Neuromuscular Hip Biomechanics and Pathology in the Athlete

Michael R. Torry, PhDa'*, Mara L. Schenker, BSa, Hal D. Martin, DOb, Doug Hogoboom, BSa, Marc J. Philippon, MDa aBiomechanics Research Laboratory, Steadman-Hawkins Research Foundation, 181 West Meadow Drive, Suite 1000, Vail, CO 81657, USA

hOklahoma Sports Science and Orthopedics, 6205 N. Santa Fe, Suite 200, Oklahoma City, OK 73118, USA

Dynamic movement occurs at the hip joint and is characterized and constrained by the anatomy of the region, including osseous, ligamen-tous, and musculotendonous structures. The majority of patients who require hip arthroscopy are young, active individuals with a history of hip or groin pain. In some athletes, the onset of hip pain may be due to a traumatic event such as a fall, tackle, or collision. However, in many sports, athletes suffer a minor hip injury or perform repetitive motions that exacerbate a chronic pathologic or congenital hip condition that leads to increased capsular laxity and labral tears over time. One of the obvious benefits of arthroscopic hip surgery in this population is that it allows the surgeon to perform procedures within the hip joint with a minimal amount of postoperative morbidity, allowing for a return to sporting activities in a shorter time period. This type of surgery is relatively new, with only a few experts advancing in the field worldwide. However, this surgery is gaining popularity among sports medicine/orthopedic surgeons, and is being performed more and more on all levels of athletes and in the nonarthritic, hip-injured population alike.

Although joint mechanics for total hip joint replacements (THR) are well described, little is known with regard to hip joint mechanics in injuries such as hip labral tears that are observed in younger athletes; and although hip arthro-scopic techniques have been developed and evolved over the last 5 years, the mechanisms of these injuries across various sports are not well understood. Moreover, rehabilitation protocols associated with hip arthroscopy remain rooted in THR theories and paradigms. It is evident from the literature that rehabilitation after hip arthroscopic surgery requires a mechanical foundation for its implementation during initial, intermediate, and return to sport/agility protocols. Without such a scientific foundation, the risk of an unsuccessful surgery or reinjury is greatly enhanced.

* Corresponding author. E-mail address: [email protected] (M.R. Torry).

0278-5919/06/$ - see front matter © 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.csm.2005.12.002 sportsmed.theclinics.com

The purpose of this article is to review the literature related to the osseous, ligamentous support as well as the neuromuscular control strategies associated with hip joint mechanics. The neuromuscular contributions to hip stability and mobility with respect to gait will be provided because the data related to gait represents the largest body of knowledge regarding hip function. Further, this article will describe the probable mechanisms of injury in sporting activities most often associated with hip injury in the young athlete.

OSSEOUS STRUCTURES CONTRIBUTING TO HIP STABILIZATION

The adult hip is a multiaxial ball-and-socket synovial joint composed of two bony structures: the femur and the acetabulum. This bony architecture provides the hip with inherent stability. Three biomechanical and anatomic geometries of the femur and acetabulum are significant to joint stability and preservation of the labrum and articular cartilage: appropriate femoral head-neck offset, acetabular anteversion, and acetabular coverage of the femoral head. Proper function of the hip joint necessitates that the amount of offset from the femoral head to the femoral neck be enough to allow a full range of motion without impinging upon the acetabular labrum. A lack of offset from the femoral head to the femoral neck has been described as a cause for femoroacetabular impingement [1]. Flexion at the hip may cause the osseous femoral head-neck junction to come into contact with the acetabular labrum, resulting in impingement [1-3]. A large femoral head can compensate for a flat head-neck junction by simulating offset and adding stability to the joint [4].

Large variations exist in the rotational axis that characterizes the relationship between the acetabular and femoral osseous structures. The range of acetabular anteversion to femoral anteversion affects the rotation of the extremity and changes from the time of birth and through mature skeletal development. The transfer of dynamic and static load to the ligamentous and osseous structures is dependent on this relationship. Abnormal distribution of force or pressure in an incongruent joint precipitates chronic or acute injury. Normal adult acetabular positioning intersects the sagittal plane at 40° and the transverse plane at 60°, opening anteriorly and laterally [5]. The acetabulum is positioned approximately 45° caudally and 15° anteriorly [6,7]. Normal anteversion of the acetabulum is essential to maintaining a normal relationship with the femoral head and is critical in avoidance of impingement [8]. Normal range of acetabular anteversion as defined by Tonnis and Heinecke [9] is 15° to 20°, decreased anteversion is 10° to 14°, and increased anteversion is 21° to 25°. An increase in external rotation is commonly found with decreased acetabular anteversion.

In addition to recognizing acetabular anteversion, it is also important to appreciate the degree of femoral head coverage provided by the acetabulum. This can be measured radiographically as the central edge angle of Wiberg, which is defined as the angle between the horizontal line through the center of the femoral head and a line tangent to the superior and inferior acetabular rims. The normal center edge angle is 30° and a decrease in this angle (dysplasia) has been associated with rapid onset of osteoarthritis [10-13]. Center edge angles of less than 20° correlate with an abnormal orientation of the acetabulum, providing less than satisfactory head coverage and load transfer.

