[
clinical commentary
]
dONaLd a. NEUmaNN, PT, PhD, FAPTA
Kinesiology o the Hip: A Focus Focus on Muscular Actions
T
he hip joint serves as a central pivot point or the body as a whole. This large ball-and-socket joint allows simultaneous, triplanar movements o the emur relative to the pelvis, as well as the trunk and pelvis relative to the emur. Liting the oot o the ground, reaching towards the oor, or rapidly rotating the trunk and pelvis while supporting the body over one limb typically demands strong and specifc activation o the hips’ surrounding musculature. Pathology that aects the strength, control, or extensibility o the hip muscles can signicantly disrupt the uidity, comort, and metabolic efciency o many routine movements involving both unctional and recreational activities. Furthermore, abnormal perormance o the muscles o the hip may alter the distribution o orces across the joint’s articular suraces, potentially causing, or at least predisposing, degenerative changes in the articular cartilage, bone, and surrounding connective tissues. Physical therapy diagnosis related to t
SYNOPSIS: The 21 muscles that cross the hip
provide both triplanar movement and stability between the emur and acetabulum. The primary intent o this clinical commentary is to review and discuss the current understanding o the specic actions o the hip muscles. Analysis o their actions is based primarily on the spatial orientation o the muscles relative to the axes o rotation at the hip. The discussion o muscle actions is organized organize d according to the 3 cardinal planes o motion. Actions are considered rom both emoralon-pelvic and pelvic-on-emor pelvic-on-emoral al perspectives, with particular attention to the role o coactivation o trunk muscles. Additional attention attention is paid to the biomechanical variables that alter the efective-
the hip and adjacent regions oten requires a solid understanding o the actions o the surrounding muscles. This knowledge is instrumental in identiying when a specic muscle muscle or muscle group is weak, painul, dominant, or tight (ie, lacks the extensibility to permit normal range o motion). Depending on the particular muscle, any one o these conditions can signicantly aect the alignment across the lumbar spine, pelvis, and emur, ultimately aecting the alignment throughout the entire lower limb. Furthermore, understanding the actions o the hip ness, orce, and torque o a given muscle action. The role o certain muscles in generating compression orce at the hip is also presented. Throughout the commentary, the kinesiology o the muscles o the hip are considered primarily rom normal but also pathological perspectives, supplemented with several clinically relevant scenarios. This overview should serve as a oundation or understanding the assessment and treatment o musculoskeletal impairments that involve not only the hip, but also the adjacent low back and knee regions. J Orthop Sports Phys Ther 2010;40(2):82-94. doi:10.2519/ jospt.2010.3025 jospt.2010 .3025 t KEY WORdS: adductor magnus, biomechanics, gluteus maximus, maximus, gluteus medius, medius, hip
muscles is undamental to interventions used to specically activate, strengthen, or stretch certain muscles. The primary purpose o this paper is to review and analyze the actions o the muscles o the hip. The discussion will include several topics associated with muscular kinesiology, including a muscle’s torque (strength) potential, moment arm (ie, leverage), cross-sectional area, overall ber direction, and line o orce relative to an axis o rotation. When available, data rom the research literature will be cited. As will be pointed out, some actions o muscles are strongly supported by rigorous research, while others are not. Line of Force The discussion o muscle action will be organized according to the 3 cardinal planes o motion o the hip: sagittal, horizontal, and rontal. For each plane o motion, a muscle’s action is based primarily on the orientation o its line o orce relative to the joint’s axis o rotation. FIGURE 1 illustrates this orientation or several muscles acting within the sagittal plane. This gure, based on a straight-line model o muscle action, stems rom the work o Dostal and others. 16,17 Using a male cadaver, the proximal and distal attachments o the muscles were careully dissected and then digitized. A straight line between the attachment points was used to represent the muscle’s line o orce. Observe in FIGURE 1, or instance, that a muscle’s line o orce that passes anterior to the joint’s medial-lateral axis o rota-
1
Proessor, Physical Therapy Department, Marquette University, Milwaukee, WI. Address correspondence to Dr Donald A. Neumann, Marquette University, Physical Therapy Department, Walter Schroeder Complex, Rm 346, PO Box 1881, Milwaukee, WI 53201-1881. E-mail:
[email protected] [email protected]
82 | february 2010 | volume 40 | number 2 | journal of orthopaedic & sports physical therapy
Sagittal Plane (From the Side)
10.0
5.0
) m c ( r o i r e f n I r o i r e p u S
G l u te u s m e d i u s (p o s t .)
G l u t e u s m a x i m u s
) t. n ( a s u i m i n m s u t e l u G
e a t la e ia c s a f
r o s n e T
s u i r o t r a S
Rectus emoris
s a o s p o i l I
0.0
s u e n i t c e P
–5.0
–10.0
B i c e p s f e m o r i s a n d s e m i t e n d i n o s u s
s i v e r b r o t c u d d A
S e m im e m b ar n o s u s
s u g n l o r t o c u d d A
Adductor magnus (post.)
s i r o m e f s u t c e R
that the rectus emoris has a 4.3-cm moment arm or fexion, along with a 0.2-cm moment arm or external rotation, and a 2.3-cm moment arm or abduction. The work by Dostal et al 16,17 is highlighted throughout this paper because it applies to all hip muscles across all 3 planes o motion. No other single source o such extensive data could be located. Extrapolating Dostal et al’s 16,17 work to the general population requires caution, however, because the data represent only 1 (male) cadaver specimen and are based on a relatively simple straight-line model. Nevertheless, the data do provide valuable insight into a critical variable that determines a muscle’s action. Additional published data o this type is needed to more adequately refect the complex shape o many muscles and the anthropometric dierences based on gender, age, body size, and natural variability. Based on inormation published in the literature and cadaver and skeletal inspection, the muscles o the hip will be designated as being primary or secondary or a given action ( TaBLE 2). Some muscles have only a marginal potential to produce a particular action, due to actors such as negligible moment arm length or small cross-sectional area. Muscles that likely have an insignicant action will not be considered in the discussion.
