Angle of Inclination: The angle of inclination of the proximal femur describes the angle within the frontal plane between the femoral neck and the medial side of the femoral shaft (Figure 12-7). At birth this angle measures about 140 to 150 degrees. Primarily because of the loading across the femoral neck during walking, this angle usually reduces to its normal adulthood value of about 125 degrees.^16,132 As depicted by the pair of red dots in Figure 12-7, this angle optimizes the alignment of the joint surfaces.

A change in the normal angle of inclination is referred to as either coxa vara or coxa valga. Coxa vara (Latin coxa, hip, + vara, to bend inward) describes an angle of inclination markedly less than 125 degrees; coxa valga (Latin valga, to bend outward) describes an angle of inclination markedly greater than 125 degrees (see Figure 12-7, B and C). These abnormal angles can significantly alter the articulation between the femoral head and the acetabulum, thereby affecting hip biomechanics. Severe malalignment may lead to dislocation or stress-induced degeneration of the joint.

Femoral Torsion: Femoral torsion describes the relative rotation (twist) between the bone’s shaft and neck. Normally, as viewed from above, the femoral neck projects about 15 degrees anterior to a medial-lateral axis through the femoral condyles.⁴³ This degree of torsion is called normal anteversion (Figure 12-8, A). In conjunction with the normal angle of inclination, an approximate 15-degree angle of anteversion affords optimal alignment and joint congruence (see alignment of red dots in Figure 12-8, A).

Femoral torsion that is markedly different from 15 degrees is considered abnormal. Torsion significantly greater than 15 degrees is called excessive anteversion (see Figure 12-8, B). In contrast, torsion significantly less than 15 degrees (i.e., approaching 0 degrees) is called retroversion (see Figure 12-8, C).

Typically a healthy infant is born with about 40 degrees of femoral anteversion.⁴³ With continued bone growth, increased weight bearing, and muscle activity, this angle usually decreases to about 15 degrees by 16 years of age. Excessive anteversion that persists into adulthood can increase the likelihood of hip dislocation, articular incongruence, increased joint contact force, and increased wear on the articular cartilage.⁵⁷ These factors may lead to secondary osteoarthritis of the hip.¹⁴⁷

Excessive anteversion in children may be associated with an abnormal gait pattern called “in-toeing.” In-toeing is a walking pattern with exaggerated posturing of hip internal rotation. The amount of in-toeing is generally related to the amount of femoral anteversion. This gait pattern apparently is a compensatory mechanism used to guide the excessively anteverted femoral head more directly into the acetabulum (Figure 12-9). In addition, Arnold and colleagues have shown that the exaggerated internally rotated position during walking serves to increase the moment arm of the important hip abductor muscles—leverage that is substantially reduced with excessive femoral anteversion.⁹ Regardless of the reason for the internal rotated position, children may, over time, develop shortening of the internal rotator muscles and various ligaments, thereby reducing external rotation range of motion. Fortunately, most children with in-toeing eventually walk normally.¹⁶³ The gait pattern typically improves with time because of a natural normalization of the anteversion or a combined structural compensation in other parts of the lower extremity, most commonly the tibia.⁶⁰ There is no evidence that nonoperative treatment can reduce excessive femoral anteversion.

Excessive femoral anteversion of 25 to 45 degrees is common in persons with cerebral palsy, and even anteversion as high as 60 to 80 degrees has been reported.^8,16 In-toeing typically persists in children with cerebral palsy who are ambulatory and usually does not resolve.¹⁴⁹

Compact and Cancellous Bone: Walking produces tension, compression, bending, shear, and torsion on the proximal femur. Many of these forces are large, exceeding one’s body weight. Throughout a lifetime, the proximal femur typically resists and absorbs these repetitive forces without incurring injury. This is accomplished by two strikingly different compositions of bone. Compact bone is very dense and unyielding, with an ability to withstand large loads. This type of bone is particularly thick in the cortex, or outer shell, of the lower femoral neck and entire shaft (Figure 12-10). These regions are subjected to large shear and torsion forces. Cancellous bone, in contrast, is relatively porous, consisting of a spongy, three-dimensional trabecular lattice, as shown in Figure 12-10. The relative elasticity of cancellous bone is ideal for repeatedly absorbing external forces. Cancellous bone tends to concentrate along lines of stress, forming trabecular networks. A medial trabecular and an arcuate trabecular network are visible within the femur shown in Figure 12-10. The overall pattern of the trabecular network changes when the proximal femur is subjected to abnormal forces over an extended time.

