How is the Force of Contraction Applied to the Tendons by the Muscle's Individual Fibers?

Image and question have been updated for clarity!

The image above is a side view of a semi-transparent skeletal muscle. The dark red lines represent individual fibers, the blue lines represent tendons. The pink area also consists of muscle fibers, but it is made transparent for the purpose of the demonstration. Please assume the fibers are properly encased in fascicles and fascia. Tendons connect the muscle to the bones. The bones can move towards the center of the image if pulled. The arrows show the flow of force during contraction along each fiber.

The muscle has grown large enough that the outer fibers protrude around the tendon (where they connect). During contraction, do the protruding outer fibers help pull the tendon towards the center of the muscle like the internal fibers in the middle of the muscle do? If so, how is the force controlled and directed to accomplish this?

It seems there exists athletes with this type of anatomy (such as very large bodybuilders) but the bio-mechanics of this type of anatomy completely confuse me. It would seem the flow of force of the outer fibers would pull the tendon away, unless the force is somehow redirected. Thank you so much for your insight on this!!!

How is the Force of Contraction Applied to the Tendons by the Muscle's Individual Fibers? - Biology

The force a muscle generates is dependent on its length and shortening velocity.

Learning Objectives

Differentiate between force-length and force-velocity of muscle contraction

Key Takeaways

Key Points

  • The force -length relationship indicates that muscles generate the greatest force when at their resting (ideal) length, and the least amount of force when shortened or stretched relative to the resting length.
  • The force-velocity relationship demonstrates that power produced is controlled by the velocity and force of muscle contraction, with an optimum power output at one third of maximum velocity.

Key Terms

  • force: Any influence that causes an object to undergo a certain change concerning its movement, direction, or geometrical construction.
  • resting length: Often the ideal length of a muscle and the length at which it can create the greatest active force.
  • power: A measure of force x velocity, a measurable output for muscle contraction
  • Force-Velocity Relationship: The relationship between the speed and force of muscle contraction, outputted as power.
  • Force-Length Relationship: The relationship between sarcomere length and force produced in the muscle, modulated by actin and myosin myofilament overlap.

Muscle Force Generation

The force a muscle generates is dependent on the length of the muscle and its shortening velocity. These two fundamental properties limit many key biomechanical properties, including running speed, strength, and jumping distance.

Force-Length Relationship

Due to the presence of titin, muscles are innately elastic. Skeletal muscles are attached to bones via tendons that maintain the muscle under a constant level of stretch called the resting length. If this attachment was removed, for example if the bicep was detached from the scapula or radius, the muscle would shorten in length.

The Ideal Length of a Sarcomere: Sarcomeres produce maximal tension when thick and thin filaments overlap between about 80 percent to 120 percent, approximately 1.6 to 2.6 micrometers.

Muscles exist in this state to optimize the force produced during contraction, which is modulated by the interlaced myofilaments of the sarcomere. When a sarcomere contracts, myosin heads attach to actin to form cross-bridges. Then, the thin filaments slide over the thick filaments as the heads pull the actin. This results in sarcomere shortening, creating the tension of the muscle contraction. If a sarcomere is stretched too far, there will be insufficient overlap of the myofilaments and the less force will be produced. If the muscle is over-contracted, the potential for further contraction is reduced, which in turn reduces the amount of force produced.

Simply put, the tension generated in skeletal muscle is a function of the magnitude of overlap between actin and myosin myofilaments.

In mammals, there is a strong overlap between the optimum and actual resting length of sarcomeres.

Force-Velocity Relationship

Force-Velocity Relationship: As velocity increases force and therefore power produced is reduced. Although force increases due to stretching with no velocity, zero power is produced. Maximum power is generated at one-third of maximum shortening velocity.

The force-velocity relationship in muscle relates the speed at which a muscle changes length with the force of this contraction and the resultant power output (force x velocity = power). The force generated by a muscle depends on the number of actin and myosin cross-bridges formed a larger number of cross-bridges results in a larger amount of force. However, cross-bridge formation is not immediate, so if myofilaments slide over each other at a faster rate the ability to form cross bridges and resultant force are both reduced.

At maximum velocity no cross-bridges can form, so no force is generated, resulting in the production of zero power (right edge of graph). The reverse is true for stretching of muscle. Although the force of the muscle is increased, there is no velocity of contraction and zero power is generated (left edge of graph). Maximum power is generated at approximately one-third of maximum shortening velocity.

Mechanics of Skeletal Muscle Contraction

Motor Unit. Each motoneuron that leaves the spinal cordinnervates multiple muscle fibers, the number depend-ing on the type of muscle. All the muscle fibers inner-vated by a single nerve fiber are called a motor unit. In general, small muscles that react rapidly and whose control must be exact have more nerve fibers for fewer muscle fibers (for instance, as few as two or three muscle fibers per motor unit in some of the laryngeal muscles). Conversely, large muscles that do not require fine control, such as the soleus muscle, may have several hundred muscle fibers in a motor unit. An average figure for all the muscles of the body is questionable, but a good guess would be about 80 to 100 muscle fibers to a motor unit.

The muscle fibers in each motor unit are not all bunched together in the muscle but overlap other motor units in microbundles of 3 to 15 fibers. This interdigita-tion allows the separate motor units to contract in support of one another rather than entirely as individ-ual segments.

Muscle Contractions of Different Force – Force Summation.

Summation means the adding together of individualtwitch contractions to increase the intensity of overall muscle contraction. Summation occurs in two ways: (1) by increasing the number of motor units contracting simultaneously, which is called multiple fiber summa-tion, and (2) by increasing the frequency of contraction,which is called frequency summation and can lead to tetanization.

