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TIPOS DE AÇÃO MUSCULAR
Luciana De Michelis Mendonça
Cinesiologia
Contribuição Prof. Giovanna Amaral
Função dos músculos
42 Secdon I Essential Topics o f Kinesiology
T A B L E 3 - 1. Major Concepts: Muscle as a Skeletal 
Stabilizer
1. Muscle morphoiogy
2. Strutturai organization of skeletal muscle
3. Connettive tissues vvithin muscle
4. Physiologic cross-sectional area
5. Pennation angle
6. Passive length-tension curve
7. Parallel and series elastic components of muscle and ten- 
don
8. Elastic and viscous properties of muscle
9. Attive length-tension curve
10. Histology of thè muscle fìber
11. Total length-tension curve
12. Isometric force and internai torque-joint angle curve devel- 
opment
13. Mechanical and physiologic properties affecting internai 
torque-joint angle curve
muscle is stimulated by thè nervous System. The relationship 
between muscle force and length and how it influences thè 
isometric torque generated about a joint are then examined. 
Table 3 - 1 is a summary of thè major concepts addressed in 
this section.
Muscle Morphoiogy: Shape and Structure
Muscle morphoiogy describes thè basic shape of a vvhole 
muscle. Muscles have many shapes, reflecting their ultimale 
function. Figure 3 - 1 shows two common shapes of muscle: 
fusiform and pennate (from thè Latin penna, meaning 
feather). Fusiform muscles, such as thè biceps bracini, have 
fìbers running parallel to each other and to thè centrai ten- 
don. In pennate muscles, thè fìbers approach thè centrai ten- 
don obliquely. Pennate muscles may be further classified as 
unipennate, bipennate, or multipennate, depending on thè 
number of similarly angled sets of fìbers that attach into thè 
centrai tendon.
The muscle fiber is thè structural unii of muscle, ranging 
in thickness from about 10 to 100 micrometers, and length 
from about 1 to 50 cm .17 Each muscle fìber is actually an 
individuai celi with multiple nuclei. The connettive tissue 
that surrounds and supports muscle serves many roles. Simi- 
lar to connettive tissue throughout other bodily structures, 
thè connettive tissue within muscle consists of fìbers embed- 
ded in an amorphous ground substance. Most fìbers are 
collagen, and thè remaining fìbers are elastin. The combina- 
tion of these two proteins provides strength, structural sup- 
port, and elasticity io muscle.
Three different, although structurally related, sets of con-
nettive tissue occur in muscle: epimysium, perimysium, and 
endomysium (Fig. 3 - 2 ) . The epimysium is a tough strutture 
that surrounds thè entire surface of thè muscle belly and 
separates it from other muscles. In essence, thè epimysium 
gives form to thè muscle belly. The epimysium contains 
tightly woven bundles of collagen fìbers that are highly resis-
tive io stretch. The perimysium lies beneath thè epimysium, 
and divides muscle into fascicles that provide a conduit for 
blood vessels and nerves. This connettive tissue, like epi-
mysium, is tough and thick and resistive to stretch. The 
endomysium surrounds individuai muscle fìbers. It is com- 
posed of a relatively dense meshwork of collagen ftbrils that 
are partly connected to thè perimysium. Through lateral 
connections to thè muscle fìber, thè endomysium conveys 
part of thè contrattile force to thè tendon.
Although thè three types of connettive tissues are de- 
scribed as separate entities, they are interwoven in such a 
way that they may be considered as a continuous sheet of 
connettive tissue. All connettive tissue that encases a mus-
cle, directly or indirectly, contributes to thè tendons of thè 
muscle.
Muscle Architecture
Each muscle and its tendons have different architecture and, 
as a consequence, are able to generate different ranges of 
force. Understanding muscle architecture allows thè predic- 
tion of thè functional role of a given muscle. Physiologic 
cross-sectional area and pennation angle are major determi- 
nants of thè range and thè force produced by thè muscle.
The physiologic cross-sectional area of a muscle reflects thè 
amount of contrattile protein available to generate force. 
Generally speaking, thè cross-sectional area (cm2) of a fusi-
form muscle is determined by dividing thè muscles volume 
(cm1) by its length (cm). A fusiform muscle with many thick 
fìbers has a greater cross-sectional area than a muscle of 
similar length and morphoiogy with fewer thinner fìbers. 
Maximal force potential o f a muscle is, therefore, proportional to 
thè sum o f thè cross-sectional area o f all thè fìbers. Under 
normal conditions, thè thicker thè muscle, thè greater thè 
force potential. Measuring thè cross-sectional area of a fusi-
form muscle is relatively simple because all fìbers run paral-
Pennate Fusiform
FIGURE 3 -1 . Two common shapes of muscle, fusiform and pen-
nate, are shown. Different shapes are formed by different fiber 
orientation relative to thè connecting tendon. (Modifìed from Wil-
liams PL: Gray’s Anatomy: The Anatomical Basis of Medicine and 
Surgery, 38th ed. New York, Churchill Livingstone, 1995.)
Funções dos músculos 
•  Mobilidade 
–  Produção de movimento 
–  Componente rotatório 
•  Estabilidade 
–  Aproximação de superfícies 
articulares 
–  Componente translatório 
•  As funções de estabilidade e 
mobilidade dependem da 
estrutura e biomecânica dos 
músculos e das articulações 
Arquitetura muscular
42 Secdon I Essential Topics o f Kinesiology
T A B L E 3 - 1. Major Concepts: Muscle as a Skeletal 
Stabilizer
1. Muscle morphoiogy
2. Strutturai organization of skeletal muscle
3. Connettive tissues vvithin muscle
4. Physiologic cross-sectional area
5. Pennation angle
6. Passive length-tension curve
7. Parallel and series elastic components of muscle and ten- 
don
8. Elastic and viscous properties of muscle
9. Attive length-tension curve
10. Histology of thè muscle fìber
11. Total length-tension curve
12. Isometric force and internai torque-joint angle curve devel- 
opment
13. Mechanical and physiologic properties affecting internai 
torque-joint angle curve
muscle is stimulated by thè nervous System. The relationship 
between muscle force and length and how it influences thè 
isometric torque generated about a joint are then examined. 
Table 3 - 1 is a summary of thè major concepts addressed in 
this section.
Muscle Morphoiogy: Shape and Structure
Muscle morphoiogy describes thè basic shape of a vvhole 
muscle. Muscles have many shapes, reflecting their ultimale 
function. Figure 3 - 1 shows two common shapes of muscle: 
fusiform and pennate (from thè Latin penna, meaning 
feather). Fusiform muscles, such as thè biceps bracini, have 
fìbers running parallel to each other and to thè centrai ten- 
don. In pennate muscles, thè fìbers approach thè centrai ten- 
don obliquely. Pennate muscles may be further classified as 
unipennate, bipennate, or multipennate, depending on thè 
number of similarly angled sets of fìbers that attach into thè 
centrai tendon.
The muscle fiber is thè structural unii of muscle, ranging 
in thickness from about 10 to 100 micrometers, and length 
from about 1 to 50 cm .17 Each muscle fìber is actually an 
individuai celi with multiple nuclei. The connettive tissue 
that surrounds and supports muscle serves many roles. Simi- 
lar to connettive tissue throughout other bodily structures, 
thè connettive tissue within muscle consists of fìbers embed- 
ded in an amorphous ground substance. Most fìbers are 
collagen, and thè remaining fìbers are elastin. The combina- 
tion of these two proteins provides strength, structural sup- 
port, and elasticity io muscle.
Three different, although structurally related, sets of con-
nettive tissue occur in muscle: epimysium, perimysium, and 
endomysium (Fig. 3 - 2 ) . The epimysium is a tough strutture 
that surrounds thè entire surface of thè muscle belly and 
separates it from other muscles. In essence, thè epimysium 
gives form to thè muscle belly. The epimysium contains 
tightly woven bundles of collagen fìbers that are highlyresis-
tive io stretch. The perimysium lies beneath thè epimysium, 
and divides muscle into fascicles that provide a conduit for 
blood vessels and nerves. This connettive tissue, like epi-
mysium, is tough and thick and resistive to stretch. The 
endomysium surrounds individuai muscle fìbers. It is com- 
posed of a relatively dense meshwork of collagen ftbrils that 
are partly connected to thè perimysium. Through lateral 
connections to thè muscle fìber, thè endomysium conveys 
part of thè contrattile force to thè tendon.
Although thè three types of connettive tissues are de- 
scribed as separate entities, they are interwoven in such a 
way that they may be considered as a continuous sheet of 
connettive tissue. All connettive tissue that encases a mus-
cle, directly or indirectly, contributes to thè tendons of thè 
muscle.
Muscle Architecture
Each muscle and its tendons have different architecture and, 
as a consequence, are able to generate different ranges of 
force. Understanding muscle architecture allows thè predic- 
tion of thè functional role of a given muscle. Physiologic 
cross-sectional area and pennation angle are major determi- 
nants of thè range and thè force produced by thè muscle.
The physiologic cross-sectional area of a muscle reflects thè 
amount of contrattile protein available to generate force. 
Generally speaking, thè cross-sectional area (cm2) of a fusi-
form muscle is determined by dividing thè muscles volume 
(cm1) by its length (cm). A fusiform muscle with many thick 
fìbers has a greater cross-sectional area than a muscle of 
similar length and morphoiogy with fewer thinner fìbers. 