muscle action Versus muscle Torque 5.0
0.0
–5.0
Posterior-Anterior (cm)
FIGURE 1. A lateral view shows the sagittal plane line o orce o several hip muscles. The axis o rotation (green circle) is directed in the medial-l ateral direction through the emoral head. The fexors are indicated by soli d arrows and the extensors by dashed arrows. The internal moment arm used by the rectus emoris is shown as a thick black line, orig inating at the axis o rotation. (For clarity, not all muscles are shown.) The lines o orce are not drawn to scale and, thereore, do not indicate a muscle’s relative orce potential. Reproduced with permission rom Neumann DA, Kinesiology o the Musculoskeletal System: Foundations or Rehabilitation, 2nd ed, Elsevier, 2010.
tion would be characterized as a fexor (such as the highlighted rectus emoris); conversely, a muscle’s line o orce passing posterior to the same axis would be characterized as an extensor. This visual perspective not only strongly suggests a
muscle’s action but, equally important, indicates the relative moment arm length (leverage) available to generate the torque or the particular action. The original data used to generate FIGURE 1 is listed in TaBLE 1.17 This table shows, or example,
Although the visual representation o FIGURE 1 is useul or assessing a muscle’s potential action within a given plane, 2 limitations must be recognized. First, the gure lacks inormation to indisputably rank the muscle’s relative torque potential within a given plane. A m uscle torque and a muscle action are indeed dierent. While a muscle action describes the potential direction o rotation o the joint ollowing its activation by the nervous system, a muscle torque describes the “strength” o the action. A muscle torque can be estimated by the product o the muscle orce (in Newtons) within a plane o interest and the muscle’s associated moment arm length (in centimeters). Both
journal of orthopaedic & sports physical therapy | volume 40 | number 2 | february 2010 | 83
[ TaBLE 1
clinical commentary
List o Moment Arm Data (cm) or the Muscles o the Hip, Categorized by Their Potential Action in the Sagittal, Horizontal, and Frontal Planes 17 *
muscle
Sgittl Plne
Horizontl Plne
Frontl Plne
Adductor brevis
F: 2.1
IR: 0.5
Ad: 7.6
Adductor longus
F: 4.1
IR: 0.7
Ad: 7.1
Adductor magnus (anterior head)
E: 1.5
ER: 0.2
Ad: 6.9
Adductor magnus (posterior head)
E: 5.8
IR: 0.4
Ad: 3.4
Biceps emoris
E: 5.4
ER: 0.6
Ad: 1.9
Gemellus inerior
E: 0.4
ER: 3.3
Ad: 0.9
Gemellus superior
E: 0.3
ER: 3.1
Ab: 0.1
Gluteus maximus
E: 4.6
ER: 2.1
Ad: 0.7
Gluteus medius (anterior fbers)
E: 0.8
IR: 2.3
Ab: 6.7
Gluteus medius (middle fbers)
E: 1.4
IR: 0.1
Ab: 6.0
Gluteus medius (posterior fbers)
E: 1.9
ER: 2.4
Ab: 4.3
Gluteus minimus (anterior fbers)
F: 1.0
IR: 1.7
Ab: 5.8
Gluteus minimus (middle fbers)
F: 0.2
ER: 0.3
Ab: 5.3
Gluteus minimus (posterior fbers)
E: 0.3
ER: 1.4
Ab: 3.9
Gracilis
F: 1.3
ER: 0.3
Ad: 7.1
Iliopsoas
F: 1.8
IR: 0.5
Ab: 0.7
Obturator externus
F: 0.7
ER: 0.4
Ad: 2.4
Obturator internus
E: 0.3
ER: 3.2
Ad: 0.7
Pectineus
F: 3.6
IR: 1.0
Ad: 3.2
Piriormis
E: 0.1
ER: 3.1
Ab: 2.1
Quadratus emoris
E: 0.2
ER: 3.4
Ad: 4.4
Rectus emoris
F: 4.3
ER: 0.2
Ab: 2.3
Sartorius
F: 4.0
ER: 0.3
Ab: 3.7
Semimembranosus
E: 4.6
IR: 0.3
Ad: 0.4
Semitendinosus
E: 5.6
IR: 0.5
Ad: 0.9
Tensor ascia latae
F: 3.9
0.0
Ab: 5.2
Abbreviations: Ab, abduction; Ad, adduction; E, extension; ER, external rotation; F, fexion; IR, internal rotation. * Muscles are presented in alphabetical order. Data are based on the male cadaver specimen being oriented in the anatomic position.
variables o orce and moment arm are equally important when estimating the potential torque output, or strength, o a muscle. Although FIGURE 1 is constructed to appreciate a muscle’s likely action and relative moment arm length, it does not indicate the muscle’s orce potential. The arrows used in the gure are not vectors and are not drawn to scale. The orientation o the arrows represents only the assumed linear direction o the orce, not its amplitude. Estimating a muscle’s orce requires other inormation, such as its cross-sectional area. The second limitation o FIGURE 1 is
that the lines o orce o the muscles and the lengths o the moment arms apply only to the anatomic position. Once moved out o this position, the variables that aect a muscle’s action and torque potential change. 8 These changes partially explain why maximal-eort torque and, in some cases, even a muscle’s action vary across the ull range o hip motion. Unless otherwise specied, the actions o the muscles o the hip discussed in this paper are based upon a contraction that has occurred rom the anatomic position. Provided the aorementioned limita-
]
tions described or FIGURE 1 are respected, the associated method o visual analysis can provide a very useul and logical mental construct or considering a muscle’s potential action, as well as peak strength, assuming maximal orce production.
