts of absolute force, although animals are capable of even greater feats. However, when we talk about relative strength humans are actually quite weak in contrast to other species. That’s why over the years we have developed tools in the workplace to make up for our basic strength deficiencies. Here are a few interesting little facts on muscle:
Fleas can jump more than 130 times their own height. If a human could do this we would be able to jump nearly 800 feet or scale large buildings in a single jump as Superman often does!
Our strongest muscle is our masseter muscle in our jaw that we use for chewing. It is calculated that these muscles can generate almost 1000 pounds of force. Please don’t try this as you will most likely shatter your teeth!
The smallest muscles in the body are found in your ear – the stapedius.
Our largest muscle is our gluteus maximus.
Unless we are unconscious our muscles are always under some tension.
Muscles under too much tension for too long form fibrous clumps known as knots. Massage is good for breaking these us.
As you can see, our muscle, especially skeletal muscle, is highly versatile. Let’s now take a further look at some of its properties.
Responsibility and Actions
Our body is capable of moving in many ways. Various systems exist within the body to facilitate this movement and the skeletal muscle system is the most obvious and abundant. Other factors that cause movement are small cilia (hairs), flagella on the surface of cells (flagella are tail like structures as in a sperm tail), gravitational force. Most of our movement is via muscle but our muscles also have other roles (albeit indirectly related to movement). Here are other responsibilities of muscle.
1. Production of Body Heat. As our muscles contract they produce heat. The more they contract the more heat they produce and vice versa. That’s why when we exercise we get hot but might need a blanket over us while lying on the couch watching TV. Our body temperature of @ 37° C. is maintained by the thousands of reactions and movements that occur daily (even at rest) and require muscle contraction. When these actions are not enough to maintain our body temperature our muscles can activate an additional mechanism to produce heat. Yes, you guessed it, shivering, which can increase heat production up to 10 times normal levels for a short period of time.
2. Body Movement. This is the most obvious function of muscle. Most of our muscles are attached to bones to be pulled in different directions to facilitate walking, swimming, cycling, etc. Later in the chapter we will look in more detail at movement specific functions such as agonists and antagonists.
3. Maintaining Posture. Remember we said earlier that unless we’re unconscious there is some degree of muscle contraction always present. Sometimes we may not even realize our muscles are contracting. But consider this: even as you sit (or stand) here and read this, your neck, back and arm muscles are all contracting to maintain your posture. When you walk they do the same.
4. Breathing. The movement of our diaphragm up and down allows us to create positive and negative pressures that permit air to move in and out of our lungs.
5. Speech. The acute and precise muscles of our face allow us to talk, gesture, etc. and ultimately communicate.
6. Heart Beat. The constant contraction and relaxation of our cardiac muscle allows us to continually circulate our blood and its nutrients.
7. Constriction of Vessels and Organs. The smooth muscle in these tissues allows us to regulate the flow of blood, food, water, feces, etc. by constricting or dilating our vessels.
8. Sight. Our ability to widen and narrow our pupils allows us to see and also prevent too much light damage from entering our eyes.
Thus our muscles perform a side variety of vital and not so vital functions. And even though we are more concerned with skeletal muscles for our studies, we should recognize these other important functions. The functions of muscle listed above involve skeletal, cardiac and smooth muscle. Let’s take a more detailed look at specific roles of skeletal muscle.
In kinesiology, most of our focus and study of muscle is concerned with the way skeletal muscle functions. It is within this role of movement that we see muscle take on many roles. These roles include action as an agonist, antagonist, stabilizer, neutralizer, and synergist. A muscle can virtually act to perform any of these roles depending upon the nature of the muscle action. We often refer to these as abilities of muscle actions.