Maximal force potential o f a muscle is, therefore, proportional to 
thè sum o f thè cross-sectional area o f all thè fìbers. Under 
normal conditions, thè thicker thè muscle, thè greater thè 
force potential. Measuring thè cross-sectional area of a fusi-
form muscle is relatively simple because all fìbers run paral-
Pennate Fusiform
FIGURE 3 -1 . Two common shapes of muscle, fusiform and pen-
nate, are shown. Different shapes are formed by different fiber 
orientation relative to thè connecting tendon. (Modifìed from Wil-
liams PL: Gray’s Anatomy: The Anatomical Basis of Medicine and 
Surgery, 38th ed. New York, Churchill Livingstone, 1995.)
Arquitetura/estrutura à função/ação
Fusiforme = transmitem 100% da força 
para o tendão
Penados = produzem mais força 
80% fibras tipo 1 (resistência)
Multipenado (maior área de secção 
transversa)
Mesmo “perdendo” força por ser 
penado, o sóleo transmite muita 
força para o tendão
44 Section I Essential Topics o f Kinesiology
FIGURE 3-3. Unipennate muscle is shown with thè muscle ftbers 
oriented at a 30-degree angle of pennation (0),
physiologic cross-sectional area. As shown in Figure 3 - 3 , a 
pennation angle of 30 degrees stili enables thè fibers to 
transfer 86% of their force through thè long axis of thè 
tendon.
Muscle and Tendon: Generation of Force
PASSIVE LENGTH-TENSION CURVE
Muscle contains contractile proteins that are embedded 
within a network of connective tissues, namely, thè epimys- 
ium, perimysium, and endomysium. Table 3 - 2 lists thè 
functions of these tissues. Connective tissues are slightly 
elastic and, like a rubber band, generate resistive force (i.e., 
tension) when elongated.
For functional rather than anatomie purposes, thè con-
nective tissues within thè muscle and tendon have been 
described as thè paraìlel elastic component and thè series elas-
tic component. Elongation or stretch of thè whole muscle 
lengthens thè connective tissue elements (Fig. 3 - 4 ) . The 
paraìlel elastic component refers to thè connective tissues
TABLE 3 - 2 . Functions of Connective Tissue 
within Muscle
1. Provides gross structure to muscle
2. Serves as a conduit for blood vessels and nerves
3. Generates passive tension by resisting stretch 
4 Assists muscle to regain shape after stretch
5. Conveys contractile force to thè tendon and across thè joint
that surround or lie paraìlel to thè proteins that cause thè 
muscle to contract. The series elastic component, in contrast, 
refers to thè connective tissues within thè tendon. Because 
thè tendon lies in series with thè contractile proteins, active 
forces produced by these proteins are transferred directly to 
thè bone and across thè joint. Stretching a muscle by ex- 
tending a joint elongates both thè paraìlel elastic component 
and thè series elastic component, generating a springlike 
resistance, or stiffness, in thè muscle. The resistance is re- 
ferred to as a passive tension because it is does not depend 
on active or volitional contraction. The concept of paraìlel 
and serial elastic components is a simplifìed description of 
thè anatomy; however, it is useful to explain thè levels of 
resistance generated by a stretched muscle.
The tendon has several unique mechanical properties. Be-
cause of thè longitudinal orientation and thickness of its 
collagen fibers, thè tendon can resist large forces that might 
otherwise damage thè muscle tissue. Muscle fibers decrease 
in diameter by as much as 90% as they blend with thè 
tendon tissue.12 As a result, thè force through a muscle fiber 
per cross-sectional area (i.e., stress) increases significantly. At 
each end of a muscle fiber is an extensive folding of thè 
plasmalemma (i.e., thè membrane surrounding thè muscle 
fiber), which interdigitates with thè connective tissue of thè 
tendon. This folding ensures that high forces can be distrib- 
uted over a large area, thus reducing thè stress on thè 
muscle.
When thè paraìlel and series elastic components are 
stretched within a muscle, a generalized passive length-tension 
curve is generated (Fig. 3 - 5 ) . The curve is similar to that 
obtained by stretching a rubber band. Approximating thè 
shape of an exponential mathematica! function, thè passive
Bone Paraìlel EC
FIGURE 3-4. Contractile components 
and elastic components (EC) that 
generate force in muscle tissue are 
shown. The contractile component 
represents thè actin and myosin 
crossbridge structures. The paraìlel 
elastic component (paraìlel to thè 
contractile component) represents 
muscle connective tissue. The series 
elastic component (in series with thè 
whole muscle) represents thè connec-
tive tissues within thè tendon. The 
paraìlel and series connective tissues 
act in a manner similar to a spring.
Contração muscular
compacted collagen fibers that attach directly or indi-
rectly to muscles, fasciae, bones, cartilage, and other
muscles. Aponeuroses distribute forces generated by
the muscle to the structures to which they are attached.1
! Parallel and Series Elastic Components of Muscle
All of the connective tissue in a muscle is intercon-
nected and constitutes the passive elastic component of
a muscle. The connective tissues that surround the mus-
cle, plus the sarcolemma, the elastic protein titin, and
other structures (i.e., nerves and blood vessels), form
the parallel elastic component of a muscle. When a mus-
cle lengthens or shortens, these tissues also lengthen or
shorten, because they function in parallel to the muscle
contractile unit. For example, the collagen fibers in the
perimysium of fusiform muscles are slack when the sar-
comeres are at rest but straighten out and become taut
as sarcomere lengths increase. As the perimysium is
lengthened, it also becomes stiffer (resistance to fur-
ther elongation increases). The increased resistance of
perimysium to elongation may prevent overstretching
of the muscle fiber bundles and may contribute to the
tension at the tendon.31 When sarcomeres shorten from
their resting position, the slack collagen fibers within
the parallel elastic component buckle (crimp) even fur-
ther. Whatever tension might have existed in the colla-
gen at rest is diminished by the shortening of the
sarcomere. Because of the many parallel elastic compo-
nents of a muscle, the increase or decrease in passive
tension can substantially affect the total tensionoutput
of a muscle. This relationship between length and ten-
sion will be addressed in the next section.
The tendon of the muscle is considered to function
in series with the contractile elements. This means that
the tendon will be under tension when the muscle
actively produces tension. When the contractile ele-
ments in a muscle actively shorten, they exert a pull on
the tendon. The pull must be of sufficient magnitude to
take up the slack (compliance) in the tendon so that
the muscle pull can be transmitted through the tendon
to the bony lever (Fig. 3-12). Fortunately, the compli-
ance (or extensibility) of the tendon is relatively small
(about 3% to 10% in human muscles). Thus, most of
the muscle force can be used for moving the bony lever
and is not dissipated stretching the tendon. The tendon
is also under tension when a muscle is controlling or
braking the motion of the lever in an eccentric con-
traction. A tendon is under reduced tension only when
a muscle is completely relaxed and in a relatively short-
ened position.
122 ! Section 1: Joint Structure and Function: Foundational Concepts
" Figure 3-10 ! Iliotibial tract. A lateral view of the left lower
limb showing the deep fascial iliotibial tract extending from the
tubercle of the iliac crest to the lateral aspect of the knee. The right
arrow represents the pull of the gluteus maximus. The left arrow rep-
resents the pull of the tensor fasciae latae.
" Figure 3-11 ! Retinacula. A. The superior and inferior reti-
nacula are shown in their normal position, in which they form a tun-
nel for the tendons from the extensor muscles of the lower leg. B.
When the retinacula are torn or removed, the tendons move anteri-
orly.
" Figure 3-12 ! Series elastic component. A. The muscle is
shown in a relaxed state with the tendon slack (crimping or buckling
of collagen fibers has occurred). The sarcomere depicted above the
muscle shows minimal overlap of thick and thin filaments and little
cross-bridge formation. B. The muscle in an actively shortened posi-
tion shows that the tendons are under tension and no crimp can be
observed. The sarcomere depicted above the muscle shows extensive
overlap of filaments and cross-bridge formation.
03Levengie(F)-03 05/14/2005 3:45 PM Page 122
Copyright © 2005 by F. A. Davis.
Contração muscular
tension. The muscle fiber will shorten (contract) if a
sufficient number of sarcomeres actively shorten and 
if either one or both ends of the muscle fiber are free 
to move. The active shortening of a muscle is called a
concentric contraction, or shortening contraction (Fig.
3-6). In contrast to a shortening contraction, in which
the thin filaments are being pulled toward the thick fil-
aments, the muscle may undergo an eccentric contrac-
tion, or lengthening contraction. In a lengthening
contraction, the thin filaments are pulled away from the
thick filaments, and cross-bridges are broken and re-
formed as the muscle lengthens. Tension is generated
by the muscle as cross-bridges are re-formed. Eccentric
contractions occur whenever a muscle actively resists
motion created by an external force (such as gravity).
The muscle fiber will not change length if the force cre-
ated by the cross-bridge cycling is matched by the exter-
nal force. The contraction of a muscle fiber without
changing length is called an isometric contraction.
116 ! Section 1: Joint Structure and Function: Foundational Concepts
" Figure 3-5 ! Cross-bridge cycle. A. Rest. Cross-bridges proj-
ect from a myosin myofilament but are not coupled with an actin
myofilament. Adenosine triphosphate (ATP) is attached near the
head of the cross-bridge; troponin covers the active sites on the actin
myofilament; and calcium ions are stored in the sarcoplasmic reticu-
lum. B. Coupling. Arrival of the muscle action potential depolarizes
the sarcolemma and T tubules; calcium ions are released and react
with troponin; and change in the shape of the troponin-calcium com-
plex uncovers active sites on actin; a cross-bridge couples with an
adjacent active site, thereby linking myosin and actin myofilaments.
