DR. DIBYENDUNARAYAN BID [PT]T H E S A R VA J A N I K C O L L E G E O F P H Y S I O T H E R A P Y,

R A M P U R A , S U R AT

Biomechanics of the

Hip Complex: 2

Function of the Hip Joint

Motion of the Femur on the Acetabulum

The motions of the hip joint are easiest to visualize as movement of the convex femoral head within the concavity of the acetabulum as the femur moves through its three degrees of freedom: flexion/extension, abduction/adduction, and medial/lateral rotation.

The femoral head will glide within the acetabulum in a direction opposite to motion of the distal end of the femur.

Flexion and extension of the femur occur from a neutral position as an almost pure spin of the femoral head around a coronal axis through the head and neck of the femur.

The head spins posteriorly in flexion and anteriorly in extension.

However, flexion and extension from other positions (e.g., in abduction or medial rotation) must include both spinning and gliding of the articular surfaces, depending on the combination of motions.

The motions of abduction/adduction and medial/lateral rotation must include both spinning and gliding of the femoral head within the acetabulum, but the intra-articular motion again occurs in a direction opposite to motion of the distal end of the femur.

As is true at most joints, the joint’s range of motion (ROM) is influenced by structural elements, as well as by whether the motion is performed actively or passively and whether passive tension in two-joint muscles is encountered or avoided.

The following ranges of passive joint motion are typical of the hip joint.

Flexion of the hip is generally about 90° with the knee extended and 120 ° when the knee is flexed and when passive tension in the two-joint hamstrings muscle group is released. Hip extension is considered to have a range of 10 ° to 30°.

Hip extension ROM appears to diminish somewhat with age, whereas flexion remains relatively unchanged.

When hip extension is combined with knee flexion, passive tension in the two-joint rectus femoris muscle may limit the movement. The femur can be abducted 45° to 50° and adducted 20° to 30°.

Abduction can be limited by the two-joint gracilis muscle and adduction limited by the tensor fascia lata (TFL) muscle and its associated iliotibial (IT) band.

Medial and lateral rotation of the hip are usually measured with the hip joint in 90° of flexion; the typical range is 42° to 50°.

Femoral anteversion is correlated with decreased range of lateral rotation and less strongly with increased range of medial rotation.8

When the femoral head is torsioned anteriorly more than normal (Fig. 10-18), lateral rotation of the femur turns the head out even more, both risking subluxation and encountering capsuloligamentous and muscular restrictions on the anterior aspect of the joint as the head presses forward.

Hip joint rotation can correspondingly be affected by retroversion of the femur, as well as by acetabular anteversion and laxity of the joint capsule.

Normal gait on level ground requires at least the following hip joint ranges:

30° flexion, 10° hyperextension, 5° of both abduction and adduction, and 5° of both medial and lateral rotation.

Walking on uneven terrain or stairs will increase the need for joint range beyond that required for level ground, as will activities such as sitting in a chair or sitting cross-legged.

Motion of the Pelvis on the Femur

Whenever the hip joint is weight-bearing, the femur is relatively fixed, and, in fact, motion of the hip joint is produced by movement of the pelvis on the femur.

At all joints, the motion between articular surfaces is the same whether the distal lever moves or the proximal lever moves.

However, the proximal lever and distal lever move in opposite directions to produce the same articular motion.

For example, elbow flexion can be a rotation of the distal forearm upward or, conversely, a rotation of the proximal humerus downward.

In examinations of the upper extremity joint complexes thus far, motion of the distal lever functionally tended to predominate, and so this apparent reversal of motions was not a point of discussion.

At the hip joint, this reversal of motion of the lever is further complicated by the horizontal orientation and shape of the pelvis (the “levers” of the hip are not in line but lie essentially perpendicular to each other).

In contrast to other joints, there is also a new set of terms to identify joint motion when the pelvis (rather than femur) is the moving segment.

