Hip Anatomy and Biomechanics in the AthletePaul E. Hughes, M.D., Jim C. Hsu, M.D., and Matthew J. Matava, M.D.

SUMMARYThe hip is a unique joint that possesses both mobility and

stability because of its anatomic configuration. This joint is cru-cial to athletic activities involving the lower extremity, such asrunning, jumping, and kicking, as well as to the generation andtransference of forces for activities involving the upper extremity.The tremendous loads that the hip withstands result from mus-cular, gravitational, and joint reaction forces inherent in weight-bearing. This article reviews the anatomy and biomechanics ofthe hip as a foundation for the evaluation and treatment of ath-letes with hip disorders.

Key Words: Hip, Anatomy, Biomechanics

ANATOMY OF THE HIP

Osseous AnatomyThe hip joint is a highly congruent ball-and-socket joint

comprising the acetabulum and femoral head. The concavityof the acetabulum develops in response to the presence of aspherical femoral head.1 Within the acetabulum of a child, thephyses of the ilium, ischium, and pubis converge as the tri-radiate cartilage. Embryologically, the acetabular fossa is dis-cernible by 8 weeks to 9 weeks of fetal development.2 By the17th week of development, the joint cavity has cleaved withthe formation of a synovial layer.2 Ossification of this physealcomplex is completed by the age of 16 years to 18 years.1

The acetabular opening is oriented in an anterior, lat-eral, and inferior direction, whereas the head of the femurfaces the acetabulum in a medial, cranial, and anterior direc-tion. The coverage of the weightbearing surface of the femo-ral head is primarily related to the degree of inferior acetab-ular tilt as measured by the center-edge angle of Wiberg.3

This angle is formed by one line connecting the lateral rim ofthe acetabulum with the center of the femoral head, and asecond line extending vertically from the center of the femo-ral head (Fig. 1). A center edge angle of less than 20° isconsidered abnormal and implies an acetabulum with a morevertical orientation with less coverage of the femoral head.

The acetabular labrum is a fibrocartilaginous structureattached to the bony acetabular rim analogous to the labrumof the shoulder. The labrum deepens the acetabulum fromless than one-half of its spherical volume to greater thanone-half of this volume, thus adding stability to the joint.4

The labrum is widest at the posteroinferior acetabular quad-rant (mean, 6.4 mm ± 1.7 mm) and thickest at the anterosu-perior quadrant (mean, 5.5 mm ± 1.5 mm).5

The hyaline articular cartilage of the acetabulum isthickest (range, 1.75 mm–2.5 mm) at the superior regionwhere the weightbearing forces are greatest, and thinnest atthe posteromedial region (range, 0.75 mm–1.25 mm).6 A cen-tral, nonarticulating depression, known as the acetabularfossa, is occupied by a fat pad known as the pulvinar. Thisfibrofatty tissue contains vascular branches from the obtura-tor artery and nerve endings from the posterior branch of theobturator nerve.7,8 The articular cartilage surface of the ac-etabulum surrounds this fossa in a horseshoe configurationwith a gap inferiorly. The transverse acetabular ligament—the inferior continuation of the acetabular labrum—traversesthis nonarticulating notch.

The ligamentum teres passes from the acetabular fossa

From the Sports Medicine Section, Washington University Department of Orthopedic

Surgery, St. Louis, MO.

Address correspondence and reprint requests to: Matthew J. Matava, M.D., Washington

University, Department of Orthopedic Surgery, Suite 11300 West Pavilion, One Barnes-

Jewish Hospital Drive, St. Louis, MO 63110. E-mail: matavam@msnotes.wustl.edu

Sports Medicine and Arthroscopy Review10:103–114 © 2002 Lippincott Williams & Wilkins, Inc., Philadelphia

103 DOI: 10.1097/01.JSA.0000017305.66476.DA

to the fovea centralis on the medial aspect of the femoralhead, slightly posterior and inferior to center. This strongtriangular-shaped structure is attached inferiorly to the trans-verse acetabular ligament and is composed of well-organizedcollagen fibers in a banded configuration. The exact functionof this ligament is unclear, though it likely assists in stabili-zation of the hip because its rupture often gives rise to symp-toms of hip instability and pain.2

The head of the femur forms approximately two-thirdsof a sphere with a diameter in the range of 45 mm to 56 mmin the adult.9 It is entirely covered with hyaline cartilageexcept for the fovea centralis. The hyaline cartilage coveringthe femoral head is thickest (approximately 2.5 mm)9 on itssuperior, medial, and slightly posterior surface. Accordingly,it is this region of the femoral head that is the primary contactpoint with the acetabulum during weightbearing.4

The neck of the femur is approximately 5 cm long inthe adult hip.4 The angle of inclination between the femoralneck and femoral shaft in the frontal plane is 125° ± 5° in theskeletally-mature hip, though this angle can be as great as150° in the newborn.4 In the axial plane, the femoral neckforms an angle of torsion with the transcondylar axis of thefemur. This angle is oriented anteriorly an average of 14° inadults.4 At the junction of the femoral neck and shaft, thegreater trochanter projects superolaterally and the lesser tro-chanter projects posteromedially. The orientation of thesebony prominences significantly influences the function of themuscles that insert on them (see below).