Center-Edge Angle: The center-edge angle is highly variable but on average measures about 35 degrees in radiographs from adults (Figure 12-13, A).^4,50 As described in the legend of Figure 12-13, a significantly lower central-edge angle reduces the acetabular coverage of the femoral head. This reduced coverage increases the risk of dislocation and, equally important, reduces the contact area within the joint.¹⁰⁸ A central-edge angle of only 15 degrees, for example, reduces normal contact area by as much as 35%.⁵⁰ During the single-limb–support phase of walking, for instance, this reduced surface area would theoretically increase joint pressure (force/area) by about 50%. Over many years of walking, this scenario may lead to premature hip osteoarthritis, often starting with degeneration of the acetabular labrum.^26,94,111

Acetabular Anteversion Angle: The acetabular anteversion angle measures the extent to which the acetabulum projects anteriorly within the horizontal plane, relative to the pelvis. Such a perspective can be measured through computed tomography. Observed from above, the acetabular anteversion angle is normally about 20 degrees (Figure 12-13, B).^4,148 Even when normal, this orientation exposes part of the anterior side of the femoral head. The thick anterior capsular ligament of the hip and the iliopsoas tendon naturally cover and support this vulnerable side of the joint. A hip demonstrating excessive acetabular anteversion is more exposed anteriorly: when anteversion is severe, the hip is more prone to anterior dislocation and associated lesions of the anterior labrum, especially at the extremes of external rotation. The likelihood of these associated pathologies increases when acetabular anteversion is combined with excessive femoral anteversion.⁹⁵

An acetabulum that projects directly laterally, or even slightly posterior-laterally, within the horizontal plane is described as being abnormally retroverted.

Close-Packed Position of the Hip: Full extension of the hip (i.e., about 20 degrees beyond the neutral position) in conjunction with slight internal rotation and slight abduction twists or “spirals” most of the fibers within the capsular ligaments to their most taut position (Figure 12-17). This position is useful therapeutically during attempts to stretch the entirety of the hip’s capsular ligaments. Because the position of full extension, slight internal rotation and abduction elongates most of the capsule, it is considered the close-packed position at the hip.¹⁶⁰ The passive tension generated especially by full extension lends stability to the joint and reduces passive accessory movement or “joint play.” The hip is one of a very few joints in the body in which the close-packed position is not also associated with the position of maximal joint congruency. The hip joint surfaces fit most congruently in about 90 degrees of flexion with moderate abduction and external rotation. In this position, most of the capsule and associated ligaments have “unraveled” to a more slackened state, adding only little passive tension to the joint.

Rotation of the Femur in the Sagittal Plane: On average, with the knee flexed, the hip flexes to about 120 degrees (Figure 12-20, A).^41,150 Tasks such as comfortably squatting or tying a shoelace typically require this amount of hip flexion.⁷⁷ Full hip flexion slackens the three primary capsular ligaments but stretches the inferior capsule and muscles such as the gluteus maximus. With the knee fully extended, hip flexion is typically limited to 70 to 80 degrees by increased tension in the hamstring muscles. Considerable variability can be expected in this movement because of high inter-subject variability in hamstring muscle flexibility.

The hip normally extends about 20 degrees beyond the neutral position.¹⁵⁰ Full hip extension increases the passive tension throughout the capsular ligaments—especially the iliofemoral ligament and the hip flexor muscles. When the knee is fully flexed during hip extension, passive tension in the stretched rectus femoris, which crosses both the hip and the knee, reduces hip extension to about the neutral position.

Rotation of the Femur in the Frontal Plane: The hip abducts on average about 40 degrees, limited primarily by the pubofemoral ligament and the adductor muscles (see Figure 12-20, B).¹⁵⁰ The hip adducts about 25 degrees beyond the neutral position.¹⁷ In addition to interference with the contralateral limb, passive tension in stretched hip abductor muscles, iliotibial band, and superior fibers of the ischiofemoral ligament limits full adduction.

Rotation of the Femur in the Horizontal Plane: The magnitude of internal and external rotation of the hip is particularly variable among subjects. On average, the hip internally rotates about 35 degrees from the neutral position (see Figure 12-20, C).^150,156 With the hip in extension, maximal internal rotation elongates the external rotator muscles, such as the piriformis, and parts of the ischiofemoral ligament.

The extended hip externally rotates on average about 45 degrees. Excessive tension in the lateral fasciculus of the iliofemoral ligament can limit full external rotation. In addition, external rotation can be limited by excessive tension in any internal rotator muscle.

Lumbopelvic Rhythm: The caudal end of the axial skeleton is firmly attached to the pelvis by way of the sacroiliac joints. As a consequence, rotation of the pelvis over the femoral heads typically changes the configuration of the lumbar spine. This important kinematic relationship is known as lumbopelvic rhythm, introduced in Chapter 9. This concept is revisited in this chapter with a focus on the kinesiology at the hip.

Figure 12-21 shows two contrasting types of lumbopelvic rhythms frequently used during pelvic-on-femoral (hip) flexion. Although the kinematics depicted are limited to the sagittal plane, the concepts can also be applied to pelvic rotations in frontal and horizontal planes. Figure 12-21, A shows an example of an ipsidirectional lumbopelvic rhythm, in which the pelvis and lumbar spine rotate in the same direction.⁹⁰ The effect of this movement is to maximize the angular displacement of the entire trunk relative to the lower extremities—an effective strategy for increasing reach of the upper extremities. The kinematics of the ipsidirectional lumbopelvic rhythm are discussed in detail in Chapter 9. In contrast, during contradirectional lumbopelvic rhythm, the pelvis rotates in one direction while the lumbar spine simultaneously rotates in the opposite direction (see Figure 12-21, B). The important consequence of this movement is that the supralumbar trunk (i.e., that part of the body located above the first lumbar vertebra) can remain nearly stationary as the pelvis rotates over the femurs. This type of rhythm is used during walking, for example, when the position of the supralumbar trunk—including the head and eyes—needs to be held relatively fixed in space, independent of the rotation of the pelvis. In this manner the lumbar spine functions as a mechanical “decoupler,” allowing the pelvis and the supralumbar trunk to move independently.¹⁵² A person with a fused lumbar spine, therefore, is unable to rotate the pelvis about the hips without a similar rotation of parts of the supralumbar trunk. This abnormal situation is readily apparent when the individual walks.