The terms for pelvic motions are used with weight-bearing hip motion because the motions of the pelvis are more apparent to the eye of the examiner and are, in fact, key to what occurs at the joints above and below the pelvis.

It must be emphasized, however, that the motion of the pelvis presented in the next sections are not new motions of the hip joint but are simply how the same three degrees of freedom for the joint are accomplished by the pelvis rather than the femur.

Anterior and Posterior Pelvic Tilt

Anterior and posterior pelvic tilt are motions of the entire pelvic ring in the sagittal plane around a coronal axis. In the normally aligned pelvis, the antero-superior iliac spines (ASISs) of the pelvis lie on a horizontal line with the posterior superior iliac spines and on a vertical line with the symphysis pubis (Fig. 10-20A).

Anterior and posterior tilting of the pelvis on the fixed femur produce hip flexion and extension, respectively. Hip joint extension through posterior tilting of the pelvis brings the symphysis pubis up and the sacrum of the pelvis closer to the femur, rather than moving the femur posteriorly on the pelvis (see Fig. 10-20B).

Hip flexion through anterior tilting of the pelvis moves the ASISs anteriorly and inferiorly; the inferior sacrum moves farther from the femur, rather than moving the femur away from the sacrum (see Fig. 10-20C).

Anterior and posterior tilting will result in flexion and extension of both hip joints simultaneously in bilateral stance or can occur at the stance hip joint alone if the opposite limb is non-weight-bearing.

Lateral Pelvic Tilt

Lateral pelvic tilt is a frontal plane motion of the entire pelvis around an antero-posterior axis. In the normally aligned pelvis, a line through the ASISs is horizontal.

In lateral tilt of the pelvis in unilateral stance, one hip joint is the pivot point or axis for motion of the opposite side of the pelvis as it elevates (pelvic hiking) or drops (pelvic drop).

If a person stands on the left limb and hikes the pelvis, the left hip joint is being abducted because the medial angle between the femur and a line through the ASISs increases (Fig. 10-21A).

If a person stands on the left leg and drops the pelvis, the left hip joint will adduct because the medial angle formed by the femur and a line through the ASISs will decrease (see Fig. 10-21B).

In descriptions of the hip joint motions that occur in unilateral stance, the hip joint of the non–weight- bearing limb is in an open chain and has no motions on it. However, the non–weight-bearing leg typically hangs straight down as the pelvis moves.

Lateral Shift of the PelvisLateral pelvic tilt can also occur in bilateral

stance. If both feet are on the ground and the hip and knee of one limb are flexed, the opposite limb is largely the weight-bearing limb and the terminology is the same as for unilateral stance.

However, if both limbs are weight-bearing, lateral tilt of the pelvis will cause the pelvis to shift to one side or the other.

With pelvic shift, the pelvis cannot hike but can only drop. Because there is a closed chain between the two weight-bearing feet and the pelvis, both hip joints will move in the frontal plane in a predictable way as the pelvic tilt (or pelvic shift) occurs.

If the pelvis is shifted to the right in bilateral stance, the left side of the pelvis will drop, the right hip joint will be adducted, and the left hip joint will be abducted (Fig. 10-22).

Anterior and Posterior Pelvic Rotation

Pelvic rotation is motion of the entire pelvic ring in the transverse plane around a vertical axis.

Although rotation can occur around a vertical axis through the middle of the pelvis in bilateral stance, it most commonly and more importantly occurs in single-limb support around the axis of the supporting hip joint.

Forward rotation of the pelvis occurs in unilateral stance when the side of the pelvis opposite to the supporting hip joint moves anteriorly (Fig. 10-23A).

Forward rotation of the pelvis produces medial rotation of the supporting hip joint. Backward rotation of the pelvis occurs when the side of the pelvis opposite the supporting hip moves posteriorly (see Fig. 10-23C).

Posterior rotation of the pelvis produces lateral rotation of the supporting hip joint.