The term agonist is used to describe the muscle when it is acting as the prime mover or prime muscle in an action. The agonist muscle creates a torque in the same direction as the joint action (or plane). In other words, the agonist is the muscle (or muscles) that are acting concentrically during the action in which their joint is involved. Sometimes there is more than one agonist in a movement and when this happens we normally extend our definitions to include the terms primary and assistant (or secondary) agonist. The muscles can also be referred to as prime movers or protagonists. From a terminology point of view there should always be a muscle and joint referred to when discussing agonists. All these terms, agonist, prime mover, and protagonist, are considered undefined unless the joint action is also described. So, for example, “in elbow flexion the agonist is the biceps brachii” is a correct statement, whereas “the biceps brachii is an agonist” is an incomplete statement. Do a biceps curl in a nice straightforward movement to illustrate the various roles of agonist, assistant agonist, etc. In this motion which is correctly described as elbow flexion, the biceps brachii and the brachialis act as the agonists, while the brachioradialis, extensor carpi radialis longus and pronator teres act as assistant agonists.
That was actually a lengthy discussion on agonists but the background information will serve you well as we consider the other terms. An antagonist is the muscle that creates resistance or torque opposite the agonist. They do this by developing force eccentrically as the agonists are contracting concentrically. Normally antagonists and agonists are positioned on the opposing sides of a joint. In the above example of elbow flexion the triceps would be the antagonists. The main role of antagonists is really to provide a breaking action to slow down the contraction especially if it involves a very powerful and fast movement like throwing a fastball. Conversely the role of agonists is to provide an acceleration movement. While agonists and antagonists are the two most obvious roles muscles play, they do play other important but less obvious roles. These other roles are often collectively termed synergistic roles with the muscles referred to as synergists. Synergists usually take on two roles referred to as stabilizers and neutralizers. Synergistic muscles are usually acting in a less obvious but cooperative way to agonists. One role is that of a stabilizer. (Sometimes referred to as fixator or supporter.) When muscles act there are numerous forces acting upon the muscle that may cause it to act in a manner that is unwanted. Stabilizer muscles usually exert their effect and force without any noticeable action movement in the muscle. Let’s take the deltoid muscle for example. The deltoid muscle has two main roles. The first and most obvious is to move the humerus. It helps lift our arms up. The second, however, is a stabilizing role as it helps press the head of the humerus into the shoulder socket. This traction action of pulling the humeral head into the socket is a stabilizing role. The stabilizing action of a muscle can usually be determined by its static contraction properties. The action of steadying bones at the joint around which they rotate is probably the most common action of stabilizing muscles.
Another important role of muscle is the action of a neutralizer. Remember we have over 400 muscles in the body and many of them are contracting in any given movement even though their roles may be minimal. Often these roles are that of a neutralizer that basically defined is a muscle that acts to prevent an unwarranted action during a movement. A common example often cited to demonstrate the neutralizer concept is that of elbow flexion. During elbow flexion (e.g. a biceps curl) the biceps brachii produce forces that cause flexion of the elbow but also supination (rotation) of the forearm. Since elbow flexion i.e. the bicep curl, is the only action wanted, something has to neutralize the supination action. This is achieved by the pronator teres which is that small muscle that runs across the inside of your elbow. Thus we can see that a muscle can have four distinct movement responsibilities. Namely, agonist, antagonist, stabilizer and neutralizer. The variability in muscle movement function means that many of our muscles can actually perform all four actions, although this is rarely the case.
Muscle Classification and Naming
You will have noticed that we have already been referring to different mucles in our discussion of architecture and role. For example, we have referred to the deltoid, pronator teres and biceps brachii. Believe it or not, muscles are named using a fairly simple and logical guide. In essence, all our muscles are named according to one of four main criteria, namely shape, location, size and function (or sometimes a combination). Some other secondary criteria are also used for selected muscles and they include origin and insertion, the number of heads on the muscle, and the orientation of the fasciculi. Shortly we will look at this naming system in more detail. First there are some other basic terms and information we should be familiar with.
(General Muscle Principles)
The basic movement of a muscle is contraction or shortening. For the most part this occurs by the muscle pulling two bones together (flexion) since the muscle is usually attached to bone on two ends. However, this is not always the case as sometimes a muscle is not attached to bone at both ends and may in fact only be attached to skin. This is the case with several muscles in our face. Regardless, the muscle has two attachment points and these are referred to as the origin and insertion.
