The Hip – Anatomy, Structure, Functions

The Hip – Anatomy, Structure, Functions

The hip region is located lateral and anterior to the gluteal region, inferior to the iliac crest, and overlying the greater trochanter of the femur, or “thigh bone”. In adults, three of the bones of the pelvis have fused into the hip bone or acetabulum which forms part of the hip region.

The hip joint, scientifically referred to as the acetabulofemoral joint (art. coxae), is the joint between the femur and acetabulum of the pelvis and its primary function is to support the weight of the body in both static (e.g., standing) and dynamic (e.g., walking or running) postures. The hip joints have very important roles in retaining balance, and for maintaining the pelvic inclination angle.

Structure

Region

The proximal femur is largely covered by muscles and, as a consequence, the greater trochanter is often the only palpable bony structure in the hip region.[rx]

Articulation

The hip joint is a synovial joint formed by the articulation of the rounded head of the femur and the cup-like acetabulum of the pelvis. It forms the primary connection between the bones of the lower limb and the axial skeleton of the trunk and pelvis. Both joint surfaces are covered with a strong but lubricated layer called articular hyaline cartilage.

The cuplike acetabulum forms at the union of three pelvic bones — the ilium, pubis, and ischium.[rx] The Y-shaped growth plate that separates them, the triradiate cartilage, is fused definitively at ages 14–16.[rx] It is a special type of spheroidal or ball and socket joint where the roughly spherical femoral head is largely contained within the acetabulum and has an average radius of curvature of 2.5 cm.[rx] The acetabulum grasps almost half the femoral ball, a grip augmented by a ring-shaped fibrocartilaginous lip, the acetabular labrum, which extends the joint beyond the equator.[rx] The joint space between the femoral head and the superior acetabulum is normally between 2 and 7 mm.[rx]

The head of the femur is attached to the shaft by a thin neck region that is often prone to fracture in the elderly, which is mainly due to the degenerative effects of osteoporosis.

The acetabulum is oriented inferiorly, laterally and anteriorly, while the femoral neck is directed superiorly, medially, and slightly anteriorly.

Articular angles

  • The transverse angle of the acetabular inlet (also called Sharp’s angle and is generally the angle referred to by acetabular angle without further specification)[rx] can be determined by measuring the angle between a line passing from the superior to the inferior acetabular rim and the horizontal plane; an angle which normally measures 51° at birth and 40° in adults, and which affects the acetabular lateral coverage of the femoral head and several other parameters.[rx]
  • The sagittal angle of the acetabular inlet is an angle between a line passing from the anterior to the posterior acetabular rim and the sagittal plane. It measures 7° at birth and increases to 17° in adults.[rx]
  • Wiberg’s center-edge angle (CE angle) is an angle between a vertical line and a line from the center of the femoral head to the most lateral part of the acetabulum,[rx] as seen on an anteroposterior radiograph.[rx]
  • The vertical-center-anterior margin angle (VCA) is an angle formed from a vertical line (V) and a line from the center of the femoral head (C) and the anterior (A) edge of the dense shadow of the subchondral bone slightly posterior to the anterior edge of the acetabulum, with the radiograph being taken from the false angle, that is, a lateral view rotated 25 degrees towards becoming frontal.[rx]
  • The articular cartilage angle (AC angle, also called acetabular index[rx] or Hilgenreiner angle) is an angle formed parallel to the weight-bearing dome, that is, the acetabular source or “roof”,[rx] and the horizontal plane,[rx] or a line connecting the corner of the triangular cartilage and the lateral acetabular rim.[rx] In normal hips in children aged between 11 and 24 months, it has been estimated to be on average 20°, ranging between 18° to 25°.[rx] It becomes progressively lower with age.[rx] Suggested cutoff values to classify the angle as abnormally increased include:
  • 30° up to 4 months of age.[rx]
  • 25° up to 2 years of age.[rx]