Pelvic rotation can occur in bilateral stance as well as unilateral stance, as is true for lateral pelvic tilt.

If both feet are bearing weight and the axis of motion occurs around a vertical axis through the center of the pelvis, the terms forward rotation and backward rotation must be used by referencing a side (e.g., forward rotation on the right and backward rotation on the left).

Coordinated Motions of the Femur,Pelvis, and Lumbar Spine

When the pelvis moves on a relatively fixed femur, there are two possible outcomes to consider. Either the head and trunk will follow the motion of the pelvis (moving the head through space) or the head will continue to remain relatively upright and vertical despite the pelvic motions.

These are open- and closed-chain responses, respectively. Each of these two situations produces very different reactions from the joints and segments proximal and distal to the hip joints and pelvis and must be examined separately.

Pelvifemoral Motion

When the femur, pelvis, and spine move in a coordinated manner to produce a larger ROM than is avail-able to one segment alone, the hip joint is participating in what will predominantly (but not exclusively) be an open-chain motion termed pelvifemoral motion.

Pelvifemoral motion can be considered analogous to scapulohumeral motion because the combination of motions at several joints serves to increase the range available to the distal segment.

In the case of scapulohumeral motion, the joints are serving the hand.

In the case of pelvifemoral motion, the joints may serve either end of the chain: the foot or head.

Pelvifemoral motion has also been referred to as pelvifemoral “rhythm,” which implies a continuous relationship between the two segments, which is arguable because the relative contributions can vary among individuals and in different activities.

Bohannon and colleagues determined that pelvic rotation contributed between 30% and 46% of the total range of a passive straight leg raise.

During active maximal hip flexion (knee flexed) in standing, Murray and colleagues found that the pelvis contributed between 8% and 32% of the total motion, with an even greater variability among individuals (9% to 53%) when a 4.53-kg ankle weight was added.

The link between hip, pelvis, and lumbar motion is the basis of using pain with active straight-leg raising as a test for severity of dysfunction in persons with low back pain.

Closed-Chain Hip Joint Function

The joints of the right and left lower limbs are part of a true closed chain when both lower limbs are weight-bearing and the chain is defined as all the segments between the right foot, up through the pelvis, and down through the left foot.

A true closed chain is formed because both ends of the chain (both feet in this example) are “fixed” and movement at any one joint in the chain invariably involves movement at one or more other links in the chain.

It is also common usage to consider that the joints of one or both lower limbs are part of a closed chain whenever a person is standing (weight-bearing) on one or both lower limbs, which leads to inappropriately considering the terms “weight-bearing” and “closed chain” to be interchangeable.

The lower limbs were weight-bearing in Example 10-1 but were effectively part of an open chain.

Consequently, weight-bearing and closed chain cannot be synonymous. How, then, do the joints of the lower extremity function in a closed chain in standing?

For the hips (and other lower limb joints) to be in a closed chain in standing, both ends of the chain (the head and the feet) must be fixed.

The feet are, in fact, fixed by weight-bearing. The head, however, is often (but not necessarily) functionally “fixed.”

Although the head is certainly free to move in space, the head most often remains upright and vertically oriented during upright activities.

The drive to keep the head upright is due, in part, to the influence of the tonic labyrinthine and optical righting reflexes that are normally evident almost immediately at birth and continue to operate throughout life.

The drive to keep the head upright and over the sacrum will effectively fix the head in relative space even though this is not structurally the case; that is, the head is functionally rather than structurally fixed.

When the head (one end of the chain) is held upright and over the feet (the other end of the chain), all the segments in the axial skeleton and lower limbs function as part of a closed chain; movement at one joint will create movement in at least one other linkage in the chain.

Consequently, in our functional closed-chain premise, hip flexion does not occur independently (which would move the head forward in space) but is accompanied by motion in one or more inter-posed segments to ensure that the head remains upright over the base of support and that the body does not become unstable.