SaGITTaL PLaNE Hip Flexors
F
IGURE 1 depicts muscles that fex
the hip and TaBLE 2 lists the actions o these and other muscles as either primary or secondary. One o the more prominent hip exor muscles is the iliopsoas. This thick muscle produces a orce across the hip, sacroiliac joint, lum bosacral junction, and lumbar spine.18,41,52 Because the muscle spans both the axial and appendicular components o the skeleton, it is a hip exor as well as a trunk exor. In addition, the psoas major aords an important element o vertical stability to the lumbar spine, especially when the hip is in ull extension and passive tension is greatest in the muscle. 52 The conjoined distal tendon o the iliacus and the psoas major crosses anterior and slightly medial to the emoral head, as it courses toward its insertion on the lesser trochanter. During this distal path, the broad tendon is deected posteriorly about 35° to 45° as it crosses the superi or pubic ramus o the pubis. With the hip in ull extension, this deection raises the tendon’s angle-o-insertion relative to the emoral head, thereby increasing the muscle’s leverage or hip exion. As the hip exes to 90°, the exion leverage becomes even greater.8 Such a parallel increase in leverage with increased exion may partially oset the muscle’s potential loss in active orce (and ultimately torque) caused by its reduced length. Theoretically, a sufciently strong and isolated bilateral contraction o any hip exor muscle will either rotate the emur toward the pelvis, the pelvis (and possi bly the trunk) towards the emur, or both actions simultaneously. These kinematics occur within the sagittal plane about a medial-lateral axis o rotation through
84 | ebruary 2010 | volume 40 | number 2 | journal o orthopaedic & sports physical therapy
contributing muscle groups.14 Rapid exTaBLE 2 ion o the hip is generally associated with abdominal muscle activation that slightly precedes the activation o the hip exor muscles Priry Secondry muscles.22 This anticipatory activation Flexors • Iliopsoas • Adductor brevis has been shown to be most dramatic and • Sartorius • Gracilis • Tensor fasciae latae • Gluteus minimus (anterior bers) consistent in the transverse abdominis, at • Rectus femoris least in healthy subjects without low back • Adductor longus • Pectineus pain.40 The consistently early activation o the transverse abdominis may reect Extensors • Gluteus maximus • Gluteus medius (middle and posterior bers) • Adductor magnus (posterior head) • Adductor magnus (anterior head) a eedorward mechanism intended to • Biceps femoris (long head) stabilize the lumbopelvic region by in• Semitendinosus • Semimembranosus creasing intra-abdominal pressure and increasing the tension in the thoracolumExternal rotators • Gluteus maximus • Gluteus medius (posterior bers) • Piriformis • Gluteus minimus (posterior bers) bar ascia.21,46 • Obturator internus • Obturator externus Without sufcient stabilization o the • Gemellus superior • Sartorius • Gemellus inferior • Biceps femoris (long head) pelvis by the abdominal muscles, a strong • Quadratus femoris contraction o the hip exor muscles may Internal rotators Not applicable • Gluteus minimus (anterior bers) inadvertently tilt the pelvis anteriorly • Gluteus medius (anterior bers) (FIGURE 2B). An excessive anterior tilt o • Tensor fasciae latae • Adductor longus the pelvis typically accentuates the lum• Adductor brevis bar lordosis. This posture may contribute • Pectineus to low back pain in some individuals. • Adductor magnus (posterior head) Although FIGURE 2B highlights the Adductors • Pectineus • Biceps femoris (long head) • Adductor longus • Gluteus maximus (posterior bers) unopposed contraction o 3 o the more • Gracilis • Quadratus femoris recognizable hip exor muscles, the same • Adductor brevis • Obturator externus principle can be applied to all hip exor • Adductor magnus (anterior and posterior heads) muscles. Any muscle that is capable o Abductors • Gluteus medius (all bers) • Piriformis • Gluteus minimus (all bers) • Sartorius exing the hip rom a emoral-on-pelvic • Tensor fasciae latae • Rectus femoris perspective has a potential to ex the hip * Each action assumes a muscle is ully activated rom the anatomic position. Several o these muscles rom a pelvic-on-emoral rotation. For may have a diferent action when they are activated outside o this reerence position. this reason, tightness o secondary hip exors, such as adductor brevis, gracilis, the emoral heads. Note that the arrow- undesired and excessive anterior tilting o and anterior bers o the gluteus minihead representing the line o orce o the the pelvis. Normally, moderate to high hip mus, would, in theory, contribute to an rectus emoris in FIGURE 1, or example, is exion eort is associated with relatively excessive anterior pelvic tilt and exaggerdirected upward, toward the pelvis. This strong activation o the abdominal mus- ated lumbar lordosis. convention is used throughout this paper cles.22 This intermuscular cooperation is and assumes that at the instant o muscle very apparent while lying supine and per- Hip Extensors contraction, the pelvis is more physically orming a straight leg raise movement. The primary hip extensors include the stabilized than the emur. I the pelvis is The abdominal muscles must generate gluteus maximus, posterior head o the inadequately stabilized by other muscles, a potent posterior pelvic tilt o sufcient adductor magnus, and the hamstrings a sufciently strong orce rom the rectus orce to neutralize the strong anterior (TaBLE 2).13,17 In the anatomic position, the emoris (or any other hip exor muscle) pelvic tilt potential o the hip exor mus- posterior head o the adductor magnus could rotate or tilt the pelvis anteriorly. cles. This synergistic activation o the ab- has the greatest moment arm or extenIn this case, the arrowhead o the rectus dominal muscles is demonstrated by the sion, ollowed closely by the semitendinoemoris would logically be pointed downrectus abdominis ( FIGURE 2a). The extent sus.17 The moment arm or both o these ward toward the relatively xed emur. to which the abdominal muscles actually extensor muscles increases as the hip is The discussion above helps to explain neutralize and prevent an anterior pelvic exed to 60°. 39 According to Winter,50 the why a person with weakened abdominal tilt is dependent on the demands o the gluteus maximus and adductor magnus muscles may demonstrate, while actively activity—or example, o liting 1 or both have the greatest cross-sectional areas o contracting the hip exors muscles, an limbs—and the relative strength o the all the primary extensors. The middle and
Muscles of the Hip, Organized According to Primary or Secondary Actions*
journal of orthopaedic & sports physical therapy | volume 40 | number 2 | february 2010 | 85
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clinical commentary
]
muscles, while simultaneously perorming a traditional passive-stretching maneuver o the hip fexor muscles, may provide an additional stretch to these Normal activation o abdominal muscles muscles. One underlying advantage o Rectus emoris this therapeutic approach is that it may actively engage and potentially educate ominis Rec tus abd Flexion the patient about controlling the biome as u s o s c P chanics o this region o the body. a Achieving near ull extension o the hips has important unctional advantages, such as increasing the metabolic eB ciency o relaxed stance and walking. 