Figure 12-22 shows pelvic-on-femoral osteokinematics at the hip, organized by plane of motion. These kinematics are all based on the contradirectional lumbopelvic rhythm. In many cases the amount of pelvic-on-femoral rotation is restricted by natural limitations of movement within the lumbar spine.

Pelvic Rotation in the Sagittal Plane: Anterior and Posterior Pelvic Tilting: Hip flexion can occur through an anterior pelvic tilt (see Figure 12-22, A). As defined in Chapter 9, a “pelvic tilt” is a short-arc, sagittal plane rotation of the pelvis relative to stationary femurs. The direction of the tilt—either anterior or posterior—is based on the direction of rotation of a point on the iliac crest. The anterior tilt of the pelvis occurs about a medial-lateral axis of rotation through both femoral heads. The associated increased lumbar lordosis offsets most of the tendency of the supralumbar trunk to follow the forward rotation of the pelvis. While sitting with 90 degrees of hip flexion, the normal adult can perform about 30 degrees of additional pelvic-on-femoral hip flexion before being restricted by a completely extended lumbar spine. Full anterior tilt of the pelvis slackens most of the ligaments of the hip, most notably the iliofemoral ligament. Marked tightness in any hip extensor muscle—such as the hamstrings—could theoretically limit the extremes of an anterior pelvic tilt. As depicted in Figure 12-22, A, however, because the knees are flexed, the partially slackened hamstring muscles would not normally produce any noticeable resistance to an anterior pelvic rotation. During standing (and with knees fully extended), however, the more elongated hamstrings are more likely to resist an anterior pelvic tilt, but the amount of resistance is usually insignificant unless the muscle is physiologically impaired and generating extreme resistance to elongation.⁹⁷

As depicted in Figure 12-22, A, the hips can be extended about 10 to 20 degrees from the 90-degree sitting posture via a posterior tilt of the pelvis. During sitting, this short-arc pelvic rotation would increase the length (and therefore tension) only minimally in the iliofemoral ligament and rectus femoris muscle. As depicted in the figure, the lumbar spine flexes, or flattens, as the pelvis posteriorly tilts.

Pelvic Rotation in the Frontal Plane: Pelvic-on-femoral rotation in the frontal and horizontal planes is best described assuming a person is standing on one limb. The weight-bearing extremity is referred to as the support hip.

Abduction of the support hip occurs by raising or “hiking” the iliac crest on the side of the nonsupport hip (see Figure 12-22, B). Assuming that the supralumbar trunk remains nearly stationary, the lumbar spine must bend in the direction opposite the rotating pelvis. A slight lateral convexity occurs within the lumbar region toward the side of the abducting hip.

Pelvic-on-femoral hip abduction is restricted to about 30 degrees, primarily because of the natural limits of lateral bending in the lumbar spine. Marked tightness in hip adductor muscles or the pubofemoral ligament can limit pelvic-on-femoral hip abduction. In the event of a marked adductor contracture, the iliac crest on the side of the nonsupport hip remains lower than the iliac crest of the support hip, which may interfering with walking.

Adduction of the support hip occurs by a lowering of the iliac crest on the side of the nonsupport hip. This motion causes a slight lateral concavity within the lumbar region on the side of the adducted hip. A hypomobile lumbar spine and/or reduced extensibility in the iliotibial band or hip abductor muscles, such as the gluteus medius, piriformis, or tensor fasciae latae, may restrict the extremes of this motion.

Pelvic Rotation in the Horizontal Plane: Pelvic-on-femoral rotation occurs in the horizontal plane about a longitudinal axis of rotation (see green circle at femoral head in Figure 12-22, C). Internal rotation of the support hip occurs as the iliac crest on the side of the nonsupport hip rotates forward in the horizontal plane. During external rotation, in contrast, the iliac crest on the side of the nonsupport hip rotates backward in the horizontal plane. If the pelvis is rotating beneath a relatively stationary trunk, the lumbar spine must rotate (or twist) in the opposite direction as the rotating pelvis. The small amount of axial rotation normally permitted in the lumbar spine significantly limits the full expression of horizontal plane rotation of the support hip. The full potential of pelvic-on-femoral rotation requires that the lumbar spine and trunk follow the rotation of the pelvis—a movement strategy more consistent with an ipsidirectional lumbopelvic rhythm.

