Biomechanics of hip complex 2

DR. DIBYENDUNARAYAN BID [PT]
T H E S A R V A 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 A T
Biomechanics
of the
Hip Complex: 2

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 Pelvis
 Lateral 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.

Biomechanics of hip complex 2

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Biomechanics of hip complex 2