The origin is sometimes referred to as the fixed end or head and is the stationary part of the muscle. By this we mean the end of the muscle that is attached to the least moveable bone in the action. This is the proximal end of the muscle and is usually closer to the axial skeleton or mid-section of the body. For example, in the biceps cure, the humerus doesn’t move but the radius does. The origin of the biceps is at the head of the humerus. The muscle insertion is the other end of the muscle. It is also referred to as the mobile end or the distal insertion. The attachment occurs on the most movable bone in the action. Using the biceps example again, the insertion occurs on the radial tubreosity (bone) causing the radius to move towards the humerus during a biceps curl. (Fig 10.1 in Seeley et al.). Greater stresses tend to be applied at the insertion site and often after strenuous activity the insertion site is more inflamed and painful. Now let’s get back to muscle naming and classification.
As we discussed, muscle is normally named using one (or more) of the following main criteria: shape, location, size and function. Additionally, some secondary criteria are often used.
shape, location, size, function, origin and insertion, the number of heads on the muscle, orientation of the fasciculi.
Depending on the text you use you will see different terminology for muscle shapes. In this text I will present the different shapes under the headings we used earlier namely, pennate, straight and circular.
Our muscles are arranged in a variety of shapes and are normally grouped into three classes as a function of the way in which the fasciculi are arranged. This is sometimes referred to as (angle of) pennation. The three arrangements are pinnate, straight (or fusiform) and circular. Contained within these three arrangements are sub-classifications of arrangements. The orientation of the muscle fasciculi (or fibers) has several implications for the muscle action. The orientation affects how much movement can occur by the muscle and indeed how much force it can produce. In terms of simple biomechanics we can consider our muscle fibers to be either in parallel arrangement (fusiform) or a feather-like arrangement (penniform). The fusiform arrangement applies to muscles whose architecture comprises mostly muscle fibers that run parallel and longitudinal. The penniform arrangement applies to muscles whose fibers are normally short, feather-like in appearancde and attach to more than one tendon. The main differences between the arrangements are that the pinnate arrangement facilitates more force production while the fusiform arrangement facilitates more movement. If you think about the big, strong muscles in your back and chest, you’ll see these have a more penniform arrangement. The muscles in your arms are more fusiform and allow more mobility but produce less force. Let’s look at some arrangement specifics in more detail.
Pennate (or penniform) fibers are usually organized in a diagonal orientation or, more specifically, feather-like orientation. Pennate comes from the Latin “Pennatus” meaning feather. In the pinnate arrangement the fasciculi can run in multiple directions and therefore pinnate fibers are further classified as unipennate, bipennate or multipennate (Use fig 10.2a in Seeley & Tate). When the muscle is strictly like a feather, meaning the fibers are divided on two sides of the same tendon, the design is referred to as bipennate. In general, the bipennate arrangement is characterized by a single tendon running between the muscle fibers which extend in a diagonal orientation from the tendon out. Both sides of the tendon appear symmetrical and it truly does resemble a feather. An example of a bipennate muscle is the rectis fe____ muscle in the quadriceps. In contrast to the bipennate muscle is the unipennate (or semipennate) muscle arrangement. In this arrangement all fasciculi are on the same side of the tendon. The tibialis posterior muscle in the leg is an example of a unipennate muscle. The final pinnate arrangement is multipennate. In this arrangement the fasciculi are arranged in multiple corations around multiple muscle tendons with the fibers again running diagonally between them. The deltoid muscle is a nice example of a multipennate muscle. Other muscles that also technically fall under the straight classification are quadrate and rhomboidal. In these orientations the muscle fibers still run in a parallel orientation but are more square and rectangular in appearance. These muscles appear as four sided and are usually flat. Muscles of this appearance usually have it reflected in their name, for example, pronator quadratus on your wrist or your rhomboid muscle on your back.