Lumbar Plexus: The lumbar plexus is formed from the ventral rami of spinal nerve roots T¹²-L⁴. This plexus gives rise to the femoral and obturator nerves (Figure 12-24, A). The femoral nerve, the largest branch of the lumbar plexus, is formed by L²-L⁴ nerve roots. Motor branches innervate most hip flexors and all knee extensors. Within the pelvis, proximal to the inguinal ligament, the femoral nerve innervates the psoas major and iliacus. Distal to the inguinal ligament, the femoral nerve innervates the sartorius, part of the pectineus, and the quadriceps muscle group. The femoral nerve has an extensive sensory distribution covering much of the skin of the anterior-medial aspect of the thigh. The sensory branches of the femoral nerve innervate the skin of the anterior-medial aspect of the lower leg, via the saphenous cutaneous nerve.

Like the femoral nerve, the obturator nerve is formed from L²-L⁴ nerve roots. Motor branches innervate the hip adductor muscles. The obturator nerve divides into anterior and posterior branches as it passes through the obturator foramen. The posterior branch innervates the obturator externus and anterior head of the adductor magnus. The anterior branch innervates part of the pectineus, the adductor brevis, the adductor longus, and the gracilis. The obturator nerve has a sensory distribution to the skin of the medial thigh.

Sacral Plexus: The sacral plexus, located on the posterior wall of the pelvis, is formed from the ventral rami of L⁴-S⁴ spinal nerve roots. Most nerves from the sacral plexus exit the pelvis via the greater sciatic foramen to innervate the posterior hip muscles (see Figure 12-24, B).

Three small nerves innervate five of the six “short external rotators” of the hip. The nerves are named simply by the muscles that they innervate. The nerve to the piriformis (S¹-S²) innervates the piriformis. External to the pelvis, the nerve to the obturator internus and gemellus superior (L⁵-S²) and the nerve to the quadratus femoris and gemellus inferior (L⁴-S¹) travel to and innervate their respective muscles.

The superior and inferior gluteal nerves are named according to their position relative to the piriformis muscle as they exit the greater sciatic notch. The superior gluteal nerve (L⁴-S¹) innervates the gluteus medius, gluteus minimus, and tensor fasciae latae. The inferior gluteal nerve (L⁵-S²) provides the sole innervation to the gluteus maximus.

The sciatic nerve, the widest and longest nerve in the body, is formed from L⁴-S³ nerve roots. This nerve exits the pelvis through the greater sciatic foramen, usually inferior to the piriformis. The sciatic nerve consists of two nerves: the tibial and the common fibular (peroneal), both enveloped in one connective tissue sheath. In the posterior thigh, the tibial portion of the sciatic nerve innervates all the biarticular muscles within the hamstring group and the posterior head of the adductor magnus. The common fibular portion of the sciatic nerve innervates the short head of the biceps femoris.

The sciatic nerve branches into separate tibial and common fibular components usually just proximal to the knee. It is not uncommon, however, that the division occurs more proximally near the pelvis. A division proximal to the greater sciatic foramen usually results in the common fibular nerve piercing the piriformis as the nerve exits the pelvis.

As a reference, the primary spinal nerve roots that supply the muscles of the lower extremity are listed in Appendix IV, Part A. In addition, Appendix IV, Parts B and C include additional reference items to help guide the clinical assessment of the functional status of the L²-S³ nerve roots.

Anatomy and Individual Action: The iliopsoas is large and long, spanning the area between the last thoracic vertebra and the proximal femur (see Figure 12-26). Anatomically, the iliopsoas consists of two muscles: the iliacus and the psoas major. The iliacus attaches on the iliac fossa and extreme lateral edge of sacrum, just over the sacroiliac joint. The psoas major attaches along the transverse processes of the last thoracic and all lumbar vertebrae, including the intervertebral discs.⁵³ The fibers of the iliacus and the psoas major usually fuse just anterior to the femoral head (see Figure 12-26, left hip). A tendon forms that anchors the muscle to the femur, on the lesser trochanter. En route to its distal insertion, the broad tendon of the iliopsoas is deflected posteriorly about 35 to 45 degrees, immediately after it crosses the superior pubic ramus. With the hip in full extension, this deflection raises the tendon’s angle-of-insertion to the femur, thereby increasing the muscle’s leverage for hip flexion.

The iliopsoas is a potent hip flexor from both a femoral-on-pelvic and pelvic-on-femoral perspective. From the anatomic position, the iliopsoas is not an effective rotator, although with the hip abducted the iliopsoas assists with external rotation.¹⁵⁷

The iliopsoas muscle produces force that crosses the lumbar and lumbosacral regions as well as the hip.5,78,154 Because of its strong anterior tilting action on the pelvis, the iliopsoas can accentuate the lumbar lordosis if the pelvis is not well stabilized by an abdominal muscle such as the rectus abdominis. The psoas major provides excellent vertical stability to the lumbar spine, especially in near full hip extension, where the passive tension in the muscle is greatest (see Chapter 10).¹⁸¹

The psoas minor lies anterior to the muscle belly of the psoas major, although it may be absent in about 40% of people.¹⁶⁰ This slender muscle attaches proximally between the twelfth thoracic and first lumbar vertebra and distally to the pelvis near the pectineal line. The psoas minor has no ability to flex the hip; isolated bilteral contraction of the psoas minor may have the ability to posteriorly tilt the pelvis.