11 Full or nearly ull hip extension allows a Reduced activation o abdominal muscles person’s line o gravity to pass just posteAnterior tilt rior to the medial-lateral axis o rotation i s R ec tus abdom in emo r f i s us t Rec through the emoral heads. Gravity, in P s oas c u s a i l this case, can assist with maintaining the I Flexion effort extended hip while standing, with little activation rom the hip extensor muscles. Because the hip’s capsular ligaments naturally become “wound up” and relatively taut in ull extension, an additional eleFIGURE 2. The synergistic action o one representative abdominal muscle (rectus abdominis) is ill ustrated while liting the right lower limb. (A) With normal activation o the abdominal muscles, the pelvis is stabilized and ment o passive extension torque, albeit prevented rom anterior tilting by the downward pull o the hip fexor muscles. (B) With reduced activation o the relatively small, may urther assist with abdominal muscles, contraction o the hip fexor muscles is shown producing a marked anterior tilt o the pelvis the ease o standing. This biomechanical (increasing the lumbar lordosis). The reduced activation in the abdominal muscle is indicated by the lighter red situation may be benecial by tempocolor. Reproduced with permission rom Neumann DA, Kinesiology o the Musculoskeletal System: Foundations or rarily reducing the metabolic demands Rehabilitation, 2nd ed, Elsevier, 2010. on the muscles but also by reducing the posterior bers o the gluteus medius and vis (FIGURE 4). A posterior tilting motion o joint reaction orces across the hips due anterior head o the adductor magnus are the pelvis is actually a short-arc, bilateral to muscle activation, at least or short considered secondary extensors. 16 (pelvic-on-emoral) hip extension move- periods. The hip extensor muscles, as a group, ment. Both right and let acetabula rotate produce the greatest torque across the in the sagittal plane, relative to the xed HORIZONTaL PLaNE hip than any other muscle group ( FIGURE emoral heads, about a medial-lateral axis o rotation. Assuming the trunk remains Hip Externl Rottors 3).10 The extensor torque is oten used to rapidly accelerate the body upward and upright during this action, the lumbar IGURE 5 shows a superior view of orward rom a position o hip fexion, spine must fex slightly, reducing its natuthe lines o orce o several external such as when pushing o into a sprint, ral lordotic posture. and internal rotators o the hip. The arising rom a deep squat, or climbing While standing, the perormance o a external rotator muscles (depicted as sola very steep hill. The position o fexion ull posterior pelvic tilt, theoretically, in- id arrows) pass generally posterior-lateral naturally augments the torque potential creases the tension in the hip’s capsular to the joint’s longitudinal (or vertical) o the hip extensor muscles. 5,23,34 Further- ligaments and hip fexor muscles. These axis o rotation. Because the vertical axis more, with the hip markedly fexed, many tissues, i tight, can potentially limit the o rotation remains roughly aligned with o the adductor muscles produce an ex- end range o an active posterior pelvic the emur, it is only truly vertical near the tension torque, thereby assisting the pri- tilt. Contraction o the abdominal mus- anatomic position. The muscles considmary hip extensors. 23 cles (acting as short-arc hip extensors, as ered as primary external rotators include With the trunk held relatively station- depicted in FIGURE 4) can, theoretically, the gluteus maximus and 5 o the 6 short ary, contraction o the hip extensors and assist other hip extensor muscles in elon- external rotators ( TaBLE 2). From the anaabdominal muscles (with the exception o gating (stretching) a tight hip capsule or tomic position, the secondary external rothe transverse abdominis 22) unctions as hip fexor muscle. For example, strongly tators include the posterior bers o the a orce-couple to posteriorly tilt the pel- coactivating the abdominal and gluteal gluteus medius and minimus, obturator A
i l I
F
86 | february 2010 | volume 40 | number 2 | journal of orthopaedic & sports physical therapy
240 220 200 180 160 ) m N ( e u q r o T
140 120 100 80
FIGURE 4. The orce-couple between representative
60 40 20 0 Extensors
Adductors
Flexors
Abductors
Internal rotators
External rotators
Muscle Group Sagittal plane
Frontal plane
Horizontal plane
hip extensors (gluteus maximus and hamstrings) and abdominal muscles (rectus abdominis and obliquus externus abdominis) is shown posteriorly tilting the pelvis while standing upright. The moment arms or each muscle group are indicated by the dark black lines. The extension at the hip stretches the ilioemoral ligament (shown as a short, curved arrow just anterior to the emoral head). Reproduced with permission rom Neumann DA, Kinesiology o the Musculoskeletal System: Foundations or Rehabilitation, 2nd ed, Elsevier, 2010.
FIGURE 3. Average maximal-eort torque (Nm) produced by
the 6 major muscle g roups o the hip (standard deviations indicated by brackets). Data were measured isokinetically at 30°/s rom 35 healthy young males, and averaged over the ull range o motion.10 Data or sagittal and rontal planes torques were obtained while standing with the hip in extension. Data or horizontal plane torques were obtained while sitting, with the hip fexed 60° and the knee fexed to 90°. Reproduced with permission rom Neumann DA, Kinesiology o the Musculoskeletal System: Foundations or Rehabilitation, 2nd ed, Elsevier, 2010.
externus, sartorius, and the long head o the biceps emoris. The obturator externus is considered a secondary rotator because its line o orce lies so close to the longitudinal axis o rotation ( FIGURE 5). In general, any muscle with a line o orce that either passes through or parallels the axis o rotation cannot develop a torque. In a ew degrees o hip internal rotation, it is likely that the line o orce o the obturator externus would indeed pass through the longitudinal axis, there by negating any torque potential in the horizontal plane. The gluteus maximus is the most potent external rotator muscle o the hip. 13 This suitably named muscle is the largest muscle o the hip, accounting or about 16% o the total cross-sectional area o all hip musulature.50 Assuming that the gluteus maximus muscle’s line o orce is directed approximately 45° with respect to
hip also likely provide an important element o mechanical stability to the acetabuloemoral articulation. Interestingly, the popular posterior surgical approach to a total hip arthroplasty used by some surgeons necessarthe rontal plane, maximal-efort activa- ily cuts through at least part o the hip’s tion would theoretically generate 71% o posterior capsule, potentially disrupting its total orce within the horizontal plane several o the short external rotator ten(based on the sine or cosine o 45°). All o dons. Studies have reported a signicant this orce could theoretically be used to reduction in the incidence o posterior hip generate an external rotation torque. dislocation when the surgeon careully The short external rotator muscles are repairs the posterior capsule and external ideally designed to produce an efective rotator tendons.15,33,48 Greater success o external rotation torque. With the slight capsulotendinous reattachment has been exception o the piriormis, the remain- more recently documented, purportedly ing short external rotators possess a as a result o using techniques that result near-horizontal line o orce. This overall in less disruption o the piriormis and orce vector makes a near-perpendicular most o the quadratus emoris. 27 intersection with the joint’s longitudinal The unctional potential o the entire (vertical) axis o rotation. This being the external rotator muscle group is most ulcase, nearly all o a given muscle’s orce is ly recognized while perorming pelvic and dedicated to producing external rotation trunk rotational activities while bearing torque. This orce is also ideally aligned weight over 1 limb. With the right emur to compress the hip joint suraces. In a held relatively xed, contraction o the manner generally similar to the inraspi- external rotators would rotate the pelvis natus and teres minor at the glenohumer- and the attached trunk to the let. This al joint, the short external rotators o the action o planting the limb and cutting
journal of orthopaedic & sports physical therapy | volume 40 | number 2 | february 2010 | 87
[
clinical commentary
Horizontal Plane (From Above) 5.0 P A d d e c uc to t i n e r l o n u s g us
G l u t e u s m i n . ( a n t . )
Ad du ct o r b re vi s ) m c ( r o i r e t s o P r o i r e t n A
O b tu ra t o r e xt e r n u s
0.0
s t ). . ( p o n i . m G l u t
Gluteus medius (post.)