C. Contraction. Linkage of a cross-bridge and an active site triggers
adenosine triphospatase (ATPase) activity of myosin; ATP splits into
adenosine diphosphate (ADP) ! PO4 ! energy; the reaction pro-
duces a transient flexion of the cross-bridge; the actin myofilament is
pulled a short distance past the myosin myofilament; and Z disks are
moved closer together. D. Recharging. The cross-bridge uncouples
from the active site and retracts; ATP is replaced on the cross-bridge.
The recoupling, flexion, uncoupling, retraction, and recharging
processes are repeated hundreds of times per second. E. Relaxation.
Cessation of excitation occurs; calcium ions are removed from the
vicinity of the actin myofilament and are returned to storage sites in
the sarcoplasmic reticulum; troponin returns to its original shape,
covering active sites on the actin myofilament; and actin and myosin
myofilaments return to the rest state. (From Smith LK, Weiss EL,
Lemkuhl LD [eds]: Brunnstrom’s Clinical Kinesiology, 5th ed.
Philadelphia, FA Davis, 1996, p 83, with permission.)
A.
B.
C.
Isometric
Concentric
Eccentric
" Figure 3-6 ! Types of muscle contraction from the perspec-
tive of change (or lack of change) in sarcomere length during the
contraction. A. Isometric contraction with no change in length. B.
Concentric or shortening muscle contraction. C. Eccentric or length-
ening muscle contraction. The top illustration in each type of con-
traction represents the beginning of the contraction, and the bottom
illustration represents the end of the contraction.
03Levengie(F)-03 05/14/2005 3:45 PM Page 116
Copyright © 2005 by F. A. Davis.
tension. The muscle fiber will shorten (contract) if a
sufficient number of sarcomeres actively shorten and 
if either one or both ends of the muscle fiber are free 
to move. The active shortening of a muscle is called a
concentric contraction, or shortening contraction (Fig.
3-6). In contrast to a shortening contraction, in which
the thin filaments are being pulled toward the thick fil-
aments, the muscle may undergo an eccentric contrac-
tion, or lengthening contraction. In a lengthening
contraction, the thin filaments are pulled away from the
thick filaments, and cross-bridges are broken and re-
formed as the muscle lengthens. Tension is generated
by the muscle as cross-bridges are re-formed. Eccentric
contractions occur whenever a muscle actively resists
motion created by an external force (such as gravity).
The muscle fiber will not change length if the force cre-
ated by the cross-bridge cycling is matched by the exter-
nal force. The contraction of a muscle fiber without
changing length is called an isometric contraction.
116 ! Section 1: Joint Structure and Function: Foundational Concepts
" Figure 3-5 ! Cross-bridge cycle. A. Rest. Cross-bridges proj-
ect from a myosin myofilament but are not coupled with an actin
myofilament. Adenosine triphosphate (ATP) is attached near the
head of the cross-bridge; troponin covers the active sites on the actin
myofilament; and calcium ions are stored in the sarcoplasmic reticu-
lum. B. Coupling. Arrival of the muscle action potential depolarizes
the sarcolemma and T tubules; calcium ions are released and react
with troponin; and change in the shape of the troponin-calcium com-
plex uncovers active sites on actin; a cross-bridge couples with an
adjacent active site, thereby linking myosin and actin myofilaments.
C. Contraction. Linkage of a cross-bridge and an active site triggers
adenosine triphospatase (ATPase) activity of myosin; ATP splits into
adenosine diphosphate (ADP) ! PO4 ! energy; the reaction pro-
duces a transient flexion of the cross-bridge; the actin myofilament is
pulled a short distance past the myosin myofilament; and Z disks are
moved closer together. D. Recharging. The cross-bridge uncouples
from the active site and retracts; ATP is replaced on the cross-bridge.
The recoupling,flexion, uncoupling, retraction, and recharging
processes are repeated hundreds of times per second. E. Relaxation.
Cessation of excitation occurs; calcium ions are removed from the
vicinity of the actin myofilament and are returned to storage sites in
the sarcoplasmic reticulum; troponin returns to its original shape,
covering active sites on the actin myofilament; and actin and myosin
myofilaments return to the rest state. (From Smith LK, Weiss EL,
Lemkuhl LD [eds]: Brunnstrom’s Clinical Kinesiology, 5th ed.
Philadelphia, FA Davis, 1996, p 83, with permission.)
A.
B.
C.
Isometric
Concentric
Eccentric
" Figure 3-6 ! Types of muscle contraction from the perspec-
tive of change (or lack of change) in sarcomere length during the
contraction. A. Isometric contraction with no change in length. B.
Concentric or shortening muscle contraction. C. Eccentric or length-
ening muscle contraction. The top illustration in each type of con-
traction represents the beginning of the contraction, and the bottom
illustration represents the end of the contraction.
03Levengie(F)-03 05/14/2005 3:45 PM Page 116
Copyright © 2005 by F. A. Davis.
Tipos de contração muscular
Isotônica
• Concêntrica: FM > forças externas
• Excêntrica: forças externas > FM
Isométrica
•Comprimento inalterado
• FM= forças externas
• encurtamento de elementos
contráteis e alongamento dos elásticos
Tipos de contração muscular
Tipo de contração muscular 
•  Contração muscular = 
ação muscular: 
– Concêntrica 
– Excêntrica 
–  Isométrica 
Contração concêntrica
Prof. Fabiano Botelho Siqueira, MSc. 
Contração concêntrica 
•  Músculo produz torque suficiente para �vencer� 
resistência (peso, gravidade, etc.) 
•  Ossos aos quais o músculo insere se movem em 
direção ao músculo 
•  Movimento ocorre na direção da ação principal do 
músculo 
 
Prof. Fabiano Botelho Siqueira, MSc. 
Contração concêntrica 
•  Músculo produz torque suficiente para �vencer� 
resistência (peso, gravidade, etc.) 
•  Ossos aos quais o músculo insere se movem em 
direção ao músculo 
•  Movimento ocorre na direção da ação principal do 
músculo 
 
tension. The muscle fiber will shorten (contract) if a
sufficient number of sarcomeres actively shorten and 
if either one or both ends of the muscle fiber are free 
to move. The active shortening of a muscle is called a
concentric contraction, or shortening contraction (Fig.
3-6). In contrast to a shortening contraction, in which
the thin filaments are being pulled toward the thick fil-
aments, the muscle may undergo an eccentric contrac-
tion, or lengthening contraction. In a lengthening
contraction, the thin filaments are pulled away from the
thick filaments, and cross-bridges are broken and re-
formed as the muscle lengthens. Tension is generated
by the muscle as cross-bridges are re-formed. Eccentric
contractions occur whenever a muscle actively resists
motion created by an external force (such as gravity).
The muscle fiber will not change length if the force cre-
ated by the cross-bridge cycling is matched by the exter-
nal force. The contraction of a muscle fiber without
changing length is called an isometric contraction.
116 ! Section 1: Joint Structure and Function: Foundational Concepts
" Figure 3-5 ! Cross-bridge cycle. A. Rest. Cross-bridges proj-
ect from a myosin myofilament but are not coupled with an actin
myofilament. Adenosine triphosphate (ATP) is attached near the
head of the cross-bridge; troponin covers the active sites on the actin
myofilament; and calcium ions are stored in the sarcoplasmic reticu-
lum. B. Coupling. Arrival of the muscle action potential depolarizes
the sarcolemma and T tubules; calcium ions are released and react
with troponin; and change in the shape of the troponin-calcium com-
plex uncovers active sites on actin; a cross-bridge couples with an
adjacent active site, thereby linking myosin and actin myofilaments.
C. Contraction. Linkage of a cross-bridge and an active site triggers
adenosine triphospatase (ATPase) activity of myosin; ATP splits into
adenosine diphosphate (ADP) ! PO4 ! energy; the reaction pro-
duces a transient flexion of the cross-bridge; the actin myofilament is
pulled a short distance past the myosin myofilament; and Z disks are
moved closer together. D. Recharging. The cross-bridge uncouples
from the active site and retracts; ATP is replaced on the cross-bridge.
The recoupling, flexion, uncoupling, retraction, and recharging
processes are repeated hundreds of times per second. E. Relaxation.
Cessation of excitation occurs; calcium ions are removed from the
vicinity of the actin myofilament and are returned to storage sites in
the sarcoplasmic reticulum; troponin returns to its original shape,
covering active sites on the actin myofilament; and actin and myosin
myofilaments return to the rest state. (From Smith LK, Weiss EL,
Lemkuhl LD [eds]: Brunnstrom’s Clinical Kinesiology, 5th ed.
Philadelphia, FA Davis, 1996, p 83, with permission.)
A.
B.
C.
Isometric
Concentric
Eccentric
" Figure 3-6 ! Types of muscle contraction from the perspec-
tive of change (or lack of change) in sarcomere length during the
contraction. A. Isometric contraction with no change in length. B.
Concentric or shortening muscle contraction. C. Eccentric or length-
ening muscle contraction. The top illustration in each type of con-
traction represents the beginning of the contraction, and the bottom
illustration represents the end of the contraction.
