iliopsoas

J Orthop Sci (2002) 7:199207

The function of the psoas major muscle: passive kinetics andmorphological studies using donated cadavers

Masaharu Yoshio1, Gen Murakami2, Toshio Sato2, Shuichi Sato3, and Seiji Noriyasu1

1 Department of Physical Therapy, School of Health Sciences, Sapporo Medical University, Minami 1 Nishi 17, Chuo-ku, Sapporo,Hokkaido 060-8556, Japan2 Department of Anatomy, School of Medicine, Sapporo Medical University, Sapporo, Japan3 Department of Physical Therapy, School of Health Sciences, Aomori University of Health and Welfare, 58-1 Mase Hamadate,Aomori 030-8505, Japan

Introduction

The majority of hemiplegic patients and almost 20% ofelderly women are able to raise their legs while in thesupine position, but have difficulty in using the reboundof this action to sit up from the supine position.22,23

Moreover, when walking, stroke patients tend to flexthe hip joint and assume an anteflexed posture.Similarly, many infants suffering from cerebral palsyappear to be humpbacked in the lumbar region whensitting.20 We speculated that a basic disorder thatappears to be common in these patients is insuffici-ent stabilization of the hip joint.21 Particularly, wehypothesized that, to maintain a posture during astanding or sitting position, the psoas major muscle(PMM), known to be the most powerful flexor and theonly muscle acting directly on the hip joint, as well as onthe vertebral column, makes a major contribution tostabilizing the hip joint rather than acting as a flexor.

Surgical procedures to diminish or destroy PMMfunction have been performed in patients whose flexioncontracture of the hip joint was suspected to becomesevere in the future.35,15,17 From the opposite viewpoint,some orthopedic surgeons have also believed in a closerelationship between PMM function and hemiplegicgait and/or sitting. Several physiologists2,7,8 haveattempted to elucidate the action of the PMM fromelectromyograms, but their results were controversialand even led to critical discrepancies, depending ontheir experimental approaches. Why is PMM functionso difficult to identify? A cue for resolving the confusionhas been given by LaBan et al.,11 Linden and Delhez,13

and Andersson and Oddsson,1 who suggested thatdifferent actions of the PMM depended on the flexionangle at the hip joint.

We considered that the kinetics of the PMM shouldbe analyzed in its dynamic phase (flexor function for thelower extremity), as well as in its static phase (fixation ofthe hip joint to maintain a sitting or standing position

Abstract This study was carried out to analyze the phasicheterogeneity in the function of the psoas major muscle(PMM) depending on the flexion angle at the hip joint.The study design was a passive kinetic experiment using 25osteoligamentous specimens with the PMM tendon. Wemeasured the flexion angle of the hip joint where the PMMtendon loses contact with the femoral head and pelvic surface.Ten osteoligamentous specimens were used for additionalmeasurements of the tensile force and pressure exerted on thePMM and/or at the bone-tendon interface when the PMMtendon was gently pulled in line with the PMM origin in thesupine position. The tension loading the PMM tendon wasmeasured at seven different angled positions of hip jointflexion (0, 15, 30, 45, 60, 75, and 90), using a load cellattached to a traction appliance. The pressure was measuredat each of eight sites along the long axis of the PMM, using apressure sensor. The PMM tendon lost contact with thefemoral head at angled positions of 14 (average) hip flexion,and lost contact with the iliopectineal eminence at positions of54 (average). The tension was stronger at angled positions of030 at the hip joint. The pressure on the femoral head andpelvic surface were stronger at positions of 030 at the hipjoint. The pressure on the femoral head was strongest at a hipflexion of 0. The tensile force markedly decreased at 4560flexion at the hip joint, while the pressure on the femoral headgradually reduced to zero in the same phases. We concludedthat the PMM works phasically: (1) as an erector of thelumbar vertebral column, as well as a stabilizer of the femoralhead in the acetabulum at 015 flexion at the hip joint; (2)less as a stabilizer, in contrast to maintaining its erector action,at 1545; and (3) as an effective flexor of the lower extrem-ity, at 4560.

