2 Musculature: Functional Groups
2.1 The Muscles of the Lower Limb: Classification
In most mammals, the upper and lower limbs share many functions and have analogous functional groups of muscles. In humans, however, the specializations of the upper limb for manipulation and the lower limb for ambulation have imposed radically different requirements upon their respective muscle groups. For instance, the shoulder girdle has considerable freedom of motion on the trunk and is acted upon by an array of muscles, but the pelvic ring is firmly fixed to the vertebral column and changes position very little relative to the trunk, and has no comparable muscles to move it. In contrast, the hip and gluteal muscles have evolved into massive and powerful movers and stabilizers of the femur, counteracting the loads imposed by support of the whole body weight on two limbs and maintaining balance and stability during bipedal locomotion; these muscles are, in aggregate, larger than their counterparts that act upon the humerus, with a significantly different arrangement and orientation.
As with the upper limb (see p. 298), the muscles of the lower limb can be classified on the basis of origin, topography, function, and innervation. Each such classification system has advantages and disadvantages, so several schemes are presented here. Segregation of muscles that act at the hip into specific functional groups is valid only for a particular joint position, because the axis of motion changes relative to the muscles as the joint is dynamically reoriented, causing abductors to become adductors, for example. Muscles surrounding the hip can be categorized topographically into an inner and outer group, relative to the pelvic girdle (see A). Muscles acting on the knee and foot can be grouped logically in an arrangement that uses both functional and topographical criteria, because these muscles tend to be clustered by functional groups into discrete compartments and act in a consistent way on joints with restricted ranges of motion. As with the upper limb, it is also instructive to categorize the lower limb’s muscles by the pattern of their innervation (see E), a pattern that reveals the underlying logic of different clinical syndromes involving nerve damage.
A The hip and gluteal muscles
B The thigh muscles
C The leg muscles
Anterior compartment • Tibialis anterior • Extensor digitorum longus • Extensor hallucis longus • Fibularis tertius Lateral compartment • Fibularis longus • Fibularis brevis Posterior compartment Superficial part • Triceps surae – Soleus – Gastrocnemius (medial and lateral heads) • Plantaris Deep part • Tibialis posterior • Flexor digitorum longus • Flexor hallucis longus • Popliteus |
D The short muscles of the foot
Dorsal muscles • Extensor digitorum brevis • Extensor hallucis brevis Plantar muscles Medial compartment • Abductor hallucis • Flexor hallucis brevis (medial and lateral heads) Lateral compartment • Abductor digiti minimi • Flexor digiti minimi brevis • Opponens digiti minimi Central compartment • Flexor digitorum brevis • Adductor hallucis (transverse and oblique heads) • Quadratus plantae • First through fourth lumbricals • First through third plantar interossei |
E Classification of muscles based on their motor innervation
All the muscles of the lower limb are supplied by branches of the lumbar plexus (T12–L4) and the sacral plexus (L5–S3). They may be supplied by short, direct branches or by long nerves emanating from the corresponding plexus (see p. 536). See also F.
Nerve or plexus | Muscles supplied |
Lumbar plexus |
|
Direct branches (muscular branches) (L1–L3) | Psoas major and minor |
Nerves of the lower limb arising from the lumbar plexus | |
Femoral nerve (L2–L4) | Iliacus, pectineus; sartorius, quadriceps femoris |
Obturator nerve (L2–L4) | Obturator externus, pectineus, adductor longus, adductor brevis, adductor magnus (deep part), adductor minimus, gracilis |
Sacral plexus |
|
Direct branches (muscular branches) (L5–S2) | Piriformis, obturator internus, gemelli, quadratus femoris |
Nerves of the lower limb arising from the sacral plexus | |
Superior gluteal nerve (L4–S1) | Tensor fasciae latae, gluteus medius and minimus |
Inferior gluteal nerve (L5–S2) | Gluteus maximus |
Sciatic nerve (L4–S3) (see F) | Adductor magnus (superficial part, tibial part), biceps femoris (long head, tibial part), biceps femoris (short head, fibular part), semimembranosus (tibial part), semitendinosus (tibial part) |
• Common fibular* nerve (L4–S2) |
|
– Deep fibular nerve | Tibialis anterior, extensor digitorum longus and brevis, fibularis tertius, extensor hallucis longus and brevis |
– Superficial fibular nerve | Fibularis longus and brevis |
• Tibial nerve (L4–S3) | Popliteus, triceps surae, plantaris, tibialis posterior, flexor digitorum longus, flexor hallucis longus |
– Medial plantar nerve | Abductor hallucis, flexor hallucis brevis (medial head), flexor digitorum brevis, first lumbrical |
– Lateral plantar nerve | Flexor hallucis brevis (lateral head), adductor hallucis, abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi, quadratus plantae, second to fourth lumbricals, first through third plantar interossei, first through fourth dorsal interossei |
* The common fibular nerve and its divisions are also referred to as “peroneal nerves.” | |
F The branches of the lumbosacral plexus that innervate the muscles of the lower limb
Right leg, lateral view. The anterior rami of the lumbar and sacral nerves, with contributions from the subcostal nerve and coccygeal nerve (not shown here), combine to form the lumbosacral plexus. While the branches arising from the lumbar plexus run anterior to the hip joint and mainly supply the muscles on the anterior and medial views of the thigh, the branches from the sacral plexus run behind the hip joint to supply the posterior thigh muscles and all the muscles of the leg and foot. The grossly visible division of the sciatic nerve into its two terminal branches (the tibial nerve and common fibular nerve) is generally located just above the knee joint, as pictured here (low division). But the nerve fibers that make up the two terminal branches become organized into bundles at a much more proximal level, where they already appear as separate nerve branches within their common fibrous sheath. In the high division pattern, the nerve divides into its terminal branches while still in the lesser pelvis (see p. 559).