Femoral neck angle

The angle between the longitudinal axes of the femoral neck and shaft called the caput-collum-diaphyseal angle or CCD angle normally measures approximately 150° in newborns and 126° in adults (coxa norma).[rx]

An abnormally small angle is known as coxa vara and an abnormally large angle as coxa valga. Because changes in the shape of the femur naturally affect the knee, coxa valga is often combined with genu varum (bow-leggedness), while coxa vara leads to genu valgum (knock-knees).[rx]

Changes in trabecular patterns due to altered CCD angle. Coxa valga leads to more compression trabeculae, coxa vara to more tension trabeculae.[rx]

Changes in the CCD angle is the result of changes in the stress patterns applied to the hip joint. Such changes, caused for example by a dislocation, change the trabecular patterns inside the bones. Two continuous trabecular systems emerging on the auricular surface of the sacroiliac joint meander and criss-cross each other down through the hip bone, the femoral head, neck, and shaft.

  • In the hip bone, one system arises on the upper part of the auricular surface to converge onto the posterior surface of the greater sciatic notch, from where its trabeculae are reflected to the inferior part of the acetabulum. The other system emerges on the lower part of the auricular surface, converges at the level of the superior gluteal line, and is reflected laterally onto the upper part of the acetabulum.
  • In the femur, the first system lines up with a system arising from the lateral part of the femoral shaft to stretch to the inferior portion of the femoral neck and head. The other system lines up with a system in the femur stretching from the medial part of the femoral shaft to the superior part of the femoral head.[rx]

On the lateral side of the hip joint, the fascia lata is strengthened to form the iliotibial tract which functions as a tension band and reduces the bending loads on the proximal part of the femur.[rx]

Capsule

The capsule attaches to the hip bone outside the acetabular hip which thus projects into the capsular space. On the femoral side, the distance between the head’s cartilaginous rim and the capsular attachment at the base of the neck is constant, which leaves a wider extracapsular part of the neck at the back than at the front.[rx][rx]

The strong but loose fibrous capsule of the hip joint permits the hip joint to have the second-largest range of movement (second only to the shoulder) and yet support the weight of the body, arms, and head.

The capsule has two sets of fibers: longitudinal and circular.

  • The circular fibers form a collar around the femoral neck called the zona orbicularis.
  • The longitudinal retinacular fibers travel along the neck and carry blood vessels.

Ligaments

Extracapsular ligaments. Anterior (left) and posterior (right) aspects of the right hip.
Intracapsular ligament. Left hip joint from within pelvis with the acetabular floor removed (left); right hip joint with capsule removed, anterior aspect (right).

The hip joint is reinforced by four ligaments, of which three are extracapsular and one intracapsular.

The extracapsular ligaments are the iliofemoral, ischiofemoral, and pubofemoral ligaments attached to the bones of the pelvis (the ilium, ischium, and pubis respectively). All three strengthen the capsule and prevent an excessive range of movement in the joint. Of these, the Y-shaped and twisted iliofemoral ligament is the strongest ligament in the human body.[rx] In the upright position, it prevents the trunk from falling backward without the need for muscular activity. In the sitting position, it becomes relaxed, thus permitting the pelvis to tilt backward into its sitting position. The iliofemoral ligament prevents excessive adduction and internal rotation of the hip. The ischiofemoral ligament prevents medial (internal) rotation while the pubofemoral ligament restricts abduction and internal rotation of the hip joint.[rx] The zona orbicularis, which lies like a collar around the most narrow part of the femoral neck, is covered by the other ligaments which partly radiate into it. The zona orbicularis acts like a buttonhole on the femoral head and assists in maintaining the contact in the joint.[rx] All three ligaments become taut when the joint is extended – this stabilizes the joint, and reduces the energy demand of muscles when standing.[rsx]