During weightbearing activities, the proximal femur is

subjected to tremendous tensile and compressive stresses, es-pecially at the intertrochanteric and subtrochanteric regions.Trabecular bone patterns develop to resist these deformingforces. Ward10 described these trabecular patterns and attrib-uted their orientation to the directional alignment of weight-bearing stress exerted on the proximal femur (Fig. 2). Thesetrabeculae consist of a primary compressive group, whicharises from the medial subtrochanteric cortex and ascendssuperiorly into the weightbearing femoral head, and a pri-mary tensile group, which spans from the foveal area of thefemoral head, through the superior femoral neck, and into thelateral subtrochanteric cortex.10 Secondary compressive, sec-ondary tensile, and a greater trochanteric group complete thepattern of trabecular orientation.10 The calcar femorale is adense plate of bone extending laterally from the posterome-dial femoral cortex to the posterior aspect of the greater tro-chanter. The calcar is thickest at its medial aspect and gradu-ally thins as it extends laterally.11

Capsular AnatomyA strong fibrous capsule encloses the hip joint that aids

in the maintenance of hip stability. Proximally, the capsuleattaches to the bony rim of the acetabulum approximately 6mm to 8 mm from the labrum.5 At its distal (femoral) inser-tion, the anterior capsule attaches to the intertrochanteric lineand greater trochanter, whereas the posterior capsule attachesjust proximal to the posterior intertrochanteric line.4 Al-though most of the capsular fibers run longitudinally parallelto the femoral neck, a smaller subset of fibers, the zona or-bicularis, encircles the femoral neck. This condensed groupof circular fibers reinforces the hoop stresses encountered bythe acetabular labrum.7 The inner surface of the capsule islined by the synovium of the hip joint. The synovial liningalso covers the acetabular fossa, labrum, and intracapsularportion of the femoral neck.4

Three extracapsular ligaments connect the pelvis andfemur to reinforce the hip capsule (Fig. 3). These ligamentsare tight with the hip in extension, and are most slack in thecombined positions of flexion, abduction, and external rota-tion.12 The iliofemoral ligament (ligament of Bigelow) is thestrongest of the three. It extends from the anterior inferior

FIG. 1. Diagram of the hip joint illustrating a center-edge angle of25° (normal greater than 20°). This angle is formed by one lineconnecting the lateral rim of the acetabulum with the center of thefemoral head, and a second line extending vertically from the centerof the femoral head.3

FIG. 2. AP radiograph of the hip illustrating the primary and sec-ondary compressive and tensile trabecular bone groups. A greatertrochanteric group completes the pattern of trabecular orientation.

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iliac spine (in two separate bands) to the anterior intertro-chanteric line in an inverted-Y configuration. The primaryrole of the iliofemoral ligament is to resist hyperextension ofthe hip.12 The pubofemoral ligament attaches proximally tothe superior pubic ramus and distally to the inferior femoralneck in order to provide resistance to hip hyperabduction.12

The ischiofemoral ligament, the thinnest of the three, extendsfrom the ischial rim of the acetabulum, across the posteroin-ferior aspect of the hip joint, to insert on the femoral neck. Itsprimary function is to stabilize the joint in extension.12

Blood SupplyThe hip joint receives its blood supply from several

sources. The acetabulum is supplied by three main arteries:the obturator, the superior gluteal, and inferior gluteal (Fig.4A). The superior gluteal artery supplies both the superiorand posterior portions of the acetabulum, and the inferiorgluteal artery supplies the inferior and posterior portions.8

The acetabular branch of the obturator artery provides theprimary blood supply to the medial aspect of the acetabulum.4

A smaller, terminal branch of the posterior division of theobturator artery, known as the foveal artery, traverses theligamentum teres to supply a small area of the femoral headaround the fovea centralis.12 The recess between the capsuleand labrum is lined with highly vascularized, loose connec-tive tissue. A group of three to four small blood vessels isfound in a circumferential pattern within the substance of thelabrum and along the labrum–bone junction.5

The medial and lateral circumflex femoral arteries sup-ply the blood flow to the majority of the femoral head andneck (Fig. 4B). The lateral epiphyseal artery, a branch of themedial circumflex artery, is of particular importance becauseit supplies more than one-half of the femoral head.13 Theascending artery, which arises from the lateral circumflexartery, pierces the joint capsule close to its femoral attach-ment and gives off multiple subsynovial retinacular arteriesthat ascend parallel to the femoral neck (Fig. 4B). Thesearteries are prone to interruption of flow in the presence ofincreased intracapsular pressure from either infection or frac-ture.14

FIG. 3. Extra-capsular ligaments reinforcing the hip capsule. A: Anterior view. B: Posterior view.

FIG. 4. Vascular supply to the hip joint. A: The blood supply to the acetabulum is provided by branches of the obturator, superior gluteal,and inferior gluteal arteries. B: The blood supply to the femoral head and neck is provided by the medial and lateral circumflex femoralarteries, both direct branches of the femoral artery.