Multiple Fiber Summation. When the central nervoussystem sends a weak signal to contract a muscle, the smaller motor units of the muscle may be stimulated in preference to the larger motor units. Then, as the strength of the signal increases, larger and larger motor units begin to be excited as well, with the largest motor units often having as much as 50 times the contractile force of the smallest units. This is called the size princi-ple. It is important, because it allows the gradations ofmuscle force during weak contraction to occur in small steps, whereas the steps become progressively greater when large amounts of force are required. The cause of this size principle is that the smaller motor units are driven by small motor nerve fibers, and the small motoneurons in the spinal cord are more excitable than the larger ones, so they naturally are excited first.

Another important feature of multiple fiber summa-tion is that the different motor units are driven asyn-chronously by the spinal cord, so that contraction alternates among motor units one after the other, thus providing smooth contraction even at low frequencies of nerve signals.

Frequency Summation and Tetanization. Figure 6–13shows the principles of frequency summation and tetanization. To the left are displayed individual twitch contractions occurring one after another at low fre-quency of stimulation. Then, as the frequency increases, there comes a point where each new contraction occurs before the preceding one is over. As a result, the second contraction is added partially to the first, so that the total strength of contraction rises progressively with increasing frequency. When the frequency reaches a critical level, the successive contractions eventually become so rapid that they fuse together, and the whole muscle contraction appears to be completely smooth and continuous, as shown in the figure. This is called tetanization. At a slightly higher frequency, the strengthof contraction reaches its maximum, so that any addi-tional increase in frequency beyond that point has no further effect in increasing contractile force. This occurs because enough calcium ions are maintained in the muscle sarcoplasm, even between action potentials, so that full contractile state is sustained without allowing any relaxation between the action potentials.

Maximum Strength of Contraction. The maximum strengthof tetanic contraction of a muscle operating at a normal muscle length averages between 3 and 4 kilograms per square centimeter of muscle, or 50 pounds per square inch. Because a quadriceps muscle can have as much as 16 square inches of muscle belly, as much as 800 pounds of tension may be applied to the patellar tendon. Thus, one can readily understand how it is possible for muscles to pull their tendons out of their insertions in bone.

Changes in Muscle Strength at the Onset of Contraction—The Staircase Effect (Treppe). When a muscle begins to con-tract after a long period of rest, its initial strength of contraction may be as little as one half its strength 10 to 50 muscle twitches later. That is, the strength of con-traction increases to a plateau, a phenomenon called the staircase effect, or treppe.

Although all the possible causes of the staircase effect are not known, it is believed to be caused primarily by increasing calcium ions in the cytosol because of the release of more and more ions from the sarcoplasmic reticulum with each successive muscle action potential and failure of the sarcoplasm to recapture the ions immediately.

Skeletal Muscle Tone. Even when muscles are at rest, acertain amount of tautness usually remains. This is called muscle tone.Because normal skeletal muscle fibers do not contract without an action potential to stimulate the fibers, skeletal muscle tone results entirely from a low rate of nerve impulses coming from the spinal cord. These, in turn, are controlled partly by signals transmitted from the brain to the appropriate spinal cord anterior motoneurons and partly by signals that originate in muscle spindles located in the muscle itself. Both of these are discussed in relation to muscle spindle and spinal cord function.

Muscle Fatigue. Prolonged and strong contraction of amuscle leads to the well-known state of muscle fatigue. Studies in athletes have shown that muscle fatigue increases in almost direct proportion to the rate of depletion of muscle glycogen. Therefore, fatigue results mainly from inability of the contractile and metabolic processes of the muscle fibers to continue supplying the same work output. However, experiments have also shown that transmission of the nerve signal through the neuromuscular junction, can diminish at least a small amount after intense pro-longed muscle activity, thus further diminishing muscle contraction. Interruption of blood flow through a con-tracting muscle leads to almost complete muscle fatigue within 1 or 2 minutes because of the loss of nutrient supply, especially loss of oxygen.

Lever Systems of the Body. Muscles operate by applyingtension to their points of insertion into bones, and the bones in turn form various types of lever systems. Figure 6–14 shows the lever system activated by the biceps muscle to lift the forearm. If we assume that a large

biceps muscle has a cross-sectional area of 6 square inches, the maximum force of contraction would be about 300 pounds. When the forearm is at right angles with the upper arm, the tendon attachment of the biceps is about 2 inches anterior to the fulcrum at the elbow, and the total length of the forearm lever is about 14 inches. Therefore, the amount of lifting power of the biceps at the hand would be only one seventh of the 300 pounds of muscle force, or about 43 pounds. When the arm is fully extended, the attachment of the biceps is much less than 2 inches anterior to the fulcrum, and the force with which the hand can be brought forward is also much less than 43 pounds.

In short, an analysis of the lever systems of the body depends on knowledge of (1) the point of muscle inser-tion, (2) its distance from the fulcrum of the lever, (3) the length of the lever arm, and (4) the position of the lever. Many types of movement are required in the body, some of which need great strength and others of which need large distances of movement. For this reason, there are many different types of muscle some are long and contract a long distance, and some are short but have large cross-sectional areas and can provide extreme strength of contraction over short dis-tances. The study of different types of muscles, lever systems, and their movements is called kinesiology and is an important scientific component of human physioanatomy.

“Positioning” of a Body Part by Contraction of Agonist and Antag-onist Muscles on Opposite Sides of a Joint—“Coactivation” of Antagonist Muscles. Virtually all body movements arecaused by simultaneous contraction of agonist and antagonist muscles on opposite sides of joints. This is called coactivation of the agonist and antagonist muscles, and it is controlled by the motor control centers of the brain and spinal cord.