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The intracapsular ligament, the ligament teres, is attached to a depression in the acetabulum (the acetabular notch) and a depression on the femoral head (the fovea of the head). It is only stretched when the hip is dislocated, and may then prevent further displacement.[rx] It is not that important as a ligament but can often be vitally important as a conduit of a small artery to the head of the femur, that is, the foveal artery.[rx] This artery is not present in everyone but can become the only blood supply to the bone in the head of the femur when the neck of the femur is fractured or disrupted by injury in childhood.[rx]

Blood supply

The hip joint is supplied with blood from the medial circumflex femoral and lateral circumflex femoral arteries, which are both usually branches of the deep artery of the thigh (profundal femoris), but there are numerous variations and one or both may also arise directly from the femoral artery. There is also a small contribution from the foveal artery, a small vessel in the ligament of the head of the femur which is a branch of the posterior division of the obturator artery, which becomes important to avoid avascular necrosis of the head of the femur when the blood supply from the medial and lateral circumflex arteries are disrupted (e.g. through fracture of the neck of the femur along their course).[rx]

The hip has two anatomically important anastomoses, the cruciate and the trochanteric anastomoses, the latter of which provides most of the blood to the head of the femur. These anastomoses exist between the femoral artery or profundal femoris and the gluteal vessels.[rx]

Muscles and movements

The hip muscles act on three mutually perpendicular main axes, all of which pass through the center of the femoral head, resulting in three degrees of freedom and three pair of principal directions: Flexion and extension around a transverse axis (left-right); lateral rotation and medial rotation around a longitudinal axis (along the thigh); and abduction and adduction around a sagittal axis (forward-backward);[rx] and a combination of these movements (i.e. circumduction, a compound movement in which the leg describes the surface of an irregular cone).[rx] Some of the hip muscles also act on either the vertebral joints or the knee joint, that with their extensive areas of origin and/or insertion, different part of individual muscles participate in very different movements, and that the range of movement varies with the position of the hip joint.[rx][rx] Additionally, the inferior and Superior Gemelli muscles assist the obturator internus and the three muscles together form the three-headed muscle known as the triceps coxae.[rx][rx]

The movements of the hip joint is thus performed by a series of muscles which are here presented in order of importance[rx] with the range of motion from the neutral zero-degree position[rx] indicated:

  • Lateral or external rotation (30° with the hip extended, 50° with the hip flexed) – gluteus maximus; quadratus femoris; obturator internus; dorsal fibers of gluteus medius and minimus; iliopsoas (including psoas major from the vertebral column); obturator externus; adductor Magnus, longus, brevis, and minimus; piriformis; and sartorius. The iliofemoral ligament inhibits lateral rotation and extension, this is why the hip can rotate laterally to a greater degree when it is flexed.
  • Medial or internal rotation (40°): anterior fibers of gluteus medius and minimus; tensor fasciae latae; the part of adductor magnus inserted into the adductor tubercle; and, with the leg abducted also the pectineus.
  • Extension or retroversion (20°): gluteus maximus (if put out of action, active standing from a sitting position is not possible, but standing and walking on a flat surface is); dorsal fibers of gluteus medius and minimus; adductor Magnus; and piriformis. Additionally, the following thigh muscles extend the hip: semimembranosus, semitendinosus, and long head of biceps femoris. The maximal extension is inhibited by the iliofemoral ligament.
  • Flexion or anteversion (140°): the hip flexors: iliopsoas (with psoas major from vertebral column); tensor fasciae latae, pectineus, adductor longus, adductor brevis, and gracious. Thigh muscles acting as hip flexors: rectus femoris and sartorius. Maximal flexion is inhibited by the thigh coming in contact with the chest.
  • Abduction (50° with hip extended, 80° with hip flexed): gluteus medius; tensor fasciae latae; gluteus maximus with its attachment at the fascia lata; gluteus minimus; piriformis; and obturator internus. Maximal abduction is inhibited by the neck of the femur coming into contact with the lateral pelvis. When the hips are flexed, this delays the impingement until a greater angle.
  • Adduction (30° with hip extended, 20° with hip flexed): adductor Magnus with adductor minimus; adductor longus, adductor brevis, gluteus maximus with its attachment at the gluteal tuberosity; gracilis (extends to the tibia); pectineus, quadratus femoris; and obturator externus. Of the thigh muscles, semitendinosus is especially involved in hip adduction. Maximal adduction is impeded by the thighs coming into contact with one another. This can be avoided by abducting the opposite leg, or having the legs alternately flexed/extended at the hip so they travel in different planes and do not intersect.