Key words Psoas major muscle Kinetics Hip joint

Offprint requests to: M. YoshioReceived: April 4, 2001 / Accepted: October 31, 2001

200 M. Yoshio et al.: Kinetic study of the psoas major muscle

against gravity). We believed that our kinetic studiesof the PMM would provide critical understanding ofhemiplegic gait and sitting, even though PMM functiontends to be given little attention in the field of physicaltherapy.

Materials and methods

Preparation of experimental specimens

Thirty-five donated cadavers (24 males and 11 females,from individuals aged over 70 years when they died)were used for the passive kinetic experiments. Twenty-five of these cadavers were used for observations asto which changes in the PMM tendon occurred duringflexion movement, while 10 cadavers (7 males and 3females) were used for measurements of tensile forceand pressure. There was no preexisting pathology, forexample, significant osteoarthritis, in any of thecadavers.

Osteoligamentous specimens were prepared forkinetic experiments on the hip joint by the followingprocedure. The trunk was amputated between the 11thand 12th thoracic vertebrae, so as to leave the originof the PMM intact. The pelvis, with the lumbar verte-brae, was then sectioned in half, and amputation wasperformed at the knee joints. Soft tissues were thenremoved from the bone, leaving only the PMM and thearticular capsules and ligaments of the hip joint. For theobservation of changes in the course of the PMMtendon (25 specimens), the articular capsule of the hipjoint was partly removed to show the contact of thetendon with the femoral head and to show the positionof the head in the acetabulum.

The specimen was placed on a hinge-like table, with atilting board attached to the base. Blocks of wood werescrewed into the pelvis and into the lumbar vertebrae,and these blocks were fixed to the hinge-like table withclamps. The line joining the anterior superior iliac spineand pubic tubercle was made parallel to the surface ofthe base table, i.e., the line was kept horizontal. Thisoperation made the so-called angle of pelvic tilt almost60, which corresponded to the standard tilt angle of thehuman pelvis in the standing position.9 Thus, the pelviswas fixed on the stable base table, while the femur,supported by the tilting board, was operated to showvarious flexion angles in the hip joint, depending on theexperiments (see below) (Fig. 1).

After observation of the morphology of the PMM ata hip joint flexion angle of 0, the musculotendinousregion of the PMM was dissected at the origin, andthe muscle belly was removed. During the preparationdescribed above, we confirmed smooth flexion move-ment (090) of the specimens at the hip joint.

Changes in contact with the PMM tendon duringflexion movement

Using 25 osteoligamentous specimens with the PMMtendon, we measured the flexion angle of the hip joint atthree points (A, B, and C in Fig. 2), A being the anglewhere the PMM tendon loses contact with the femoralhead, B, the angle where the articular surface of thefemoral head becomes invisible alongside the tendonand disappears into the acetabulum (here, the tendonnever contacts with the femoral head), and C, the anglewhere the tendon loses contact with the iliopectinealeminence to float on the pelvic surface (here, the tendonshows an almost straight course from the lessertrochanter to the origin at the lumbar spine). Finally,the angle at the so-called pulley (i.e., bony structure16),which corresponds to the highest point of theiliopectineal eminence, was measured. In a certainrange of hip flexion, i.e., almost between the angles Band C, the PMM tendon turns backward to the lessertrochanter at this pulley, showing an obtuse angle.

Measurement of tension and pressure

We conducted a passive kinetics study using tenosteoligamentous specimens. The angle of the hip jointwas set at seven different angled positions (0, 15, 30,45, 60, 75, and 90), step by step, by adjusting thehinge-like, tilting board, the angle depending on theexperiment. The tension loading the PMM tendon wasmeasured at each angle of hip joint flexion, using a loadcell (Kyowa Electronic Instruments, Tokyo, Japan;