Anteversion of the femur is also important in maintaining proper static and dynamic mechanics in the hip. Anteversion of the femur diminishes with age. A healthy 1 year old has an average anteversion of 31°. This anteversion decreases to 24° at 8 years and to 15° by 15 years [14]. The McKibbin instability index is based on the sum of the angles of the femoral and acetabular anteversion. This ratio will affect range of motion. The sum of the angles of femoral and acetabular anteversion predicts instability for summed angles of 60° or more and predicts low instability for angles of less than 20°. The authors found that, of 290 hips tested, 38% had a low and 6% had a high index.

LIGAMENTOUS STRUCTURES CONTRIBUTING TO HIP STABILIZATION

The hip capsule is comprised of a series of ligaments, which can be subdivided into functional and anatomic components. The five primary ligaments discussed in the hip are the iliofemoral (lateral and medial arms), pubofemoral, ischiofemoral, the ligamentum teres femoris, and the ligamentum obicularis. The collagen structure of the hip as demonstrated by electron microscopy is similar to that of the shoulder and the elbow [15].

The iliofemoral ligament (also referred to as the Y-ligament of Bigelow) is the largest of the ligaments and reinforces the capsule anteriorly. Originating at the anterior superior iliac spine (ASIS) and the acetabular rim, it inserts at the intertrochanteric line and the front of the greater trochanter. The ischiofemoral ligament supports the capsule posteriorly, fastening the ischial portion of the acetabular rim to the neck of the femur, medial to the base of the greater trochanter. The pubofemoral ligament reinforces the capsule inferiorly, extending from the superior pubic ramus and acetabular rim to the lower femoral neck. These ligaments are connected to each other by the circular ligamentum obicu-laris, which circumvents the femoral neck. The ligamentum teres femoris originates at the acetabular notch from the transverse acetabular ligament, and inserts in the fovea of the femoral head.

The function of these ligaments has been well described in terms of limiting ranges of motion. There is debate in the literature over which ligament might limit what motion. Most authors agree that the iliofemoral ligament limits extension [16], the pubofemoral ligament limits abduction, and the ischiofemoral ligament limits internal rotation. It is thought that with an elongated or surgically resected iliofemoral ligament, the ligamentum teres has a limiting effect on external rotation. There is debate regarding the ligament limitation in other motions and debate as to what role is played by the functional subdivisions of each ligament (such as the lateral and medial iliofemoral ligament) [17]. The ligamentum orbicularis appears to be overlooked as a major key in stability of the hip joint. Traditionally, the ligamentum orbicularis was thought to be relevant only to extension by tightening the posterior capsule [18]. It now appears to play a vital role in stability, particularly in the area where the lateral arm of the iliofemoral ligament and the orbicularis merge together and continue over the anterosuperior portion of the capsule.

Although studies have described independent motions limited by the ligaments, it is believed that they do not function independently. The ligament complex surrounding the hip acts to stabilize the hip in all ranges of motion. Fuss and Bacher [17] discussed three varieties of interconnections between the ligaments as they form the capsule: parallel fibers either join and become one ligament, join and intermingle though separate ligaments, or join by fusing at the borders (pilema, confluens and conjunction fibrarum, respectively). Fuss and Bacher performed a kinematic study on 10 intact pelves secured to a table mount. The ligaments of the hip were removed except for the iliofemoral ligament (medial and lateral arm). The hip was taken through extension, abduction, adduction and internal/external rotation movements (as guided by a grid) and the motion of the ligament was recorded. In many hips, the iliofemoral ligament appeared to lock when the hip was in pure terminal extension without rotation. The ligament moved to the lateral aspect of the femoral head in abduction or external rotation unlocking the major anterior structure. The pubofemoral ligament contribution to the capsular structures is thought to play a role in controlling this motion.

Certain in vivo studies have illustrated the importance of the ligamentous structures in providing stability to the hip joint [19-22]. While standing, the body's center of gravity lies just posterior to the axis of the hip in the sagittal plane, which causes the pelvis to tilt posteriorly on the femoral head [19]. This tilt is opposed by the tensile forces from the stretching of the anterior capsule, implying that the energy required to stand stationary should be compensated by the ligaments without muscular contribution [19]. Gait involves ranges of motion in all three planes. The force for motion is derived from the musculature of the lower limbs, although stability could not be maintained without the liga-mentous capsule. Abnormal functioning of the iliofemoral ligament has been identified as a cause for coxa sultans [20]. Owing to the relatively large tensile forces of the ligaments of the capsule, dislocation of the hip requires high impact forces, except in children, due to their relatively shallow acetabulum [21,22].