Other muscle orientations that typically fall under the pinnate classification include triangular or fan shaped muscles although in all reality this shape of muscle should be classified as multipennate. Fan shaped muscles are pinnate in arrangement and actually resemble quite closely the shape of a “hand fan.” These muscles are relatively flat and the fibers project outwards from a narrow attachment almost resembling a garden bush. So they have a narrow attachment at one end and a wide attachment at the other. Our pectoralis major muscle on our chest is a nice example of a triangular muscle.
Straight (Fusiform) Muscle
There are multiple terms used interchangeably to refer to straight muscle. You will see the terms straight, fusiform and longitudinal and although they are all commonly used, there are subtle variations. In general, straight muscles have the fasciculi organized parallel to the long axis of the muscle. Strictly speaking this description really refers to a longitudinal muscle arrangement. The santorius and hyoid muscle are examples of longitudinal muscles. A fusiform muscle still contains parallel fibers running longitudinally, however, fasiform muscles are more tapered at either end. These muscles can be long or short in length in contrast to straight muscles which are usually only long. Examples of the fusiform orientation are abundant and include our biceps brachii and gastroznemuis. A general distinguishing appearance is that fusiform muscles are more spindle-like in appearance versus straight muscles which are very thin rectangular in appearance. Therefore the term longitudinal is the more obsolete reference of the three terms.
The final shape classification of muscle is the orbicular muscle (see figure 10.2 Seeley). Sometimes referred to as circular, the orbital muscles are circular in orientation. The muscle fibers are organized in a circular shape and are a unique characteristic of sphincter muscles and of course the eye muscles. The main role of these muscles is to open and close to permit light, fluid, etc. to enter or exit.
The next classification for naming muscle is muscle location. Using a conventional anatomical approach, we can refer to a particular body region, AKA location. For example, in our arm we have several “brachial” structures. We have the brachial artery, the brachial plexis, the brachioradialis and, of course, the biceps brachii. The biceps term tells us this is a two headed muscle in the “brachial” region thereby using its location to determine its name. Gluteus is a term for buttock and pectoralis is a term for chest. This allows us to use the location to determine the muscle name. In addition, these terms imply a body location and inform us of where a particular muscle can be found. You will see shortly that combining location and function is a common method for naming muscles as it not only defines muscle by location but also by function.
The fourth main category for classifying muscle is muscle function, i.e. what does the muscle do for a movement pattern in its primary role? As we know, muscles can move in many ways, e.g., flexion, abduction, or elevation. Sometimes this descriptive term is added to the muscle to give the muscle a name. In using this approach the movement term is usually used in combination with another naming descriptor. For example, the erector spinae. Erector is the term to imply erect, or elevate or lift, while “spinae” is the location. Collectively, these terms tell us that the muscle is on the back or spine and it erects or lifts. Another example would be the flexor carpi radialis. Flexor is the movement or function, carpi is the area (hand) location and “radialis” provides additional location information. This is an effective method for naming muscles in that it typically provides us with a little bit more information on the muscle itself.
The other remaining categories that are less common are orientation of the fasciculi, origin and insertion and the number of muscle heads.
— Orientation of fasciculi. The two main orientation terms are rectus and oblique. These terms are used to describe a muscle that has a straight muscle fasciculi orientation (rectus) or an angular muscle orientation (oblique). Examples are the rectus abdominus and the rectus oblique.
— Origin and insertion. This nomenclature is used to describe a muscle based on its starting (origin) and ending (insertion) point. For example, the brachioradialis originates on our arm (brachium) and ends or inserts onto the radius. The sternecleidomastoid is another example.
— Number of heads. This nomenclature is used to describe the number of heads on a particular muscle and is usually used in connection with location. For example, biceps means two heads, while triceps means three heads. The addition of the term brachii to both gives us location.
In all, we have seven means of naming muscles but four categories – size, location, shape and function – are mainly used.
Muscle origins and insertions
We have already looked at the basic definitions of the terms origin and insertion. But let us take a quick recap. The origin of a muscle is usually considered the starting point and occurs closest to the midline of the body. It is also the least movable end of the muscle in terms of range of motion. The insertion or ending point is further away from the midline and is often referred to as the distal point (proximal is used for origin). This attachment site is usually the most movable location in terms of range of motion and usually is the area of the muscle that experiences the greater stress during movement. Remember, for the most part muscle starts on one bone and ends on another and it is this arrangement that allows for skeletal movement. In general, the long bones have greater distances between origins and insertions and this distance is also what allows our sweeping locomotion movements.