In any instance in which there is normal or abnormal pelvic motion during weight-bearing and the head must remain upright, compensatory motions of the lumbar spine will occur if available.

This does not rule out the need for compensation at additional joints as well, but the lumbar spine tends to be the “first line of defense.” As we examine the other joints of the lower extremity and move on to posture and gait, other compensatory motions will be encountered and discussed.

Table 10-1 presents the compensatory motions of the lumbar spine that accompany given motions of the pelvis and hip joint in a functional closed chain.

Hip Joint Musculature

There have been numerous studies of the muscles of the hip joint. Most confirm underlying principles of muscle physiology seen at the other joints we have examined so far.

That is, hip joint muscles work best in the middle of their contractile range or on a slight stretch (at so-called optimal length-tension); two-joint muscles generate greatest force when not required to shorten over both joints simultaneously; and tension generation is optimal with eccentric contractions, followed by isometric and then concentric contractions.

The muscles of the hip joint make their most important contributions to function during weight-bearing.

In weight-bearing, the muscles are called on to move or support the HAT (approximately two thirds of body weight) rather than the weight of one lower limb (approximately one sixth of body weight).

Consequently, the hip joint muscles adapt their structure to the required function, as can be seen in their large areas of attachment, their length, and their large cross-section.

The alignment of the hip joint muscles and the large ROM available at the hip joint result in muscle functions that are strongly influenced by hip joint position.

For example, the adductor muscles may be hip flexors in the neutral hip joint but will be hip extensors when the hip joint is already flexed.

Delp and colleagues used computer modeling to determine that the torque-generating capability of the medial rotators increased with increased hip flexion, whereas the torque-generating capacity of the lateral rotators decreased with increasing hip flexion.

They similarly determined that the piriformis muscle was a lateral rotator at 0 of hip flexion but a medial rotator at 90 of hip flexion. Such inversions of function are found in a few muscles at the shoulder (the clavicular portion of the pectoralis major, for example), but are fairly common in the hip joint.

As a consequence, results of various studies may appear to be contradictory, but, in fact, testing conditions explain differing results. Some gender-related differences also have been found that explain differential findings.

It is best to examine muscle action at the hip joint in the context of specific functions such as single-limb support, posture, and gait. The next section will briefly review muscle function, but we will leave more detailed analyses for later in this and other chapters.

Although the traditional action of each muscle on the distal femoral segment is described for the most part, it must be emphasized that any of the muscles is as likely (or more likely) to produce joint action by moving the proximal pelvic segment instead.

Flexors

The flexors of the hip joint function primarily as mobility muscles in open-chain function; that is, they function primarily to bring the swinging limb forward during ambulation or in various sports. The flexors may function secondarily to resist strong hip extension forces that occur as the body passes over the weight-bearing foot.

Nine muscles have action lines crossing the anterior aspect of the hip joint. Of these, the primary muscles of hip flexion are the iliopsoas, rectus femoris, TFL, and sartorius.

The iliopsoas muscle is considered to be the most important of the primary hip flexors. It consists of two separate muscles, the iliacus muscle and the psoas major muscle, both of which attach to the femur by a common tendon. The two components of the iliopsoas muscle have many points of origin, including the iliac fossa and the disks, bodies, and transverse processes of the lumbar vertebrae.

Given the attachments of the psoas major muscle to the anterior vertebrae and the iliacus muscle to the iliac fossa, activity of or passive tension in these muscles would anteriorly tilt the pelvis (iliacus muscle) and, apparently, pull the lumbar vertebrae anteriorly into flexion (psoas major muscle).

In closed-chain function (head vertical), however, these muscles seem to create a paradoxical lumbar lordosis (lumbar extension) that results from the body’s attempt to keep the head over the sacrum with anterior pelvic tilt and lower lumbar flexion.