03Levengie(F)-03 05/14/2005 3:45 PM Page 116
Copyright © 2005 by F. A. Davis.
Tipos de contração 
•  Concêntrica 
–  Actina desliza sobre 
miosina 
–  Músculo encurta 
–  Tensão se 
desenvolve 
Contração concêntrica
Contração isométrica
Prof. Fabiano Botelho Siqueira, MSc. 
Contração isométrica 
•  Torque produzido pelo músculo igual torque 
produzido por forças externas ou contrárias 
ao movimento 
•  Não há movimento visível no comprimento 
muscular 
Prof. Fabiano Botelho Siqueira, MSc. 
Contração isométrica 
•  Torque produzido pelo músculo igual torque 
produzido por forças externas ou contrárias 
ao movimento 
•  Não há movimento visível no comprimento 
muscular 
tension. The muscle fiber will shorten (contract) if a
sufficient number of sarcomeres actively shorten and 
if either one or both ends of the muscle fiber are free 
to move. The active shortening of a muscle is called a
concentric contraction, or shortening contraction (Fig.
3-6). In contrast to a shortening contraction, in which
the thin filaments are being pulled toward the thick fil-
aments, the muscle may undergo an eccentric contrac-
tion, or lengthening contraction. In a lengthening
contraction, the thin filaments are pulled away from the
thick filaments, and cross-bridges are broken and re-
formed as the muscle lengthens. Tension is generated
by the muscle as cross-bridges are re-formed. Eccentric
contractions occur whenever a muscle actively resists
motion created by an external force (such as gravity).
The muscle fiber will not change length if the force cre-
ated by the cross-bridge cycling is matched by the exter-
nal force. The contraction of a muscle fiber without
changing length is called an isometric contraction.
116 ! Section 1: Joint Structure and Function: Foundational Concepts
" Figure 3-5 ! Cross-bridge cycle. A. Rest. Cross-bridges proj-
ect from a myosin myofilament but are not coupled with an actin
myofilament. Adenosine triphosphate (ATP) is attached near the
head of the cross-bridge; troponin covers the active sites on the actin
myofilament; and calcium ions are stored in the sarcoplasmic reticu-
lum. B. Coupling. Arrival of the muscle action potential depolarizes
the sarcolemma and T tubules; calcium ions are released and react
with troponin; and change in the shape of the troponin-calcium com-
plex uncovers active sites on actin; a cross-bridge couples with an
adjacent active site, thereby linking myosin and actin myofilaments.
C. Contraction. Linkage of a cross-bridge and an active site triggersadenosine triphospatase (ATPase) activity of myosin; ATP splits into
adenosine diphosphate (ADP) ! PO4 ! energy; the reaction pro-
duces a transient flexion of the cross-bridge; the actin myofilament is
pulled a short distance past the myosin myofilament; and Z disks are
moved closer together. D. Recharging. The cross-bridge uncouples
from the active site and retracts; ATP is replaced on the cross-bridge.
The recoupling, flexion, uncoupling, retraction, and recharging
processes are repeated hundreds of times per second. E. Relaxation.
Cessation of excitation occurs; calcium ions are removed from the
vicinity of the actin myofilament and are returned to storage sites in
the sarcoplasmic reticulum; troponin returns to its original shape,
covering active sites on the actin myofilament; and actin and myosin
myofilaments return to the rest state. (From Smith LK, Weiss EL,
Lemkuhl LD [eds]: Brunnstrom’s Clinical Kinesiology, 5th ed.
Philadelphia, FA Davis, 1996, p 83, with permission.)
A.
B.
C.
Isometric
Concentric
Eccentric
" Figure 3-6 ! Types of muscle contraction from the perspec-
tive of change (or lack of change) in sarcomere length during the
contraction. A. Isometric contraction with no change in length. B.
Concentric or shortening muscle contraction. C. Eccentric or length-
ening muscle contraction. The top illustration in each type of con-
traction represents the beginning of the contraction, and the bottom
illustration represents the end of the contraction.
03Levengie(F)-03 05/14/2005 3:45 PM Page 116
Copyright © 2005 by F. A. Davis.
Tipos de contração 
•  Isométrica 
–  Actina desliza sobre 
miosina até certo 
ponto 
–  Músculo encurta, mas 
não de maneira visível 
–  Tensão se desenvolve 
Contração isométrica
Contração excêntrica
Prof. Fabiano Botelho Siqueira, MSc. 
Concentração excêntrica 
•  Torque muscular insuficiente para vencer 
resistência. 
•  Músculo alonga - pontes quebram. 
•  Extremidades do músculo se afastam, músculo 
controla, freia movimento. 
•  Movimento ocorre em direção oposta a da ação 
principal do músculo. 
Prof. Fabiano Botelho Siqueira, MSc. 
Concentração excêntrica 
•  Torque muscular insuficiente para vencer 
resistência. 
•  Músculo alonga - pontes quebram. 
•  Extremidades do músculo se afastam, músculo 
controla, freia movimento. 
•  Movimento ocorre em direção oposta a da ação 
principal do músculo. 
tension. The muscle fiber will shorten (contract) if a
sufficient number of sarcomeres actively shorten and 
if either one or both ends of the muscle fiber are free 
to move. The active shortening of a muscle is called a
concentric contraction, or shortening contraction (Fig.
3-6). In contrast to a shortening contraction, in which
the thin filaments are being pulled toward the thick fil-
aments, the muscle may undergo an eccentric contrac-
tion, or lengthening contraction. In a lengthening
contraction, the thin filaments are pulled away from the
thick filaments, and cross-bridges are broken and re-
formed as the muscle lengthens. Tension is generated
by the muscle as cross-bridges are re-formed. Eccentric
contractions occur whenever a muscle actively resists
motion created by an external force (such as gravity).
The muscle fiber will not change length if the force cre-
ated by the cross-bridge cycling is matched by the exter-
nal force. The contraction of a muscle fiber without
changing length is called an isometric contraction.
116 ! Section 1: Joint Structure and Function: Foundational Concepts
" Figure 3-5 ! Cross-bridge cycle. A. Rest. Cross-bridges proj-
ect from a myosin myofilament but are not coupled with an actin
myofilament. Adenosine triphosphate (ATP) is attached near the
head of the cross-bridge; troponin covers the active sites on the actin
myofilament; and calcium ions are stored in the sarcoplasmic reticu-
lum. B. Coupling. Arrival of the muscle action potential depolarizes
the sarcolemma and T tubules; calcium ions are released and react
with troponin; and change in the shape of the troponin-calcium com-
plex uncovers active sites on actin; a cross-bridge couples with an
adjacent active site, thereby linking myosin and actin myofilaments.
C. Contraction. Linkage of a cross-bridge and an active site triggers
adenosine triphospatase (ATPase) activity of myosin; ATP splits into
adenosine diphosphate (ADP) ! PO4 ! energy; the reaction pro-
duces a transient flexion of the cross-bridge; the actin myofilament is
pulled a short distance past the myosin myofilament; and Z disks are
moved closer together. D. Recharging. The cross-bridge uncouples
from the active site and retracts; ATP is replaced on the cross-bridge.
The recoupling, flexion, uncoupling, retraction, and recharging
processes are repeated hundreds of times per second. E. Relaxation.
Cessation of excitation occurs; calcium ions are removed from the
vicinity of the actin myofilament and are returned to storage sites in
the sarcoplasmic reticulum; troponin returns to its original shape,
covering active sites on the actin myofilament; and actin and myosin
myofilaments return to the rest state. (From Smith LK, Weiss EL,
Lemkuhl LD [eds]: Brunnstrom’s Clinical Kinesiology, 5th ed.
Philadelphia, FA Davis, 1996, p 83, with permission.)
A.
B.
C.
Isometric
Concentric
Eccentric
" Figure 3-6 ! Types of muscle contraction from the perspec-
tive of change (or lack of change) in sarcomere length during the
contraction. A. Isometric contraction with no change in length. B.
Concentric or shortening muscle contraction. C. Eccentric or length-
ening muscle contraction. The top illustration in each type of con-
traction represents the beginning of the contraction, and the bottom
illustration represents the end of the contraction.
03Levengie(F)-03 05/14/2005 3:45 PM Page 116
Copyright © 2005 by F. A. Davis.
Tipos de contração 
•  Excêntrica 
–  Actina afasta da 
miosina 
–  Músculo alonga 
–  Tensão desenvolve 
Contração excêntrica
Outros tipos de contração
•Isocinética
•velocidade constante
•Econcêntrica
•comprimento inalterado
•movimento articular visível
•músculos biarticulares
Simular uma contração isométrica, 
uma concêntrica e outra excêntrica
50 Section I Essential Topìcs o j Kinesiology
FIGURE 3-14. Relationship between muscle load (extemal resis- 
tance) and maximal shortening velocity. (Velocity is equal to ihe 
slope of thè dotted line.) At a no load condition, a muscle is 
capable of shortening at a high velocity. As a muscle becomes 
progressively loaded, thè maximal shortening velocity decreases. 
Eventually, at some very large load, thè muscle is incapable of 
shortening and thè velocity is 0. (Redrawn from McComas AJ: 
Skeletal Muscle: Form & Function. Champaign, IL, Human Kinet- 
ics, 1996.)
cle’s strength. This information may be required to deter-
mine thè suitability of a person for a certain task at thè 
workplace, especially if thè task requires a criticai internai 
torque to be produced at certain joint angles.