G l u te u s m e d . ( a n t .)
mor is Quadra tus fe
Gemellus sup. –5.0
Obturator internus
i s r m f o i r i P
u s i m a x m u s t e u l G
5.0
Gemellus inferior
0.0
–5.0
Medial-Lateral (cm) FIGURE 5. A superior view depicts the horizontal plane line of force of several muscles that cross the hip. The
longitudinal axis of rotation (blue circle) passes through the femoral head in a superior-inferior direction. The external rotators are indicated by solid arrows and the internal rotators by dashed arrows. For clarity, not all muscles are shown. The lines of force are not drawn to scale and, therefore, do not i ndicate a muscle’s relative force potential. Reproduced with permission from Neumann DA, Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 2nd ed, Elsevier, 2010.
to the opposite side is a natural way to horizontal plane actions o entire or, more abruptly change direction while running. oten, parts o, external rotator muscles. The gluteus maximus appears uniquely Data indicate that the piriormis, postedesigned to perorm this action. With the rior bers o the gluteus minimus, and right limb securely planted, a strong con- the anterior bers o the gluteus maxitraction o the gluteus maximus would, in mus reverse their rotary action and betheory, generate a very eective extension come internal hip rotators as the hip is and external rotation torque about the signicantly exed. 13,17 This concept can right hip, helping to provide the neces- be elucidated with the aid o a skeleton sary thrust to the combined cutting-and- model and a piece o string designed to propulsion action. Dynamic stability o mimic the line o orce o a muscle. Conthe hip during this high-velocity rotation sider the piriormis. With the hip in ull may be one o the primary unctions o extension, afxing the proximal and disthe short external rotators. tal attachments o the string to the skelComputer modeling and biomechani- eton results in a muscular line o orce cal studies demonstrate that the sagittal that is posterior to the longitudinal axis plane position o the hip can reverse the o rotation. With the hip exed to at least
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90° to 100°, the string now migrates to the opposite side o the longitudinal axis (which has moved with the exed emur) to a position that would theoretically produce internal rotation. Using 4 cadaveric hip specimens and a computerized musculoskeletal model, Delp et al 13 reported that the piriormis possesses an external rotation moment arm o 2.9 cm with the hip in 0° o exion but a 1.4-cm internal rotation moment arm with the hip in 90° o exion. Assuming, or example, a nearmaximum contractile orce o 200 N, the muscle would theoretically produce 5.8 Nm o external rotation torque with the hip in neutral extension, but 2.8 Nm o internal rotation torque with the hip in 90° o exion. The exact point at which the 3 aorementioned traditional external rotator muscle bers actually switch their rotary action is not ully understood, and this certainly varies between muscles, portions o a muscle, and subjects. Delp et al13 provide data on the varying rotational moment arms throughout a sagittal plane arc or only a ew muscles, including the gluteus maximus. FIGURE 6a to 6C shows the changing rotational moment arms or this muscle’s anterior, mid-posterior, and extreme posterior bers across an arc o 0° to 90° o exion. As depicted in FIGURE 6a, considering both the model and the cadaver data, the gluteus maximus anterior bers have an overall external rotation moment arm in a position o 0° o exion. These same bers, however, appear to switch their rotation action by about 45° o exion; although the switch may only result in unctionally signicant internal rotation torque at exion angles greater than 60° to 70°. The midposterior and extreme posterior bers o the gluteus maximus ( FIGURE 6B and 6C) maintain an external rotation moment arm throughout virtually the entire measured range o exion. The rotational (horizontal plane) potential o the external rotator muscles as a unction o the sagittal plane position o the hip requires a careul review o the entire set o data published by Delp et al. 13
88 | february 2010 | volume 40 | number 2 | journal of orthopaedic & sports physical therapy
Gluteus Maximus A. Anterior fibers
B. Mid/posterior fibers
C. Extreme posterior fibers
60 R I
40
) m n ( m 20 o i t m a r t o A 0 R t p n e i H m –20 o M R E
–40 –60 0
20
40
60
80 90 0
20
40
60
80 90 0
20
40
60
80 90
Hip Flexion Angle (deg) Model
Hip 1
Hip 2
Hip 3
Hip 4
FIGURE 6. Horizontal plane rotational moment arms (in millimeters) or 3 sets o bers o the gluteus maximus, plotted as a unction o fexion (in degrees) o the hip. Abbreviations: IR, internal rotation moment arm; ER, external rotation moment arm. The 0° fexion angle on the horizontal axis marks the anatomic (neutral) position o the hip. Graph created rom data published by Delp et al, using 4 hip specimens and a computer model. 13
Gluteus Medius A. Anterior fibers
B. Posterior fibers
60 R I
40 ) m n ( m o i t m a t r o A R t p n e i H m o M
R E
A potential switch, or reversal, in a muscle’s rotation action could aect the method used or its therapeutic stretching. Consider the piriormis, reportedly an external rotator in ull extension but an internal rotator at 90° or more o exion.13 Restrictions in the extensibility o this muscle are typically described as limiting passive hip internal rotation, and possibly compressing the underlying sciatic nerve. A traditional method or stretching a tight piriormis is to com bine ull exion and external rotation o the hip, typically perormed with the knee exed. Because the piriormis is actually an internal rotator in a position o marked hip exion, incorporating external rotation into the stretch appears to be a rational approach. In a study on the sacroiliac joint, Snijders et al42 have shown that cross-legged sitting, which combines exion and external rotation o the hip, increases the length o the piriormis by 21% as compared to its length in upright standing.
