Chapter 10
The Muscular System
369
10
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
Muscles of the Posterior Compartment of the Thigh
Hamstrings
The hamstrings are fleshy muscles of the posterior thigh (biceps femoris, semitendinosus, and
semimembranosus). They cross both the hip and knee joints and are prime movers of thigh extension and knee
flexion. The group has a common origin site and is innervated by sciatic nerve (actually two nerves, the tibial
and common fibular nerves wrapped in a common sheath). Ability of hamstrings to act on one of the two joints
spanned depends on which joint is fixed—if knee is fixed (extended), they extend hip; if hip is extended, they
flex knee. However, when hamstrings are stretched, they tend to restrict full accomplishment of antagonistic
movement; e.g., if knees are fully extended, it is difficult to flex the hip fully (and touch your toes), and when
the thigh is fully flexed as in kicking a football, it is almost impossible to extend the knee fully at the same time
(without considerable practice). Name of this muscle group comes from old butchers’ practice of using their
tendons to hang hams for smoking. “Pulled hamstrings” are common sports injuries in those who run very hard,
e.g., football halfbacks.
Biceps femoris
(
biceps
5
two heads)
Most lateral muscle of the
group; arises from two
heads
O—ischial tuberosity
(long head); linea aspera,
lateral supracondylar
line, and distal femur
(short head)
I—common tendon
passes downward and
laterally (forming lateral
border of popliteal
fossa) to insert into head
of fibula and lateral
condyle of tibia
Extends thigh and flexes
knee
; laterally rotates leg,
especially when knee is
flexed
Sciatic nerve—tibial
nerve to long head,
common fibular nerve
to short head (L
5
–S
2
)
Semitendinosus
(sem
0
e-ten
0
dı˘-no
9
sus)
(
semi
5
half;
tendinosus
5
tendon)
Lies medial to biceps
femoris; although its name
suggests that this muscle is
largely tendinous, it is quite
fleshy; its long slender
tendon begins about two-
thirds of the way down
thigh
O—ischial tuberosity in
common with long head
of biceps femoris
I—medial aspect of
upper tibial shaft
Extends thigh and
flexes knee
; with
semimembranosus,
medially rotates leg
Sciatic nerve—tibial
nerve portion (L
5
–S
2
)
Semimembranosus
(sem
0
e-mem
0
-brah-
no
9
sus) (
membranosus
5
membrane)
Deep to semitendinosus
O—ischial tuberosity
I—medial condyle
of tibia; via oblique
popliteal ligament to
lateral condyle of femur
Extends thigh and flexes
knee
; medially rotates leg
Sciatic nerve—tibial
nerve portion (L
5
–S
2
)
MUSCLE GALLERY
Table 10.14
(continued)
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