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Joint Innervation

Similar to its vascular supply, the hip joint receivesmultiple innervations primarily involving the hip capsule.The posterior articular nerve, a branch of the nerve to thequadratus femoris,15 provides the most extensive nerve sup-ply to the hip joint, including the posterior and inferior re-gions of the capsule and the ischiofemoral ligament.4 Supe-riorly, the hip capsule is innervated by the superior glutealnerve. Anterior innervation of the capsule is provided, pri-marily, by direct branches of the femoral nerve. However, theanteromedial and anteroinferior regions are supplied by themedial articular nerve, which arises from the anterior divisionof the obturator nerve.4 The ligamentum teres is innervatedby the posterior branch of the obturator nerve.7 Sensory nerveend organs and ramified free nerve endings are found in theacetabular labrum, suggesting that the labrum may providenociceptive and proprioceptive feedback to and from the hipjoint.16

Muscles of the Hip: Their Actions and Innervations

Almost two dozen muscles act on the hip joint to pro-duce six fundamental motions: flexion, extension, abduction,adduction, internal rotation, and external rotation (Table 1).Many of these muscles have several actions on the hip jointthat depend on joint position. Furthermore, in the case ofmuscles spanning both the hip and the knee, the position ofthe knee affects the function of the muscles on the hip.

The strongest hip flexor is a muscular complex knownas the iliopsoas, which is comprised of the psoas major, psoasminor, and iliacus muscles.4 This muscle group originatesfrom the transverse processes of the 12th thoracic through thefifth lumbar vertebrae, anterior surface of the iliac crest, andanterior sacrum. These three muscles merge distally to forma singular tendinous insertion on the lesser trochanter.4 Otherhip flexors include the rectus femoris and the sartorius,though they are clearly secondary to the iliopsoas in regard toforce generation.4

TABLE 1. Muscles active on the hip joint: their origin, insertion, and innervation

Action Muscle Origin Insertion Innervation

Flexion Iliopsoas (iliacus,psoas major, psoasminor)

T12–L5 transverse processes,iliac crest, and sacrum

Lesser trochanter Femoral nerve

Rectus femoris AIIS and anterosuperioracetabulum

Superior patella Femoral nerve (L2–L4)

Tensor fascia latae ASIS and iliac crest Iliotibial tract Superior gluteal nerve (L4, L5)Sartorius ASIS Anteromedial tibial plateau Femoral nerve (L2, L3)

Extension Gluteus maximus Outer cortex of ilium, posteriorsacrum and coccyx

Posterior iliotibial tract and glutealtuberosity

Inferior gluteal nerve (L5, S1,S2)

Biceps femoris Ischial tuberosity Fibular head and posterolateraltibial plateau

Tibial branch of sciatic nerve(L5, S1, S2)

Semimembranosus Ischial tuberosity Posteromedial tibial plateau Tibial branch of sciatic nerve(L5, S1, S2)

Semitendinosus Ischial tuberosity Anteromedial tibial plateau Tibial branch of sciatic nerve(L5, S1, S2)

Abduction Gluteus medius Anterior gluteal line Lateral surface of greater trochanter Superior gluteal nerve (L4, L5,S1)

Gluteus minimus Outer cortex of ilium Anterior surface of greatertrochanter

Superior gluteal nerve (L5, S1)

Tensor fascia latae ASIS and iliac crest Iliotibial tract Superior gluteal nerve (L4, L5)Adduction Adductor magnus Inferior pubic ramus, ischial

tuberosityGluteal tuberosity and adductor

tubercle of medial femurObturator nerve (L2, L3) and

sciatic nerve (L2–L4)Adductor longus Body of pubis Middle third of linea aspera Obturator nerve (L2–L4)Adductor brevis Inferior ramus and body of

pubisProximal linea aspera and pectineal

lineObturator nerve (L2–L4)

Internal rotation Gluteus medius Anterior gluteal line Lateral surface of greater trochanter Superior gluteal nerve (L4, L5,S1)

Gluteus minimus Outer cortex of ilium Anterior surface of greatertrochanter

Superior gluteal nerve (L5, S1)

Tensor fascia latae ASIS and iliac crest Illiotibial tract Superior gluteal nerve (L4, L5)External rotation Obturator internus Inner surface of obturator

membraneMedial greater trochanter Nerve to obturator internus (L5,

S1)Obturator externus Outer surface of obturator

membrane, pubic ramus, andischium

Trochanteric fossa Obturator nerve (L3, L4)

Superior gemellus Ischial spine Posterior greater trochanter Nerve to obturator internus (L5,S1)

Inferior gemellus Ischial tuberosity Posterior greater trochanter Nerve to quadratus femorisPiriformis Anterior surface of sacrum and

sacrotuberous ligamentPosterosuperior greater trochanter Ventral rami of S1 and S2

Quadratus femoris Lateral border of ischialtuberosity

Quadrate tubercle Nerve to quadratus femoris

Adapted from Robbins.4

AIIS � anterior inferior iliac spine; ASIS � anterior superior iliac spine.

