Human feet are everted so that their soles lie fully on the
ground during ambulation. What muscle is developmentally
unique to humans, inserts into the base of the fi fth metatarsal,
and assists in eversion (or pronation)?
(A) Extensor digitorum longus
(B) Extensor digitorum brevis
(C) Fibularis longus
(D) Fibularis tertius
(E) Flexor digiti minimi brevis
The answer is D: Fibularis tertius. The fibularis (peroneus)
tertius muscle arises off the distal portion of the extensor
digitorum longus muscle and attaches to the dorsal surface
of the base of the fifth metatarsal. This muscle is rarely
found in other primates and is functionally linked to effi cient
terrestrial bipedalism. Though this muscle may be highly variable
in human specimens, it is important in eversion of the
foot during ambulation and may have a proprioceptive role
to guard against excessive inversion of the foot at the ankle
joint. Choice A (Extensor digitorum longus) is incorrect.
The extensor digitorum longus arises off the superiolateral
aspect of the tibia and interosseous membrane and inserts
into the middle and distal phalanges of the lateral four digits.
This muscle extends the lateral four digits and dorsiflexes
the ankle. It does not insert into the base of the fifth metatarsal
or assist in eversion. It does give rise to the fi bularis
tertius muscle distally. Choice B (Extensor digitorum brevis)
is incorrect. The extensor digitorum brevis is an intrinsic foot
muscle that inserts into the middle phalanges of the lateral
four digits to extend these toes. It does not insert into the
base of the fi fth metatarsal or assist in eversion. Choice C
(Fibularis longus) is incorrect. The fi bularis longus muscle
arises off the proximal aspect of the fi bula and inserts into
the base of the fi rst metatarsal and medial cuneiform bone.
This muscle is important in eversion of the foot; however,
it can be eliminated due to its medial insertion in the foot.
Choice E (Flexor digiti minimi brevis) is incorrect. The fl exor
digiti minimi brevis arises from the base of the fi fth metatarsal
and inserts into the base of the proximal phalanx of the fi fth
digit. It does not participate in eversion of the foot, so this
answer can be easily eliminated.
ground during ambulation. What muscle is developmentally
unique to humans, inserts into the base of the fi fth metatarsal,
and assists in eversion (or pronation)?
(A) Extensor digitorum longus
(B) Extensor digitorum brevis
(C) Fibularis longus
(D) Fibularis tertius
(E) Flexor digiti minimi brevis
The answer is D: Fibularis tertius. The fibularis (peroneus)
tertius muscle arises off the distal portion of the extensor
digitorum longus muscle and attaches to the dorsal surface
of the base of the fifth metatarsal. This muscle is rarely
found in other primates and is functionally linked to effi cient
terrestrial bipedalism. Though this muscle may be highly variable
in human specimens, it is important in eversion of the
foot during ambulation and may have a proprioceptive role
to guard against excessive inversion of the foot at the ankle
joint. Choice A (Extensor digitorum longus) is incorrect.
The extensor digitorum longus arises off the superiolateral
aspect of the tibia and interosseous membrane and inserts
into the middle and distal phalanges of the lateral four digits.
This muscle extends the lateral four digits and dorsiflexes
the ankle. It does not insert into the base of the fifth metatarsal
or assist in eversion. It does give rise to the fi bularis
tertius muscle distally. Choice B (Extensor digitorum brevis)
is incorrect. The extensor digitorum brevis is an intrinsic foot
muscle that inserts into the middle phalanges of the lateral
four digits to extend these toes. It does not insert into the
base of the fi fth metatarsal or assist in eversion. Choice C
(Fibularis longus) is incorrect. The fi bularis longus muscle
arises off the proximal aspect of the fi bula and inserts into
the base of the fi rst metatarsal and medial cuneiform bone.
This muscle is important in eversion of the foot; however,
it can be eliminated due to its medial insertion in the foot.
Choice E (Flexor digiti minimi brevis) is incorrect. The fl exor
digiti minimi brevis arises from the base of the fi fth metatarsal
and inserts into the base of the proximal phalanx of the fi fth
digit. It does not participate in eversion of the foot, so this
answer can be easily eliminated.
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