The sartorius, the longest muscle in the body, originates at the anterior-superior iliac spine (see Figure 12-26). This thin, fusiform muscle courses distally and medially across the thigh to attach on the medial surface of the proximal tibia (see Figure 13-7). The name sartorius is based on the Latin root sartor, referring to a tailor’s position of cross-legged sitting. This name describes the muscle’s combined action of hip flexion, external rotation, and abduction.

The tensor fasciae latae attaches to the ilium just lateral to the sartorius (see Figure 12-26). This relatively short muscle attaches distally to the proximal part of the iliotibial band. The band extends distally across the knee to attach to the lateral tubercle of the tibia.

The iliotibial band is a component of a more extensive connective tissue known as the fascia lata of the thigh.¹⁶⁰ Laterally, the fascia lata is thickened by attachments from the tensor fasciae latae and the gluteus maximus. At multiple locations the fascia lata turns inward between muscles, forming distinct fascial sheets known as intermuscular septa. These septa partition each of the main muscle groups of the thigh according to innervation. The intermuscular septa of the thigh ultimately attach to the linea aspera on the posterior surface of the femur, along with attachments of most of the adductor muscles and several of the vasti muscles (components of the quadriceps).

From the anatomic position, the tensor fasciae latae is a primary flexor and abductor of the hip. The muscle is also a secondary internal rotator.¹³⁶ As indicated by its name, the tensor fasciae latae increases tension throughout the fascia lata. Tension passed inferiorly through the iliotibial band may help stabilize the extended knee. Repetitive tension within the iliotibial band may cause inflammation at its insertion site near the lateral tubercle of the tibia. Stretching a tightened iliotibial band with the knee extended often incorporates various combinations of hip adduction and extension.⁴⁸

The proximal part of the rectus femoris emerges between the limbs of an inverted V formed by the sartorius and tensor fasciae latae (see Figure 12-26). This large bipennate-shaped muscle has its proximal attachment on the anterior-inferior iliac spine and along the superior rim of the acetabulum and in the joint capsule.¹⁶⁰ Along with the other members of the quadriceps, the rectus femoris attaches to the tibia via the patellar tendon. The rectus femoris is responsible for about one third of the total isometric, flexion torque at the hip.¹⁰⁵ In addition, the rectus femoris is a primary knee extensor. The combined two-joint actions of this important muscle are considered in Chapter 13. The anatomy and function of the pectineus and adductor longus are described in the section on the adductors of the hip.

Overall Function:

Functional Anatomy: The adductor muscle group occupies the medial quadrant of the thigh. Topographically, the adductor muscles are organized into three layers (Figure 12-31). The pectineus, adductor longus, and gracilis occupy the superficial layer. Proximally, these muscles attach along the superior and inferior pubic ramus and adjacent body of the pubis. Distally, the pectineus and the adductor longus attach to the posterior surface of the femur—near and along varying regions of the linea aspera. The long and slender gracilis attaches distally to the medial side of the proximal tibia (see Figure 13-7). The middle layer of the adductor group is occupied by the triangular-shaped adductor brevis. The adductor brevis attaches to the pelvis on the inferior pubic ramus and to the femur along the proximal one third of the linea aspera.

The deep layer of the adductor group is occupied by the massive, triangular adductor magnus (see Figure 12-26, left side, and Figure 12-37, right side). This large muscle attaches primarily from the ischial ramus and part of the ischial tuberosity. From its proximal attachment the adductor magnus forms anterior and posterior heads.

The anterior head of the adductor magnus has two sets of fibers: horizontal and oblique. The relatively small (and often poorly defined) set of horizontally directed fibers crosses from the inferior pubic ramus to the extreme proximal end of the linea aspera, often called the adductor minimus. The larger obliquely directed fibers run from the ischial ramus to nearly the entire length of the linea aspera, as far distally as the medial supracondylar line. Both parts of the anterior head are innervated by the obturator nerve, which is typical of the adductor muscles.

The posterior head of the adductor magnus consists of a thick mass of the fibers arising from the region of the pelvis adjacent to the ischial tuberosity. From this posterior attachment the fibers run vertically and attach as a tendon on the adductor tubercle on the medial side of the distal femur. The posterior head of the adductor magnus is innervated by the tibial branch of the sciatic nerve, as are most hamstring muscles. Because of a location, innervation, and action similar to those of the hamstring muscles, the posterior head is often referred to as the extensor head of the adductor magnus.

Overall Function: The line of force of the adductors approaches the hip from many different orientations. Functionally, therefore, the adductor muscles produce torques in all three planes at the hip.^36,119 The following section considers the primary actions of the adductors in the frontal and sagittal planes. The action of these muscles as secondary internal rotators is discussed later in this chapter.