Hip Internl Rottors
20
0 –20 –40 –60 0
20
40
60
80
90
0
20
40
60
80
90
Hip Flexion Angle (deg) Model
Hip 1
Hip 2
Hip 3
Hip 4
FIGURE 7. Horizontal plane rotational moment arms (in mill imeters) or 2 sets o bers o the gluteus medius, plotted as a unction o fexion (in degrees) o the hip. Abbreviations: IR, internal rotation moment arm; ER, external rotation moment arm. The 0° fexion angle on the horizontal axis marks the anatomic (neutral) position o the hip. Graph created rom data published by Delp et al, using 4 hip specimens and a computer model. 13
When reviewed or the gluteus maximus, as a whole, this large muscle is a potent external rotator, most notably at hip angles lower than 45° to 60° o exion. There is, however, a noticeable shit in rotation potential that avors greater internal rota-
tion (or less external rotation) leverage at higher hip exion angles, but only or the more anterior components o the muscle. Most o the gluteus maximus muscle maintains an external rotation moment arm throughout 0° to 90° o exion.
In sharp contrast to the external rotators, no muscle with any potential to internally rotate the hip lies even close to the horizontal plane. From the anatomic position, thereore, it is difcult to assign any muscle as a primary internal rotator o the hip. 17 Several secondary internal rotators exist, however, including the anterior bers o the gluteus minimus and the gluteus medius, tensor asciae latae, adductor longus, adductor brevis, pectineus, and posterior head o the adductor magnus13,17 (FIGURE 5). Note that in contrast to most traditional sources,26,44 Dostal et al’s 17 data listed in TaBLE 2 show that the tensor ascia latae has zero horizontal plane leverage, at least while standing upright in the anatomic position. Because the overall orientation o the internal rotator muscles is positioned closer to the vertical than horizontal position, these muscles possess a ar greater biomechanical potential to generate torque in the sagittal and rontal
journal of orthopaedic & sports physical therapy | volume 40 | number 2 | february 2010 | 89
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clinical commentary
planes than in the horizontal plane. The rather distinct biomechanical contrast in the rotary potential o the external rotator and internal rotator muscles is curious and interesting. The reasons or the dierences may be related to the unique unctional demands o human movement (walking, running, or crawling). With the hip fexed 90°, the internal rotation torque potential o the internal rotator muscles dramatically increases.13,17,31 With the help o a skeleton model and piece o string, it may be instructive to mimic the line o orce o an internal rotator muscle such as the anterior bers o the gluteus medius. Flexing the hip close to 90° reorients the muscle’s line o orce rom nearly parallel to nearly perpendicular to the longitudinal axis o rotation at the hip. (This occurs because the longitudinal axis o rotation remains nearly parallel with the shat o the repositioned emur.) FIGURE 7 shows the changing horizontal plane moment arms or the anterior and posterior bers o the gluteus medius as the hip is fexed rom 0° to 90°. 13 As depicted in FIGURE 7a, the anterior bers are only marginal internal rotators at 0° o fexion, but experience an 8-old increase in internal rotation leverage by 90° o fexion. Based on these data, an assumed nearmaximum contraction orce o 200 N o the anterior bers o the gluteus medius would theoretically produce 1.4 Nm o internal rotation torque at neutral extension but 11.6 Nm o internal rotation torque at 90° o fexion. 13 (In live humans, such a large increase in torque at 90° o fexion may not actually occur due to the potential loss in active peak orce created by the shortened muscle bers.) FIGURE 7a indicates that in a position o only 20° to 25° o hip fexion, the internal rotation moment arm o the anterior bers o the gluteus medius would at least double. Although speculation, an exaggerated anterior pelvic tilt posture could theoretically predispose one to excessive internal rotating posturing o the hip joint. Surprisingly, very little live human
Frontal Plane (From Behind) G l u t e u s m e d i u s
10.0
G lu t e u s m i n i m u s
5.0
P i r i f
) m c ( r o i r e f n I r o i r e p u S
o r m i s G l u t e u s m a x i m u s
s u i r o t r a S
e a t l a e i a c s a f
r o s n e T
0.0
–5.0
Adductor brevis
P e c t i n e u s
A d d u c t o r l o n g u s
is Quad. femor
B i c e p s fe m o ri s
G r a c li i s
s u n g a m r o t c u d d A
–10.0
5.0
. ) t s o p (
A d d u c t o r m a g n u s ( a n t . )
0.0
–5.0
Medial-Lateral (cm) FIGURE 8. A posterior view depicts the frontal plane line of force of several muscles that cross the hip. The axis of
rotation (purple circle) is directed in the anterior-posterior direction through the femoral head. The abductors are indicated by solid arrows and the adductors by dashed arrows. For clarity, not all muscles are shown. The lines of force are not drawn to scale and, therefore, do not indicate a muscle’s relative force potential. Reproduced with permission from Neumann DA, Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 2nd ed, Elsevier, 2010.