The role of the iliopsoas muscle in hip flexion may be particularly critical when hip flexion from a sitting position is required. Smith and colleagues62 proposed that the hip cannot be flexed beyond 90 when the iliopsoas muscle is paralyzed, because the other hip flexor muscles are effectively actively insufficient in that position.

Basmajian and DeLuca summarized the often contradictory evidence of many investigations by concluding that both segments of the iliopsoas muscle are active in various stages of hip flexion. The moment arm (MA) of the iliopsoas muscle for medial or lateral rotation is very small and probably not functionally relevant.

The rectus femoris muscle is the only portion of the quadriceps muscle that crosses both the hip joint and knee joint. It originates on the anterior inferior iliac spine and inserts by way of a common tendon into the tibial tuberosity.

The rectus femoris muscle flexes the hip joint and extends the knee joint. Because it is a two-joint hip flexor, the position of the knee during hip flexion will affect its ability to generate force at the hip.

Simultaneous hip flexion and knee extension considerably shorten this muscle and increase the likelihood of active insufficiency.

Consequently, the rectus femoris muscle makes its best contribution to hip flexion when the knee is maintained in flexion.

The sartorius muscle is a straplike muscle originating on the ASIS. It crosses the anterior aspect of the femur to insert into the upper portion of the medial aspect of the tibia.

The sartorius muscle is considered to be a flexor, abductor, and lateral rotator of the hip, as well as a flexor and medial rotator of the knee.

Wheatley and Jahnke proposed that the sartorius muscle, although a two-joint muscle, should be relatively unaffected by the position of the knee, given the relatively small proportional change in length with increased knee flexion.

Its function is probably most important when the knee and hip need to be flexed simultaneously (as in climbing stairs), but its small cross-section argues against a unique or critical role at the hip joint.

The TFL muscle originates more laterally than the sartorius muscle. Its origin is on the anterolateral lip of the iliac crest. The muscle fibers extend only about one fourth of the way down the lateral aspect of the thigh before inserting into the IT band. The IT band or IT tract is the thickened lateral portion of the fascia lata of the hip and thigh.

The IT band attaches proximally to the iliac crest lateral to the TFL muscle. After the tensor attaches to the IT band, the IT band continues distally on the lateral thigh to insert into the lateral condyle of the tibia.

The TFL muscle is considered to flex, abduct, and medially rotate the femur at the hip, although the TFL’s contribution to hip abduction may be dependent on simultaneous hip flexion.

The most important contribution of the TFL muscle may be in maintaining tension in the IT band. The IT band assists in relieving the femur of some of the tensile stresses imposed on the shaft by weight-bearing forces.

Because bone more effectively resists compressive than tensile stresses, reduction of tensile stresses is important in maintaining integrity of the bone.

Functionally, it appears that the TFL muscle and IT band are expendable. The IT band may be removed and used for autogenous fascial transplants without any evident change in active or passive hip or knee function.

Excessive tension in the IT band may also contribute to reduced hip adduction ROM when the hip is extended.

Gajdosik and colleagues performed the Ober test, presumed to test tension in the IT band, on men and women without impairments. They found an average passive hip adduction of 9 for men and 4 for women when both the hip and knee were extended.

When the knee was flexed during the maneuver, the hip remained in 4 of abduction for men and 6 of abduction for women, which implied that there was greater tension in the lateral hip joint structures (potentially with the IT band as a key factor) when the knee was flexed.

The Ober test presumably moves the IT band from its position anterior to the greater trochanter to a position posterior to the greater trochanter by extending the hip. Movement of the IT band anteriorly and posteriorly over the greater trochanter during functional activities has been implicated in “snapping hip” syndrome and in inflammation of the trochanteric bursa.

The secondary hip flexors are the pectineus, adductor longus, adductor magnus, and the gracilis muscles.

These muscles are described in the next section because they are predominantly adductors of the hip. Each, however, is capable of contributing to hip joint flexion, but that contribution is dependent on hip joint position.