Ilium

The ilium is the uppermost and largest bone of the pelvis.

Key Points

The ilium articulates with the ischium, sacrum, and pubis.

The ilium is divisible into the body and the ala, or wing.

The body and ala are separated by the arcuate line on the posterior surface and the margin of the acetabulum on the inferior surface.

The iliac crest is on the upper margin of the ala.

The external surface of the ilium is partly articular (relating to a joint) and partly non-articular.

Bi-iliac width is an anatomical term referring to the widest measure of the pelvis between the outer edges of the upper iliac bones.

Key Terms

acetabulum: A concave structure formed from three bones in the pelvis that articulates with the femoral head to form the hip joint.

ilium: The upper and widest of the three bones that make up each side of the hipbone and pelvis.

pelvis: The large compound bone structure at the base of the spine that supports the legs. It consists of hip bone, sacrum, and coccyx.

The ilium is the uppermost and largest bone of the pelvis. It articulates with the sacrum, ischium, and pubis.

The human ilium is divisible into the body and the ala, or wing. These two parts are separated on the superior surface by a curved line known as the arcuate line, and on the inferior surface by the margin of the acetabulum.

This is a photo of an anatomical model of the ilium, the uppermost bone of the pelvis that extends laterally, with its major parts labeled as the iliac crest, acetabulum, ischium, and pubis. 

Ilium: The ilium is the uppermost bone of the pelvis and extends laterally.

The body contributes to the formation of the acetabulum, a concave structure where the head of the femur articulates to form the hip joint. The internal surface of the body forms part of the wall of the lesser pelvis and gives origin to some fibers of the obturator internus muscle.

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Ala

The ala or wing, is the large expanded portion that extends laterally. It has an external and internal surface, a crest (illiac crest), and an anterior and posterior border.

The external surface is smooth and divided by three lines: the posterior, anterior, and inferior gluteal lines. The gluteus maximus originates from the external surface adjacent to the posterior gluteal line, the gluteus medius adjacent to the anterior line, and the gluteus minimus from the inferior line.

The internal surface of the ala is smooth and concave and forms the iliac fossa. The sacrospinalis and multifidus muscles of the back originate in the posterior.

In humans, bi-iliac width is an anatomical term referring to the widest measure of the pelvis between the outer edges of the upper iliac bones. The bi-iliac width measure is helpful in obstetrics because a pelvis that is significantly too small or too large can have obstetrical complications. It is also used by anthropologists to estimate body mass.

Ischium

The ischium forms the lower and posterior portion of the hip bones of the pelvis.

Key Points

The ischium articulates with the ilium and the pubis.

The ischium is divided into the body, superior ramus of the ischium, and inferior ramus of the ischium.

The ischial tuberosity, which supports weight when sitting, is located on the ischium.

Key Terms

ischium: The most inferior of the three bones that make up each side of the pelvis.

The ischium forms the lower and back part of the hip bone. Situated below the ilium and behind the pubis, the superior portion of this bone forms approximately one third of the acetabulum, which articulates with the femoral head to form the hip joint.

This is a drawing of the left hip joint that is opened by removing the floor of the acetabulum from within the pelvis. The ischium is labeled at bottom left of the ilium. The head of the femur can be seen in the acetabulum. 