2.2 The Hip and Gluteal Muscles: The Inner Hip Muscles
A Schematic of the inner hip muscles
Iliopsoas |
|
Origin: | • ① Psoas major (superficial layer): lateral surfaces of the T12 vertebral body, the L1–L4 vertebral bodies, and the associated intervertebral disks • ① Psoas major (deep layer): costal processes of the L1–L5 vertebrae • ② Iliacus: the iliac fossa |
Insertion: | Common insertion on the lesser trochanter of the femur as the iliopsoas ③ (psoas minor inserts into the iliopectineal arch, not depicted; see B) |
Action: | • Hip joint: flexion and external rotation • Lumbar spine: unilateral contraction (with the femur fixed) bends the trunk laterally to the same side, bilateral contraction raises the trunk from the supine position |
Innervation: | Femoral nerve (iliacus) and direct branches from the lumbar plexus (psoas) (L1–L4) |
Properties and clinical aspects of the iliopsoas muscle
The iliopsoas is classified as a hip flexor, along with the rectus femoris, sartorius, and tensor fasciae latae. It is the most powerful flexor, its long vertical travel making it an important muscle for standing, walking, and running. As a typical postural muscle with a preponderance of slow-twitch red (type I) fibers, however, the iliopsoas is inherently susceptible to pathological shortening (particularly in older patients with a sedentary lifestyle or chronic immobilization conditions) and requires regular stretching to maintain normal tone (see pp. 58 and 505). Shortening (contracture) of the hip flexors leads to
• increased anterior pelvic tilt,
• increased lumbar lordosis, and
• limitation of hip extension.
Unilateral shortening of the iliopsoas, in which the ilium on the affected side is tilted forward, can be diagnosed with the Thomas maneuver (see p. 438). This condition leads to pelvic torsion, in which the pelvis becomes twisted upon itself. This mainly alters the function of the sacroiliac joints but also compromises the intervertebral joints and the lumbosacral junction (increased lordosis of the lumbar spine with degenerative changes in the vertebral bodies, see p. 132). Patients with bilateral iliopsoas weakness or paralysis are unable to raise the trunk from the supine position, despite intact abdominal muscles, without using their arms and are greatly limited in their ability to walk and climb stairs without assistance.
B The inner hip muscles
Anterior view.
Note: In the lower extremity, the directional terms “posterior” and “anterior” are better used than “dorsal” and “ventral”; for background, see rotation of the extremities on p. 21.
The psoas major unites with the iliacus at the level of the inguinal ligament to form a conjoined muscle, the iliopsoas. Approximately 50% of the population also has a psoas minor muscle (as shown here), which arises from the T12 and L1 vertebrae and inserts into the iliopectineal arch (iliac fascia).
2.3 The Hip and Gluteal Muscles: The Outer Hip Muscles
A Schematic of the vertically oriented outer hip muscles
① Gluteus maximus | |
Origin: | Lateral part of the dorsal surface of the sacrum, posterior part of the gluteal surface of the ilium (behind the posterior gluteal line), also from the thoracolumbar fascia and sacrotuberous ligament |
Insertion: | • Upper fibers: iliotibial tract • Lower fibers: gluteal tuberosity |
Action: | • Entire muscle: extends and externally rotates the hip, stabilizes the hip in both the sagittal and coronal planes • Upper fibers: abduction • Lower fibers: adduction |
Innervation: | Inferior gluteal nerve (L5–S2) |
② Gluteus medius | |
Origin: | Gluteal surface of the ilium (below the iliac crest between the anterior and posterior gluteal line) |
Insertion: | Lateral surface of the greater trochanter of the femur |
Action: | • Entire muscle: abducts the hip, stabilizes the pelvis in the coronal plane • Anterior part: flexion and internal rotation • Posterior part: extension and external rotation |
Innervation: | Superior gluteal nerve (L4–S1) |
③ Gluteus minimus | |
Origin: | Gluteal surface of the ilium (below the origin of gluteus medius) |
Insertion: | Anterolateral surface of the greater trochanter of the femur |
Action: | • Entire muscle: abducts the hip, stabilizes the pelvis in the coronal plane • Anterior part: flexion and internal rotation • Posterior part: extension and external rotation |
Innervation: | Superior gluteal nerve (L4–S1) |
④ Tensor fasciae latae | |
Origin: | Anterior superior iliac spine |
Insertion: | Iliotibial tract |
Action: | • Tenses the fascia lata • Abduction, flexion, and internal rotation of the hip joint |
Innervation: | Superior gluteal nerve (L4–S1) |
