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Eleven of the twenty-two muscles of the ankle and foot are intrinsic;
that is, they are located entirely within the foot. The other eleven
are extrinsic; they have distal tendon attachments on the foot but
are otherwise located outside it (Table 8.1). A twelveth extrinsic
muscle, the plantaris, has been omitted because it is a vestige
often absent in human beings. When present it assists the ankle
extensor muscles.
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Characteristics
and Functions of Individual Muscles
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This muscle (Figure 8.21a) lies along the full length of the
anterior surface of the tibia from the lateral condyle down to the
medial aspect of the tarsometatarsal region. Approximately one-half to
two-thirds of the way down the leg it becomes tendinous. The tendon
passes in front of the medial malleolus on its way to the first
cuneiform. The muscle dorsiflexes the ankle
and foot, and supinates (inverts and
adducts) the tarsal joints when the foot is dorsiflexed. In
EMG studies it was found that one-half the subjects in free standing
had activity in the tibialis anterior that went away when the subjects
leaned forward. The muscle is also active during the initial contact
phase of walking, allowing the foot to be lowered to the ground
in a controlled manner. The muscle may be palpated on the anterior
surface of the leg just lateral to the tibia.
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Extensor Digitorum
Longus
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This muscle (Figures 8.21b and 8.22) extends
the four lesser toes. It also dorsiflexes
both the ankle and the tarsal joints and helps
evert and abduct the latter. It is a penniform muscle, situated
lateral to the tibialis anterior muscle in the upper part of the
leg and lateral to the extensor hallucis longus in the lower part.
Just in front of the ankle joint the tendon divides into four tendons,
one for each of the lesser toes. The muscle may be palpated on the anterior
surface of the ankle and the dorsal surface of the foot, lateral
to the tendon of the extensor hallucis longus.
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Extensor Hallucis
Longus
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This muscle (see Figure 8.21a) extends
and hyperextends the great toe. It also dorsiflexes
the ankle and the tarsal joints. Like the preceding muscle,
it is penniform in structure. Its upper portion lies beneath the
tibialis anterior and extensor digitorum longus, but about halfway
down the leg the tendon emerges between these two muscles, thus
becoming superficial. After it reaches the ankle, the tendon slants
medially across the dorsal surface of the foot to the top of the
great toe. It may be palpated on the dorsal surface of the foot
and great toe.
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This muscle (see Figure 8.22) dorsiflexes
and pronates (everts and abducts) the tarsal joints and dorsiflexes the ankle. It is a small
muscle that lies lateral to the extensor digitorum longus, sometimes
described as the fifth tendon of the latter muscle. It may be palpated
on the dorsal surface of the foot close to the base of the fifth
metatarsal.
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This muscle (see Figure 8.22) plantar
flexes, everts, and abducts the tarsal joints, and plantar flexes the ankle. It also is
most active during the propulsive phase of walking. It is situated
superficially on the lateral aspect of the leg with its distal tendon
passing behind the lateral malleolus and proceeding forward and
downward to the margin of the foot, where it passes behind the tuberosity
of the fifth metatarsal. At this point it turns under the foot,
passes through the peroneal groove of the cuboid, and slants forward
across the plantar surface of the foot to its attachment at the
base of the first metatarsal and first cuneiform, not far from the
attachment of the tibialis anterior. The muscle belly may be palpated
on the lateral surface of the lower half of the leg and just above
and behind the lateral malleolus.
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This muscle (see Figures 8.21a and 8.22) plantar
flexes and everts and abducts the tarsal joints and helps plantar flex the ankle. It is
a penniform muscle, lying beneath the peroneus longus on the lower
half of the lateral aspect of the leg. Its tendon passes behind
the lateral malleolus immediately anterior to the tendon of longus
and continues forward just above the longus tendon to its attachment
on the tuberosity of the fifth metatarsal, below the attachment
of the peroneus tertius. It may be palpated on the lateral margin
of the foot, just posterior to the base of the fifth metatarsal.
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Gastrocnemius (Also Listed with the Knee Muscles)
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This is a powerful fast-twitch fiber muscle (Figure 8.23) for plantar flexing the foot at the ankle joint. It
is the most superficial muscle on the back of the leg and can be
seen as two bulges in the upper part of the calf when it is well
developed. Its two heads, together with the soleus, constitute the
triceps surae. The lateral and medial portions of the muscle remain
distinct from each other as far down as the middle of the back of
the leg. Then they fuse to form the broad tendon of Achilles.
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The most familiar function of this muscle is to enable one to rise on the toes. It
has also been shown to be active in most individuals during normal
relaxed standing. The muscle has a large angle of pull, approximately
90 degrees when the foot is in its fundamental position. Its internal
structure and its leverage combine to make it an exceedingly powerful
muscle. The gastrocnemius is more active with the knee extended
than with a flexed knee. It is most active in closed kinetic chain
activity in the standing position but shows a moderate level of
activity when resisted in seated, prone, and supine positions (Carlsson
et al. 2001; Tamaki et al. 1997). It may be palpated in the calf
of the leg and on the back of the ankle.