NEUROMUSCULAR FACTORS CONTRIBUTING TO HIP STABILIZATION

Maintaining an appropriate femoral head position within the joint capsule and labral complex is paramount to normal hip function and failure in this mechanism can lead to debilitating labral and cartilage compression in active individuals. Thus, hip congruency, although affected by, is not solely dependent upon the femoral head-acetabular bony and labral constituents for complete hip stabilization. The ligaments described above and the muscles that cross the hip joint contribute and provide for articular congruency (ie, proper joint rotation of the femoral head within the acetabular-labral complex) and maintain articular stabilization (ie, limit translations of the femoral head within the acetabular-labral complex). To accomplish this, muscles that cross the hip must act as force regulators across a very wide range of motions by regulating their stiffness. Muscular stiffness is determined by a complex neural feedback control system. A highly regulated hierarchy of neuromuscular control strategies begins with the activation of the single fiber and progresses to the mechanical properties of the whole muscle. Discussing the exact mechanisms that are involved in this neuro-mechanical hierarchy is beyond the scope of this article, but a few of the more pertinent aspects are listed briefly below:

1. Muscle stiffness is regulated by muscle activation frequency (ie, temporal summation) [23].

2. Muscle stiffness is regulated by muscle fiber recruitment (ie, spatial summation) [24].

3. Muscle stiffness is regulated by the sarcomere length-tension relationship [25].

4. Muscle stiffness is regulated by sarcomere force-velocity relationship [26].

5. Muscle stiffness is regulated by passive sarcomere length tension relationships [27].

6. Intrafusal and extrafusal (muscle spindle) fibers feedback mechanisms [28].

7. Muscle force and moment regulation by skeletal muscle architecture [29,30].

The first six points relate a specific muscle's function primarily to its intrinsic properties and are standard across all skeletal muscles. However, point 7, muscle stiffness regulation by skeletal muscle architecture (ie, the physical arrangement of the muscle fibers within a specific muscle) is of substantial importance at the hip given the large, "irregular" shaped muscles that cross this joint, and much work has been recently constructed in this area [31,32]. Functionally, the force generated by a muscle is proportional to its physiologic cross-sectional area (PCSA). The total excursion of a muscle is determined by its fiber length. Traditionally, fiber length were determined by dissection methods and histologic analysis; but recently, newer MRI-based technologies have been used with great success and detail [31,33,34]. Thus, from a muscle design perspective, muscle architecture results in muscle function based on unique fiber arrangements. Mechanical properties of many of the larger muscles surrounding the hip have been characterized and are presented in Table 1. Although detailed studies of muscles architecture have been conducted for the lower extremity [34], these studies often omit many of the smaller muscles (eg, pirifirmis, superior and inferior gemullus and obturator internus and externus) that cross the hip.

Because many of the hip muscles involve very complex geometric architectures, determining their exact mechanical influence on hip function is difficult. Computer modeling techniques enhanced by computer tomography (CT) and MRI are some of the newer techniques of estimating the complex hip muscular actions. These methods have allowed researchers to reconstruct the hip muscle geometry with "lumped parameter muscle models," where each muscles is represented by a single line of action estimated from a centroid of the muscles taken from the a 3D reconstruction via an MRI image [31,33,34]. These "lumped parameter muscle models," however, only allow for a one length of muscle fiber and moment arm to be estimated for each muscle path [31,33,34].

Table 1

Muscle-tendon parameters for the hip muscles

Peak

Optimal

Tendon

Physiological

muscle

fiber

Pennation

slack

Tendon

cross-sectional

force

length

angle

length

length/fiber

Muscle

Area (cm2)

(N)

(cm)

(degrees)

(cm)

length

Gluteus medius 1

22.0

550

5.4

8

7.8

1.4

Gluteus medius 2

15.2

380

8.4

0

5.3

0.6

Gluteus medius 3

17.4

435

6.5

19

5.3

0.8

Gluteus minimus 1

7.2

180

6.8

10

1.6

0.2

Gluteus minimus 2

7.6

190

5.6

0

2.6

0.5

Gluteus minimus 3

8.6

215

3.8

1

5.1

1.3

Gluteus maximus 1

15.2

380

14.2

5

12.5

0.9

Gluteus maximus 2

22.0

550

14.7

0

12.7

0.9

Gluteus maximus 3

14.8

370

14.4

5

14.5

1.0

Adductor magnus 1

13.8

345

8.7

5

6.0

0.7

Adductor magnus 2

12.4

310

12.1

3

13.0

1.0

Adductor magnus 3

17.8

445

13.1

5

26.0

2.0

Adductor longus

16.8

420

13.8

6

11.0

0.8

Adductor brevis

11.4

285

13.3

0

2.0

0.2

Pectineus

7.0

175

13.3

0

0.1

0.1

Iliacus

17.2

430

10.0

7

9.0

0.9

Psoas

14.8

370

10.4

8

13.0

1.3

Quadratus femoris

10.2

255

5.4

0

2.4

0.4

Gemelli

4.4

110

2.4

0

3.9

1.6

Piriformis

11.8

295

2.6

10

11.5

4.4

Rectus femoris

12.8

780

8.4

5

34.6

4.0

Semimembranosus

16.9

1030

8.0

15

35.9

4.5

Semitendinosus

5.4

330

20.1

5

26.2

1.3

Biceps femoris (lh)

11.8

720

10.9

0

34.1

3.1

Gracilis

1.8

110

35.2

3

14.0

0.4

Sartorius

1.7

105

57.9

0

4.0

0.1

Tensor fasciae latae

2.5

155

9.5

3

42.5

4.5

Optimal muscle fiber length is defined as the number of sarcomeres in series, and has been shown to be a major component of maximal velocity of shortening during a contraction [26]. Muscle belly fiber lengths can be determined by methods described by Veeger et al [82], where the distance between the most proximal and most distal musculotendinous conjunctions are measured in situ then removed, macerated, and measured again via calibrated microscopic examination.