Overall Function: An “ideal” primary internal rotator muscle of the hip would theoretically be oriented in the horizontal plane during standing, at some linear distance from the longitudinal or vertical axis of rotation at the hip. From the anatomic position, however, there are no primary internal rotators because no muscle is oriented even close to the horizontal plane. Several secondary internal rotators exist, however, including the anterior fibers of the gluteus minimus and the gluteus medius, tensor fasciae latae, adductor longus, adductor brevis, and pectineus. The horizontal line of force of many of these muscles is depicted in Figure 12-34.^36,91,104 The anatomy of each of the internal rotators is described in other sections (see Figures 12-26 and 12-41).

With the hip approaching 90 degrees of flexion, the internal rotation torque potential of the internal rotator muscles dramatically increases.33,36,104 This becomes clear with the help of a skeleton model and piece of string to mimic the line of force of muscles, such as the anterior fibers of the gluteus minimus or gluteus medius. Flexing the hip close to 90 degrees reorients the line of force of these muscles from nearly parallel to nearly perpendicular to the longitudinal axis of rotation at the hip. This occurs because the longitudinal axis of rotation remains parallel with the shaft of the repositioned femur. Delp and co-workers have reported that the internal rotation moment arm of the anterior part of the gluteus medius, for example, increases eightfold between 0 and 90 degrees of flexion.³³ Even some external rotator muscles (such as the piriformis, anterior fibers of the gluteus maximus, and posterior fibers of the gluteus minimus) switch actions and become internal rotators beyond about 90 degrees of flexion.³³ These changes in muscle action help explain why maximal-effort internal rotation torque in healthy persons has been shown to be about 50% greater with the hip flexed rather than extended.⁹⁹

The kinesiologic phenomenon just described also helps explain the excessively internally rotated and flexed (“crouched”) gait pattern often observed in persons with cerebral palsy.^62,166 With poor active control of hip extension (especially combined with contracture of the hip flexor muscles), the flexed posture of the hip exaggerates the internal rotation torque potential of many muscles of the hip.^6,7,33 This gait pattern may be better controlled by enhanced activation of the gluteus maximus, a potent extensor and external rotator.⁶

Biomechanics of the Adductor Muscles as Internal Rotators of the Hip: In general, most of the adductor muscles are capable of producing a modest internal rotation torque at the hip when the body is in or near the anatomic position.36,91,104 This action, however, may be difficult to reconcile considering that most adductors attach to the posterior side of the femur along the linea aspera. With normal anatomy of the hip, a shortening of these muscles would appear to rotate the femur externally instead of internally. What must be considered, however, is the effect that the natural anterior bowing of the femoral shaft has on the line of force of the muscles. Bowing places much of the linea aspera anterior to the longitudinal axis of rotation at the hip (Figure 12-35, A). As depicted in Figure 12-35, B, the horizontal force component of an adductor muscle, such as the adductor longus, lies anterior to the axis of rotation. Force from this muscle therefore acts with a moment arm to produce internal rotation.

Anatomy and Individual Action: The primary hip extensors are the gluteus maximus, the hamstrings (i.e., the long head of the biceps femoris, the semitendinosus, and the semimembranosus), and the posterior head of the adductor magnus (Figure 12-37).³⁶ The posterior fibers of the gluteus medius and anterior fibers of the adductor magnus are secondary extensors. With the hip flexed to at least about 70 degrees and beyond, most adductors muscles (with the possible exception of the pectineus) are capable of assisting with hip extension.

The gluteus maximus has numerous proximal attachments from the posterior side of the ilium, sacrum, coccyx, sacrotuberous and posterior sacroiliac ligaments, and adjacent fascia. The muscle attaches into the iliotibial band of the fascia lata (along with the tensor fasciae latae), and the gluteal tuberosity on the femur. The gluteus maximus is a primary extensor and external rotator of the hip.

The hamstring muscles have their proximal attachment on the posterior side of the ischial tuberosity and attach distally to the tibia and fibula. Based on these attachments, the hamstrings extend the hip and flex the knee. The anatomy and function of the posterior head of the adductor magnus is described under the section on adductors of the hip.

Figure 12-25 depicts the line of force of the primary hip extensors. In the extended position the posterior head of the adductor magnus has the greatest moment arm for extension. The adductor magnus and the gluteus maximus have the greatest cross-sectional areas of all the extensors.¹⁷⁶

Overall Function:

Anatomy and Individual Action: The primary hip abductor muscles are the gluteus medius, gluteus minimus, and tensor fasciae latae.²³ The piriformis and sartorius are considered secondary hip abductors.

The gluteus medius attaches on the external surface of the ilium above the anterior gluteal line. The muscle attaches distally on the lateral aspect of the greater trochanter (see Figure 12-37). The distal attachment provides the gluteus medius with the greatest abductor moment arm of all the abductor muscles (see Figure 12-29). The gluteus medius is also the largest of the hip abductor muscles, occupying about 60% of the total abductor cross-sectional area.²³ The broad and fan-shaped gluteus medius has been considered as having three functional sets of fibers: anterior, middle, and posterior.^23,159 All fibers contribute to abduction of the hip; however, from the anatomic position the anterior fibers also produce internal rotation, and the posterior fibers also produce extension and external rotation. These actions may change considerably when muscle activation is initiated from outside the anatomic position.⁸

The gluteus minimus lies deep and slightly anterior to the gluteus medius. This muscle attaches proximally on the ilium—between the anterior and inferior gluteal lines—and distally on the anterior-lateral aspect of the greater trochanter (Figure 12-41). The muscle also attaches into the superior capsule of the joint.¹⁶⁹ These attachments may retract this part of the capsule away from the joint during motion—a mechanism that may prevent capsular impingement.