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The gluteus maximus, a large and powerful muscle thatinserts on the posterolateral iliotibial tract and gluteal tuber-osity, is the chief extensor of the hip.4 The three hamstringmuscles—the biceps femoris, semimembranosus, and semi-tendinosus—originate at the ischial tuberosity and cross theknee joint to insert on the posteromedial tibial plateau (semi-membranosus and semitendinosus) and fibular head (bicepsfemoris). These three muscles also function as hip extensorswhen the knee is in an extended position.4

The main abductors of the hip are the gluteus mediusand gluteus minimus, both originating from the outer cortexof the ilium and inserting on the greater trochanter.4 Com-promise in the function of the hip abductors can lead to aTrendelenburg gait pattern manifested as a compensatory up-per-body shift towards the involved side in order to maintainthe center of gravity over the compromised hip joint, thuspreventing pelvic drop.17

The primary hip adductors are the adductor longus,adductor brevis, and adductor magnus.4 These muscles origi-nate from the inferior pubic rami, ischial tuberosity, and pu-bis, with insertion sites on the adductor tubercle (adductormagnus) and along the linea aspera located on the medialaspect of the femur.4

Although no muscle group acts as the primary internalrotator of the hip, the tensor fascia latae, anterior portion ofthe gluteus medius, and gluteus minimus work in concert tocause internal hip rotation as a secondary function.4 Externalhip rotation is produced by a group of small muscles thatoriginate at the pelvis and insert primarily along the posterioraspect of the greater trochanter and proximal femur. Thesemuscles include the obturator internus, obturator externus,superior gemellus, inferior gemellus, piriformis, and quadra-tus femoris.4

BIOMECHANICS OF THE HIP WITH IMPLICATIONS FORTHE ATHLETE

There is considerable literature addressing hip biome-chanics during static weightbearing conditions (e.g., single-and double-leg stance) and dynamic situations (e.g., walkingand stair-climbing). Despite the relevant role played by thehip joint in various athletic pursuits, there has been relativelylittle written pertaining to hip biomechanics in the athleticpopulation.

Hip Motion

The hip possesses great stability because of its ana-tomic congruity, and also has considerable mobility withinsix degrees of freedom. Hip range of motion is greatest in thesagittal plane. Active hip flexion is 120° with the knee flexedand 90° with the knee fully extended.4,18 Passive hip flexionis approximately 140° with the knee flexed. Active hip ex-tension is 10° to 20°, and passive extension is as much as30°.4,18,19 Tightness of the rectus femoris or the iliofemoralligament can limit hip extension when the knee is flexed.Normal hip abduction is at least 50° and adduction 30° (lim-ited by the opposite extremity and the tensor fascia

lata).4,18,19 Internal and external rotation of the flexed hipmay range from 0° to 70° and 0° to 90°, respectively.20 In-ternal rotation is limited by the short external rotator muscles(obturator internus and externus, superior and inferior ge-melli, quadratus femoris, and piriformis) and the ischiofemo-ral ligament. External rotation is limited by the lateral band ofthe iliofemoral ligament, the pubofemoral ligament, the in-ternal rotator muscles, and the degree of femoral neck ante-version.4 Ultimately, the degree of hip motion in each planeis dependent upon the overall flexibility of the athlete. Cer-tain sports, such as gymnastics, demand more hip flexibilitythan other sports, such as marathon running. As people age,there is a progressive decrease in the range of ambulatory hipmotion because of a corresponding decrease in stridelength.4,21

Gait Cycle: Walking

The hip joint has the primary role of lower extremityadvancement during gait. The arc of hip motion during thewalking cycle is approximately 40° to 50°, with an average of30° to 40° of flexion and 5° to 10° of extension.22–24 Thewalking gait cycle consists of two phases: the stance phase,which is 60% of the cycle, and the swing phase, which con-stitutes the remaining 40%.20,25 The stance phase is consid-ered the period during which the foot is on the ground, be-ginning at initial contact and ending at toe-off. There is adouble-support phase in which both feet are on the ground forapproximately 20% of the total gait cycle.25 It is this portionof the gait cycle that defines walking. Perry25 has furthersubdivided the stance phase into five secondary phases: initialcontact, loading response, midstance, terminal stance andpreswing (Fig. 5). The swing phase is defined as the periodduring which the foot is in the air, beginning at toe-off andending at initial heel contact. This phase has also been sub-divided into three secondary phases: initial swing, midswing,and terminal swing24,25 (Fig. 5).

During the double-support phase of walking, the body’scenter of gravity falls just posterior to the axis of the hip jointin the sagittal plane, creating a slight posterior tilt of thepelvis on the femoral head.24 This rotatory tilt is counterbal-anced by the passive tension of the anterior hip capsule andextracapsular ligaments. As a result, little or no muscle ac-tivity is needed to maintain this sagittal plane equilibrium.24

Both internal and external rotational motions occur atthe pelvis, femur, and tibia during the walking cycle. Totalpelvic rotation spans a 10° arc.24 Maximum internal rotationis seen at initial ground contact, reaching a maximum atapproximately 15% of the gait cycle.24 Maximum externalrotation occurs at toe-off.24 The pelvis also demonstrates ro-tatory motion in the sagittal (anterior tilt) and coronal (pelvicdrop) planes of 4° and 7°, respectively (Fig. 6).24,26

Muscle activity during the gait cycle is a coordinatedsequence of events resulting in the smooth transition fromone phase to the next. At initial ground contact, the ham-strings and gluteus maximus contract to aid in hip extension.At midstance, the abductors stabilize the pelvis with the