Tendon slack length is typically measured in situ prior to dissection and after muscular tissue separation. Tendon slack length represents the noncontractile element of the musculotendinous unit and each bundle's tendon slack length is usually quantified (cm) via calibrated microscopic examination.

Pennation angle of muscle fibers represents the angle or direction of pull between the insertion and origin of the muscles. These angles are noted in situ and prior to dissection and the angle of pull can be measured with a goniometer. Of note, how researchers determine individual muscle bundles within each broad fan shaped muscle is subject to much debate. For instance, most hip anatomic studies have divided the gluteus medius into at least three separate bundles based on the broad anatomic insertion sites across the pelvic-iliac crest. Similarly, some authors have combined the illiacus and psoas; while others separate their functions.

Physiologic cross-sectional area of muscle is defined as the number of sarcomeres in parallel and is reported to be directly related to the amount of tension a muscle can produce [26] (muscle mass + fiber length) / pennation angle).

Data from Wickiewicz TL, Roy RR, Powell PL, et al. Muscle architecture of the human lower limb. Clin Orthop 1983;179:275-83; Brand RA, Pedersen DR, Friederich JA. The sensitivity of muscle force predictions to changes in physiologic cross-sectional area. J Biomech 1986;19(8):589-96; Friederich JA, Brand RA. Muscle fiber architecture in the human lower limb. J Biomech 1990;23(1):91-5.

Because muscle moment arms and fiber length may be different within the resting geometry of a muscle, or may change over a given range of motion for a specific muscle, using single lines of action to represent these actions may over-or underestimate each muscle's force generating capacity given a dynamic movement [31,33,34]. Moreover, Herzog and Keurs [36] have shown that lumped parameter models do not accurately predict in vivo force-velocity behaviors for muscles with complex geometries. To illustrate this point further, Blemker and Delp [32] developed a mathematical model of the hip joint in which the complex geometries of the major muscles of the hip over a specified range of hip flexion and extension were estimated from an MRI of a single subject. This technique allowed the researchers to reconstruct and characterize the complex 3D geometries of the hip musculature and to represent each muscle with multiple muscle fibers with varying fiber lengths and with each fiber possessing its own moment arm. This 3D model highlighted the diverse behaviors (please see Figs. 6A-L and 7A-L in Blemker and Delp, Aunals Biomedical Engineering, 2005, pp. 668-9) among individual muscle fibers within a specific hip muscle as well as illustrated the changing roles specific fibers of a particular hip muscle may have while undergoing flexion and extension [31,33,34]. The considerable change in fiber moment arms within each muscle indicates that the force generating capacity of a muscle may in fact change with different femoral, pelvic, or lumbar motions. This is also evident from the work of Arnold et al [37], who suggested that during upright standing with normal femoral anteversion, the medial hamstrings, adductor brevis, adductor longus, pectineus, and ischiocondylar portion of the adductor magnus produce internal rotation via hip internal rotation moments; the gracilis and proximal portion of the adductor magnus produce external hip rotation moments; and, the middle and distal portions of the adductor magnus have negligible rotation moments. When the hip is rotated more than 20°, or when the knee is flexed more than 30°, the rotational moment arms of the semimembranosus and semitendinosus switch from internal to external [37]. The gracilis also becomes more external with hip internal rotation and knee flexion and the moment arm of the ischiocondylar portion of the adductor magnus becomes less internal with internal hip rotation.

FUNCTIONAL ANALYSIS OF HIP BIOMECHANICS

In vivo estimates of hip mechanics for dynamic activities have been attempted using optical capture, accelerometer, or goniometric methods. Optical methods employ high speed cameras to capture the 3D motion of reflective markers that are placed on pertinent and relative boney landmarks of the subjects. These systems produce 3D trajectories of the markers, which used to estimate internal joint centers and determine segment motions, velocities, and accelerations. These kinematic parameters are then combined with subject's anthropometric inertial data and external forces to yield external reaction forces and moments. These external forces and moments are then used to estimate internal joint reaction forces and internal "muscle" moments. The internal muscles moments must generate equal and opposite forces to the externally measured moments, and are composed of the muscle contraction, passive soft tissues, and joint reaction forces. However, using the inverse dynamics solution only yields net muscle moments, and these cannot be decomposed into individual muscle contributions to the motion without appropriate assumptions to obtain an equal number of unknowns and equations; or by employing an optimization scheme. Optimization methods assume that the force distribution among the muscles is made by applying an objective function (usually based on a physical property of a muscle). Early hip models [35] were limited in that they assumed muscles

were single bundles represented by straight muscle paths that possess similar fibers lengths with the same moment arms over the cross-section of a large muscle. Today, more sophisticated models [38] have employed more precise muscle paths with better defined "wrapping functions" to deflect muscles path around pertinent anatomic structures and more specific fiber length parameters for individual muscle bundles within the complex geometry of a whole muscle. These advancements have contributed to the understanding of the functional roles for the individual muscles surrounding the hip, as they more closely represent the true functional geometry of those muscles in vivo.