The position of each separate part of the body, such as an arm or a leg, is determined by the relative degrees of contraction of the agonist and antagonist sets of muscles. For instance, let us assume that an arm or a leg is to be placed in a midrange position. To achieve this, agonist and antagonist muscles are excited about equally. Remember that an elongated muscle contracts with more force than a shortened muscle, which was demonstrated in Figure 6–9, showing maximum strength of contraction at full functional muscle length and almost no strength of contraction at half normal length. Therefore, the elongated muscle on one side of a joint can contract with far greater force than the shorter muscle on the opposite side. As an arm or leg moves toward its midposition, the strength of the longer muscle decreases, whereas the strength of the shorter muscle increases until the two strengths equal each other. At this point, movement of the arm or leg stops. Thus, by varying the ratios of the degree of activation of the agonist and antagonist muscles, the nervous system directs the positioning of the arm or leg.

Stiffness of cat soleus muscle and tendon during activation of part of muscle

Experiments have been carried out on the soleus muscle and its tendon in the anesthetized cat. Measurements of isometric tension and muscle stiffness were made during contraction of whole or part of the muscle in response to stimulation of ventral root filaments. In an attempt to determine the distribution of tension in different portions of the tendon during activation of only part of the muscle, the free tendon of insertion was split longitudinally into two halves and a strain gauge attached to each piece. From a large number of measurements, it was found that the mean fraction of tension recorded in one-half of the tendon remained about the same, over a wide range of tensions. However, the scatter of values, which increased as the portion of muscle contracting was reduced, was greater than expected if muscle fibers were randomly distributed throughout the muscle. Measurements of muscle and tendon stiffness were made from length and tension changes during stretch of the actively contracting muscle. Ventral root stimulation that engaged 20% or more of the muscle yielded a value for tendon compliance (0.09 mm/N), which was the same as for stimulating the whole muscle. This result suggested that for contraction of portions as small as 20% of the muscle, fibers were effectively attached to the whole tendon, indicating that tendinous attachments of individual muscle fibers ran independent of one another over only a short distance and were bound together over most of their remaining course. It was concluded that groups of muscle fibers selected by stimulation of ventral root filaments are not entirely randomly distributed throughout the muscle. However, for groups representing larger fractions of the total tension, (greater than 20%) the distribution is uniform enough and the connections between their tendinous attachments firm enough for the force applied by such a group to act through a tendon compliance, which is the same as that seen by the whole muscle.

How is the Force of Contraction Applied to the Tendons by the Muscle's Individual Fibers? - Biology

In this chapter, we introduce the basic traits of the skeletal muscles, their positions in the body, how they attach to bones, and how they maneuver the joints when they contract. We will also look at how muscles change shape in different movements. Tendons are also introduced, with a focus on their characteristics and how they influence the surface form. This basic information will be elaborated on in the following chapters on the muscle groups of different regions of the body.

Muscles, along with the subcutaneous layer of adipose (fatty) tissue, define the overall shape of the figure, “fleshing out” its structure and giving substance and character to the body. One of the many challenges in drawing the figure is to depict surface forms changing in various poses. Understanding the basic placement of muscles and how they stretch and compress in different movements will give you, as an artist, the advantage of knowing what occurs beneath the skin (and how that) influences what you see on the surface.

Figurative artists from centuries past up to the present have known the value of studying the human muscular system. When you view figurative works by painters and sculptors such as Michelangelo, Artemesia Gentileschi, Auguste Rodin, Peter Paul Rubens, and others, it’s evident that these artists were well aware of anatomical forms and utilized that information to serve their artistic vision. Their knowledge of anatomy never overpowered their personal style or aesthetics&mdashit only enhanced their work. Learning about the muscular system thus opens a creative door to many possible artistic options, whether you pursue an exacting anatomical realism, exaggerate bodily forms to create interesting visual dynamics, or explore a more expressive interpretation of the human form.

Let’s begin by looking at the entire muscular system. The following drawings show the male and female figures in both anterior and posterior views.





Muscles are situated within the body in two basic layers: the superficial muscle layer (also known as externus or superficialis) and the deep muscle layer (also called internus or profundus). Many figurative artists become familiar with the superficial layer when learning the basic anatomical forms of the body. With only a few exceptions, such as the sacrospinalis muscle of the back, the muscles of the deep layer do not influence the surface forms and usually are not visible. Some anatomy books refer to a middle layer, called the intermediate layer, located in the lower arm, foot, and torso.

One way to become familiar with muscles is to categorize them into groups wherever possible. Besides being identified by the layer to which they belong, muscles can be grouped in a number of other ways, including the following:

·&emspBy their function or action (e.g., flexor group, extensor group, adductor group)

·&emspBy their location in the body or by reference to other anatomical forms (e.g., gluteal group, abdominal group, pectoral group, scapula group, radial group, thenar group, peroneal group)

·&emspBy compartment, because muscles are separated into different compartments by deep fascia called intermuscular septa (e.g., anterior compartment, posterior compartment, medial compartment)

·&emspBy colloquial (common) names (e.g., thumb group, inner thigh group, upper thigh group, hamstring group)

Some muscles, such as the sartorius muscle of the upper leg, do not belong to any of these categories. These muscles assist in various movements while remaining independent of any group.

Skeletal Muscles

Beyond the groupings given above, muscles are classified as belonging to three basic types: cardiac muscle (pertaining to the heart), smooth muscles (usually affiliated with the tubular structures of the body, such as the arteries, colon, and bronchial tubes, as well as the iris of the eye), and skeletal muscles. As their name implies, the skeletal muscles attach to bones. It is the skeletal muscles that most interest artists because they are instrumental in creating bodily movement and because their shapes are often easy to see beneath the surface of the body.