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gluteus medius and gluteus minimus, providing lateral stabi-lization into terminal stance. The gluteus maximus, gluteusmedius, and probably the gluteus minimus increase activationintensity throughout the loading response of the stance phaseand then taper off by the end of midstance.4,25 The posteriorfibers of the tensor fascia lata exhibit moderate activity at theonset of the loading response (25% of manual muscle test-ing).27 The anterior fibers activate later and to a lesser degree(10% of manual muscle testing), but their activity persistsinto terminal stance.28,29 The role of the iliotibial band duringthe walking cycle is based on functional interpretations ofprevious investigators who suggested that this structureacts as a dynamic tension band, or guy wire, which can re-duce femoral diaphyseal bending stresses by as much as30%.30–33

In the transition from stance to swing, the hip flexors(iliopsoas, rectus femoris, sartorius, and anterior fibers of theiliotibial band) advance the limb from preswing to midswing.The rectus femoris is the first muscle to activate in preswing.It is followed by contractions of the sartorius and the iliacusin early swing. The gracilis and other adductors are alsoactive during initial swing, though only for a short time.20,25

The hip adductors and hamstrings activate during thetransition from the swing to the stance phase because the

dynamic forces controlling the limb tend to flex and abductthe hip.25 These muscles also function to control accelerationof the hip joint to ensure the precise placement of the foot onthe support surface in anticipation of stance. The concentric–eccentric coactivation of agonist–antagonist muscle groupsduring the transition of joint directional motion helps to sta-bilize the hip but also increases the forces on the joint. Themuscle activities, motion measurements, and their timing dur-ing all eight phases of gait are summarized in Table 2.

Gait Cycle: RunningAn understanding of the walking gait cycle is useful

because it can be used to compare the differences betweenwalking, jogging, and running. During the walking cycle, onefoot is on the ground at any one time. As the pace of walkingbecomes progressively faster, the single-stance phase length-ens in proportion to the degree of shortening of the double-stance phase. Running is defined as occurring when thedouble-stance phase is omitted during the gait cycle.34 It is atthis point that a float phase, a period of non-support whereboth feet are off the ground, occurs that lasts approximately30% of the gait cycle. The total period of weightbearing isalso reduced in running, compared with walking, such thatthe swing phase constitutes 70% of the running gait cycle and

FIG. 6. Rotatory motion of the hip joint in the (A) sagittal, (B) coronal, and (C) transverse planes.

FIG. 5. The walking gait cycle. Adapted from Perry.25

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the stance phase makes up the remaining 30%35 (Fig. 7). Thisis in contrast to the 40% to 60% swing–stance ratio notedduring walking.

One of the other significant differences between thewalking and running cycle is the decrease in total cycle time.An average walker takes 120 steps per minute with a gaitcycle time of 1 second.34 A jogger traveling at a pace of 6mph has a cycle time of 0.7 seconds, whereas a runner mov-ing at a pace of 12 mph has a gait cycle lasting only 0.6seconds. Therefore, the duration of the run cycle is 60%, andthe jog cycle is 70% of the duration of the walk cycle.36 Thisreduction in total weightbearing time during running impliesa corresponding reduction in the time allocated for shockabsorption, deceleration, foot stabilization, and acceleration.

Several subtle changes occur in the running gait cyclewhen the velocity of running increases. First, there is anincrease in total hip range of motion.37 Second, the center ofgravity of the leg approaches the hip because of the increaseddegree of knee flexion. Therefore, less torque is needed tobring the leg forward during swing despite the higher angularvelocity and acceleration.37 Third, the rectus femoris demon-strates more activity as a hip flexor in the swing phase than a

knee extensor in the stance phase when the velocity of gaitincreases.37

During running, the forward propulsion of the bodyoccurs from the swinging leg and arm motion rather thanfrom the stance limb, as verified by electromyographic stud-ies.38,39 The concentric contractions that propel the body for-ward with the greatest amplitude are the hip flexors (iliopsoasand rectus femoris) and knee extensors (vastus intermedius,vastus medialis, and vastus lateralis) during late swing.38

Many of the biomechanical events described duringrunning occur simultaneously to produce a coordinated se-quence of movements in both the upper and lower extremi-ties. The body’s center of gravity reaches peak height duringthe float phase. There is also a slight forward lean throughoutthe running cycle, primarily because of increased hip flexion.The differences in flexion–extension and abduction–adduction of the hip during walking, jogging, and running areillustrated in Figure 8.

Before initial contact, there is a reversal of hip flex-ion, rapid knee extension, and dorsiflexion of the ankle.These events prepare the body for impact at the terminal floatphase. Once impact occurs, there is a ground reaction force of

FIG. 7. The running gait cycle. Adaptedfrom Montgomery.38

TABLE 2. Eight phases of the gait cycle

Phase of Gait Hip Position Active Muscle(s)Occurrence

During Cycle (%)

StanceInitial contact 30° of flexion Hamstrings and gluteus maximus 0–2Loading response 30° of flexion Hamstrings and gluteus maximus 0–10

5° to 10° of adduction5° to 10° internal rotation

Midstance 0° of flexion–extensionneutral abduction–adduction

Gluteus medius, gluteus minimus, and tensor fascia lata 10–30

Terminal stance 10° of extension Iliacus 30–50Preswing 0° of flexion–extension Iliacus and adductor longus 50–60

30° of external rotationSwing

Initial swing 20° of flexion5° of abduction

Iliopsoas, rectus femoris, gracilis and sartorius 60–73

Midswing 20° to 30° of flexion Iliopsoas, gracilis, and sartorius 73–87Terminal swing 30° of flexion Hamstrings and gluteus maximus 87–100

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approximately 150% to 200% body weight, a forward shearforce of 50% body weight, and a medial shear force of 10%body weight.34 These forces are distributed throughout thejoints of the lower extremity.