All the fibers of the gluteus minimus contribute to abduction; the more anterior fibers also contribute to internal rotation and flexion.^23,85 The gluteus minimus is smaller than the gluteus medius, occupying about 20% of the total abductor cross-sectional area.²³

The tensor fasciae latae is the smallest of the three primary hip abductors, occupying about 11% of the total abductor cross-sectional area.²³ The anatomy of the tensor fasciae latae is discussed elsewhere in this text.

It is interesting to note that all the hip abductor muscles have an action as either internal or external rotators of the hip. The production of a pure frontal plane abduction torque therefore requires that the abductors completely neutralize one another’s horizontal plane torque potential.

Hip Abductor Mechanism: Control of Frontal Plane Stability of the Pelvis during Walking: The abduction torque produced by the hip abductor muscles is essential to the control of the frontal plane pelvic-on-femoral kinematics during walking. During most of the stance phase, the hip abductors stabilize the pelvis over the relatively fixed femur (see Figure 12-36).^71,72 During the stance phase, therefore, the hip abductor muscles have a role in controlling the pelvis in the frontal plane and, as discussed earlier, the horizontal plane.

The abduction torque produced by the hip abductor muscles is particularly important during the single-limb—support phase of gait. During this phase the opposite leg is off the ground and swinging forward. Without adequate abduction torque on the stance limb, the pelvis and trunk may drop uncontrollably toward the side of the swinging limb. The activation of the hip abductor muscle can be easily appreciated by palpating the gluteus medius just superior to the greater trochanter. The right muscle, for example, becomes firm as the left leg lifts off the ground.

The frontal plane stabilizing function of hip abductor muscles is a very important component of walking. Furthermore, the force produced by the abductors during stance accounts for most of the compressive forces generated between the acetabulum and femoral head.

Hip Abductor Mechanism: Role in the Production of Compression Force at the Hip: Figure 12-42 shows the major factors involved with maintaining frontal plane stability of the right hip during single-limb-support, similar to that required during the mid-stance phase of walking. The forces created by active hip abductors and body weight create opposing torques that control the position and stability of the pelvis (within the frontal plane) over the femoral head. During single-limb support, the pelvis is comparable to a seesaw, with its fulcrum represented by the femoral head. When the seesaw is balanced, the counterclockwise (internal) torque produced by the right hip abductor force (HAF) equals the clockwise (external) torque caused by body weight (BW). Balance of opposing torques is called static rotary equilibrium.

During single-limb support, the hip abductor muscles—in particular the gluteus medius—produce most of the compression force across the hip.³¹ This important point is demonstrated by the model in Figure 12-42.^121,123 Note that the internal moment arm (D) used by the hip abductor muscles is about half the length of the external moment arm (D₁) used by body weight.¹²⁷ Given this length disparity, the hip abductor muscles must produce a force twice that of body weight in order to achieve stability during single-limb support. On every step, therefore, the acetabulum is pulled against the femoral head by the combined forces produced by the hip abductor muscles and the pull of body weight. To achieve static linear equilibrium, this downward force is counteracted by a joint reaction force (JRF) of equal magnitude but oriented in nearly the opposite direction (see Figure 12-42). The joint reaction force is directed 10 to 15 degrees off vertical—an angle that is strongly influenced by the orientation of the hip abductor muscle force vector.⁷²

The sample data supplied in Figure 12-42 show how to estimate the approximate magnitude of the hip abductor force and hip joint reaction force. (For simplicity, it is assumed that all forces act vertically, as shown in the seesaw model.) As shown in the calculations, an upward-directed joint reaction force (JRF) of 1873.8 N (421.3 lb) occurs when a person weighing 760.6 N (171 lb) is in single-limb support over the right limb. This reaction force is about 2.5 times body weight, 66% of which comes from the hip abductor muscles. During walking, the joint reaction force is even greater because of the acceleration of the pelvis over the femoral head. Data based on computer modeling or direct measurements from strain gauges implanted into a hip prosthesis show that joint compression forces reach three times body weight during walking.^71,161 These forces can increase to at least 5 or 6 times body weight while running or ascending and descending stairs or ramps.^12,153 Even ordinary daily functional activities can generate surprisingly large hip joint forces.⁶⁴ In general, these forces serve important physiologic functions, such as stabilizing the femoral head within the acetabulum, assisting in the nutrition of the articular cartilage, and providing the stimulus for normal development and shape of joint structure in the growing child. The articular cartilage and trabecular bone normally protect the joint by safely dispersing large forces. A hip with arthritis, however, may no longer be able to provide this protection.