research could be located that measured be due to the increased leverage o some the maximal-eort, internal rotation o the internal rotator muscles (such as torque throughout a ull range o hip the anterior bers o the gluteus medius, fexion. One isokinetic study reported as depicted in FIGURE 7a), but also to a that maximal-eort internal rotation reversal o rotary action o some o the torque in healthy persons increased by traditional external rotators, such as the about 50% with the hip fexed, as a com- piriormis, or posterior bers o the glupared to extended. 30 This increased in- teus medius ( FIGURE 7B). The position o ternal rotation torque with fexion may hip fexion, thereore, aects the relative
90 | february 2010 | volume 40 | number 2 | journal of orthopaedic & sports physical therapy
torque potential o both the internal and external rotator muscles, with a global eect o biasing a greater relative increase in internal rotation torque. The actual dierences in maximal-eort torque production between the rotator groups at any given point within the range o the sagittal plane motion are not known. Interestingly, FIGURE 3 shows that the maximal-eort torques are nearly equal or the internal and external rotators; however, the data were collected with the hip fexed to 60°. 10 Maximal-eort contractions rom these muscle groups with the hip ully extended should, in theory, result in a signicant torque bias that a vors the external rotators; although this conjecture cannot be supported by in vivo research. The clinical signicance o an internal rotation torque bias with greater hip fexion has been extensively described in the literature related to the study o the excessively internally rotated and fexed (“crouched”) gait pattern in some persons with cerebral palsy. 13,19 With poor control or weakness o hip extensor muscles, the typically fexed posture o the hip exaggerates the internal rotation torque potential o many muscles o the hip. 2,5,13 This gait pattern may be better controlled by enhanced activat ion o the external rotator, abductor, and hip extensor muscles. A similar body o research is evolving that suggests a similar pattern o hip muscle weakness may be associ ated with the pathomech anics o musculoskeletal disorders o the knee, such as patelloemoral joint pain syndrome and noncontact injury to the anterior cruciate ligament in adolescent emales.9,32,49
FRONTaL PLaNE Hip adductors he primary adductors of the hip include the pectineus, adductor longus, gracilis, adductor brevis, and adductor magnus (both anterior and posterior heads). Secondary adductors include the biceps emoris (long head), the
T
gluteus maximus (especially the posterior sor (posterior) side o the medial-lateral bers), quadratus emoris, and obturator axis o rotation o the hip, by which these externus (TaBLE 2) (FIGURE 8).16,17 muscles gain leverage as hip extensors. The primary adductor muscles have The specic point at which the adductor relatively avorable leverage or adduc- muscles change leverage has not been tion o the hip, averaging almost 6 cm. 17 thoroughly investigated, although this This leverage is available or the pro- concept is discussed in papers by Dosduction o adduction torque rom both tal et al 16,17 and Hoy.23 Further research, emoral-on-pelvic and pelvic-on-emoral such as that published by Delp et al 13 and perspectives. Although rigorous study o Arnold et al, 2-5 is needed to veriy more the adductor muscles highlighting these specically the fexion and extension le2 movement perspectives is lacking in verage o the adductors muscles throughthe literature, consider the ollowing out a wide arc o sagittal plane motion. possibility. During rapid or complex The bidirectional sagittal plane torque movements involving both lower ex- potential o most o the adductor muscles tremities, it is likely that many o the is useul or powering cyclic activities adductor muscles are bilaterally and such as sprinting, bicycling, or descendsimultaneously active to control both ing and rising rom a deep squat. When emoral-on-pelvic and pelvic-on-emoral the hip is fexed, the adductor muscles are hip movements. Consider, or example, mechanically prepared to augment the a soccer player rmly planting her let other extensor muscles. In contrast, when oot as she kicks a soccer ball let-othe hip is closer to ull extension, the y are center using the right oot. To varying mechanically prepared to augment the levels, the contracting right adductor other hip fexors. The nearly constant muscles are capable o fexing, adduct- triplanar biomechanical demand placed ing, and internally rotating the right hip on the adductors muscles throughout a (emur relative to the pelvis) as a way to wide range o hip positions may partially accelerate the ball in the intended direc- explain their relatively high susceptibility tion. As part o this action, the planted to strain injury. let hip may be actively adducting and internal rotating slightly rom a pelvic- Hip abductors on-emoral perspective, driven through The primary hip abductor muscles inconcentric activation o the let adduc- clude all bers o the gluteus medius and tor muscles. Such an action likely also gluteus minimus, and the tensor asciae requires eccentric activation o the let latae (TaBLE 2).12 The piriormis, sartorius, gluteus medius, which is well suited to and rectus emoris are considered seconddecelerate and control the aoremen- ary hip abductors. The abductor muscles tioned pelvic-on-emoral motions. pass lateral to the anterior-posterior axis In addition to producing adduction o rotation o the hip ( FIGURE 8). torque at the hip joint, the adductor musThe gluteus medius is the largest o cles are considered important fexors or the hip abductors, accounting or about extensors o the hip. 17,34 Regardless o hip 60% o the total abductor muscle crossposition, the adductor magnus (especially sectional area. 12 The muscle attaches disthe posterior head) is an eective exten- tally to the lateral and superior-posterior sor o the hip, similar to the hamstring aspects o the greater trochanter. 38 This muscles. Most other adductor muscles, distal attachment, in combination with however, are considered fexors rom the its proximal attachments on the upper anatomic (extended) position ( TaBLE 1). and more fared portion o the ilium, Once the hip is fexed beyond about 40° provides the muscle with the largest abto 70° o hip fexion, the line o orce o duction moment arm o all the abductor the adductor muscles (except the adduc- muscles (TaBLE 1).17 tor magnus) appears to cross to the extenThe broad, an-shaped gluteus medius
journal of orthopaedic & sports physical therapy | volume 40 | number 2 | february 2010 | 91
[
clinical commentary
is oten subdivided unctionally into 3 sets o bers: anterior, middle, and posterior (TaBLE 1).12,17,43 All bers contribute to abduction o the hip; however, rom the anatomic position, the anterior bers also produce modest internal rotation and the posterior bers produce extension and external rotation. As described earlier in this paper, however, the strength and even direction o this muscle’s horizontal plane actions can change when the muscle is activated rom varying degrees o hip fexion.4 The gluteus minimus lies immediately deep and just anterior to the gluteus medius, attaching distally to the anterior-lateral aspect o the greater trochanter.38 The tendon o the gluteus minimus also attaches to the anterior and superior capsule o the joint. 6,44,47 Perhaps this secondary attachment may help retract the capsule rom the joint at the extremes o motion, possi bly preventing capsular impin gement. Magnetic resonance imaging and other clinical observations suggest that tears or degenerative changes at the point o attachment o the gluteus minimus (and medius) may be a source o pain oten and, perhaps, incorrectly diagnosed as trochanteric bursitis.51 The gluteus minimus is smaller than the gluteus medius, accounting or about 20% o the total abductor muscle crosssectional area.12 Similar to the gluteus medius, the an-shaped gluteus minimus has been described unctionally as possessing 3 sets o bers. 13,17 All bers cause abduction, and the more anterior bers also contribute to internal rotation, most notably when the hip is fexed. 12,29 Some authors consider the posterior bers as secondary external rotators. 17,43 The tensor asciae latae is the smallest o the 3 primary hip abductors, accounting or about 11% o the total abductor muscle cross-sectional area. 12 This muscle arises rom the outer lip o the iliac crest, just lateral to the anterior-superior iliac spine. Distally, the tensor ascia latae blends with the iliotibial band. Contraction o the hip abductor mus-
