Muscles of the Knee Joint

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The Knee Extensor Muscles. The rectus fem-oris originates from the pelvis and flexes (bends) the hip joint. It is inserted into the patella and can straighten the knee with the help of the powerful tendon that extends from the patella to the tibia. Three other large extensor muscles, the vastus muscles, are inserted into the knee joint.

The Knee Flexor Muscles: The Hamstrings. All three hamstring muscles originate from the ischial tuberosity and run toward the knee. The biceps femoris muscle is inserted into the head of the fibula. It can rotate the lower leg so that the foot points laterally. The semitendinosus and semimembranosus muscles are inserted into the medial tibial condyle and can rotate the lower leg medially. The distance between the origin and insertion of these hip extensors and knee flexors varies greatly, depending on the angle of the hip and knee joint. Shortened hamstring muscles at the back of the thigh result in

TABLE 12-2 The Main Muscles Producing Knee Movement

Function

Muscle

Proximal Attachment

Distal Attachment

Innervation

Action

Extensors

Rectus femoris

Anterior superior iliac spine

Base of patella and via

Femoral nerve (L2-L4)

Extends leg at knee joint;

and groove superior to

patella ligament to tibial

also stabilizes hip joint

acetabulum

tuberosity

and helps iliopsoas to flex

thigh

Vastus lateralis

Greater trochanter and

Base of patella and via

Femoral nerve (L2-L4)

Extends leg at knee joint

lateral lip of linea aspera

patella ligament to tibial

of femur

tuberosity

Vastus medialis

Intertrochanteric line and

Base of patella and via

Femoral nerve (L2-L4)

Extends leg at knee joint

medial lip of linea aspera

patella ligament to tibial

of femur

tuberosity

Vastus intermedius

Anterior and lateral surfaces

Base of patella and via

Femoral nerve (L2-L4)

Extends leg at knee joint

of femur

patella ligament to tibial

tuberosity

Flexors

Biceps femoris

Long head: ischial tuberosity

Lateral head of fibula;

Sciatic nerve

Flexes and rotates leg

Short head: lateral lip of

tendon is split at this

Long head: tibial division

laterally; long head

linea aspera and lateral

site by fibular collateral

(L5-S2)

extends the thigh

supracondylar line

ligament of knee joint

Short head: common

fibular division (L5-S2)

Semitendinosus

Ischial tuberosity

Medial surface of superior

Tibial division of sciatic

Extends thigh; flexes and

part of tibia

nerve (L5-S2)

rotates leg medially

Semimembranosus

Ischial tuberosity

Posterior part of medial

Tibial division of sciatic

Extends thigh; flexes and

condyle of tibia

nerve (L5-S2)

rotates leg medially

Popliteus

Lateral surface of lateral

Posterior surface of tibia,

Tibial nerve (L4-S1)

Weakly flexes knee

condyle of femur and

superior to soleal line

lateral meniscus

Medial

Sartorius

Anterior superior iliac spine

Superior part of medial

Femoral nerve (L2-L3)

Flexes, abducts, and laterally

rotators

and superior part of

surface of tibia

rotates thigh at hip joint

notch inferior to it

Gracilis

Body and inferior ramus of

Superior part of medial

Obturator nerve (L2 and

Adducts thigh, flexes leg,

pubis

surface of tibia

L3)

and helps rotate leg

medially

Lateral

Tensor fasciae

Anterior superior iliac spine

Superior part of medial

Superior gluteal nerve

Abducts, medially rotates,

rotator

latae

and anterior part of

surface of tibia

(L4-L5)

and flexes thigh; helps

external lip of iliac crest

keep knee extended;

stabilizes trunk on thigh

Tibialis anterior

Extensor hallucis logus Extensor digitorum longus

Superficial fibular nerve

Anterior intermuscular septum of leg

Fibularis [peroneus] brevis

Fibularis [peroneus] longus

Posterior intermuscular septum of leg

Fibula Flexor hallucis longus

Tibialis anterior

Deep fascia of leg Anterior tibial artery

Deep fascia of leg Anterior tibial artery

Deep fibular nerve

Flexor digitorum longus Great saphenous vein Interrosseous membrane

Tibialis posterior Posterior tibial artery Gastrocnemius, medical head Tibial nerve

Fibular artery'

Small saphenous vein Figure 12-6 Three compartments of the leg.

a stationary hip. When a person is unable to tip the pelvis forward, he or she will try to compensate for this by bending forward at the lumbar spine. Many back problems in athletes and other patients are caused by shortened hamstring muscles.

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