⑤ Piriformis | |
Origin: | Pelvic surface of the sacrum |
Insertion: | Apex of the greater trochanter of the femur |
Action: | • External rotation, abduction, and extension of the hip joint • Stabilizes the hip joint |
Innervation: | Direct branches from the sacral plexus (S1, S2) |
B Schematic of the horizontally oriented outer hip muscles
① Obturator internus | |
Origin: | Inner surface of the obturator membrane and its bony boundaries |
Insertion: | Medial surface of the greater trochanter |
Action: | External rotation, adduction, and extension of the hip joint (also active in abduction, depending on the position of the joint) |
Innervation: | Direct branches from the sacral plexus (L5, S1) |
② Gemelli | |
Origin: | • Gemellus superior: ischial spine • Gemellus inferior: ischial tuberosity |
Insertion: | Jointly with obturator internus tendon (medial surface, greater trochanter) |
Action: | External rotation, adduction, and extension of the hip joint (also active in abduction, depending on the position of the joint) |
Innervation: | Direct branches from the sacral plexus (L5, S1) |
③ Quadratus femoris | |
Origin: | Lateral border of the ischial tuberosity |
Insertion: | Intertrochanteric crest of the femur |
Action: | External rotation and adduction of the hip joint |
Innervation: | Direct branches from the sacral plexus (L5, S1) |
C The outer hip muscles: superficial layer
Right side, posterior view.
Note the position of the gluteus maximus muscle in relation to the axis of hip abduction and adduction. While the fibers of the gluteus maximus that run above the axis and insert on the tibia via the iliotibial tract are active in abducting the hip joint, the muscle fibers that run below the axis are active in adduction.
D The outer hip muscles: deep layer
Right side, posterior view.
a With the gluteus maximus removed.
b With the gluteus medius removed.
If there is weakness or paralysis of the small gluteal muscles (gluteus medius and minimus), the pelvis can no longer be stabilized in the coronal plane and will tilt toward the unaffected side (positive Trendelenburg sign, see also p. 542).
2.4 The Hip and Gluteal Muscles: The Adductor Group
A Schematic of the adductors
① Obturator externus | |
Origin: | Outer surface of the obturator membrane and its bony boundaries |
Insertion: | Trochanteric fossa of the femur |
Action: | • Adduction and external rotation of the hip joint • Stabilizes the pelvis in the sagittal plane |
Innervation: | Obturator nerve (L3, L4) |
② Pectineus | |
Origin: | Pecten pubis |
Insertion: | Pectineal line and the proximal linea aspera of the femur |
Action: | • Adduction, external rotation, and slight flexion of the hip joint • Stabilizes the pelvis in the coronal and sagittal planes |
Innervation: | Femoral nerve (L1-L4), obturator nerve (L2-L4) |
③ Adductor longus | |
Origin: | Superior pubic ramus and anterior side of the symphysis |
Insertion: | Linea aspera: medial lip in the middle third of the femur |
Action: | • Adduction and flexion (up to 70°) of the hip joint (extends the hip past 80° of flexion) • Stabilizes the pelvis in the coronal and sagittal planes |
Innervation: | Obturator nerve (L2–L4) |
④ Adductor brevis | |
Origin: | Inferior pubic ramus |
Insertion: | Linea aspera: medial lip in the upper third of the femur |
Action: | • Adduction and flexion (up to 70°) of the hip joint (extends the hip past 80° of flexion) • Stabilizes the pelvis in the coronal and sagittal planes |
Innervation: | Obturator nerve (L2, L3) |
⑤ Adductor magnus | |
Origin: | Inferior pubic ramus, ischial ramus, and ischial tuberosity |
Insertion: | • Deep part (“fleshy insertion”): medial lip of linea aspera • Superficial part (“tendinous insertion”): adductor tubercle of femur |
Actions: | • Adduction, extension, and slight flexion of the hip joint (the tendinous insertion is also active in internal rotation) • Stabilizes the pelvis in the coronal and sagittal planes |
Innervation: | • Deep part: obturator nerve (L2–L4) • Superficial part: tibial nerve (L4) |
⑥ Adductor minimus (upper division of adductor magnus) | |
Origin: | Inferior pubic ramus |
Insertion: | Medial lip of the linea aspera |
Action: | Adduction, external rotation, and slight flexion of the hip joint |
Innervation: | Obturator nerve (L2–L4) |
⑦ Gracilis | |
Origin: | Inferior pubic ramus below the pubic symphysis |
Insertion: | Medial border of the tibial tuberosity (along with the tendons of sartorius and semitendinosus) |
Action: | • Hip joint: adduction and flexion • Knee joint: flexion and internal rotation |
Innervation: | Obturator nerve (L2, L3) |