Maximal Abduction Torque Varies According to Hip Joint Angle: The unique relationship between a muscle group’s internal torque and joint angle provides insight into the functional demands naturally placed on the muscles. The shape of the plot depicted in Figure 12-43, for example, clearly shows that the abductor muscles produce their peak torque (greatest strength) when elongated.¹²⁷ The maximal torque is produced when the hip is slightly adducted or in a neutral (0-degree) hip position. This frontal plane hip angle naturally occurs when the body is in or close to the single-limb support phase of walking: precisely when these muscles are needed to provide frontal plane stability to the hip. In essence, the abductor muscles have their greatest torque reserve at muscle lengths that correspond to their greatest functional demands.

The adducted position of the hip also increases the passive tension in the naturally stiff iliotibial band. This passive tension, although likely relatively small, can nevertheless augment the abduction torque required during the single-limb support phase of walking.¹²⁸

In contrast, hip abduction torque potential is least at the near–fully shortened muscle length that corresponds to 40 degrees of abduction (see Figure 12-43). Ironically, the near–maximally abducted hip is in the position traditionally suggested for manually testing the strength of the hip abductors.⁸⁰

Functional Anatomy of the “Short External Rotators”: The six “short external rotators” of the hip are the piriformis, obturator internus, gemellus superior, gemellus inferior, quadratus femoris, and obturator externus (see Figures 12-14, 12-37, and 12-41). The line of force of these muscles is oriented primarily in the horizontal plane. This orientation is optimal for the production of external rotation torque, as most of the force component of each muscle has a perpendicular intersection with the vertical axis of rotation. In a manner similar to the infraspinatus and teres minor at the shoulder, the short external rotators also provide stability to the posterior side of the joint.

The piriformis attaches proximally on the anterior surface of the sacrum, among the ventral sacral foramina (see Figure 12-26). Exiting the pelvis posteriorly through the greater sciatic foramen, the piriformis attaches to the superior aspect of the greater trochanter (see Figure 12-41).

In addition to the action of external rotation, the piriformis is a secondary hip abductor. Both actions are apparent in the muscle’s line of force relative to the axis of rotation at the hip (see Figures 12-29 and 12-34).

The sciatic nerve usually exits the pelvis inferior to the piriformis. As described earlier in this chapter, the sciatic nerve may pass through the belly of the piriformis. A shortened or “tight” piriformis may compress and irritate the sciatic nerve, a condition known as “piriformis syndrome.”

The obturator internus muscle arises from the internal side of the obturator membrane and from the adjacent ilium (see Figure 12-41). From this origin, the fibers converge to a central tendon after exiting the pelvis through lesser sciatic foramen. This notch, which is lined with hyaline cartilage, functions as a pulley by deflecting the tendon of the obturator internus by about 130 degrees on its approach to the trochanteric fossa of the femur (Figure 12-44, A).¹⁶⁰ With the femur firmly fixed during standing, strong contraction of this muscle rotates the pelvis (and superimposed trunk) relative to the femoral head (see Figure 12-44, B). In addition to rotating the pelvis, the force produced by the obturator internus compresses the surfaces of the joint, thereby providing an element of dynamic stability to the hip.

The gemellus superior and inferior (from the Latin root geminus, meaning twins) are two, small, nearly identically sized muscles with proximal attachments on either side of the lesser sciatic notch (see Figure 12-41). Each muscle blends in with the central tendon of the obturator internus for a common attachment to the femur. Immediately below the gemellus inferior is the quadratus femoris muscle. This flat muscle arises from the external side of the ischial tuberosity and inserts on the posterior side of the proximal femur.

The obturator externus muscle arises from the external side of the obturator membrane and adjacent ilium (see Figure 12-14). The belly of this muscle is visible from the anterior side of the pelvis after removal of the adductor longus and pectineus muscles (see Figure 12-26, left side). The muscle attaches posteriorly on the femur at the trochanteric fossa (see Figure 12-6). (Based on its location and innervation, the obturator externus is more anatomically associated with the adductor group of muscles, rather than the other five short external rotators. The obturator externus is innervated by nerve roots that originate from the lumbar plexus [via the obturator nerve], as are most of the other adductor muscles. The other small external rotators, in contrast, are innervated through the sacral plexus, with nerve roots as low as S².)

Overall Function: The functional potential of the external rotators is most evident during pelvic-on-femoral rotation. Consider, for example, the external rotator muscles contracting to rotate the pelvis over the femur (Figure 12-45). With the right lower extremity firmly in contact with the ground, contraction of the right external rotators accelerates the anterior side of the pelvis and attached trunk to the left—contralateral to the fixed femur. This action of planting a foot and “cutting” to the opposite side is the natural way to abruptly change direction while running. As indicated in Figure 12-45, activation of the right gluteus maximus, for instance, is very capable of imparting both the extension and external rotation thrust to the hip during this action. If needed, the external rotation torque can be decelerated by eccentric action of internal rotator muscles. Extremely rapid eccentric activation of the adductor longus or brevis, for example, may be used to decelerate the contralateral-directed pelvic rotation—an action that may cause “strain” injury to these muscles. The mechanism of injury may partially explain the relatively high incidence of adductor muscle strain during many sporting activities that involve rapid rotation of the pelvis and trunk while running.