All of the muscles of the lower extremity increase ac-tivity as the pace of running increases.38,39 There is also asignificant increase in the percentage of the gait cycle dem-onstrating muscle activity as the running pace increases. Thisphenomenon is necessary for the maintenance of joint stabil-ity throughout the lower extremity.39 The degree of eccentricmuscle contraction about the hip also increases as the pace ofrunning increases.

Muscle activity about the hip during running has beenindividually quantified as in the walking cycle (Fig. 9). Thehip abductors possess the same period of activity at all gaitspeeds. They become active in late swing and remain activefor 50% of the stance phase.39 The peak power of the hipabductors is approximately 1 watt/kg.40 The hip abductorsfunction to stabilize the stance leg hemipelvis at the time ofinitial contact to prevent excessive sagging of the swing leghemipelvis. The hip adductors are active during the last thirdof the stance phase in walking but are active throughout theentire stance and swing phases of jogging and running.39

The adductor magnus, gluteus maximus, and tensor fas-cia lata are all active during the loading response of the stancephase to stabilize the hip when there is forward momen-tum38,39 (Fig. 9). Thus, as the lower extremity accepts the

body’s weight, the muscles about the hip and knee contract tostabilize these joints. At heel strike, there is a rapid extensionof the hip, flexion of the knee, and dorsiflexion of the ankle.Internal rotation of the entire limb produces calcaneal ever-sion and a subsequent unlocking of the transverse tarsaljoints. This results in a flexible foot that helps absorb energyat impact.

Adduction of the hip also occurs at the time of impact.Once the swinging leg has passed the stance leg and thecenter of gravity is in front of the stance leg, the pelvis beginsto externally rotate, which is initiated by the swinging leg.39

This external pelvic rotation causes progressive inversion ofthe calcaneus and subsequent stabilization of the transversetarsal joint and long arch of the foot. This hindfoot stabiliza-tion lasts for half of the stance phase before the onset of knee

FIG. 8. Comparative differences of hip motion during walking, jog-ging, and running. A: Sagittal plane motion (flexion and extension).B: Coronal plane motion (abduction and adduction). Adapted fromMann.34

FIG. 9. EMG data of selected muscles acting on the hip duringrunning. MMT � manual muscle testing. Adapted from Montgom-ery.38

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extension and ankle plantar flexion, which mark the begin-ning of push-off.39

The long head of the biceps acts to initiate hip exten-sion in the stance phase of running as the center of gravitymoves in front of the knee (Fig. 9). There is also progressiveabduction of the hip as the joint extends. Progressive hipextension during the stance phase is accomplished throughthe synergistic function of both the hamstrings and gluteusmaximus muscles.39 The rectus femoris, iliopsoas, tensor fas-cia lata, and adductor magnus are all active to control hipextension and prepare the hip for flexion. The action of thesemuscles during the loading phase also helps with hip jointstability in both the sagittal and coronal planes.38

The hamstrings have longer periods of activity in theswing and stance phases of jogging and running, comparedwith walking. The hamstrings are active for the last 50% ofthe swing phase in jogging and for the last 25% of the swingphase in running.39 They are active during the initial 50% ofthe stance phase of both jogging and running, and they func-tion synergistically with the gluteus maximus to bring aboutrapid hip joint extension during running.39 The short head ofthe biceps acts primarily to control the knee and has little orno function at the hip.

During the swing phase of the running cycle, there areconcurrent concentric and eccentric contractions of the vari-ous muscles controlling the hip.38 In midswing, the iliopsoasand rectus femoris exhibit peak concentric activity as hipflexors (Fig. 9). The tensor fascia lata and adductor magnusalso assist in hip flexion and function to stabilize the pelvis.The semimembranosus and long head of the biceps contracteccentrically to control hip flexion. The gluteus maximusassists the hamstrings in this function, but only during a rapidrunning pace.38 In late swing, the semimembranosus, longhead of the biceps, and gluteus maximus are all eccentricallyactive in initiating hip extension in order to limit hip flexion(with assistance from the adductor magnus, which can extendthe hip from a flexed position). The semimembranosus andlong head of the biceps act primarily on the hip and do notassist the short head of the biceps in producing knee flex-ion.34,38,39 The rectus femoris eccentrically controls kneeflexion while concentrically flexing the hip during the swingphase.