The main function of skeletal muscles is to shorten, or contract, their overall shape to produce movement. To better understand how they accomplish this, we need to walk through muscles’ basic internal structure. Muscles are composed of a series of elongated muscle fibers (muscle cells) that are grouped together in muscle fiber bundles, or fascicles. Even though muscle fibers are parallel to each other, they can be short, long, circular, fan shaped, or obliquely positioned on a tendon.


Arrangements of muscle fibers

Muscle Architecture

The varying lengths of muscle fibers affect the way muscles perform, with longer muscle fibers producing a greater range of movement while shorter fibers generate more power in movement. Muscle architecture is the term usually applied to the arrangement of muscle fibers. There are several muscle-architecture classifications: parallel, pennate, convergent/triangular, circular, spiral, and biventer. More information about each category, including examples of muscles belonging to each, appears in the following table.

Muscles’ Internal Structure

Within each individual muscle fiber are elongated, rodlike strands, called myofibrils, that run parallel to each other and extend to the entire length of the muscle fiber. Myofibrils, in turn, contain a series of smaller units called sarcomeres, which are positioned end to end inside the whole myofibril. And within each sarcomere are microscopic threads called myofilaments. The myofilaments are composed of contractile proteins of two different types&mdashmyosin and actin&mdashand are specialized for contraction.

Contraction can be explained, somewhat simplistically, like this: When the sarcomere units receive a signal (an electrical impulse) from the central nervous system, the actin and myosin filaments slide along each other, an action referred to as the sliding filament mechanism/theory. This creates the dynamic force needed for the contraction of a muscle. When a muscle contracts, its fibers shorten toward the center of the muscle. This is called the line of pull, or pulling force, and when enough pulling force is exerted on the muscle attachments on bones, it lifts or pulls the bones, creating movement.

Muscle Attachments (Tendons)

In most cases, a muscle attaches to two different bones (though sometimes more), generating movement, called joint action, at the joint between the bones when it contracts. The places where the muscle attaches are called the origin and insertion sites, and these beginning and ending points are generally on different bones because if they were on the same bone, the muscle would simply lock in place when trying to contract. There are exceptions to this rule, particularly regarding the muscles of the face, but we’ll deal with those separately.

The muscles do not attach directly to bones rather, the attachments are made via fibrous connective tissues called muscle tendons. The tendon at the muscle’s origin site is called the tendon of origin, or fixed attachment, because the bone to which it attaches stays more or less stationary, or fixed, during movement. The tendon at the muscle’s insertion site is referred to as the tendon of insertion, or mobile attachment, since it is connected to the bone that moves when the muscle is contracting. The following drawing, depicts the sartorius muscle of the upper leg, showing the tendon of origin on the upper end of the muscle and the tendon of insertion on the lower end.


Right upper leg, anterior view

Patterns of skeletal muscle attachments are, however, somewhat varied: Some have multiple origins and a single insertion while others have a single origin and multiple insertions. In some cases, it is hard to decide which is the fixed attachment and which the mobile attachment because the muscle changes roles during various movements. Some anatomists have therefore abandoned the terms origin and insertion, replacing them with proximal attachment and distal attachment, which identify the attachments according to their placement on the body rather than their role in movement. In this book, however, I follow traditional practice, referring to muscle attachments as origins and insertions.


How much you want to learn about muscle attachments is up to you, but this knowledge can be advantageous for an artist. Medical illustrators and forensic artists of course need to know the locations of attachments for superficial- and deep-layer muscles, but any artist whose intention is to create realistic (as oppose to stylized) figures, who draws from memory, or who works with figural movement will also find this information beneficial, because without this awareness, the muscular forms might appear inaccurate or possibly distorted. When certain tendons become prominent, then a fairly accurate depiction of how these tendons connect to bone is essential for the overall dynamics of that particular region of the body. An example is the tendon of insertion of the sternocleidomastoid (sternal portion) as it inserts into the upper part of the sternum this tendon projects quite strongly when the head is rotated. If the tendon is erroneously placed in the wrong location, it might cause that part of the figure to look peculiar.

Any depiction of muscle attachments, including those presented here, should be used as an approximate reference. Human beings are physically diverse, and through many dissections anatomists have found that there are slight variations in the locations of muscle attachments. So figurative artists should know approximately where the two (or more) ends of a muscle attach rather than worrying about the attachments’ precise location.

As I alluded to earlier, most facial muscles work differently from the skeletal muscles of the rest of the body. Except for the muscles controlling the mandible (lower jaw), facial muscles do not move any bones when their fibers contract because, aside from the mandible, the cranium consists of fused bones. Instead, they move soft-tissue forms, creating facial expressions. Similarities and differences between skeletal and facial muscle attachments are summarized in the following table.

Skeletal versus Facial Muscle Attachments



Origin: Muscle attaches to a bone with a tendon.

Origin: Muscle attaches to a bone with a tendon.

Insertion: Same muscle attaches to a different bone with a different tendon.

Insertion: Same muscle attaches into a soft-tissue structure such as skin, fascia, subcutaneous tissue, or another facial muscle.

Action: When the muscle contracts, the second bone moves.

Action: When the muscle contracts, the soft-tissue region moves, possibly creating facial movement.