Lateral view of ischium: Left hip joint, opened by removing the floor of the acetabulum from within the pelvis. The ischium is labeled at the bottom left of the ilium.

The ischium is divisible into three portions; the body, and the superior and inferior rami.

The body contains a prominent spine that is the origin for the gemellus superior muscle. Two indentations run parallel to the spine—superiorly,the greater sciatic notch and, inferiorly, the lesser sciatic notch, through which key nervous and vascular vessels pass.

The superior ramus of the ischium extends inferiorly and posteriorly from the body. It is the partial origin for the obturator internus and obturator externus muscles.

Posteriorly the ramus forms a large swelling termed the tuberosity of the ischium, or ischial tuberosity, which supports weight while sitting and is the origin for the gemellus inferior and adductor magnus muscles.

Dorsally, the ramus contributes to the obturator foramen, a large opening in the pelvis through which key nervous and vascular vessels pass.

The inferior ramus of the ischium is thin and flattened and ascends from the superior ramus of the ischium to join the inferior ramus of the pubis. It is the partial origin for the gracillis and adductor magnus muscles.

This is a photo of an anatomical model of the ilium. The iliac crest, acetabulum, ischium, and pubis are shown. The ischium is located below the ilium and behind the pubis. 

Ischium: The ischium is located below the ilium and behind the pubis.

Pubis

The pubis is the lowest and most anterior portion of the hip bones of the pelvis.

Key Points

  • The most anterior portion of the pubis, the pubic symphysis, is where the two hip bones of the pelvis are fused together.
  • The pubis has a body, a superior ramus, and an inferior ramus.
  • The body of the pubis contributes to the lunate surface and acetabular fossa in the acetabulum.

Key Terms

  • pubic symphysis: A cartilaginous joint between the two bones of the pubis.

The pubic bone forms the anterior region of the pelvis and contributes to the acetabulum, which articulates with the femoral head to form the hip joint.

The pubic bone is covered by a layer of fat that is covered by the mons pubis. The two pubic bones joint anteriorly through the pubic symphysis, a cartilaginous joint.

The pubic bone articulates with the ilium and the ischium on each hip. The internal surface forms the wall of the lesser pelvis and is the point of origin for a portion of the obturator internus muscle. The pubic bone is divisible into a body, a superior ramus, and an inferior ramus.

The Body of the Pubic Bone

This is a color illustration of the body of a pubic bone. It identifies the sacrum, ilium, ischium, pubis body, superior ramus, inferior ramus, pubic symphisis, acetabulum, obturator foramen, coccyx, and linea terminalis. 

Body of pubic bone: (1) sacrum, (2) ilium, (3) ischium, (4) pubis: 4a-body; 4b-superior ramus; 4c-inferior ramus, (5) pubic symphysis, (6) acetabulum, (7) obturator foramen, (8) coccyx, (red dotted line) linea terminalis.

The body of the pubis is a wide, strong, medial, and flat portion of the pubic bone that unites with the pubic symphisis.

The rough superior edge of the corpus, known as the pubic crest, ends laterally in the pubic tubercle. This tubercle, found roughly 3 cm from the pubic symphysis, is a distinctive feature on the lower part of the abdominal wall and is useful when attempting to localize the superficial inguinal ring and the femoral canal of the inguinal canal.

Its internal surface enters into the formation of the wall of the lesser pelvis and gives origin to a portion of the obturator internus muscle.

Superior Pubic Ramus

The superior pubic ramus is one-third of the pubic bone. It forms a portion of the obturator foramen and extends from the body to the median plane where it articulates with its counterpart from the opposite side. It is described in two portions, which are a medial flattened part and a narrow lateral prismoid portion.

Inferior Pubic Ramus

The inferior pubic ramus is a thin and flat bone that makes up one third of the pubis. It passes laterally and downward from the medial end of the superior ramus, and becomes narrower as it descends and joins with the inferior ramus of the ischium below the obturator foramen.