At the beginning of the float phase, there is rapid hipflexion by the iliopsoas and rectus femoris and correspondingpassive knee flexion and ankle dorsiflexion.39 In mid-float,hip flexion is at its peak and active knee extension begins.Rapid adduction of the hip occurs during the last half ofswing. At terminal swing, there is a rapid reversal of hipflexion, the initiation of hip extension, and knee extension.39

During the last 25% of swing, the hip extensors, hamstrings,quadriceps, and ankle plantar flexors are all active to preparefor initial ground contact.39

Hip Forces During ActivityWhen standing with both feet on the ground, each hip

experiences a load of approximately one-third to one-half ofbody weight.41 This force increases to approximately 2.5

times to 4 times body weight when standing on one leg. Withsingle-leg stance, counterbalancing forces maintain the pelvislevel. The force of contraction of the hip abductors (gluteusmedius and gluteus minimus) balances the forces acting onthe hip from the contraction of other muscles and from gravi-tational forces. This pull of the abductors must be more thanthat of body weight because the distance from the hip’s centerto the body’s center of mass is about 2.5 times longer thanthat of the lever arm of the abductors (Fig. 10). However,Inman28 noted that it is not only the abductors that helpstabilize the pelvis in single-leg stance, but the forces of thetensor fascia lata and iliotibial band, as well.

During the swing phase of gait, the hip experiencesvarious forces resulting from limb acceleration and decelera-tion and from the effect of muscle contractions (eccentric andconcentric) and ligamentous constraints. Ground reactionforces that are present during stance are not a factor duringthe swing phase. The forces acting on the hip are also largerthan expected in the swing phase. This is because the body

FIG. 10. Forces exerted on the hip joint during single-leg stance inorder to maintain pelvic equilibrium. The force of contraction of thehip abductors (M) balances the force exerted by the remaining bodymass (K). S5 � center of gravity of the mass of the remaining partof the body acting on the hip; h � lever arm of force M; h� � leverarm of force K; R � resultant of forces K and M. Adapted fromPauwels.31

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rolls over the femoral head during this phase so the abductorsdo not have to work as hard as in single-leg stance. Also, asthe extremity increases in mass, greater force is needed togenerate the angular velocity required for forward propul-sion.19 Therefore, a greater degree of muscle pull is required,which generates greater compressive force on the femoralhead. Clinically, patients with osteoarthritis develop a com-pensatory gait pattern to lessen this compressive effect. Byshifting the ipsilateral trunk and pelvic center of mass overthe hip, there is a subsequent decrease in the abductor mo-ment arm, which reduces the degree of force required fromthe abductors to maintain the pelvis level. This results in anoverall reduction in the joint reactive force on the hip joint.Correspondingly, a loss of 1 lb. of body weight can lessen theforce acting on the hip by approximately 3 lb.42

The contact pressure in the hip during walking gait isapproximately 5 Mpa, which is roughly 25 times the amountof pressure in a car tire.43 With loaded walking (carrying asmuch as 20 kg), hip joint loads can reach 8 times bodyweight.43 Considering a contact area of the hip of approxi-mately 2 cm,2 the compressive force can reach a maximum of65 Mpa during joint loading.42,43 In the dynamic setting, thehip experiences forces from a number of sources, includingthe ground reaction force, gravity, body acceleration, andmuscle contraction. Running can produce loads between 4.5times to 6 times body weight, even though the abductorswork less than during walking.44,45 This is because of theincrease in the ground reaction force associated with running.Forces up to 7.5 times body weight act on the hip during fastwalking and ascending or descending stairs.46 It is thoughtthat the synovial fluid in the healthy hip helps withstand 90%of this load so that the articular cartilage matrix does not haveto be subjected to these extremely large compressive forces.6

These loads on the hip change in direct proportion to thechange in speed and effort exerted by the subject duringactivity.

How the hip joint, and particularly the proximal femur,responds or adapts to these loads has been an area of inves-tigation for many years. It has been proposed that the lateral(or superior) femoral neck is loaded in tension, and the me-dial (or inferior) aspect is loaded in compression. Wolff47,48

championed the idea that the proximal femur experiences anaverage, or habitual, loading condition during normal weight-bearing activities. This was conceived as a bending momentthat occurs during single-leg stance whereby the superior as-pect of the femoral neck receives tensile stress while theinferior aspect undergoes compressive stress.49 This conceptis strongly rooted in the seemingly obvious conclusion thatthe trabecular patterns observed in thin coronal sections onAP roentgenograms are adaptations along the lines of stressthat are typically transmitted across the hip.48–51 This con-ception was important in the original formulation of Wolff’sLaw of mechanically-mediated bone adaptation.

These and other studies that have described the habitualpresence of a bending moment across the hip may not beaccurate. This is because they are not based on direct in vivoforce measurements, but on calculated, or assumed, magni-

tudes and directions of muscle or gravitational and inertialforces.31,42,46,52,53 Under this theory, the lateral trabeculae ofthe femoral neck would be expected to be at a right angle tothe medial trabeculae if one was loaded in tension and theother in compression. However, these trabecular lines are at a60° angle to one another in the coronal plane.54 The lateraltrabeculae are also directly in line with the abductors, tensorfascia lata, and iliopsoas muscles, which would imply that themedial and lateral trabecular systems are actually both undera degree of compression. This concept is supported by acadaveric study by Skedros et al.,55 who showed that thecollagen fibers of the cortical bone in the lateral and medialaspect of the femoral neck are both obliquely oriented, thusimplying a habitual compressive loading pattern. A hypoth-esis to explain this finding is that the musculature around thehip (particularly the abductors) acts to compress the femoralhead into the acetabulum, creating a net compressive force onthe superior and inferior aspect of the femoral neck. If thesuperior aspect of the femoral neck were only under tensionthen there would be no need for bone to form, because theuncalcified cartilage has the same tensile strength as bonewith less weight per unit volume.55 According to Wolff’sLaw, bone will form where stresses are applied to it. In thisway, the bone strives toward an optimized structure for bothmobility and strength.