Tendon Landmarks

Tendons come in a variety of shapes, including cordlike forms, flat wide sheaths, and thin flat strips. Although the subcutaneous layer of adipose (fatty) tissue obscures many tendons, a few that are shaped like elongated cords do make occasional appearances on the surface form. This generally occurs when a tendon’s muscle is contracting, pulling the tendon close to the skin. Indicating these tendons in figural studies can create a sense of dynamic tension, but the trick is to avoid making the tendons too obvious. For example, when depicting a series of tendons, such as those appearing on the dorsal (back) side of the hand, treat each one slightly differently: One or two can be prominent, the others lightly suggested otherwise, they might look like stiff spaghetti strands glued on the hand.

Cordlike fibrous structures are generally known as tendons, but broad flat sheathings of fibrous material are identified as aponeuroses (sing., aponeurosis). These wider sheets, which cover larger areas for muscle attachment, can be seen, for example, in the latissimus dorsi and external oblique muscles of the torso and abdominal regions. The following three drawings entitled Tendons and Aponeuroses&mdashSurface Form Landmarks, depict the front torso and arms, back torso and arms, and three views of the legs, showing the basic locations of the tendons of the main superficial muscles. Key bony landmarks and triangular surface-form characteristics are also shown.


Torso and arms, anterior view

ORANGE: Tendons and aponeuroses

GREEN: Triangular depressions and projections on surface form


Torso and arms, posterior view

ORANGE: Tendons and aponeuroses

GREEN: Triangular depressions and projections on surface form


Upper and lower leg, three views

ORANGE: Tendons and aponeuroses

GREEN: Triangular depressions and projections on surface form

Tendons of the Sternocleidomastoid Muscle of the Neck

Tendons from each of the sternal portions of the sternocleidomastoid (SCM) muscle attach into the manubrium of the sternum. Between them is the suprasternal notch (pit of the neck). When the head turns sideways in a rotational movement, one of the tendons becomes quite prominent on the surface form, as can be seen in the following portrait study.


Sanguine and brown pastel pencils and white chalk on toned paper.

Tendons of the Dorsal Side of the Hand

The four tendons of the extensor digitorum muscle of the lower arm insert into the four fingers. They are most easily seen on the surface when the fingers are spread apart forcefully. Because the skin on the dorsal side (back) of the hand is very thin, these tendons sometimes appear, though subtly, when a hand is more relaxed, depending on the position of the hand and the way the light source is illuminating it. Again, when drawing a tendon, avoid emphasizing both sides of the tendon in heavy, dark lines, as this will give the tendon a flat look. One side should be emphasized in a soft, tonal line while the other side is indicated in a lighter value (or with white chalk if drawing on a toned paper surface) to achieve a more natural, organic look, as in the following life study. If there is tension in the tendons, then by all means accentuate them&mdashbut be careful not to overdo it.


Sanguine and brown pastel pencils, charcoal, and white chalk on toned paper.

Tendons of the Anterior Region of the Lower Arm

The tendons of the various flexor muscles of the lower arm are usually seen on the surface when the hand clenches in a fist, as shown in the following life study. When the hand relaxes, the tendons become harder to detect.


Sanguine and brown pastel pencils, charcoal, and white chalk on toned paper.

Tendons of the Hamstring Muscles

The tendons of the hamstring muscles attach on both sides of the popliteal fossa, located on the back of the knee. When the knee bends, these tendons become much more visible on the surface, as can be seen in the bent left leg in the following life study. The tendon of the biceps femoris muscle becomes especially prominent. This muscle is located on the outer side of the upper leg, and its tendon attaches into the head of the fibula bone, which is positioned on the outer side of the lower leg.


Charcoal pencil, sanguine and brown pastel pencils, and white chalk on toned paper.

The Achilles Tendon

The Achilles tendon&mdashnamed for the warrior of Greek mythology who was killed by an arrow shot into his heel&mdashis the tendon of the gastrocnemius and soleus muscles of the lower leg. It inserts into the heel bone (calcaneus) and appears on the surface form, sometimes quite prominently, as a thick, ropelike structure. The Achilles tendons are clearly visible in the following life study.

Graphite pencil, ballpoint pen, colored pencil, and white chalk on toned paper.

Tendons of the Dorsal Side of the Foot

The four tendons of the extensor digitorum longus muscle of the lower leg attach into each of the four lesser toes. When the toes are spread apart in a forceful manner, these tendons become very obvious on the surface form. A slender but prominent tendon from the extensor hallucis longus (long extender of the great toe) inserts directly into the great toe. This tendon becomes particularly noticeable when the large toe points upward.

A ropelike tendon on the lower part of the tibialis anterior muscle (shin muscle) of the lower leg can be seen only when the foot is in certain positions. Many artists confuse this muscle with the great toe’s tendon and draw a continuous line from the tibialis anterior muscle directly into the great toe. This is fine for gesture studies, but in more detailed renderings you should try to distinguish the subtle separation between the two tendons near the ankle. In the life study below, I’ve slightly exaggerated the tendons to show their placement on the surface more clearly.

Graphite pencil, colored pencil, and white chalk on toned paper.

Muscle Contraction

Muscle contraction is also known as muscle action or muscle tension. Muscles can shorten their muscle fibers, lengthen them from a contracted state, or stabilize them at the same length. All these actions produce tension within the muscle fibers. When a muscle is not in any state of contraction, it is said to be relaxed, or in a “resting state.”

Muscle contractions can activate movement (initiating joint action), control the tempo of a movement (accelerating or slowing down joint action), or prevent unwanted movement by stabilizing a joint. The two basic categories of muscle contraction are called dynamic (isotonic) and static (isometric). Let’s look at each.