Kapandji noted that these muscles contribute to flexion only up to 40 to 50 of hip flexion.

Once the femur is superior to the point of origin of a muscle, the muscle will become an extensor of the hip joint. The gracilis, a two-joint muscle, is active as a hip flexor when the knee is extended but not when the knee is flexed.

Adductors

The hip adductor muscle group is generally considered to include the pectineus, adductor brevis, adductor longus, adductor magnus, and the gracilis muscles. The adductors are located anteromedially.

The adductors longus, brevis, and magnus muscles arise in a group from the body and inferior ramus of the pubis to insert along the linea aspera.

The gracilis muscle is the only two-joint adductor. It originates on the symphysis pubis and pubic arch and inserts on the medial surface of the shaft of the tibia.

The contribution of the adductor muscles to hip joint function has been debated for many years.

One of the reasons for debate is a question as to the degree to which the flexed, adducted, and medially rotated posture assumed by many individuals with cerebral palsy is attributable to adductor spasticity.

Arnold and Delp (using kinematic data from children with cerebral palsy and excessive medial rotation of the hip, and a “deformable femur” model) concluded that, in the normal hip in standing, the adductor brevis, adductor longus, pectineus,

and posterior adductor magnus muscles had only small MAs for medial rotation, whereas the gracilis and anterior adductor magnus muscles had small MAs for lateral rotation.

With excessive femoral anteversion, the MAs of the adductor brevis, pectineus and the middle gluteus magnus muscles switched from medial rotatory to lateral rotatory lines of pull. After examining the changes in MAs with femoral anteversion or combined hip medial rotation and knee flexion, the investigators concluded that the adductors were unlikely to have a strong influence on the medially rotated hip position during the gait cycle.26

Basmajian and DeLuca believed that the variability in study findings for the adductors supported the theory of Janda and Stará that the adductors function not as prime movers but by reflex response to gait activities.

As shall be seen in our discussion of muscle function in bilateral stance, the adductors may be synergists to the abductor muscles when both feet are on the ground, enhancing side-to-side stabilization of the pelvis.

Although the role of the adductor muscles may be less clear than that of other hip muscle groups, the relative importance of the adductors should not be underestimated.

The adductors as a group contribute 22.5% to the total muscle mass of the lower extremity, in comparison with only 18.4% for the flexors and 14.9% for the abductors.

The adductors are also capable of generating a maximum isometric torque greater than that of the abductors.

Extensors

The one-joint gluteus maximus muscle and the two-joint hamstrings muscle group are the primary hip joint extensors.

These muscles may receive assistance from the posterior fibers of the gluteus medius, from the posterior adductor magnus muscle, and from the piriformis muscle.

The gluteus maximus is a large, quadrangular muscle that originates from the posterior sacrum, dorsal sacroiliac ligaments, sacrotuberous ligament, and a small portion of the ilium.

The gluteus maximus crosses the sacroiliac joint before its most superior fibers insert into the IT band (as do the fibers of the TFL muscle) and its inferior fibers insert into the gluteal tuberosity.

The gluteus maximus is the largest of the lower extremity muscles; this muscle alone constituting 12.8% of the total muscle mass of the lower extremity.

The maximus is a strong hip extensor that appears to be active primarily against a resistance greater than the weight of the limb.

Its MA for hip extension is considerably longer than that of either the hamstrings or the adductor magnus muscles and is maximal in the neutral hip joint position.

A favorable length-tension relationship, however, allows it to exert its peak extensor moment at 70° of hip flexion.

The segments of the maximus have a substantial capacity to laterally rotate the femur, although the MAs for lateral rotation diminish with increased hip flexion.

The three two-joint extensors are the long head of the biceps femoris, the semitendinosus, and the semi-membranosus muscles, known collectively as the ham-strings.

Each of these three muscles originates on the ischial tuberosity. The biceps femoris crosses the posterior femur to insert into the head of the fibula and lateral aspect of the lateral tibial condyle.