False and True Pelves

The false (greater) pelvis is larger and superior to the true (lesser) pelvis where the pelvic inlet is located.

Key Points

Some believe that the false pelvis is actually part of the abdominal cavity and therefore that the true pelvis is the only true portion of the pelvis.

The true pelvis contains the pelvic inlet and is a short, curved canal, deeper on its posterior than on its anterior wall.

The true pelvis contains the pelvic colon, rectum, bladder, and some of the reproductive organs.

The false pelvis supports the intestines (specifically, the ileum and sigmoid colon) and transmits part of their weight to the anterior wall of the abdomen.

Key Terms

true pelvis: Bounded in front and below by the pubic symphysis and the superior rami of the pubis; above and behind, by the sacrum and coccyx; and laterally, by a broad, smooth, quadrangular area of bone, corresponding to the inner surfaces of the body and superior ramus of the ischium, and the part of the ilium below the arcuate line.

false pelvis: Bounded on either side by the ilium; in front it is incomplete, presenting a wide interval between the anterior borders of the ilia; behind is a deep notch on either side between the ilium and the base of the sacrum.

There is some disagreement as to what constitutes the pelvis. Depending on what is included in the description these groupings are often termed true (lesser) or false (greater) pelves.

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The True Pelvis

The true (or lesser) pelvis is bounded in front and below by the pubic symphysis and the superior rami of the pubis; above and behind, by the sacrum and coccyx; and laterally, by a broad, smooth, quadrangular area of bone, corresponding to the inner surfaces of the body and superior ramus of the ischium, and the part of the ilium below the arcuate line.

This cavity is a short, curved canal, deeper on its posterior than on its anterior wall, and contains the pelvic inlet. Some consider this region to be the entirety of the pelvic cavity. Others define the pelvic cavity as the larger space including the false greater pelvis, just above the pelvic inlet.

This is a photo of the greater and lesser pelvis The greater pelvis is highlighted in yellow and is larger and superior to the lesser pelvis (highlighted in red) where the pelvic inlet is located. 

Greater and lesser pelvis: The greater pelvis (yellow) is larger and superior to the lesser pelvis (red) where the pelvic inlet is located.

The true pelvis contains the pelvic colon, rectum, bladder, and some of the reproductive organs. The rectum is at the back, in the curve of the sacrum and coccyx; the bladder is in front, behind the pubic symphysis.

In the female, the uterus and vagina occupy the interval between these viscera. The pelvic splanchnic nerves arising at S2–S4 are in the lesser pelvis.

The False Pelvis

The false (or greater) pelvis is bounded on either side by the ilium. In front it is incomplete, presenting a wide interval between the anterior borders of the ilia; behind is a deep notch on either side between the ilium and the base of the sacrum.

Some consider this region to be part of the pelvic cavity, while others consider it part of the abdominal cavity (hence the name false pelvis). Others compromise by referring to the area as the abdominopelvic cavity.

The false pelvis supports the intestines (specifically, the ileum and sigmoid colon), and transmits part of their weight to the anterior wall of the abdomen.

Comparison of Female and Male Pelves

The female pelvis has evolved to its maximum width for childbirth and the male pelvis has been optimized for bipedal locomotion.

Key Points

The pelvis is one of the most useful skeletal elements for differentiating between males and females.

Female pelves are larger and wider than male pelves and have a rounder pelvic inlet.

Male iliac crests are higher than females, causing their false pelves to look taller and narrower.

The male sacrum is longer, narrower, straighter, and has a pronounced sacral promontory relative to the female sacrum.

The angle between the inferior pubic rami is acute (70 degrees) in men, but obtuse (90–100 degrees) in women. Accordingly, the angle is called the subpubic angle in men and the pubic arch in women.

Key Terms

pelvis: The large compound bone structure at the base of the spine that supports the legs. It consists of hip bone, sacrum, and coccyx.