Fig. 1. The experimental design. Medial view of left-sidedosteoligamentous specimen. The pelvis (P) is fixed on the basetable (BT) with screw devices (asterisks), while the femur (F)lies on the tilting board (TB). The traction appliance (TA) isconnected to the psoas major muscle (PMM) tendon. The loadcell (LC) for the tension measurement is interposed at theconnection. The operator holds a supporting handle of thepressure sensor with his right hand

201M. Yoshio et al.: Kinetic study of the psoas major muscle

LU-20-KSB34D; instrumental errors in measurementwithin plus or minus 0.5%; Figs. 1, 3, 4) attached to atraction appliance. The load cell is a strain gauge thatsenses only forces that affect its strain column at thecenter. A clip attached to the tip of the load cell wasscrewed into the PMM tendon. By holding the grip ofthe traction appliance, the tendon was gently pulled inline with the PMM origin, and the tension was measuredwhen the femoral shaft rose to approximately 1cmabove the base table (Fig. 1). The tension measure-ments were performed eight times at each angle, be-cause the pressure measurement (see below), done atthe eight different measurement sites (see below), wasaccompanied by the tension measurement.

Measurement of the pressure at the tendon-boneinterface (i.e., between these sliding surfaces) was con-

ducted at the same time as the tension measurement. Apressure sensor (Kyowa Electronic Instruments; PS-5KA; instrumental errors in measurement within plusor minus 1.0%; Figs. 3, 4) attached to a steel handle wasinserted into the tendon-bone interface to measure thepressure of the PMM tendon acting upon the bone sur-faces (Figs. 3, 4). The pressure sensor is a subminiaturestrain gauge. For each of the seven angled position ofhip joint flexion, pressure was measured at each of eightmeasurement sites along the long axis of the PMM: 2cm

Fig. 2AC. Observations of the attachment of the left-sidedPMM tendon to the bony surface; A, in the extended position;B, in slight flexion; C, in greater flexion. F, Femur; P, pelvis;star, PMM tendon; thin arrows, femoral head; thick arrows,pelvic surface

Fig. 3. Measurement of the pressure on the bony surface.Ventromedial view of the left-sided osteoligamentousspecimen. The PMM tendon (star) and the lesser trochanter(asterisk) are clearly seen. The traction appliance (TA), loadcell (LC) for the tension measurement, connector, and PMMtendon are arranged and connected sequentially. The pressuresensor (arrow with PS) is inserted under the tendon. F, Femur;P, pelvis

Fig. 4. System for the measurement of tension and pressure


(s2) and 1cm (s1) upward from the level of theiliopectineal eminence (a point where the long axis andtransverse line crossed); at the level of the iliopectinealeminence (s0); and 1cm (i1), 2cm (i2), 3cm (i3), 4cm(i4), and 5cm (i5) downward from the eminence level,respectively (Fig. 5).

The tension and pressure data obtained from the twosensors were transferred via a sensor interface board(Kyowa Electronic Instruments; PCD-100A-1A) to apersonal computer (Fig. 4). The data were analyzed foreach flexion angle and measurement site.

Morphological observations and measurements ofthe ten specimens used for measurement

Because the femur, PMM tendon, and hip joint work asa lever-like system comprising a fulcrum and two pointsof action, the tension and pressure would be committedto morphometric parameters concerning these lever-like structures. In order to evaluate which morphome-tric parameters influence the tension and/or pressure,the following measurements were made on the PMM,pelvis, and femur: (1) weight and length of the femur;(2) diameter of the femoral head; (3) the distance fromthe pectineal eminence to the lesser trochanter; (4) thedistance from the lesser trochanter to the most cranialpoint of the femoral head; (5) the neck-shaft angle andfemoral antetorsion; (6) the width and thickness of thePMM tendon at each measurement site (Fig. 5); and (7)the angle between the distal and proximal courses of thePMM tendon at the so-called pulley at the measurementsite.