The other two hamstrings insert on the medial aspect of the tibia. All three muscles extend the hip with or without resistance, as well as serving as important knee flexors.

The hamstrings increase their MA for hip extension as the hip flexes to 35 and decrease it thereafter. This is somewhat in contrast to the MA of the gluteus maximus that is maximal at neutral position and decreases with any hip flexion thereafter.

Regardless of these changes in MA with joint position, the MA of the combined hamstrings for hip extension is smaller than that of the gluteus maximus at all points in the hip flexion/extension ROM. As two-joint muscles, the role of the hamstrings in hip extension is also strongly influenced by knee position.

Chleboun and colleagues (using ultrasonography) determined that the MA for the long head of the biceps femoris was greater for hip extension than for knee flexion, with hip position affecting its excursion capability more than did knee position.69

Although these investigators reported only on the long head of the biceps femoris, the anatomy of the medial hamstrings (semimembranosus and semitendinosus) makes it likely that these muscles have similar attributes.

If the hip is extended and the knee is flexed to 90 or more, the hamstrings may not be able to contribute much to hip extension force because of active insufficiency or approaching active insufficiency.

Extension forces in the hip increase by 30% if the knee is extended during hip extension.

The optimal length-tension relationship for the long head of the biceps is estimated to be at 90 of hip flexion and 90 of knee flexion,69 and it is likely that the medial ham-strings show similar behavior.

The medial hamstrings have a small MA for medial rotation in the neutral hip but appear to switch to lateral rotators with hip flexion or knee flexion.

The biceps femoris appears to con-tribute to lateral rotation of the hip.

Abductors

Active abduction of the hip is brought about predominantly by the gluteus medius and the gluteus minimus muscles.

The superior fibers of the gluteus maximus and the sartorius muscles may assist when the hip is abducted against strong resistance.

The TFL muscle is given variable credit for its contribution and may be effective as an abductor only during simultaneous hip flexion.

The gluteus medius originates on the lateral surface of the wing of the ilium and inserts into the greater trochanter, beneath the gluteus maximus.

The gluteus medius has anterior, middle, and posterior parts that function asynchronously during movement at the hip.

Analogous to the deltoid muscle of the glenohumeral joint, the anterior fibers of the gluteus medius are active in hip flexion, whereas the posterior fibers function during extension.

In the neutral hip, the posterior portion of the medius will produce a lateral rotatory moment, whereas the middle and anterior have small medial rotatory moments.

In hip flexion, all portions medially rotate the hip.All portions of the muscle abduct, regardless of

hip joint position.

The gluteus minimus muscle lies deep to the gluteus medius, arising from the outer surface of the ilium with its fibers converging on an aponeurosis that ends in a tendon on the greater trochanter. The minimus is consistently an abductor and flexor of the hip, with its rotator function dependent on hip position. How-ever, the minimus is a medial rotator in hip flexion.71

There appears to be consensus that the gluteus minimus commonly has a tendinous insertion into the joint capsule as it passes to the greater trochanter. It is hypothesized that this attachment retracts the capsule during hip abduction to prevent entrapment or tightens the capsule to add to the gluteus minimus’s primary function of stabilizing the femoral head in the acetabulum.

The gluteus minimus and medius muscles function together to either abduct the femur (distal level free) or, more important, to stabilize the pelvis (and super-imposed HAT) in unilateral stance against the effects of gravity.

As will be presented later, the gluteus medius and minimus muscles will offset the gravitation adduction torque on the pelvis (pelvis drop) around the stance hip. The abductors are physiologically designed to work most effectively in a neutral or slightly adducted hip (slightly lengthened abductors).73,74

Isometric abduction torque in the neutral hip position is 82% greater than abduction torque when the hip is in 25 of abduction (shortened abductors).6

Lateral Rotators

Six short muscles have lateral rotation as a primary function. These muscles are the obturator internus and externus, the gemellus superior and inferior, the quadratus femoris, and the piriformis muscles.