Implications for Athletic Activity

There is a paucity of literature pertaining to the biome-chanics of the hip during sports other than running. One suchstudy56 calculated the loads on the hip joint in various skiingactivities by combining video data and a simple musclemodel with an accepted mathematical method. For alpineskiing, the calculated loads on the hip joint were 4.1 times to7.8 times body weight.56 The loads were less for long turnson flatter hills, and greater for short turns on steeper inclines.For cross-country skiing (without ski pole) the calculatedloads on the hip were 4.0 times to 4.6 times body weight.Mogul skiing was associated with loads on the hip that weremuch higher, ranging from 8.3 times to 12.4 times bodyweight.56

Landing is an activity common to many athletic activi-ties. Devita and Skelly57 investigated the effects of landingstiffness on the lower extremity. A soft landing occurs withthe knee flexed greater than 90°, and a stiff landing occurswith the knee flexed less than 90°. These authors found thatthe hip and knee sustain higher ground reaction forces andabsorb more energy with soft landings, whereas the ankleexperiences higher ground reaction forces and absorbs moreenergy with stiff landings. They attributed these findings tothe greater degree of hip and knee flexion associated with asoft landing technique that results in a greater degree of en-ergy absorption by these two joints. The highest muscle mo-ment and power calculations were found in the hip joint inboth landing techniques. The hip extensor moment workseccentrically to reduce hip flexion velocity, especially in stifflandings. The greater extensor moment in the stiff landing

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causes a greater reduction in hip flexion velocity and pro-duces a more erect body configuration at impact. A primaryfactor in successfully performing the stiff landing is to initiatethe impact phase with a more erect body posture. This re-duces the moment arms of the external forces, which accel-erates joint flexion, especially at the hip and knee. The flexormoment of the hip helps rotate the trunk and thigh forward(hip flexion) from a vertically aligned position in preparationfor landing in order to reduce the impact stress on the spine.DeVita and Skelly57 have shown that the reduced momentarms partially counteract the increase in external forces suchthat in their study, a stiff landing was performed with only a13% increase in loads over all joints even though the groundreaction force increased 23%. This data identifies the criticalrole of the extensor moment at the hip in late descent inmodifying landing stiffness and the magnitude of the impactforces by adjusting the body posture at impact.

With an increase in height of landing comes an increasein the work done by the knee and hip extensors comparedwith the ankle plantar flexors.57,58 This finding suggests thatthe muscles supporting the ankle are less capable of energyabsorption than the muscles supporting the hip and knee. Asthe height of the landing increases, lower extremity posturebecomes more flexed, and the hip extensors become increas-ingly involved in energy dissipation.59 There is also a shift inenergy absorption from distal to proximal muscle groups asthe mechanical demands on the lower extremity increase, asin an increase in landing height.59

The biomechanics governing the forces involved withlanding may be sport-specific. McNitt-Gray60 found thatgymnasts had lower average maximum ground reactionforces when landing from 3 different heights compared withrecreational athletes. The gymnasts also reached a peakground reaction force value 6.3 msec faster in the landingphase than recreational athletes at all landing heights. It ap-pears that gymnasts are less sensitive to increases in landingheights than recreational athletes because of their ability toreduce maximum ground reaction force values during land-ing. This ability to reduce landing stresses may result in gym-nasts being less susceptible to injury because of their apparentability to attenuate impact forces.60,61

Clinical Implications for AthletesDecreased hip motion or muscular weakness about the

hip could potentially affect athletic performance and put theathlete at risk for injury. Fixed flexion contractures of the hiphave been implicated as a cause of low back pain because ofthe increased lordotic strains placed on the lumbar spine.62,63

Professional ice hockey players have been shown to lackapproximately 10° of hip extension.64 Yet this has not beenshown to predispose these athletes to injury. On the contrary,the relatively poor flexibility noted in some long-distancerunners appears to be beneficial in that they exhibit a moreeconomical gait pattern compared to more flexible runners.This phenomenon is thought to result from the reduced mus-cular demands needed to stabilize a relatively stiff hip.65,66

Muscular imbalance about the hip has been shown to

place ice hockey players at risk for adductor strains. Tyler etal.67 analyzed hip abduction and adduction strength in 81professional hockey players before two consecutive seasons.Adduction strength was found to be 18% lower in playerswho subsequently sustained an adductor strain compared withadduction strength of the uninjured players. Moreover, aplayer was 17 times more likely to sustain an adductor strainif his adductor strength was less than 80% of his abductorstrength.67 It remains to be determined whether preseasonstrength training is effective in reducing the incidence ofadductor strains in athletes identified at risk.

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