]
tant unctional role o the hip abductor muscles occurs during the single-limb 110 support phase o walking. The external 100 (gravitational) adduction torque about ) 90 the hip dramatically increases within the m N ( e 80 rontal plane as soon as the contralateral Right hip u q r o limb leaves the ground. 24 The hip abduc T 70 Left hip tors respond by generating an abduction 60 torque about the stance hip that stabilizes 50 the pelvis relative to the emur. 24 In addi40 tion, these same muscles may be required 30 to produce a smaller, but at times neces10 30 –10 0 20 40 sary, internal rotation torque about the Hip Angle (deg) stance hip to rotate the pelvis in the same direction as the advancing contralateral FIGURE 9. Maximal-efort isometric hip abduction “swing” limb. Interestingly, both the glutorque as a unction o rontal plane range o teus medius and minimus (and possibly abduction in 30 healthy persons.37 The –10° angle the tensor ascia latae) are capable o on the horizontal axis o the graph represents the combining abduction and internal rotaadducted position where the muscles are at their longest length. Reproduced with permission rom tion torque at the hip. Neumann DA, Kinesiology o the Musculoskeletal The orce produced by the hip abducSystem: Foundations or Rehabilitation, 2nd ed, tor muscles to maintain rontal plane Elsevier, 2010. stability during single-limb support accles with the pelvis stabilized in the ron- counts or most o the compressive orce tal plane can produce emoral-on-pelvic generated between the acetabulum and hip abduction. Clinically, the torque o emoral head. This important point is an abducting emur is oten resisted to demonstrated by the model in FIGURE 10, measure the abduction torque o the hip which assumes a person i s standing only abductors as a whole. FIGURE 9 shows a on the stance (right) limb. The moment plot o the maximum-eort isometrical- arm (D) used by the hip abductor musly produced torque o the right and let cles is about hal the length o the moabductor muscles in a sample o young ment arm (D1) used by body weight (W). 37 healthy adults. 37 Note that the plot is Given the dierences in moment arm essentially linear, with the least torque lengths, the hip abductor muscles must produced at 40° o abduction when the produce a orce (M) about twice that o muscles are at their near ully shortened superincumbent body weight to achieve (contracted) length. Paradoxically, this rontal plane stability while standing position is most oten used to manu- on the 1 limb. The acetabulum is pulled ally test the maximal strength o the hip down against the emoral head not only abductors. 26 by the orce o the activated hip abductor muscles, but also by the gravitational FIGURE 9 also shows that the greatest peak hip abductor torque occurs when pull o body weight. When added, these the abductor muscles are nearly maxi- 2 inerior-directed orces theoretically mally elongated, in a position o 10° o equal about 2.5 to 3 times one’s ull body adduction. 37 This rontal plane position weight.25 It is noteworthy that about 66% corresponds generally to the position o this orce is created by the hip abduco the hip joint when the body is in its tor muscles. To achieve static equilibrium single-limb support phase o walking, about the stance hip, these downward exactly when these muscles are required orces are counteracted by a joint reacto generate rontal plane stability o the tion orce (see “J” in FIGURE 10) o equal hip. magnitude but oriented in nearly the op As implied above, the most impor- posite direction as the muscle orce. The 120
92 | february 2010 | volume 40 | number 2 | journal of orthopaedic & sports physical therapy
M
D
D1
W
= J
M×D Internal torque
W × D1 External torque
These orces can increase to 5 or 6 times body weight while running or ascending and descending stairs.7,45 Even ordinary unctional activities or exercises can create joint orces that greatly exceed body weight.20 Normally, joint orces have important unctions, such as stabilizing the emoral head within the acetabulum and providing the stimulus or normal growth and development o the hip in the growing child. Many joint protection principles taught to patients with ailing (or potentially ailing) biologic or prosthetic hip joints are based on an understanding o the rontal plane biomechanics described in FIGURE 10.1,28,35,36
CLOSING COmmENTS
FIGURE 10. A rontal plane model
shows how the orce produced by the right hip abductor muscles (indicated in red as M) stabilizes the pelvis while standing only on the right limb. The right hip i s shown with a prosthesis. The pelvis-and-trunk are assumed to be in static equilibrium about the right hip. The counterclockwise torque (solid circle) is the product o the hip abductor orce (M) times its moment arm (D); the clockwise torque (dashed circle) is the product o superincumbent body weight (W) times its moment arm (D 1). Because the system is in equilibrium, the torques in the rontal plane are equal in magnitude and opposite in direction: M D = W D . A joint reaction orce (J) is directed 1 through the hip joint. Reproduced and modied with permission rom Neumann DA, Kinesiology o the Musculoskeletal System: Foundations or Rehabilitation, 2nd ed, Elsevier, 2010.
joint reaction orce is directed about 15° rom vertical, an angle that is strongly infuenced by the line o orce o the hip abductor muscles. 25 The biomechanics described in FIGURE 10 is based on a person simply standing statically on 1 limb. While walking, however, the joint reaction orce is even greater, due to the acceleration o the pelvis over the emoral head. Data based on computer modeling or direct measurements rom strain gauges implanted into a hip prosthesis show that joint reaction (compression) orces reach at least 3 times body weight while walking. 24,45
lthough great strides have been made over the last several decades, there is still much to be learned about how muscles o the hip act in isolation and, espec ially, in groups. Muscle actions are currently best understood when activated rom the anatomic position. What is needed, however, is a greater understanding o how a muscle’s action (and strength) changes when activated outside the anatomic position. This knowledge would provide clinicians with a more thorough and realistic appreciation o the potential actions o the muscles that cross the hip. Ultimately, this level o understanding will improve the ability to diagnose, understand, and treat impairments based on the abnormal unctioning o hip muscles. t
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The author would like to thank Jeremy Karman, PT, for his careful review of some of the clinical issues described in this paper. Acknowledgements:
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clinical commentary
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mORE INFORmaTION
VIEW Videos on JOSPT ’s Website Videos posted with select articles on the Journal ’s website ( www.jospt.org) show how conditions are diagnosed and interventions performed. For a list of available videos, click on “COLLECTIONS” in the navigation bar in the left-hand column of the home page, select “Media”, check “Video”, and click “Browse”. A list of articles with videos will be displayed.
94 | february 2010 | volume 40 | number 2 | journal of orthopaedic & sports physical therapy
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