Other muscles that have fibers posterior to the axis of motion at the hip (the posterior fibers of the gluteus medius and minimus and the gluteus maximus) may produce lateral rotation combined with the primary action of the muscle (although it has already been noted that the lateral rotatory function of these muscles decreases or becomes medial with increased hip flexion).

Of the primary lateral rotators, each inserts either on or in the vicinity of the greater trochanter (Fig. 10-29).

The obturator internus muscle originates from the inside (posterior aspect) of the obturator foramen and emerges through the lesser sciatic foramen to insert on the medial aspect (inside) of the greater trochanter.

The gemellus superior and gemellus inferior muscles arise from the ischium of the pelvis, just above and just below the point at which the obturator internus passes through the lesser sciatic notch.

Both gemelli follow and blend with the obturator internus tendon to insert with the internus tendon into the greater trochanter.

The obturator externus muscle is sometimes considered to be an anteromedial muscle of the thigh because it originates on the external (anterior) surface of the obturator foramen. However, it crosses the posterior aspect of the hip joint and inserts on the medial aspect of the greater trochanter in the trochanteric fossa.

The quadratus femoris muscle is a small quadrangular muscle that originates on the ischial tuberosity and inserts on the posterior femur between the greater and lesser trochanters.

The piriformis muscle originates largely on the anterior surface of the sacrum, passes through the greater sciatic notch, and follows the inferior border of the posterior gluteus medius to insert above the other lateral rotators into the medial aspect of the greater trochanter.

The piriformis and gluteus maximus are the only two muscles that cross the sacroiliac joint. The sciatic nerve, the largest nerve in the body, enters the gluteal region just inferior to the piriformis muscle.

The lateral rotator muscles are positioned to per-form their rotatory function effectively, given the nearly perpendicular orientation to the shaft of the femur (see Fig. 10-29).

However, exploration of function of these muscles has been restricted because of the relatively limited access to electromyography (EMG) surface or wire electrodes.

Like their rotator cuff counterpart at the glenohumeral joint, these muscles would certainly appear to be effective joint compressors because their combined action line parallels the head and neck of the femur.

Using modeling, Delp and colleagues60 determined that the obturator internus, like the gluteal muscles, decreased its MA for lateral rotation with increased hip flexion.

The piriformis was estimated to have a large MA for lateral rotation with the hip joint at 0 but switched to a medial rotator with half the MA when the hip reached 90 of flexion.

The obturator externus and quadratus femoris were the only lateral rotators that did not diminish their MA for lateral rotation with increased hip joint flexion.

Hypothetically, the lines of pull of the deep one-joint lateral rotators should make them ideal tonic stabilizers of the joint during most weight-bearing and non–weight-bearing hip joint activities.

Although their ability to perform lateral rotation may decrease with hip flexion in some instances, the action lines of these muscles should remain largely compressive (parallel to the femoral neck) throughout the hip joint ROM.

Medial Rotators

There are no muscles with the primary function of producing medial rotation of the hip joint. The more consistent medial rotators are the anterior portion of the gluteus medius, gluteus minimus, and the TFL muscles. Although controversial, the weight of evidence appears to support the adductor muscles as medial rotators of the joint, with the possible exception of the gracilis muscle.

The ability of hip joint muscles to shift function with changing position of the hip joint is evident when medial rotation of the hip is examined.

There is a trend toward increased medial rotation torques (or decreased lateral rotation torques) with increased hip flexion among many of the hip joint muscles, with three times more medial rotation torque in the flexed hip than in the extended hip.

Delp and colleagues, although clear as to the limitations of their modeling, suggested that the medial rotation that accompanies a “crouched” gait seen in many individuals with cerebral palsy may be attributable more to hip flexion than to adductor spasticity.

End of Part - 2