The document describes the anatomy and functions of the muscles in the third and fourth layers of the foot, specifically focusing on the flexor hallucis brevis, adductor hallucis, and flexor digiti minimi brevis muscles, as well as the dorsal and plantar interossei. It details their origins, insertions, actions, and innervations. This information provides an understanding of how these muscles contribute to foot movement and stability.

1. MUSCLO-SKELETAL SYSTEM
FOOT ANATOMY

3. THIRD LAYER
There are three muscles in the third
layer in the sole of the foot:
–two (flexor hallucis brevis and
adductor hallucis) are associated
with the great toe;
–the third (flexor digiti minimi
brevis) is associated with the little
toe.

5. FLEXOR HALLUCIS BREVIS
• The flexor hallucis brevis muscle has two
tendinous heads of origin:
–the lateral head originates from the plantar
surfaces of the cuboid, behind the groove
for fibularis longus, and adjacent surface of
the lateral cuneiform;
–the medial head originates from the tendon
of the tibialis posterior muscle as it passes
into the sole of the foot.

6. FLEXOR HALLUCIS BREVIS
• The medial and lateral heads unite and
give rise to a muscle belly, which itself is
separated into medial and lateral parts
adjacent to the plantar surface of
metatarsal I.
• Each part of the muscle gives rise to a
tendon that inserts either on the lateral
or medial side of the base of the
proximal phalanx of the great toe.

8. FLEXOR HALLUCIS BREVIS
• ACTION: Flexor hallucis brevis
flexes the metatarsophalangeal
joint of the great toe
• INNERVATION: Flexor hallucis
brevis is innervated by the
medial plantar nerve.

9. ADDUCTOR HALLUCIS
MUSCLE
• The adductor hallucis muscle
originates by two muscular heads:
1. Transverse head
2. Oblique head
• which join near their ends to insert
into the lateral side of the base of
the proximal phalanx of the great
toe

11. ADDUCTOR HALLUCIS
MUSCLE
• the transverse head originates from the
plantar ligaments associated with the
metatarsophalangeal joints of the lateral
three toes
• the muscle crosses the sole of the foot
transversely from lateral to medial and
joins the oblique head near the base of
the great toe

13. ADDUCTOR HALLUCIS
MUSCLE
• the oblique head is larger than the
transverse head and originates from the
plantar surfaces of the bases of
metatarsals II to IV and from the sheath
covering the fibularis longus muscle
• this head passes obliquely through the
sole of the foot and joins the transverse
head.

14. ADDUCTOR HALLUCIS
MUSCLE
• The tendon inserts to the Lateral side of
base of proximal phalanx of great toe.
• ACTION: Adductor hallucis adducts the
great toe at the metatarsophalangeal
joint
• INNERVATION: Adductor hallucis is
innervated by the lateral plantar nerve.

16. FLEXOR DIGITI MINIMI
BREVIS
• The flexor digiti minimi brevis muscle
originates from the plantar surface of
the base of metatarsal V and
adjacent sheath of the fibularis
longus tendon.
• It inserts on the lateral side of the
base of the proximal phalanx of the
little toe

18. FLEXOR DIGITI MINIMI
BREVIS
• ACTION: Flexor digiti minimi brevis
flexes the little toe at the
metatarsophalangeal joint
• INNERVATION: Flexor digiti minimi
brevis is innervated by the lateral
plantar nerve.

20. FOURTH LAYER
• There are two muscle groups in
the deepest muscle layer in the
sole of the foot:
1. dorsal interossei (4)
2. plantar interossei (3)

22. DORSAL INTEROSSEI
• The four dorsal interossei are the most
superior muscles in the sole of the foot,
which are located between the
metatarsals.
• All four muscles are bipennate (arises
from two metatarsals) and originate
from the sides of adjacent metatarsals.

23. DORSAL INTEROSSEI
• The tendons of the dorsal interossei
insert into the free margin of the
dorsal expansions and base of the
proximal phalanges of the toes.
• The great and little toes have their
own abductors (abductor hallucis and
abductor digiti minimi) in the first
layer of muscles in the sole of the
foot.

25. DORSAL INTEROSSEI
• ACTION: Abduct digits two to four and
flex the metatarsophalangeal joints.
• INNERVATION: The dorsal interossei
are innervated by the lateral plantar
nerve.
• The first and second dorsal interossei
also receive branches on their superior
surfaces from the deep fibular nerve.

26. PLANTAR INTEROSSEI
• The three plantar interossei adduct the
third, fourth, and little toes toward the
long axis through the second toe.
• Each plantar interosseous muscle
originates from the medial side of its
associated metatarsal and inserts into the
medial free margin of the dorsal
expansion and base of the proximal
phalanx.

29. PLANTAR INTEROSSEI
• ACTION: Adduct digits three to five
and flex the metatarsophalangeal
joints.
• INNERVATION: All are innervated
by the lateral plantar nerve.