MUSCLE AS A SKELETAL MOVER: FORCE 
M0DULATI0N
The previous section considers how an isometrically acti- 
vated muscle can stabilize thè skeletal System; this next sec-
tion considers how muscles actively grade forces while 
changing lengths, which is necessary to move thè skeletal 
System. Active grading of muscle force requires a mechanism 
to control excitation of muscle tissue. The nervous system 
acts as a “controller” that can vary thè activation of muscle 
according to thè particular demands of thè task. For exam- 
ple, if thè task is to point accurately at a small target, thè 
controller must be able to make split-second adjustments in 
activation levels io a relatively small number of muscle fi- 
bers. With this control strategy, thè pointing finger does not 
veer off course when extemal perturbations or resistance are 
imposed. If thè task is to produce a forceful motion, thè 
controller must then rapidly and efficiently adivate large 
numbers of muscle fibers.
Understanding thè role of muscle activationin generating 
movement begins with an appreciation of how muscle force 
is modulated while thè muscle is either shortening or length- 
ening. The ways in which force is graded by neural activa-
tion are explored. The reduction in force that occurs with 
muscular fatigue is examined. Finally, thè use of electromy-
ography as a tool for understanding muscle activation during 
movement is introduced.
Moduiating Force Through Concentric or 
Eccentric Activation: Force-Velocity 
Relationship
The nervous System stimulates a muscle to generate or resisi 
a force by concentric, eccentric, or isometric activation. Dur-
ing concentric activation, thè muscle shortens (contracts); 
during eccentric activation, thè muscle elongates; and during 
isometric activation, thè length of thè muscle remains Con-
stant. During concentric and eccentric activation, thè rate o j 
change of length is significanti related to thè muscle’s maxi-
mal force potential. During concentric activation, for exam- 
ple, thè muscle contracts at a maximum velocity when thè 
load is negligible (Fig. 3 -1 4 ) . As thè load increases, thè 
maximal contraction velocity of thè muscle decreases. At 
some point, a very large load results in a contraction velocity 
of zero (i.e., thè isometric state).
Eccentric activation needs to be considered separately 
from concentric activation. With eccentric activation, a load 
that barely exceeds thè isometric force level causes thè mus-
cle to lengthen slowly. Speed of lengthening increases as a 
greater load is applied. There is a maximal load that thè 
muscle cannot resist, and beyond this load level thè muscle 
uncontrollably lengthens.
The theoretical force-velocity curve for muscle across con-
centric, isometric, and eccentric activations is often shown 
with thè force on thè Y (vertical) axis and shortening and 
lengthening velocity on thè X (horizontal) axis (Fig. 3 -1 5 ) . 
In generai, during a maximal effort concentric activation, thè 
amount of muscle force is inversely proportional to thè veloc-
ity of muscle shortening. During a maximal effort eccentric 
activation, thè muscle force is, to a point, directly proportional 
to thè velocity of muscle lengthening. The clinical expression 
of a force-velocity relationship of muscle is a torque-joint
FIGURE 3-15. Theoretic force-velocity curve of an activated muscle 
is shown. Concentric activation is shown on thè righi and eccentric 
activation on thè left. Isometric activation occurs at thè zero veloc-
ity point on thè graph.
44 Section I Essential Topics o f Kinesiology
FIGURE 3-3. Unipennate muscle is shown with thè muscle ftbers 
oriented at a 30-degree angle of pennation (0),
physiologic cross-sectional area. As shown in Figure 3 - 3 , a 
pennation angle of 30 degrees stili enables thè fibers to 
transfer 86% of their force through thè long axis of thè 
tendon.
Muscle and Tendon: Generation of Force
PASSIVE LENGTH-TENSION CURVE
Muscle contains contractile proteins that are embedded 
within a network of connective tissues, namely, thè epimys- 
ium, perimysium, and endomysium. Table 3 - 2 lists thè 
functions of these tissues. Connective tissues are slightly 
elastic and, like a rubber band, generate resistive force (i.e., 
tension) when elongated.
For functional rather than anatomie purposes, thè con-
nective tissues within thè muscle and tendon have been 
described as thè paraìlel elastic component and thè series elas-
tic component. Elongation or stretch of thè whole muscle 
lengthens thè connective tissue elements (Fig. 3 - 4 ) . The 
paraìlel elastic component refers to thè connective tissues
TABLE 3 - 2 . Functions of Connective Tissue 
within Muscle
1. Provides gross structure to muscle
2. Serves as a conduit for blood vessels and nerves
3. Generates passive tension by resisting stretch 
4 Assists muscle to regain shape after stretch
5. Conveys contractile force to thè tendon and across thè joint
that surround or lie paraìlel to thè proteins that cause thè 
muscle to contract. The series elastic component, in contrast, 
refers to thè connective tissues within thè tendon. Because 
thè tendon lies in series with thè contractile proteins, active 
forces produced by these proteins are transferred directly to 
thè bone and across thè joint. Stretching a muscle by ex- 
tending a joint elongates both thè paraìlel elastic component 
and thè series elastic component, generating a springlike 
resistance, or stiffness, in thè muscle. The resistance is re- 
ferred to as a passive tension because it is does not depend 
on active or volitional contraction. The concept of paraìlel 
and serial elastic components is a simplifìed description of 
thè anatomy; however, it is useful to explain thè levels of 
resistance generated by a stretched muscle.
The tendon has several unique mechanical properties. Be-
cause of thè longitudinal orientation and thickness of its 
collagen fibers, thè tendon can resist large forces that might 
otherwise damage thè muscle tissue. Muscle fibers decrease 
in diameter by as much as 90% as they blend with thè 
tendon tissue.12 As a result, thè force through a muscle fiber 
per cross-sectional area (i.e., stress) increases significantly. At 
each end of a muscle fiber is an extensive folding of thè 
plasmalemma (i.e., thè membrane surrounding thè muscle 
fiber), which interdigitates with thè connective tissue of thè 
tendon. This folding ensures that high forces can be distrib- 
uted over a large area, thus reducing thè stress on thè 
muscle.
When thè paraìlel and series elastic components are 
stretched within a muscle, a generalized passive length-tension 
curve is generated (Fig. 3 - 5 ) . The curve is similar to that 
obtained by stretching a rubber band. Approximating thè 
shape of an exponential mathematica! function, thè passive
Bone Paraìlel EC
FIGURE 3-4. Contractile components 
and elastic components (EC) that 
generate force in muscle tissue are 
shown. The contractile component 
represents thè actin and myosin 
crossbridge structures. The paraìlel 
elastic component (paraìlel to thè 
contractile component) represents 
muscle connective tissue. The series 
elastic component (in series with thè 
whole muscle) represents thè connec-
tive tissues within thè tendon. The 
paraìlel and series connective tissues 
act in a manner similar to a spring.
Tecido Conectivo
44 Section I Essential Topics o f Kinesiology
FIGURE 3-3. Unipennate muscle is shown with thè muscle ftbers 
oriented at a 30-degree angle of pennation (0),
physiologic cross-sectional area. As shown in Figure 3 - 3 , a 
pennation angle of 30 degrees stili enables thè fibers to 
transfer 86% of their force through thè long axis of thè 
tendon.
Muscle and Tendon: Generation of Force
PASSIVE LENGTH-TENSION CURVE
Muscle contains contractile proteins that are embedded 
within a network of connective tissues, namely, thè epimys- 
ium, perimysium, and endomysium. Table 3 - 2 lists thè 
functions of these tissues. Connective tissues are slightly 
elastic and, like a rubber band, generate resistive force (i.e., 
tension) when elongated.
For functional rather than anatomie purposes, thè con-
nective tissues within thè muscle and tendon have been 
described as thè paraìlel elastic component and thè series elas-
tic component. Elongation or stretch of thè whole muscle 
lengthens thè connective tissue elements (Fig. 3 - 4 ) . The 
paraìlel elastic component refers to thè connective tissues
TABLE 3 - 2 . Functions of Connective Tissue 
within Muscle
1. Provides gross structure to muscle
2. Serves as a conduit for blood vessels and nerves
3. Generates passive tension by resisting stretch 
4 Assists muscle to regain shape after stretch
5. Conveys contractile force to thè tendon and across thè joint
that surround or lie paraìlel to thè proteins that cause thè 
muscle to contract. The series elastic component, in contrast, 
refers to thè connective tissues within thè tendon. Because 
thè tendon lies in series with thè contractile proteins,active 
forces produced by these proteins are transferred directly to 
thè bone and across thè joint. Stretching a muscle by ex- 
tending a joint elongates both thè paraìlel elastic component 
and thè series elastic component, generating a springlike 
resistance, or stiffness, in thè muscle. The resistance is re- 
ferred to as a passive tension because it is does not depend 
on active or volitional contraction. The concept of paraìlel 
and serial elastic components is a simplifìed description of 
thè anatomy; however, it is useful to explain thè levels of 
resistance generated by a stretched muscle.
The tendon has several unique mechanical properties. Be-
cause of thè longitudinal orientation and thickness of its 
collagen fibers, thè tendon can resist large forces that might 
otherwise damage thè muscle tissue. Muscle fibers decrease 
in diameter by as much as 90% as they blend with thè 
tendon tissue.12 As a result, thè force through a muscle fiber 
per cross-sectional area (i.e., stress) increases significantly. At 
each end of a muscle fiber is an extensive folding of thè 
plasmalemma (i.e., thè membrane surrounding thè muscle 
fiber), which interdigitates with thè connective tissue of thè 
tendon. This folding ensures that high forces can be distrib- 
uted over a large area, thus reducing thè stress on thè 
muscle.