Results

Changes in contact with the PMM tendon duringflexion movement

In the 25 specimens examined, the PMM tendon lostcontact with the femoral head at angled positions of719 of hip flexion (average, 14; angle A; Fig. 2A).Conversely, at less angled positions (less than 7 in allspecimens), the tendon was in contact with the femoralhead and turned its course backward along the cur-vature of the head. After the flexion angle increased,the PMM tendon lost contact with the iliopectinealeminence, to float on the pelvic surface at positions of4267 (average, 54; angle C; Fig. 2C). In the latterpositions, the tendon showed an almost straight coursefrom the lesser trochanter to the origin at the lumbarspine. In every specimen, at a certain angled positionbetween the two angles A and C, the articular surfaceof the femoral head became invisible alongside thetendon and disappeared into the acetabulum. When thearticular surface disappeared, the flexion angle rangedfrom 25 to 53 (average, 39; angle B; Fig. 2B). Almostbetween the two angles A and C (absolutely betweenthe angles B and C), the PMM tendon was in contactwith the pelvic surface and turned backward on thepulley at the iliopectineal eminence. In this situation,the tendon did not push the femoral head. Con-sequently, the PMM tendon pushed the femoral headonto the acetabulum at the beginning of the flexion (atless than 14 flexion, on average), and then the tendontook a load on the pelvic surface instead of the femoralhead, especially onto the pulley, at a flexion position ofless than 54, on average. On hip joint flexion of 67 (themaximum of angle C), the actual contact at the tendon-bone interface was reduced to a pin-point, or even lost.The angle at the pulley, which corresponded to thechange in the tendon course at angles of about 1560,ranged from 119 to 135 (average, 124). The angle atthe pulley showed no distinct relationship with theabove mentioned three angles A, B, and C (Table 1).

Tensile force exerted on the PMM tendon

Reproducibility of experiments. The absolute tensionvalues, measured at each of seven angled positions (0,15, 30, 45, 60, 75, and 90), were different in theeight repeated experiments. The deviation in the valuesranged from 0.5 to 1.0kgf at each angle in all tenspecimens. This deviation corresponded to a smallpart (less than 1/10) of the maximum amplitude offluctuation in tensions measured at the seven differentangles (Fig. 6). Therefore, we regarded the deviationduring the repeated experiments as negligible. More-over, these eight repeated experiments clearly showed acommon pattern (Fig. 6).

Fig. 5. Measurement sites for the pressure at the tendon-boneinterface. Ventral view of the right half of the specimen. Theeight measurement sites (s2, s1, s0, i1, i2, i3, i4, and i5: also seetext) are put along the long axis of the PMM (thick black line).In this case, the tendon changed its course at the s0 pulley atthe highest level of the iliopectineal eminence


Tensile forces: general and individual observations. Thetension was large at the angled positions of 0, 15, and30, while it was weak at positions of 60 and 75. Themaximum tension was found at 15 in six of the tenspecimens, while the other four specimens showedmaximum tension at the 30 position (Fig. 7). Of the tenspecimens, specimen no. 749 required the strongestforce to pull up the femur from the 0, 15, and 30positions. This specimen was characterized by arelatively large angle of the tendon course at the pulley(129; the maximum was 135: see above, under theheading Changes in contact with the PMM tendonduring flexion movement.) Between 0 and 15, thePMM was fully extended, and its tendon was in contactwith both the pelvis and the femoral head at sitess0i4.

The tension showed a large decrease between anglepositions 30 and 60. Notably, sideslip of the tendonwas consistently observed during the process of de-creasing tensile force, especially at and over 45 offlexion. Between the 60 and 90 positions, five speci-mens showed a constant decrease of tension as the angleof flexion increased toward 90, while the tension inthe other five specimens slightly increased or remainedalmost constant. Six specimens showed mini-mum tension at 90, three at 60, and one at 75. Theminimum tension ranged from 2.0 to 6.8kgf. Theminimum tension ranged from 20% to 66% (average,43%) of the maximum tension. Specimen no. 745,showing the largest percentage (67%), produced arelatively large tension at any angled position and had amoderate angle at the pulley (123), whereas specimenno. 773, showing the smallest percentage (20%), wascharacterized by a distinct decrease of tension at the