Anderson and Pandy [38] developed a muscle model that included select hip musculature to analyze a complete gait cycle. This model contained 54 independent muscles, and the results estimated each muscle's contribution to the support phase of gait. A muscle's potential for generating support was described by its contribution to the vertical ground reaction force per unit of muscle force. Of the hip muscles, the gluteus medius, maximus, and minimus provided the majority of the support in first 0% to 30% of stance (Fig. 1A) . From foot flat to just after contralateral toe-off (eg, 10-50% of stance), the gluteus maximus and posterior medius/minimus contributed significantly to the vertical ground reaction force. With assistance from joints and bones to gravity, the anterior and posterior gluteus medius/minimus generated nearly all the support evident in midstance. Posterior gluteus medius/minumus provided support throughout midstance, while the anterior gluteus medius/minimus contributed only toward the end of midstance (Fig. 1B). Interestingly, the iliopsoas developed substantial forces during late stance, but this muscle did not make substantial contributions to support [38].

The study of Anderson and Pandy [38] has shown that the muscular actions of the gluteus medius and minimus depend strongly on body positions. Anterior gluteus medius/minimus developed forces as large as the posterior gluteus

Fig. 1. (A-C) Individual muscles contributions to support during gait from heel strike (HS) to toe-off (TO). Here, support is represented by the shaded gray area, which is the vertical ground reaction force. Symbols used to represent muscles in the figure are: DF, ankle dorsiflexors; GAS, gastrocnemius; GMEDP, posterior gluteus medius/minimus; GMAX, medial and lateral portions of the gluteus maxumus; GMEDA, anterior gluteus medius/minimus; SOL, soleus; VAS, vasti. In this figure the gluteus maximus contributes the most muscle force to supprt in early stance; The posterior gluteus medius/minimus contributes notable force throughout the stance phase. In later stance, the anterior gluteus medius/minimus is most effective at maintaining support during gait. The passive resistance of the skeleton to the force of gravity was less then 50% of body weight through out stance, suggesting that muscles are the most important parameter to support the body during gait. Of these muscles, the hip gluteus maximus contributed the most force to support, followed by the vasti, gluteus medius/minimus, and soleus/gastrocnenius of the body compared with all other muscles during gait. Unfortunately, the mechanical roles of the smaller hip muscles such as the pectineus, pirifirmis, superior and inferior gemullus, and obturator internus and externus were not included in this model. (From Anderson FC, Pandy MG. Individual muscle contributions to support in normal walking. Gait Posture 2003;17(2):159-69; with permission.)

Fig. 2. Contributions of individual muscle groups to the net vertical acceleration of the center of mass of the walking model. Muscle symbols used are: CDF, dorsiflexors of the contralateral limb; CGAS, contralateral gastrocnemius; CGMAX, contralateral gluteus maximus; CGMEDA, contrlateral anterior gluteus medius/minimus; CGMEDP, contralateral posterior gluteus medius/ minimus; CLIG, ligaments of contralateral limb; CSOL, soleus of contralateral limb; GAS, gastrocnemius; GMAX, medial and lateral portions of the gluteus maxumus; GMEDA, anterior gluteus medius/minimus; GMEDP, posterior gluteus medius/minimus; SOL, soleus; VAS, vasti. Only muscles that on the limb in contact with the ground contributed to the vertical acceleration of the center of mass. (From Anderson FC, Pandy MG. Individual muscle contributions to support in normal walking. Gait Posture 2003;17(2):159-69; with permission.)

medius, yet the anterior gluteus medius contributed very little to support during early stance. The reason for this is that the anterior gluteus medius possesses a moment arm at the hip that acts to flex the hip as well as abduct it. These two actions oppose one another and prevent the anterior gluteus medius from generating support in early stance no matter how large its force. As the hip extends during mid and late stance phase, the anterior gluteus medius moment arm falls close to zero. The muscle becomes more of a pure abductor and its action more closely resembles the actions of the posterior gluteus medius. The value of the study by Anderson and Pandy [38] is that this study estimated true muscles forces (N) for each muscle (Fig. 2), offering considerably more information then one can derive from electromyography (EMG) alone or from inverse dynamic analysis techniques.