When thè paraìlel and series elastic components are 
stretched within a muscle, a generalized passive length-tension 
curve is generated (Fig. 3 - 5 ) . The curve is similar to that 
obtained by stretching a rubber band. Approximating thè 
shape of an exponential mathematica! function, thè passive
Bone Paraìlel EC
FIGURE 3-4. Contractile components 
and elastic components (EC) that 
generate force in muscle tissue are 
shown. The contractile component 
represents thè actin and myosin 
crossbridge structures. The paraìlel 
elastic component (paraìlel to thè 
contractile component) represents 
muscle connective tissue. The series 
elastic component (in series with thè 
whole muscle) represents thè connec-
tive tissues within thè tendon. The 
paraìlel and series connective tissues 
act in a manner similar to a spring.
Tecido Conectivo
muscles working over the joints of the body produces
the movements we use for daily activities, work, play,
and sport. Unfortunately, some of the movements may
cause injury to the muscles and tendons. The following
case identifies a common muscle injury. Throughout
this chapter, you will see how the structure and function
of the muscles can be applied to this clinical situation.
3-1 Patient Case
Vik Patel, a 50-year-old man, was playing softball one summer
evening. He was trying to catch a fly ball when he stepped back
with his right foot and slipped slightly. As his foot slipped, the
motion at the ankle was dorsiflexion and the ankle plantar flexor
muscles were contracting as he tried to push off so that he could
run forward. At the moment of trying to push off, he felt a twinge
of pain in the right calf muscle. Vik states that he has pain in the
calf muscle and along the Achilles tendon when he tries to stand
on his toes and when he does calf-stretching exercises. After eval-
uation of Vik, it appears that he might have strained the calf mus-
cle or caused some tendinitis.
Elements of Muscle Structure
Skeletal muscles are composed of muscle tissue (con-
tractile) and connective tissue (noncontractile). The
muscle tissue has the ability to develop tension in
response to chemical, electrical, or mechanical stimuli.
The connective tissue, on the other hand, develops ten-
sion in response to passive loading.1 The properties of
these tissues and the way in which they are interrelated
give muscles their unique characteristics.
Composition of a Muscle Fiber
! Contractile Proteins
A skeletal muscle is composed of many thousands of
muscle fibers. A single muscle contains many fascicles,
a group of muscle fibers (cells) surrounded by connec-
tive tissue (Fig. 3-1A). The arrangement, number, size,
and type of these fibers may vary from muscle to mus-
cle,2,3 but each fiber is a single muscle cell that is
enclosed in a cell membrane called the sarcolemma
(see Fig. 3-1B). Like other cells in the body, the muscle
fiber is composed of cytoplasm, which in a muscle is
called sarcoplasm. The sarcoplasm contains myofibrils
(see Fig. 3-1C), which are the contractile structures of a
muscle fiber and nonmyofibrillar structures such as
ribosomes, glycogen, and mitochondria, which are
required for cell metabolism.
The myofibril is composed of thick myofilaments
composed of the protein myosin and thin filaments
composed of the protein actin (see Fig. 3-1D). The
interaction of these two myofilaments is essential for a
muscle contraction to occur. The thin myofilaments are
formed by two chainlike strings of actin molecules
wound around each other. Molecules of the globular
protein troponin are found in notches between the two
actin strings and the protein tropomyosin is attached to
each troponin molecule (Fig. 3-2A). The troponin and
tropomyosin molecules control the binding of actin
and myosin myofilaments.
Each of the myosin molecules has globular enlarge-
ments called head groups (see Fig. 3-2B).4 The head
groups, which are able to swivel and are the binding
sites for attachment to the actin, play a critical role
in muscle contraction and relaxation. When the entire
myofibril is viewed through a microscope, the alterna-
tion of thick (myosin) and thin (actin) myofilaments
forms a distinctive striped pattern, as seen in Figure
3-1D. Therefore, skeletal muscle is often called stria-
ted muscle. A schematic representation of the ordering
of the myofilaments in a myofibril is presented in
Figure 3-3.
! Structural Proteins
The muscle fiber also consists of several structural pro-
teins (see Patel and Lieber5 for a review of these pro-
teins). Some of these proteins (intermediate filaments)
provide a structural scaffold for the muscle fiber,
whereas others (e.g., desmin) may be involved in the
transmission of force along the fiber and to adjoining
fibers. One protein, titin, has a particularly important
role maintaining the position of the thick filament dur-
ing a muscle contraction and in the development of
passive tension.6,7 Titin is a large protein that is
attached along the thick filament and spans the gap
from the thick filament to the Z lines (Fig. 3-4). More
114 ! Section 1: Joint Structure and Function: Foundational Concepts
" Figure 3-1 ! Composition of a muscle fiber. A. Groups of
muscle fibers form bundles called fascicles. B. The muscle fiber is
enclosed in a cell membrane called the sarcolemma. C. The muscle
fiber contains myofibrillar structures called myofibrils. D. The
myofibril is composed of thick myosin and thin actin myofilaments.
03Levengie(F)-03 05/14/2005 3:45 PM Page 114
Copyright © 2005 by F. A. Davis.
Prof. Fabiano Botelho Siqueira, MSc. 
Tecido conectivo 
 
•  Elementos elásticos em paralelo 
–  Endomísio 
–  Perimísio 
–  Epimísio 
 
Elementos elásticos em 
série (EES) 
 
•  Tendão 
•  Tensão é aumentada 
quando o músculo 
contrai e quando o 
músculo é alongado 
Tecido Conectivo
compacted collagen fibers that attach directly or indi-
rectly to muscles, fasciae, bones, cartilage, and other
muscles. Aponeuroses distribute forces generated by
the muscle to the structures to which they are attached.1
! Parallel and Series Elastic Components of Muscle
All of the connective tissue in a muscle is intercon-
nected and constitutes the passive elastic component of
a muscle. The connective tissues that surround the mus-
cle, plus the sarcolemma, the elastic protein titin, and
other structures (i.e., nerves and blood vessels), form
the parallel elastic component of a muscle. When a mus-
cle lengthens or shortens, these tissues also lengthen or
shorten, because they function in parallel to the muscle
contractile unit. For example, the collagen fibers in the
perimysium of fusiform muscles are slack when the sar-
comeres areat rest but straighten out and become taut
as sarcomere lengths increase. As the perimysium is
lengthened, it also becomes stiffer (resistance to fur-
ther elongation increases). The increased resistance of
perimysium to elongation may prevent overstretching
of the muscle fiber bundles and may contribute to the
tension at the tendon.31 When sarcomeres shorten from
their resting position, the slack collagen fibers within
the parallel elastic component buckle (crimp) even fur-
ther. Whatever tension might have existed in the colla-
gen at rest is diminished by the shortening of the
sarcomere. Because of the many parallel elastic compo-
nents of a muscle, the increase or decrease in passive
tension can substantially affect the total tension output
of a muscle. This relationship between length and ten-
sion will be addressed in the next section.
The tendon of the muscle is considered to function
in series with the contractile elements. This means that
the tendon will be under tension when the muscle
actively produces tension. When the contractile ele-
ments in a muscle actively shorten, they exert a pull on
the tendon. The pull must be of sufficient magnitude to
take up the slack (compliance) in the tendon so that
the muscle pull can be transmitted through the tendon
to the bony lever (Fig. 3-12). Fortunately, the compli-
ance (or extensibility) of the tendon is relatively small
(about 3% to 10% in human muscles). Thus, most of
the muscle force can be used for moving the bony lever
and is not dissipated stretching the tendon. The tendon
is also under tension when a muscle is controlling or
braking the motion of the lever in an eccentric con-
traction. A tendon is under reduced tension only when
a muscle is completely relaxed and in a relatively short-
ened position.
122 ! Section 1: Joint Structure and Function: Foundational Concepts
" Figure 3-10 ! Iliotibial tract. A lateral view of the left lower
limb showing the deep fascial iliotibial tract extending from the
tubercle of the iliac crest to the lateral aspect of the knee. The right
arrow represents the pull of the gluteus maximus. The left arrow rep-
resents the pull of the tensor fasciae latae.
" Figure 3-11 ! Retinacula. A. The superior and inferior reti-
nacula are shown in their normal position, in which they form a tun-
nel for the tendons from the extensor muscles of the lower leg. B.
When the retinacula are torn or removed, the tendons move anteri-
orly.
" Figure 3-12 ! Series elastic component. A. The muscle is
shown in a relaxed state with the tendon slack (crimping or buckling
of collagen fibers has occurred). The sarcomere depicted above the
muscle shows minimal overlap of thick and thin filaments and little
cross-bridge formation. B. The muscle in an actively shortened posi-
tion shows that the tendons are under tension and no crimp can be
observed. The sarcomere depicted above the muscle shows extensive
overlap of filaments and cross-bridge formation.
03Levengie(F)-03 05/14/2005 3:45 PM Page 122
Copyright © 2005 by F. A. Davis.
Tensão ativaTensão ativa 
•  Tensão produzida pelo elemento 
contrátil do músculo 
•  Fatores que aumentam a tensão 
ativa 
–  Grande número de pontes actina-
miosina 
–  Músculos com maior secção 
transversal 
–  Aumento na freqüência do impulso 
que chega às fibras musculares 
Tensão passiva
Prof. Fabiano Botelho Siqueira, MSc. 