Fig. 6. Reproducibility of measurements of the tensile force.The tensile force varied depending on the flexion angle (090), but the deviation in the eight measurement sites rangedfrom 0.5 to 1.0kgf at each angle. Moreover, these eightexperiments clearly showed a common pattern

Fig. 7. Changes in tensile force exerted on the PMM tendondepending on the different flexion angles

Table 1. Changes in attachment of the psoas major muscle(PMM) tendon during flexion movement

No. Pulley A B C

1 119 17 42 522 119 17 53 583 119 15 48 534 119 8 30 535 120 9 40 586 120 19 42 527 121 13 40 558 121 7 46 679 122 15 53 56

10 122 13 25 5911 122 13 44 5912 122 9 29 4513 123 16 46 4914 123 7 33 4215 125 17 47 5516 126 18 34 4917 126 16 34 5218 127 17 34 4719 127 14 29 4320 128 14 30 5121 128 18 36 5622 128 18 33 5323 129 17 45 5824 130 9 48 6425 135 13 35 53Average 124.0 14.0 39.0 53.6SD 4.2 3.8 8.0 5.9

Values are degreesA, The angle where the PMM tendon detaches from the femoral head;B, the angle where the articular surface of the femoral head becomesinvisible alongside the tendon and disappears into the acetabulum; C,the angle where the tendon detaches from the iliopectineal eminenceto float on the pelvic surface


greater angled position. Specimens nos. 754 and 758showed paradoxical alterations of tension, i.e., increasesof tension at the 75 and 90 positions.

Pressure on the bony surface

The measurement of pressure was conducted not onlyat seven different angles of the hip joint, i. e., 090positions but also at eight different sites, i.e., s2, s1, s0,i1, i2, i3, i4, and i5 (Fig. 5). Thus, at each measurementsite, the pressure measurement was performed seventimes.

Reproducibility of experiments. Special attention had tobe paid to sideslip of the PMM tendon, which was oftenunavoidable at an angle of 45 or more. We monitoredthe slipping tendon (if present) to keep the sensor justunder the tendon. A similar problem occurred becauseof variation of the bony surface: some specimens carrieda slight groove for the tendon on the bony surface,usually at measurement sites s0, i1, and i2, while othershad no groove.

Pressure: general observations and individual differ-ences. The maximum pressure was consistently foundat the 0 angled position, while the minimum pressurewas seen at positions over 60. Concerning the differ-ence in results obtained at different measurement sites,we noted two observations, as follows. First, individualdifferences were evident at the pelvic surface (s2, s1,and s0), in contrast to an almost constant pattern foundat the surface of the femoral head (i1, i2, and i3).Second, the above-stated relation between the flexionangle and pressure was modified, slightly or greatly,when pressure was measured at the pelvic surface ratherthan being measured at the surface of the femoral head.

The data obtained at s0, representing the pressure onthe pelvic surface, are shown in Fig. 8, and the data at i3,representing the pressure on the femoral head, areshown in Fig. 9. At the lesser angles of flexion (0 and15), the femoral head, rather than the real pulley,played the role of pulley to turn the tendon backward atthe site s0. At the lesser angled positions, the PMMtendon was attached to and curved along the femoralhead at i1, i2, and i3. Along the pelvic surface, includingthe s0 pulley, the tendon showed a straight course at thebeginning of flexion. Accordingly, the tendon pushedthe femoral head onto the acetabulum, especiallyonto its posterosuperior aspect. Moreover, the tendoncaused femur abduction, because the lesser trochanteris located in the medial part of the turning point of thetendon course: e.g., site i3 on the femoral head at thelesser angled positions.

At the measurement site s0, variations betweenspecimens were evident at any angled position. Some

specimens showed a constant decrease of pressure whenthe hip angle increased from 0 to 90. Others exhibiteddistinct fluctuations of pressure, including a flat orincreased pattern, at smaller flexion angles, i.e.,between 0 and 45. However, at positions over 60, thepressure decreased considerably, to nearly zero level inall specimens. The exception was specimen no. 754,which showed very low values and almost no change inpressure at any angled position. This specimen carriedthe smallest femoral head and had a relatively largeangle at the s0 pulley (130).