Dynamic Muscle Contraction

When a muscle changes length during a specific movement, either shortening its muscle fibers or lengthening them, this action is known as dynamic muscle contraction, or as isotonic contraction, dynamic muscle tension, or dynamic movement. There are two different types of dynamic muscle contraction: concentric and eccentric. Basically, concentric contractions shorten the muscle fibers and eccentric contractions lengthen them. For many years, these opposing actions were commonly referred to as squash and stretch in the animation industry, but there are other names for the actions, including stretching and compressing and extension and contraction.


Concentric and eccentric contractions

When a muscle is in a state of concentric contraction, the muscle fibers shorten toward their centers, and, in the process, pull a bone in a certain direction, causing movement at a joint. This type of action usually occurs in the “up phase” of a movement, as when lifting a barbell.

When a muscle is in a state of eccentric contraction, the muscle fibers lengthen from the contracted state and the muscle is returned to its resting length. This type of action usually occurs in the “down phase” of a movement, as when lowering a barbell.

Don’t confuse eccentric contraction with the state of rest, however. During eccentric contraction, the muscle fibers lengthen from a contracted state in a controlled manner, slowing down the movement against the influence of gravity. This smoothing out of a movement is known as the “braking force.” For example, when lifting a weight, the biceps brachii and brachialis muscles shorten their fibers (concentric contraction) to lift the forearm and hand holding the weight in an upward direction (the up phase). Then, when the weight is being lowered (the down phase), the biceps and brachialis lengthen their muscle fibers but in a controlled way that resists gravity and thus prevents the forearm from slamming down. Even though the muscle fibers are lengthening, there is tension within the muscle. The drawings on this page illustrate concentric/eccentric phases of dynamic muscle contraction.

When depicting any active or semi-active pose, you should try to locate any muscles that are in a state of compression or stretching. Visual clues include

·&emspcompact shape of a muscle

·&emspone muscle pressing against another muscle

·&emspa tendon protruding close to the surface due to tension within its muscle

I made the study at right from the ancient marble sculpture called Laocoön and His Sons (or The Laocoön Group), housed in the Vatican Museum, which depicts a man and his two sons writhing in agony as they are attacked by serpents. The focus of my study is the central figure, whose anguished, twisting action exemplifies the energetic dynamics of stretching and compression. I always recommend drawing directly from figurative sculpture, whether ancient or contemporary, whenever possible. The three-dimensional anatomical forms of stone or bronze statues appear so much more clearly than they do in photographs of sculptures. Plus, you can usually walk around the sculpture and draw it from different viewpoints.


Sanguine and brown pastel pencils and white chalk on toned paper.

Static Muscle Contraction

In static contraction&mdashalso called isometric contraction&mdasha muscle increases tension within its muscle fibers but does not change its length, thereby remaining stationary. No movement occurs at any joint, and in fact, this type of contraction stops movement altogether. Static contraction prepares muscles for possible action, as when a sprinter adopts a stationary position before taking off in a race. It is essential for maintaining posture (otherwise, gravitational forces would pull us down) and is activated when holding heavy objects stationary, as shown in next drawing. It also occurs when a muscle needs to stabilize a joint when movement is not wanted.



No matter how relaxed a person may look when standing or sitting, his or her muscles are still contracting to some degree to maintain the position and to prevent the person from being pulled down by gravity. Artist’s models have to maintain each pose for a length of time without moving or shifting their weight, so their bodies are always in a state of static contraction while posing. They can usually hold difficult action poses for a short time, but for longer poses they need to make sure their weight is balanced. The mark of a good model is being able to do a long pose and make it look interesting&mdashdynamic or lyrical, not symmetrical or stiff&mdashand to hold that pose (usually for a twenty-minute interval) without moving or twitching. In interpreting the pose, the artist will try to convey the tension or relaxation of the various anatomical forms, as I did in the following life study.


Colored pastel pencil, charcoal pencil, and white chalk on toned paper.

Muscles’ Differing Roles

When a particular movement occurs at a joint, several muscles in the vicinity of the joint participate. Although the main function of every muscle is to contract its muscle fibers, a muscle can play different roles at different times during a movement or series of movements. Generally speaking, there are four different roles muscles can play: agonist (prime mover), antagonist, synergist (assistor), and stabilizer (fixator).

Agonist (Prime Mover) and Antagonist

The muscle that is mainly responsible for activating a bone or body part for a specified movement is known as the agonist, or prime mover. When the agonist muscle contracts, it becomes a more compact version of itself. As this action is occurring, another muscle&mdashthe antagonist&mdashhas to act in opposition. This muscle, usually positioned on the side of the bone or body part opposite from the agonist, has to stretch its muscle fibers to yield to the agonist muscle’s contraction. This relationship can be clearly seen in the flexion and extension of the lower arm at the elbow joint, shown in the following drawing.


Upper and lower left arm, anterior view of scapula with arm moving sideways of torso

What happens is this: When the biceps brachii (located on the anterior portion of the upper arm) contracts its muscle fibers, it lifts up the lower arm, becoming a bulging shape. In this action, the biceps brachii is considered the agonist or prime mover (with the brachialis and brachioradialis acting as synergists, or assistors). (Some experts think that the brachialis, along with the biceps, is also a prime mover when it lifts the lower arm.) The antagonist muscle in this action is the triceps brachii, which is positioned on the posterior region of the upper arm. While the biceps is contracting, the triceps stretches its muscle fibers so that the elbow joint and lower arm can move freely, without interference. When the forearm is lowered, the biceps and triceps reverse their actions, switching roles. The triceps, which now contracts its muscle fibers, becomes the prime mover in lowering the forearm, while the biceps is now the antagonist, stretching its muscle fibers. (If only the lower arm is being moved, the humerus and scapula bones remain stationary, with the shoulder joint stabilized by the rhomboids, the trapezius, and the rotator cuff muscles of the scapula.)