Tensão passiva 
•  Tensão produzida pelo elemento passivo não contrátil 
•  Tensão passiva do tecido conectivo pode ser produzida 
pelo encurtamento ou alongamento muscular 
Prof. Fabiano Botelho Siqueira, MSc. 
Tensão passiva 
•  Tensão produzida pelo elemento passivo não contrátil 
•  Tensão passiva do tecido conectivo pode ser produzida 
pelo encurtamento ou alongamento muscular 
Chapter 3 Muscle: The Ultimate Force Generator in thè Body 45
Increasing stretch
FIGURE 3 -5 . A generalized passive length-tension curve is shown. 
As a muscle is progressively stretched, thè tissue is slack during its 
irutial shortened lengths until it reaches a criticai length where it 
begins to generate tension. Beyond this criticai length, thè tension 
builds as an exponential function.
elements within thè muscle begin generating passive tension 
after thè criticai length where all of thè relaxed (i.e., slack) 
tissue has been brought to an initial level of tension. After 
this criticai length has been reached, tension progressively 
increases until it reaches levels of extremely high stiffness. At 
higher tension, thè tissue fails. The simple passive length- 
tension curve represents an important component of force- 
generating capability in muscle and tendon tissue. This capa-
bility is especially important ai very long lengths where 
muscle fibers begin to lose their active force-generating capa-
bility. Passive tension stabilizes skeletal structures against 
gravity and responds to perturbations and other imposed 
loads. Passive elongation of thè Achilles tendon of thè ankle 
during thè downstroke of bicycle pedaling, for example, al- 
lows for transmittal of hip and muscular forces to thè bicycle 
crank.6 This capability, however, is limited because of thè 
slow adaptability of thè tissue to rapidly changing extemal 
forces and because of thè significant amount of initial 
lengthening that must occur before tissue can generate suffi- 
cient passive tension.
Stretched muscle tissue exhibits thè properties of elasticity 
and viscosity. Both properties influence thè amount and rate 
of passive tension developed within a stretched muscle. A 
stretched muscle exhibits elasticity because it can temporarily 
store pari of thè energy that created thè stretch. Stored 
energy, ahhough relatively slight when compared with thè 
full force potential of thè muscle, helps prevent a muscle 
from being damaged during maximal elongation. Viscosity, in 
this context, describes thè rate-dependent resistance encoun- 
tered between thè surfaces of adjacent fluid-like tissues. Vis-
cosity is rate dependent; thus, a muscle’s internai resistance 
to elongation increases with thè rate of stretch. Viscosity
helps protect a muscle from being damaged by a quick and 
forceful stretch. The viscous properties of muscle prolong 
thè application of force to allow a more graduai elongation, 
reducing thè risk of tissue rupture. In summary, both elastic-
ity and viscosity serve as damping mechanisms that protect 
thè stracanai components of thè muscle and tendon.
ACTIVE LENGTH-TENSION CURVE
Muscle tissue is uniquely designed to generate force actively 
in response to a stimulus from thè nervous System. This 
section describes thè means for generating active force. Ac-
tive force is produced by thè muscle fiber. Ultimately, active 
force and passive tension must be transmitted io thè skeletal 
structures. The interaction between active and passive forces 
is explored in thè next section.
As explained earlier, muscle fibers constitute thè basic 
functional element of muscle. Furthermore, each muscle fi-
ber, or celi, is composed of many tiny strands called myofi- 
brils. Myofibrils are thè contractile elements of thè muscle 
fiber and have a distinctive structure. Each myofibril is 1 io 
2 micrometers in diameter and consists of many myofila- 
ments. The primary structures within myofilaments are two 
types of proteins: actin and myosin. The regular organization 
of myofilaments produces thè characteristic banded appear- 
ance of thè myofibril as seen under thè microscope (Fig. 3 — 
6). The actin and myosin physically interact through cross- 
bridges (i.e., projections from thè myosin filamenti and 
other connective structures. By way of thè endomysium, 
myofibrils ultimately connect with thè tendon. This elegant 
connective web, formed between myofilaments and connec-
tive tissues, allows force to be evenly distributed throughout 
muscle and efficiently transmitted to skeletal structures.
Upon inspection of thè muscle fiber, a distinctive light 
and dark banding is apparent (Fig. 3 - 7 ) .The dark bands, 
thè A-bands, correspond to thè presence of myosin— thè 
thick filaments. Myosin also contains projections, called 
cross-bridges, which are arranged in pairs (Fig. 3 - 8 ) . The 
light bands, thè I-bands, contain actin— thè thin filaments 
(see Fig. 3 - 7 ) . In a resting muscle fiber, actin filaments 
partially overlap myosin filaments. Under an electron micro-
scope, thè bands reveal a more complex pattern that consists 
of H bands, M lines, and Z discs (Table 3 -3 ) .
The banding pattern repeats along thè length of thè mus-
TABLE 3 - 3 . Regions Within a Sarcomere
A bands Dark bands caused by presence of thick myosin 
filament
I bands Light bands caused by presence of thin actin fila-
ment
H band Region within A band where actin and myosin do 
not overlap.
M lines Mid region thickening of thick myosin filament in 
thè center of H band
Z discs Region where successive actin filaments mesh to- 
gether. Z disc helps anchor thè thin filaments.
Relação comprimento - tensão
Relação comprimento-tensão 
•  Há um comprimento muscular ideal (�ótimo�) no 
qual um músculo produz máxima tensão 
•  Comprimento ótimo 
–  Aproximadamente comprimento de repouso 
•  Filamentos de actina/miosina estão posicionados de 
tal forma que um número de pontes podem ser 
formadas 
•  Um conceito importante 
–  Insuficiência 
Muscle Function
Muscle Tension
The most important characteristic of a muscle is its abil-
ity to develop tension and to exert a force on the bony
lever. Tension can be either active or passive, and the
total tension that a muscle can develop includes both
active and passive components. Total tension, which
was identified in Chapter 1 as Fms, is a vector quantity
that has (1) magnitude, (2) two points of application
(at the proximal and distal muscle attachments), (3) an
action line, and (4) direction of pull. The point of
application, action line, and direction of pull were the
major part of the discussion of muscle force in Chapter
1, but we now need to turn our attention to the deter-
minants of the component called magnitude of the
muscle force, or the total muscle tension.
! Passive Tension
Passive tension refers to tension developed in the par-
allel elastic component of the muscle. Passive tension in
the parallel elastic component is created by lengthen-
ing the muscle beyond the slack length of the tissues.
The parallel elastic component may add to the active
tension produced by the muscle when the muscle is
lengthened, or it may become slack and not contribute
to the total tension when the muscle is shortened. The
total tension that develops during an active contraction
of a muscle is a combination of the passive (noncon-
tractile) tension added to the active (contractile) ten-
sion (Fig. 3-13) .
Continuing Exploration: Passive Muscle Stiffness
Passive muscle stiffness is an important property of
skeletal muscle. The passive stiffness of an isolated
muscle (not connected to bones and joints) is the
slope of the passive length-tension relationship. The
steeper the slope is, the greater is the stiffness in 
the muscle. The passive stiffness of a muscle attached
to bone and crossing a joint is the slope of the
torque-angle relationship (Fig. 3-14). Titin is the pri-
mary structure of the muscle that accounts for the
stiffness of the muscle (see Lieber28 for a review of
the role of titin). On the other hand, the connective
tissues in and around the muscle (perimysium and
endomysium) are responsible for the extent to
which the muscle can be elongated.31 This is often
referred to as the muscle extensibility or flexibility.
! Active Tension
Active tension refers to tension developed by the con-
tractile elements of the muscle. Active tension in a mus-
cle is initiated by cross-bridge formation and movement
of the thick and thin filaments. The amount of active
tension that a muscle can generate depends on neural
factors and mechanical properties of the muscle fibers.
The neural factors that can modulate the amount of
active tension include the frequency, number, and size
of motor units that are firing. The mechanical proper-
ties of muscle that determine the active tension are the
isometric length-tension relationship and the force-
velocity relationship.
! Isometric Length-Tension Relationship
One of the most fundamental concepts in muscle phys-
iology is the direct relationship between isometric ten-
sion development in a muscle fiber and the length of
the sarcomeres in a muscle fiber.32 The identification of
this relationship was, and continues to be, the primary
evidence supporting the sliding filament theory of mus-
cle contraction. The isometric sarcomere length-
tension relationship was experimentally determined
with isolated single muscle fibers under very controlled
circumstances. There is an optimal sarcomere length at
which a muscle fiber is capable of developing maximal
isometric tension (see Fig. 3-13). Muscle fibers develop
maximal isometric tension at optimal sarcomere length
because the thick and thin filaments are positioned so
that the maximum number of cross-bridges within the
sarcomere can be formed. If the muscle fiber is length-
ened or shortened beyond optimal length, the amount
of active tension that the muscle fiber is able to gener-
ate when stimulated decreases (see Fig. 3-13). When a
muscle fiber is lengthened beyond optimal length,
there is less overlap between the thick and thin fila-
ments and consequently fewer possibilities for cross-
bridge formation. However, the passive elastic tension
in the parallel component may be increased when the
muscle is elongated. This passive tension is added to
the active tension, resulting in the total tension (see
Fig. 3-13).