At site i3, all specimens demonstrated a constantdecrease of pressure when hip flexion increased. Inparticular, the pressure on the femoral head decreasedmarkedly at the lesser angled positions, i.e., 0, 15, and30 of hip flexion. Moreover, in contrast to the dataobtained at s0, the pressure was very weak, being almostnonexistent, at angles greater than at 45. The exception

Fig. 8. Changes in pressure on the pelvic surface at s0depending on the different flexion angles

Fig. 9. Changes in pressure on the femoral head at i3depending on different flexion angles


to the very weak pressure obtained at large angles wasfound in specimen no. 750. This specimen was chara-cterized by: (1) a large femoral head; (2) a thin but widetendon at site i3; (3) the smallest neck-shaft angle(120); (4) the largest femoral antetorsion (30, rangingfrom 13 to 30); and (5) a relatively small angle at thepulley (123). The pressure at i3 was usually greaterthan that at s0 at the lesser angled positions, in which i3provided a pulley for the PMM tendon instead of s0.

Morphometric evaluations of the ten specimens usedfor measurements

No significant variations were seen in the topographicrelationships between the PMM tendon, femoral head,and pelvis; the size of the femoral head (its diameterranged from 46 to 56mm); on the localization of thehead on the pelvis (e.g., the distance from theiliopectineal eminence to the edge along the acetabularlabium ranged from 12 to 20mm); or the localization ofthe head in the femur (e.g., the distance from the lessertrochanter to the most cranial point of the head rangedfrom 86 (no. 744) to 108mm (no. 773). Likewise, thedistance from the iliopectineal eminence to the lessertrochanter did not show a significant variation (max,99mm; min, 90mm; average, 94mm). Specimens nos.754 and 759 with the minimum value (90mm), hadweaker tension and pressure, whereas the largestdistance (99mm in specimen no. 750) was associatedwith relatively strong pressure, at over 60.

The weight (510755g) and length (360455mm)of the femur showed relatively distinct individualdifferences. The specimen with the largest femur(specimen no. 773) showed a moderate pressure anda relatively strong tension, while the smallest femur(specimen no. 758) exhibited a moderate level ofpressure and relatively weak tension. The neck-shaftangle and femoral antetorsion also varied, but theywere limited to the full range for the Japanese popu-lation. However, the minimum neck-shaft angle (120 inspecimen no. 750) was associated with no pressureobtained at positions over 60, whereas specimen no.744, with the maximum neck-shaft angle (135), showedmoderate pressure and tension.

Discussion

The present kinetic experiments and observations ofthe topographic anatomy of the PMM tendon and bonysurface revealed that, during flexion movement at thehip joint with increasing angles, the tensile force on thePMM gradually decreased, after showing a maximum at15 or 30 of flexion. This peak of the tension seemedto correspond to a phase in which the PMM tendon

attaches to and pushes the femoral head. Therefore, thetensile force was exhausted in greater part by thefixation of the femoral head and partly by the flexionmovement. In the next phase, the pressure on thefemoral head, represented by the data at site i3,decreased markedly during the increasing flexionmovement between 30 and 45. This decrease inpressure seemed to correspond to a phase in which thePMM tendon loses contact with the femoral head, andthen with the iliopectineal eminence (site s0, or thepulley). In eight sites for the measurement of pressure,the strongest force was measured at i1i3 (i.e., on thefemoral head), and the next strongest force was exertedon the pulley at which the PMM tendon changes itscourse in this phase of flexion. Therefore, the femoralhead seems to work as an actual pulley to receivestronger pressure than the iliopectineal eminence andto guide the tendon backward. Overall, we consideredthat, in the first step of hip flexion (030), the tensileforce generated by the PMM is sacrificed for thetentative (or phasic) insertion at the femoral headand/or the pulley. Here, the force is used for thestabilization of the femoral head into the acetabulum.Simultaneously, owing to this tentative insertion(s), thePMM would tend to act as an erector of the lumbarvertebral column rather than as a flexor of the hip joint,sometimes even rather than as a stabilizer of the joint.