Synergists and Stabilizers

As mentioned, a muscle usually does not move a bone or other body structure all by itself other muscles assist in the process. These synergist muscles, or assistor muscles, provide additional pull near the prime mover’s tendon of insertion. They also help prevent any unwanted actions that could occur during a particular movement.

While a particular movement occurs, other muscles may act as stabilizers, or fixators, holding a bone firmly in place&mdashusually the bone where the prime mover muscle originates. This prevents any unwanted movement from occurring so that the agonist and synergists can act more efficiently. When stabilizer muscles hold a bone stationary, their muscle fibers contract but do not shorten&mdashthe type of contraction called static or isometric contraction.

The Influence of Force on Anatomical Forms

The term force refers to energy that produces, modifies, or restrains movement of bodily components. There are two basic types: internal force, created within the body, and external force, produced outside the body. Internal force results from muscle contractions. When the nervous system sends a message to a muscle, telling it to contract, a tension is generated, creating movement at a joint. But it’s not just the muscle and bone that are affected. Soft-tissue anatomical forms such as tendons, ligaments, the subcutaneous layer, and the skin are also influenced by internal force and may change shape in any of the following ways, also schematized in the following drawing:

·&emspCompression of forms, which occurs during the contraction of a muscle, making it shorter or compressed, or when a form is pressing against an object

·&emspStretching of forms, or tension, which occurs when a muscle stretches or when forms are pulling in opposite directions

·&emspBending of forms, which occurs when two forms press against each other

·&emspTwisting or spiraling of forms, also called torsion or torque, which usually occurs when one structure twists in one direction while another twists in a different direction


Black arrows indicate the direction of the force influencing the anatomical form.

Combinations of these actions (called combined loading) can also occur, as when a figure or bodily structure twists while bending or stretches while twisting. There are countless variations, depending on the type of action taking place. When you draw the figure in an active pose, you’ll observe that many forms are influenced by the dynamics of force, and you can interpret that dynamism in your own unique way, infusing the forms and lines of the body with vitality and energy.

The gesture drawing called Study of Male Figure in Dynamic Twisting Movement, conveys the tremendous amount of energy the model is exhibiting in this pose. The twisting action at the waist region shows both tension and compression, and the swinging of the hair contributes to the figure’s overall rhythm. The gesture drawing Study of a Female Figure Bending, shows the anatomical forms stretching and compressing throughout.


Black Conté crayon on newsprint.


Black and brown Conté crayon on newsprint.

Of the external forces affecting the body, gravity may be the most important, because it is always at work, pulling everything&mdashincluding human bodies&mdashtoward the center of the earth. Certain muscles contract without moving any joints (isometric contraction) to counter the force of gravity and help hold the body upright. But there are other external forces that can strongly affect the body, making it difficult to maintain equilibrium. These include forces of nature (a turbulent wind or a large wave), physical impact with external objects, and combat with another body (wrestling, boxing).

When the body is performing an action requiring forceful exertion, such as pushing a heavy cart or lifting a heavy box, the external impact is channeled as tension, which is also created internally within the muscles. So external and internal forces can affect the human body simultaneously&mdashand, in fact, they do so almost constantly. Illustrators, storyboard artists, comic book artists, and animators (traditional and digital) all depict figures in various forms of movement being affected by external and internal forces of every kind.

Steps in Contraction

There are at least 2 fiber types in skeletal muscle:

TYPE 1, SLOW, RED MUSCLES (e.g. long muscles of back):
long latency
adapted for slow posture maintaining contractions and have moderate diameter
high oxidative capacity
large blood supply

TYPE IIB, FAST, WHITE MUSCLES (e.g. hand muscles):
short latency
adapted for fine, skilled movements and have large diameter
low oxidative capacity
less blood supply

Muscle differentiates from mesoderm
First muscular activity recorded in 8 week-embryo
Recognizable contractions observed after 16-18 fetal weeks when:
* nerve fibers to muscle are developed
* nerve fibers have achieved contact with muscle cells (myo-neural junction) and neurotransmission apparatus (acetylcholine/cholinesterase) has developed

Cross-innervation experiments:
* a specific substance is secreted at nerve endings, or
* a pattern of nerve impulses on muscle fibers acts to cetermine contraction velocity

Skeleto-muscular (M-S) development at adolescence

M-S development involves several systems:
Muscle: size, strength, metabolism, power
CNS: coordination of motor activity, voluntary, autonomic, motivation, fatigue
Respiration: provides O2, removes CO2
Circulation: circulates O2, nutrients
Temperature regulation: during exercise X20 heat production than at rest
Stress: corticoids
Hormones: insulin, GH, IGF-I, T3, Calcitonin, PTH, androgens, estrogens

Muscle fiber number is virtually fixed at birth
The increase in mass or Hypertrophy (sometimes as much as 50%) is due to increase in length and cross-sectional area of muscle fibers due to an increase in the number of myofibrils (from 75 to over 1000)
Capacity for plasticity and regeneration in response to neural, hormonal and nutritional influences

Development of muscles cells
Reader, pp287-293

With early development:
mesodermal origin:
*myoblasts (no distinguishable features)
*4th month, myotubules (myofibrils, some motor activity)
*increased myosin, actin, Ca++ channels

With further development:
*1-7 years, slow growth
*8-17 years, accelerated growth
*18-25 years, slow growth
*increased number of myofibrils
*increased number of nuclei
*hyperplasia, hypertrophy
*no built-in time limit to thickness/strength potential (increased physical activities, gymnastics)

At birth, all limb muscles have same contraction velocity
After birth, velocity differentiates in fast and slow muscles depending on innervation, ACh, AChE levels/activity, increased electrolytes, metabolic changes

Development of myoneural junction (MJ)
*MJ number increases with development
*increased acetylcholine (ACh) levels

Before birth, Ach sensitivity is spread through length of muscle
After birth, it is localized to MJ

Power output = rate of doing work
Work: moving force through a distance

Contraction may be:
isometric: muscle does not shorten but produces force
isotonic: force remains constant but muscle shortens

How is the Force of Contraction Applied to the Tendons by the Muscle's Individual Fibers? - Biology

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited.

Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

Critical Thinking Questions

Q. What would happen to skeletal muscle if the epimysium were destroyed?

A. Muscles would lose their integrity during powerful movements, resulting in muscle damage.

Q. Describe how tendons facilitate body movement.

A. When a muscle contracts, the force of movement is transmitted through the tendon, which pulls on the bone to produce skeletal movement.

Q. What are the five primary functions of skeletal muscle?

A. Produce movement of the skeleton, maintain posture and body position, support soft tissues, encircle openings of the digestive, urinary, and other tracts, and maintain body temperature.

Q. What are the opposite roles of voltage-gated sodium channels and voltage-gated potassium channels?

A. The opening of voltage-gated sodium channels, followed by the influx of Na + , transmits an Action Potential after the membrane has sufficiently depolarized. The delayed opening of potassium channels allows K + to exit the cell, to repolarize the membrane.

Bobbert MF, Van Ingen Schenau GJ (1990) Isokinetic plantar flexion: experimental results and model calculations. J Biomech 23:105–119

Bobbert MF, Brand C, De Haan A, Huijing PA, Van Ingen Schenau GJ, Rijnsburger WH, Woittiez RD (1986a) Series elasticity of tendinous structures of rat EDL. J Physiol (Lond) 377:89P

Bobbert MF, Huijing PA, Van Ingen Schenau GJ (1986b) A model of the human triceps surae muscle-tendon complex applied to jumping. J Biomech 19:887–898

Ettema GJC, Huijing PA (1989) Properties of the tendinous structures and series elastic component of EDL muscle-tendon complexes of the rat. J Biomech 22:1209–1215

Gordon AM, Huxley AF, Julian FJ (1966) The variation in isometric tension with sarcomere length in vertebrate muscle fibers. J Physiol (Lond) 184:170–192

Huijing PA (1985) Architecture of the human gastrocnemius muscle and some functional consequences. Acta Anat 123:101–107

Huijing PA (1988) Determinants of length range of active force exertion. In: Harris G, Walker C (eds) Proceedings of the Annual Conference of IEEE Engineering in Medicine and Biology Society, vol 10, part 4, Publishing Service IEEE, New York, pp 1665–1666

ter Keurs HEDJ, Iwazumi T, Pollack GH (1978) The sarcomere length-tension relationship in skeletal muscle. J Gen Physiol 72:574–592

ter Keurs HEJD, Luff AR, Luff SE (1981) The relationship of force to sarcomere length and filament lengths of rat extensor digitorum muscle. J Physiol (Lond) 317:24P

Otten E (1988) Concepts and models of functional architecture in skeletal muscle. Exerc Sports Sci Rev 16:89–137

Stephens JA, Reinking RM, Stuart DS (1975) The motor units of cat medial gastrocnemius: electrical and mechanical properties as a function of muscle length. J Morphol 146:495–512

Stephenson DG, Stewart AW, Wilson GJ (1989) Dissociation of force from myofibrillar MgATPase and stiffness at short sarcomere lengths in rat and toad skeletal muscle. J Physiol (Lond) 410:351–366

Walker SM, Schrodt GR (1974) I segment length and thin filament periods in skeletal muscle fibers of the rhesus monkey and the human. Anat Rec 178:63–82

Woittiez RD, Huijing PA, Boom HBK, Rozendal RH (1984) A three dimensional muscle model: a quantified relation between form and function of skeletal muscles. J Morphol 182:95–113

Woittiez RD, Brand C, De Haan A, Hollander AP, Huijing PA, Van der Tak R, Rijnsburger WH (1987) A multipurpose muscle ergometer. J Biomech 20:215–218


Until recently only few clinical studies existed on the treatment of muscle injuries, and thus, the current treatment principles of muscle injuries were mostly based on experimental studies. However, with the recent surge of the published RCTs on injured skeletal muscle some foundation of knowledge can be derived from these well executed studies.

Clinically, the first aid of muscle injuries follows the RICE principle (Rest, Ice, Compression and Elevation), the principle common to the treatment of any soft tissue trauma. The objective of the use of the RICE is to stop the intramuscular bleeding and thereby limit the progression of the muscle injury to a minimum. Clinical examination should be carried out immediately after the trauma and 1𠄳 days thereafter, at which point the imaging modalities (MRI or ultrasound) can provide useful insights into the severity of the injury and the injury classified according to the new classification scheme ( Tab. 1 ). During the first few days after the injury, a short period of immobilization accelerates the formation of granulation tissue at the site of injury, but it should be noted that the duration of reduced activity (immobilization) ought to be limited only until the scar reaches sufficient strength to bear the muscle-contraction induced pulling forces without re-rupture. At this point, gradual mobilization should be started followed by a progressively intensified exercise program to optimize the healing by restoring the strength of the injured muscle, preventing the muscle atrophy, the loss of strength and the extensibility, all of which can follow prolonged immobilization. Based on the current knowledge, the rehabilitation program should consist of progressive agility and trunk stabilization exercises as well as exercised tailored to lengthen (eccentric exercises) the injured skeletal muscle9.