Chapter 3: Muscle Structure and Function ! 123
" Figure 3-13 ! The skeletal muscle sarcomere length-tension
relationship. Active, passive, and the total curves are shown. The
plateau of the active curve signifies optimal sarcomere length where
maximum active tension is developed. Isometric tension decreases as
the muscle is lengthened because fewer cross-bridges are able to be
formed. Tension decreases as the muscle is shortened because of
interdigitation of the thin filaments. The increase in passive tension
with elongation of the muscle is shown by the dashed line. Passive
plus active tension results in the total amount of tension developed by
the muscle fiber.
03Levengie(F)-03 05/14/2005 3:45 PM Page 123
Copyright © 2005 by F. A. Davis.
Relação ideal entre comprimento do 
músculo e tensão
68 
RELAÇÃO IDEAL ENTRE COMPRIMENTO DO MÚSCULO E TENSÃO 
•  �A� (posição encurtada): 
filamentos de actina e miosina 
muito próximos limitando a 
capacidade de gerar tensão ativa. 
•  �B��(comprimento ótimo): 
comprimento ideal de geração de 
tensão ativa. 
•  �C� (posição alongada): 
filamentos de actina e miosina 
muito distantes e prejudicando a 
capacidade de gerar tensão ativa 
SAHRMANN, Shirley. A. (2005) 
EDUCAÇÃO FÍSICA E FISIOTERAPIA 
A 
B 
C 
69 
Relação comprimento- tensão 
§  É a relação entre o desenvolvimento de tensão e o comprimento 
do músculo. 
 
•  Tensão máxima no comprimento ótimo. 
•  O comprimento ótimo está próximo do que é conhecido como 
comprimento de repouso (nem muito alongado e nem muito 
encurtado). Varia de indivíduo para indivíduo, de acordo com as 
características de suas atividades. 
•  Comprimentos maiores ou menores que o ótimo, cada vez menos 
oportunidade de sobreposição entre actina e miosina, resultando 
em menor potencial para se gerar força. 
Relação comprimento - tensão
Adaptação anatômica do comprimento 
do músculo
70 
ADAPTAÇÃO ANATÔMICA DO COMPRIMENTO DO MÚSCULO 
•  O músculo com 
cumprimento de repouso 
próximo de alongado 
desenvolve tensão máxima 
em posições onde ele esteja 
mais alongado. 
•  Quando este músculo fica 
em posição de 
encurtamento sua tensão é 
menor. 
•  Músculos treinados em posições alongadas desviam a curva de 
comprimento e tensão para a direita. 
•  Em posição de encurtamento,estes músculos desenvolvem uma menor 
tensão ativa quando comparados com músculos encurtados. 
comprimento
71 
Relação comprimento- tensão 
•  O número de sarcômeros não é fixo e, no músculo adulto, 
pode aumentar ou diminuir. 
•  A regulação do número de sarcômeros é uma adaptação às 
mudanças no comprimento funcional de um músculo. 
•  Para treino de músculos adaptados fora do comprimento 
fisiológico, o treinamento deve ser aplicado fora do 
comprimento habitual. Ex: músculos alongados deverão 
ser treinados na posição fisiológica para encurtada. 
Relação comprimento - tensão
Muscle Function
Muscle Tension
The most important characteristic of a muscle is its abil-
ity to develop tension and to exert a force on the bony
lever. Tension can be either active or passive, and the
total tension that a muscle can develop includes both
active and passive components. Total tension, which
was identified in Chapter 1 as Fms, is a vector quantity
that has (1) magnitude, (2) two points of application
(at the proximal and distal muscle attachments), (3) an
action line, and (4) direction of pull. The point of
application, action line, and direction of pull were the
major part of the discussion of muscle force in Chapter
1, but we now need to turn our attention to the deter-
minants of the component called magnitude of the
muscle force, or the total muscle tension.
! Passive Tension
Passive tension refers to tension developed in the par-
allel elastic component of the muscle. Passive tension in
the parallel elastic component is created by lengthen-
ing the muscle beyond the slack length of the tissues.
The parallel elastic component may add to the active
tension produced by the muscle when the muscle is
lengthened, or it may become slack and not contribute
to the total tension when the muscle is shortened. The
total tension that develops during an active contraction
of a muscle is a combination of the passive (noncon-
tractile) tension added to the active (contractile) ten-
sion (Fig. 3-13) .
Continuing Exploration: Passive Muscle Stiffness
Passive muscle stiffness is an important property of
skeletal muscle. The passive stiffness of an isolated
muscle (not connected to bones and joints) is the
slope of the passive length-tension relationship. The
steeper the slope is, the greater is the stiffness in 
the muscle. The passive stiffness of a muscle attached
to bone and crossing a joint is the slope of the
torque-angle relationship (Fig. 3-14). Titin is the pri-
mary structure of the muscle that accounts for the
stiffness of the muscle (see Lieber28 for a review of
the role of titin). On the other hand, the connective
tissues in and around the muscle (perimysium and
endomysium) are responsible for the extent to
which the muscle can be elongated.31 This is often
referred to as the muscle extensibility or flexibility.
! Active Tension
Active tension refers to tension developed by the con-
tractile elements of the muscle. Active tension in a mus-
cle is initiated by cross-bridge formation and movement
of the thick and thin filaments. The amount of active
tension that a muscle can generate depends on neural
factors and mechanical properties of the muscle fibers.
The neural factors that can modulate the amount of
active tension include the frequency, number, and size
of motor units that are firing. The mechanical proper-
ties of muscle that determine the active tension are the
isometric length-tension relationship and the force-
velocity relationship.
! Isometric Length-Tension Relationship
One of the most fundamental concepts in muscle phys-
iology is the direct relationship between isometric ten-
sion development in a muscle fiber and the length of
the sarcomeres in a muscle fiber.32 The identification of
this relationship was, and continues to be, the primary
evidence supporting the sliding filament theory of mus-
cle contraction. The isometric sarcomere length-
tension relationship was experimentally determined
with isolated single muscle fibers under very controlled
circumstances. There is an optimal sarcomere length at
which a muscle fiber is capable of developing maximal
isometric tension (see Fig. 3-13). Muscle fibers develop
maximal isometric tension at optimal sarcomere length
because the thick and thin filaments are positioned so
that the maximum number of cross-bridges within the
sarcomere can be formed. If the muscle fiber is length-
ened or shortened beyond optimal length, the amount
of active tension that the muscle fiber is able to gener-
ate when stimulated decreases (see Fig. 3-13). When a
muscle fiber is lengthened beyond optimal length,
there is less overlap between the thick and thin fila-
ments and consequently fewer possibilities for cross-
bridge formation. However, the passive elastic tension
in the parallel component may be increased when the
muscle is elongated. This passive tension is added to
the active tension, resulting in the total tension (see
Fig. 3-13).
Chapter 3: Muscle Structure and Function ! 123
" Figure 3-13 ! The skeletal muscle sarcomere length-tension
relationship. Active, passive, and the total curves are shown. The
plateau of the active curve signifies optimal sarcomere length where
maximum active tension is developed. Isometric tension decreases as
the muscle is lengthened because fewer cross-bridges are able to be
formed. Tension decreases as the muscle is shortened because of
interdigitation of the thin filaments. The increase in passive tension
with elongation of the muscle is shown by the dashed line. Passive
plus active tension results in the total amount of tension developed by
the muscle fiber.
03Levengie(F)-03 05/14/2005 3:45 PM Page 123
Copyright © 2005 by F. A. Davis.
Muscle Function
Muscle Tension
The most important characteristic of a muscle is its abil-
ity to develop tension and to exert a force on the bony
lever. Tension can be either active or passive, and the
total tension that a muscle can develop includes both
active and passive components. Total tension, which
was identified in Chapter 1 as Fms, is a vector quantity
that has (1) magnitude, (2) two points of application
(at the proximal and distal muscle attachments), (3) an
action line, and (4) direction of pull. The point of
application, action line, and direction of pull were the
major part of the discussion of muscle force in Chapter
1, but we now need to turn our attention to the deter-
minants of the component called magnitude of the
muscle force, or the total muscle tension.
! Passive Tension
Passive tension refers to tension developed in the par-
allel elastic component of the muscle. Passive tension in
the parallel elastic component is created by lengthen-
ing the muscle beyond the slack length of the tissues.
The parallel elastic component may add to the active
tension produced by the muscle when the muscle is
lengthened, or it may become slack and not contribute
to the total tension when the muscle is shortened. The
total tension that develops during an active contraction
of a muscle is a combination of the passive (noncon-
tractile) tension added to the active (contractile) ten-
sion (Fig. 3-13) .
Continuing Exploration: Passive Muscle Stiffness
Passive muscle stiffness is an important property of
skeletal muscle. The passive stiffness of an isolated
muscle (not connected to bones and joints) is the
slope of the passive length-tension relationship. The
steeper the slope is, the greater is the stiffness in 
the muscle. The passive stiffness of a muscle attached
to bone and crossing a joint is the slope of the
torque-angle relationship (Fig. 3-14). Titin is the pri-
mary structure of the muscle that accounts for the
stiffness of the muscle (see Lieber28 for a review of
the role of titin). On the other hand, the connective
tissues in and around the muscle (perimysium and
endomysium) are responsible for the extent to
which the muscle can be elongated.31 This is often
referred to as the muscle extensibility or flexibility.
! Active Tension
Active tension refers to