Studies have been published that examine the specific forces encountered in walking, climbing stairs, skiing, and in routine daily activities [39-42]. Variance of forces rises from incongruence of the femoral head to the acetabulum and the hip muscles that control these motions. It is estimated that the hip endures forces ranging from one-third of the body weight with double leg support to five times the body weight during running [43,44]. The asymmetry between the femoral head and the acetabulum allocates weight to multiple areas. This incongruence is inherent to the hip and necessary for sustaining normal function [45].

HIP JOINT REACTION FORCES

During gait, a stride will take the hip through an average of 40° to 50° of motion (30°-40° of flexion and 5°-10° of extension) [19,46]. The force from weight bearing in the acetabulum during gait is biphasic with peaks in force occurring at heel strike and toe off. Areas of contact form two columns of force on the anterior and posterior rims, joining together in the superior aspect of the fossa [47]. As more force is applied to the hip, the areas enlarge as the femoral head settles deeper in the acetabulum. The areas of most frequent weight bearing are also associated with the stiffest and thickest articular cartilage [47].

The result of the forces transferred across the hip can be visualized radio-graphically in the femoral neck as Ward's triangle [48]. This triangle is outlined by cortical and tensile trabecular osseous formations in the femoral neck. Tensile forces are generated in the medial subtrochanteric cortex and applied into the weightbearing portion of the femoral head [48]. Cortical forces span from the foveal area of the femoral head through the superior femoral neck to the subtrochanteric cortex [48]. In hips with a neck shaft angle of greater than 125° (coxa valga), compressive trabeculae are more prominent due to the increased compressive forces accounted for by the deformity of the femur. In hips with a neck shaft angle of less than 125° (coxa varus), tensile trabeculae are more prominent due to the increased tensile stresses [49].

ELECTROMYOGRAPHY OF HIP MUSCULATURE

EMG is a technique used to measure the electrical input (excitation) of a specific muscle. Considerable literature regarding EMG of the hip musculature for walking, climbing stairs, and various sporting motions has been reported. Due to space limitations and the completeness of data content, only the EMG of hip muscles during gait are presented below. Although EMG studies are valuable in determining which and when individual muscles are active, it is important to note that EMG cannot provide information regarding the amount of force a specific muscle is producing. This limitation of EMG underscores the importance of computer modeling techniques in understanding hip mechanics during functional activities and in understanding the basic mechanics associated with hip stabilization and the interaction of bony geometries and the actual muscle forces that stabilize the hip joint.

Pectineus, Pirifirmis, Superior and Inferior Gemullus, and Obturator Internus and Externus Muscles

Studies on the muscles of the hip joint have typically neglected the roles of the deep musculature (Pectineus, Piriformis, Superior and Inferior Gemullus and Obturator Internus and Externus) because of their inaccessibility and their proximity to femoral vessels. Thus, the functional roles of these muscles have been debated [50-52] with little direct evidence to support opposing views. These muscles are often thought to be the "rotator cuff" muscles for the hip, and many studies in the canine models have supported their roles in "fine tuning" hip motions [53]. However, unlike the glenohumeral joint, the human hip is considered a more stable joint via its bony articulations requiring less muscular stabilization. To this end, many authors have suggested that the small PCSA of these deep muscles combined with their small moment arms (eg, pectineus moment arms during gait has been estimated at less then 9 mm for stance phase of gait) are negligible in providing any "meaningful" forces for maintaining hip stability. Nevertheless, clinical views of the function of the pectineus make this muscle's role in hip function more important then one would ascertain from its small size and moment arm. Lamb and Pollock [54] suggested that pectineus overactivity is the major cause of flexion deformity of the hip in children with cerebral palsy. Arnold and Delp [37] have shown that the pectineus posses a internal moment arm during the upright standing position; but this muscle can posses a small external hip rotation moment when walking with an exaggerated internal thigh rotation (as noted in Fig. 7 of Arnold and Delp) [37]. These computational results correspond well with EMG profiles during gait in healthy persons. The pectineus is moderately active at mid-heel strike to mid toe-off, functioning to limit femoral abduction and contributing to femoral medial rotation. Some minor activity is also present during the swing phase [55].

Assessing the functional EMG of the pirifirmis, superior and inferior gemul-lus, and obturator internus and externus) has proven difficult given their anatomic locations and relative inaccessibility and their proximity to femoral vessels. However, new technologies such as dynamic MRI combined with computer modeling and simulation may offer some exciting advancements in understanding the functional roles of these muscles in the years to come.

Iliopsoas

Based on the anatomic insertion and origins of the iliopsoas, it is the only muscle that has the anatomic prerequisites to simultaneously and directly contribute to stability and movement of the trunk, pelvis, and leg. This muscle has two major portions (the iliacus and the psoas). These two portions have separate innervations, which makes selective activation of each portion feasible for any given movement. However, only a few studies have attempted to define and differentiate the function roles of the iliacus and psoas independently and simultaneously [56,57].