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Like the gastrocnemius, this muscle (Figure 8.24) plantar flexes the foot at the ankle joint. It
lies beneath the gastrocnemius, except along the lateral aspect
of the lower half of the calf where a portion of it lies lateral
to the upper part of the Achilles tendon. Its fibers are inserted
into the Achilles tendon in a bipenniform manner. It is predominantly
comprised of slow-twitch fibers. In an EMG study of the leg muscles,
when the subjects balanced on one foot, the soleus was consistently
more active than the gastrocnemius. In another study the soleus
was most active in minimal contractions and when the foot was in
a dorsiflexed position. This seems to imply that it was especially
active in the reduction of dorsiflexion. Campbell and coworkers,
using fine wire electrodes, have shown that the medial part of the
soleus is a strong dynamic and static plantar flexor, whereas the
lateral part is primarily a stabilizer (Basmajian and DeLuca 1985).
The muscle may be palpated slightly lateral to and below the lateral
bulge of the gastrocnemius.
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This muscle (Figure 8.25) plantar flexes
the tarsal joints and helps plantar flex the ankle. It participates
in supination (inversion and adduction) when
the foot is plantar flexed. It is the deepest muscle on the back
of the leg. The main part of the muscle covers the intermuscular
septum between the tibia and the fibula. In the lower front of the
leg its tendon slants across the medial side of the ankle, passes
behind the medial malleolus and above the sustentaculum tali, and
then turns under the foot around the medial margin of the navicular
bone to insert into its underside. The muscle is penniform in structure.
Because of its direction of pull and its numerous attachments on
the plantar surface of the tarsal bones, an important function of
this muscle appears to be maintenance of the longitudinal arch in
a reserve capacity such as is needed in a weak foot.
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Flexor Digitorum
Longus
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This muscle (Figure 8.26) flexes the
four lesser toes, plantar flexes and helps invert and adduct the
tarsal joints, and helps plantar flex the ankle. It is situated
on the medial side of the back of the leg behind the tibia. Penniform
in structure, its distal tendon passes behind the medial malleolus
between the tendons of the tibialis posterior and flexor hallucis
longus. Beneath the tarsal bones it divides into four tendons that
go to the distal phalanx of each of the four lesser toes.
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Flexor Hallucis
Longus
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This muscle (Figure 8.26) flexes the
great toe, plantar flexes and helps invert and adduct the tarsal
joints, and helps plantar flex the ankle. It is situated on
the lateral side of the back of the leg, behind the fibula and the
lateral portion of the tibia. The fibers unite with the distal tendon
in a penniform manner. The tendon crosses behind the ankle to the
medial side, passes behind and beneath the sustentaculum tali, and
runs forward under the medial margin of the foot to the distal phalanx
of the great toe. It is the most posterior of the three tendons
that pass behind the medial malleolus. One of its important functions
is to provide the push-off in walking, running, and jumping. It
may be palpated on the medial border of the calcaneal tendon close
to the calcaneus.
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Intrinsic Muscles
of the Foot
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These muscles (Figures 8.27, 8.28, and 8.29) will be treated
as a group rather than individually. There are eleven of these small
muscles or muscle groups. All but one, the extensor digitorum brevis,
are on the plantar surface and are usually described as being arranged
in four layers. The dorsal interossei muscles, although included
in the deepest layer, are situated between the metatarsal bones
rather than on either surface (see Figure 8.21b). The extensor digitorum
brevis, which includes the hallucis although the latter is sometimes
described as a separate muscle, is situated on the dorsal surface
of the foot (see Figure 8.21a). With the exception of the lumbricales and the quadratus
plantae, which help flex the lesser toes, the names of these
muscles indicate their functions. As one might expect, these intrinsic
muscles are much more highly developed in primitive people than
in people who habitually wear shoes.
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Various research investigations have shown that the intrinsic
muscles act as a functional unit, have a significant role in stabilization
of the foot during propulsion, and tend to show more activity in
feet that are habitually pronated. They do not show activity during
relaxed standing in either normal or pronated feet and are not active
in the normal static support of the
longitudinal arches. They do, however, show definite activity in
voluntary attempts to increase the height of the arches. They are
also definitely active in the movement of rising on the toes.
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On the plantar surface of the foot the muscles are covered by
fascia (Figure 8.30), divided into medial, central, and lateral
portions. The central portion, known as the plantar aponeurosis,
is particularly strong and fibrous. It extends under the whole length
of the foot, connecting the tuberosity of the calcaneus with the
bases of the proximal phalanges of the five toes. This is an exceedingly
strong band that serves as an effective binding rod for the longitudinal
arch.
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