Different Sex, Different Pelvis

Like the skull, the pelvis is highly useful for determining a skeleton’s biological sex. A wide pelvis is beneficial for childbirth, however, a narrow pelvis is beneficial for locomotion when walking upright. These conflicting demands are often termed the obstetrical dilemma.

The female pelvis has evolved to its maximum width for childbirth—a wider pelvis would make women unable to walk. In contrast, human male pelves are not constrained by the need to give birth and therefore are optimized for bipedal locomotion.

Diagnostic Criteria

There are several diagnostic criteria for differentiating a male from a female pelvis:

  • The female pelvis is larger and broader than the male pelvis, which is taller (owing to a higher iliac crest), narrower, and more compact.
  • The distance between the ischium bones is small in males. This causes the sides of the male pelvis to converge from the inlet to the outlet, whereas the sides of the female pelvis are wider apart. This results in the female inlet being large and oval in shape, while the male inlet is more heart-shaped.
  • The angle between the inferior pubic rami is acute (70 degrees) in men, but obtuse (90–100 degrees) in women. Accordingly, the angle is called the subpubic angle in men and the pubic arch in women.
  • The greater sciatic notch is wider in females.
  • The ischial spines and tuberosities are heavier and project farther into the pelvic cavity in males.
  • The male sacrum is long, narrow, straighter, and has a pronounced sacral promontory. The female sacrum is shorter, wider, more curved posteriorly, and has a less pronounced promontory.
  • The acetabula are wider apart and face more medially in females than in males. This change in the angle of the femoral head gives the female gait its characteristic swinging of hips.

Pelvic Structure and Childbearing

The human pelvis has evolved to be narrow enough for efficient upright locomotion, while still being wide enough to facilitate childbirth.

Key Points

The human pelvis is narrower and smaller than that of our closest living relatives, the apes. This narrower pelvis enables efficient bipedal locomotion.

Since the pelvis is vital for both efficient locomotion and childbirth, natural selection has been forced to strike a compromise between a wide pelvis to facilitate birthing large-brained infants and having a narrow pelvis to increase locomotive efficiency.

The human head and brain is also larger in proportion to the rest of the body in comparison to apes. The female pelvis has evolved to be as wide as possible, to make childbirth easier, without becoming so wide as to make bipedal locomotion too inefficient.

The male pelvis is narrower, since they do not have to give birth. Men therefore have greater locomotive efficiency.

Key Terms

bipedal locomotion: Walking upright; a form of terrestrial locomotion where an organism moves by means of its two rear limbs, or legs.

pubic symphysis: A cartilaginous joint between the two bones of the pubis.

Human sex differences in pelvic shape have emerged over the course of human evolution. The human pelvis is narrower and smaller than that of our closest living relatives, the apes. These changes in the pelvis enable bipedal locomotion or upright walking.

This narrowing of the pelvis has also affected the way humans give birth, as a narrow pelvis makes it more difficult for an infant to move through the birth canal. This problem is compounded because as human pelvises became smaller, the heads of infants became larger to accommodate increased brain size.

Since the pelvis is vital for both efficient locomotion and childbirth, natural selection has been forced to strike a compromise between a wide pelvis to facilitate birthing large-brained infants and having a narrow pelvis to increase locomotive efficiency. This compromise has been referred to as the obstetrical dilemma. Thus, the female pelvis has evolved to be as wide as possible, to make childbirth easier, without becoming so wide as to make bipedal locomotion too inefficient.

Additionally, the female pubic symphysis, which is the cartilaginous joint connecting the left and right side of the pelvis, is remodeled by hormones released during pregnancy, allowing it to stretch during childbirth.

Male pelves are not constrained by the issue of childbirth, and thus are narrower and more optimal for bipedal locomotion. Wider hips in females cause an increased valgus angle, which is the angle between the femur and lower leg. This increases the risk of torsional knee injuries.

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