When one begins to search the literature for precise information about the actions and functions of the iliopsoas (or psoas and the iliacus independently), the only point that is agreed upon is that this muscle is a flexor of the hip and probably has some influence on the lumbar vertebrae and pelvis in maintaining appropriate postures. Thus, there is some disagreement in the EMG information of this muscle, partly resulting from different techniques and the difficulty in measuring EMG in this muscle due to its location and pennation. Andersson et al [57] found both muscles are inactive during ipsilateral leg extension; whereas, contralateral leg extension resulted in selective recruitment of the iliacus alone. Andersson et al also noted that both muscles are active during maximal thigh abduction, but no postural activity is noted for either psoas or iliacus during standing at ease or with the whole trunk flexed 30° forward at the hip [57]. These postural positions also did not recruit the psoas or iliacus after loads up to 34 kg were added. In summary, Andersson et al concluded that the iliacus primarily stabilizes the motions between the hip and pelvis, whereas the psoas assists in stabilizing the lumbar spine in the frontal when a heavy load is applied to the contralateral side.

Iliacus

Attempts to measure EMG of the iliacus alone have shown notable activity throughout flexion of the hip during the "sit-up in the supine position" [56]. LaBan et al [58], however, found that there was little or no activity in the iliacus during the first 30° of hip flexion, but these authors noted activity during a sit-up from the "hook-lying" position. Greenlaw and Basmajian [56] further reported both medial and lateral rotation of the hip joint may produce some slight iliacus activity, whether the hip joint is passively or actively held in any of the extended, semiflexed, or flexed positions.

Psoas Major

Direct recordings from the psoas muscle are generally similar to those measured from the iliacus with a few noted exceptions. There is slight activity during relaxed standing and strong activity during flexion in many postures [57]. Also, slight to moderate activity in abduction and lateral rotation (depending on the degree of accompanying hip flexion) [57] is present, with no activity during most medial rotations and little activity during most other conditions involving the thigh [56,57]. Nachemson [59] concluded that the psoas has a significant role in maintaining upright postures.

Gluteus Maximus

Karlsson andJonsson [60]concluded that the gluteus maximus was active during extension of the thigh at the hip joint, lateral rotation, abduction against heavy resistance when the thigh is flexed to 90°, and adduction against resistance that holds the thigh abducted. The studies of Joseph and Williams [61] show that the gluteus maximus is not an important postural muscle but it exhibited moderate activity when bending forward and when straightening up from the toe-touching position [61]. In positions in which one leg sustains most of the weight, the ipsilateral gluteus maximus is active. Joseph and Williams [61] also found that, during standing, rotation of the trunk activates the muscle that is contralateral to the direction of rotation (ie, corresponding to lateral rotation of the thigh).

Gluteus Medius and Minimus

The finding of Joseph and Williams [61] that the gluteus medius and minimus are quiescent during relaxed standing serve to confirmed that these abductors prevent the Trendelenburg sign, during abduction of the thigh and in medial rotation. The Gluteus medius' and minumus' role(s) in medial rotation was confirmed by Greenlaw [62], who reported triphasic activity for gluteus medius and biphasic activity for gluteus minimus during each cycle of walking. Houtz and Fischer [63] concluded that the activity in all the glutei was minimal in bicycle pedaling (Fig. 14.5). During elevation (flexion) of the thigh in erect posture, Goto et al [64] found that the anterior part of the gluteus medius was also active in the initial stage only.

Tensor Fasciae Latae

Wheatley and Jahnke [65], Carlsoo and Fohlin [66], Goto et al [64], and Car-valho et al [67] found moderate activity in this muscle during flexion, medial rotation, and abduction of the hip joint. Duchenne [68] reported that the power of tensor fasciae latae as a rotator in response to faradic stimulation is weak. Carlsoo and Fohlin [66] argued the rotary influence of tensor fasciae latae affect at the knee, finding no activity. Greenlaw [62] found the muscle was active biphasically during each stride of the gait cycle. Unlike the glutei, tensor fasciae latae was active during bicycling, showing their greatest activity during the hip flexion phases [63].

Adductors of the Hip Joint

Janda and Vele [69], andJanda and Stara [70] investigated the role(s) of the hip adductors in children and adults during flexion and extension of both the hip and the knee, with and without resistance. They showed that the adductors were activated during flexion or extension of the knee, and became more active with resistance in children. Similarly, adults exhibited activity during flexion of the knee, but only a minority was active during extension compared with children. Janda and Stara [70] stated that this response of the adductors is related to postural control, and suggested that these muscles are facilitated through reflexes of the gait pattern rather than being called upon as prime movers.

De Sousa and Vitti [71] investigated the adductor longus and magnus during movements of the hip joint. During adduction, the longus was always active while the magnus is was almost always silent unless acting against resistance. Both muscles were shown to be active during medial thigh rotation but not during lateral rotation of the hip with the upper fibers of the adductor magnus showing the greatest activity.

Greenlaw [62] examined subjects during both fast test movements and various postures and locomotions. When standing on one foot, the adductors on that side remained silent. Medial thigh rotation recruited all the adductors. During walking, these adductors showed different types of phasic activity. There is marked difference between the two parts of the adductors magnus: the upper, possessing a pure adductor role and was active throughout the whole gait cycle, while adductor brevis and longus showed triphasic periods with the main peaks occurring at toe-off [62].