Determining the origin and insertion of the muscle is really something you must learn and memorize and then recall through manual practice. Since every muscle basically has an origin and insertion there are literally thousands in the body. For our purposes in applied kinesiology we will look only at the major locomotion muscles of the upper and lower body and consider their origins and insertions. They are summarized and presented in table form.
Upper body muscle origins and insertions
biceps brachii, triceps, pectoralis, barachioradialis, deltoid, rectus abdominus, latissimus dorsi.
Lower body muscle origins and insertions
quadriceps (vastus medialis oblique, rectus femoris, vastus lateralis, vastus intermedius) sartorius, tibialis anterior, gastrocnemius, soleus, hamstrings (biceps femoris, semitendinosus, semimembranosus).
Muscles Acting on the Arm (see Figures ##.## — ##.## )
process, and scapular
Spinous processes of T7-L5; sacrum and iliac crest; inferior angle of scapula in some people
Clavicle, sternum, superior six costal cartilages, and abdominal aponeurosis
Medial crest of intertubercular groove
Lateral crest of intertubercular groove
Medial and lateral pectoral
Flexes and extends shoulder;
abducts and medially and laterally rotates arm
Adducts and medially rotates arm; extends shoulder
Flexes shoulder; adducts and medially rotates arm; extends shoulder from flexed position
Muscles Acting on the Forearm (see Figures ##.## and ##.##)
Long head –
Short head – coracoid process
Long head – infraglenoid tubercle on the lateral border of scapula
Lateral head – lateral and posterior surface of humerus
Medial head – posterior humerus
ridge of humerus
Radial tuberosity and aponeurosis of biceps brachii
Olecranon process of ulna
Styloid process of radius
Flexes shoulder and elbow; supinates forearm and hand
Extends elbow; extends shoulder and adducts arm
Muscles of the Thigh (see Figures ##.## and ##.##)
Rectis femoris –
anterior inferior iliac spine
Vastus lateralis –
greater trochanter and linea aspera of femur
Vastus intermedius –
body of femur
Vastus medialis –
linea aspera of femur
Anterior superior iliac spine
Long head – ischial
Short head – femur
Patella and onto tibial tuberosity through
Medial side of tibial tuberosity
Head of fibula
Medial condyle of tibia
and collateral ligament
Long head – tibial
Short head – common fibular
Extends knee; rectus femoris
also flexes hip
Flexes hip and knee; rotates thigh laterally and leg medially
Flexes knee; laterally rotates leg; extends hip
Flexes knee; medially rotates leg; tenses capsule of knee joint;
Flexes knee; medially rotates leg; extends hip
Muscles Acting of the Leg acting on the Leg, Ankle and Foot (see Figures ##.## — ##.## )
Medial and lateral condyles
Fibula and tibia
Through calcaneal (Achilles) tendon to calcaneus
tendon to calcaneus
Plantar flexes foot; flexes knee
Plantar flexes foot
1. Identify how the following muscles are classified and whether they are penniform or fusiform. Know also where they are located!
a. Pectoralis major
b. Teres minor
c. Extensor digitorum
d. Serratus anterior
e. Trapezius major
g. Gluteus maximus
h. Depressor labii inferioris
I. Lattisimus dorsi
j. Adductor magnus
2. Muscle Considerations in Movement
Can you differentiate between and give examples of the following:
a. Smooth Muscle
b. Cardiac Muscle
c. Striated Muscle
Identify and describe four common characteristics for all muscle types:
Identify and describe 5 factors that influence muscle contraction for
What do we mean when we use the terms voluntary and involuntary?
Muscles are typically classified according to four criteria. Identify the four criteria and provide a muscular example to support your answer!
What is the basic difference between a fusiform and a penniform muscle ?
Differentiate between the parallel and series elastic component.
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