SPORT-SPECIFIC MECHANISMS OF HIP INJURIES IN THE ATHLETE

As arthroscopic treatments of the hip continue to evolve, there is an increasing need to understand the basic performance biomechanics of the hip joint. This information is important, as it can provide the foundation by which joint function, pathology, and therapeutic modalities can be evaluated. There are a number of recent studies that have applied different approaches to study the hip biomechanics, particularily in THR. However, there is clearly a void in the amount of literature related to the function, and pathology of the normal or injured, nonarthritic hip. Thus, the remainder of this article will offer our understanding as to how these injuries result in athletes. It is important to keep in mind that a majority of athletes undergoing hip arthroscopy have a complex injury pattern, with damage to the acetabular labrum, capsular structure, and cartilage surfaces. To ascertain the specific injury sequence and pattern(s) of cause and effect, significant research still needs to be performed.

Golf

During the downswing of a right-handed golfer, the right hip is forced into external rotation during axial loading. This movement tends to push the femoral head anteriorly, and over time may lead to focal anterior capsular laxity and stretching of the iliofemoral ligament [72,73]. Subsequent joint instability may result leading to increased translation of the ball in the socket. Labral tears, particularly in the anterosuperior weight-bearing region of the acetabulum, may follow. The labrum has been shown to function as a physiologic seal, stabilizing the femoral head in the acetabulum [74,75]. In a further propagation of the injury, labral tear leads to reduction in seal function; increased translation of the femoral head may result. In addition, an unpublished report by Bharam et al (70th Annual Meeting of the American Academy of Orthopaedic Surgeons) showed that chondral delamination in the area adjacent to the labral tear is a frequent finding in golfers.

Taekwondo

In martial arts, particularly taekwondo, a good kick can be performed well above an athlete's head. The proper positioning for a taekwondo side kick places the stance leg in 90° of external rotation. The stance leg must then sustain significant loads while the opposite leg performs the kick. Similar to the mechanism in golfers, the forced external rotation and axial loading in the stance leg (not the kicking leg) may cause anterior capsular laxity and elongation of the iliofemoral ligament. As a result of the increased translation of the femoral head with respect to the acetabulum, labral and chondral injuries may follow.

Ballet/Figure Skating

Elite ballet dancers and figure skaters perform the extremes of rotational movement during their routines. Flexibility of the lower extremities is crucial for success. Some athletes excel at these sports due to their generalized ligamentous laxity; yet, despite this apparent advantage, they may also suffer from symptoms of hip instability. Other ballet dancers and figure skaters may suffer from instability secondary to repeated hip rotation and focal capsular laxity. Hip laxity has been reported in a ballet dancer to be the cause of atraumatic dislocation of the hip [76]. A very common finding in ballet dancers and figure skaters undergoing hip arthroscopic surgery is capsular laxity with associated labral tear

Injuries to the ligamentum teres are also common in ballet dancers and figure skaters. This ligament connects the margins of the acetabular notch and transverse ligament to the fovea capitus on the femoral head. It is thought to function as a secondary stabilizer to external hip rotation [77]. In athletes with hip instability, the ligamentum teres is under increased stress to help stabilize the joint. Tears to the ligament often result.

Ice Hockey

Hockey players may suffer from traumatic hip injuries after direct blows to the greater trochanter. Isolated labral tears and chondral injuries from simple mechanical shearing are commonly found in these patients [78]. In addition to trauma, hockey players can suffer from overuse-type hip injuries. While skating, significant flexion, abduction, and slight external rotation forces are present at the hip. As a goalie, the hip sustains significant flexion and internal rotation forces. In flexion and abduction or flexion and internal rotation, any morphologic abnormality at the femoral head-neck junction would hit the antero-superior labrum and the acetabular rim. This abnormality is found in patients with cam-type femoroacetabular impingement [1,2,79] and is a very common finding in elite hockey players undergoing hip arthroscopy. Whether this is a subtle developmental deformity exacerbated by sport or whether there is a unique mechanism for the development of cam-type impingement in athletes is still not known.

Running

Although most cases of hip instability are present in athletes whose sports demand excessive rotational movements, runners may also present with subtle anterior hip instability [80]. In the stride phase of high-level extensive running, repeated hip hyperextension may stretch the anterior capsule and iliofemoral ligament. The resulting microinstability may subtly increase femoral head translation, and with repeated insults, cause labral tear and chondral injury.

During running, when the foot contacts the ground the femur is in an abducted position in relation to the pelvis. Thus, the gluteus medius and tensor fascia latae are eccentrically loaded. As the running support phase progresses, these muscles must then contract as abduction occurs at the hip. Thus, it is believed that gluteus medius weakness may lead to decreased thigh control manifesting in increased thigh adduction and internal femoral rotation. These changes may predispose the runner to several pathologic conditions including iliotibial band syndrome at the knee [81].

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Osteoarthritis

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