Kimura et al.10 have reported that the human PMMcontains many red muscle fibers, which performpersistent tonic contraction. Their result seemed tosupport the conclusion that the human PMM is not onlyused to flex the hip joint but that it also has a uniqueantigravity function in erect bipeds. This idea is alsopartly supported by Santaguida and McGill,18 whodescribed in detail which portion of the PMM isresponsible for the different movements of the lumbarvertebral column, according to magnetic resonanceimaging (MRI) observations.

Lyons and Peterson14 reported a critical angle atwhich so-called snapping occurs when the hip joint isextended from a flexed position to nearly 45. Thesnapping seems to be caused by a sudden change inthe course of the PMM tendon, such as the slidingor sideslip seen during the present experiments. Weconsider that the snapping represents a marked changeof pressure on the bony surface, especially on thepulley. Before the snapping (i.e., at the greater flexionangle), owing to the straighter course of the tendon,the PMM is likely to act more effectively as a hip flexor.At a greater flexion angle (6090), no or almost nopressure, as measured in the present experiment,indicated that the PMM tendon actually has no contactwith, and therefore has no action on, the pelvis andfemoral head. However, if the iliacus muscle shifts thePMM tendon laterally and fixes it onto the pelvic


surface, the PMM tendon is likely to alter its positiongradually to avoid the snapping, although the iliacuswas not included in our experimental design. Moreover,we considered that a flexion angle of over 60 at the hipjoint was outside the activities of daily living (ADL),because, within a flexion angle of 6090, the PMMtendon seems to be checked by the femoral nerve, ves-sels, and an innominate ligament in front of the muscle.

Routine X-ray examinations of a person in a relaxedsitting position (with flexed lumbar vertebral column),as well as in a strained sitting position (with a straighterlumbar vertebral column) showed not nearly 90flexion, but almost 60 flexion at the hip joint (Fig. 10).Flexion of over 60 seems to happen in limitedsituations, such as during sports training, e.g., in raisinga thigh close to the abdominal wall.

Previous results, taken together with the presentresults, led us to hypothesize that there were threephases of PMM function, depending on different anglesof hip joint flexion: (1) as an erector of the lumbarvertebral column, as well as a stabilizer of the femoralhead onto the acetabulum (015); (2) exertingdecreased stabilizing action, in contrast to maintainingthe erector action (1545): and (3) as an effectiveflexor of the lower extremity at the hip joint (4560).

Our results also suggested that the largest tensile forcegenerated by the PMM was used for phase 1, the nextlargest was for phase 2, and the smallest force might beexerted for flexion, i.e., phase 3. We consider that thePMM function as a hip stabilizer is overshadowed bythe action of stabilizing (or erecting) the lumbar verte-bral column, and the former action has been ignored, incontrast to the latter, which has been well investi-gated.1,13 A limited account of the stabilizing action wasfound only in the study of Glauber and Vizkelety.6 Theydescribed the pressure of the active anterior acetabu-lar wall, although they had no comment about theflexion angle. Classical kinetic studies using cadav-ers,12,19 as well as electrophysiological experiments,11

have described whether or not the hip joint or the lum-bar vertebral column moves when the PMM works.However, the action of stabilizing the hip joint, a staticaction, is unidentifiable in view of a dynamic action, i.e.,whether it moves or not. This phasic heterogeneity isthe reason that the PMM function has not been noted orhas been difficult to understand.

Because the present experimental condition cor-responds to flexion movement in the supine position,we could apply the results, especially the phasicheterogeneity of PMM function, to an evaluation andrehabilitation program, to be carried out with subjectsin a supine position, for the early management of strokepatients. We consider that the strongest tensile forceloading onto the PMM, at 015 of flexion, shouldbe reduced by the therapist in the early stage ofrehabilitation. Even in a person in good health, thePMM hardly acts to raise the thigh or trunk in thesupine position when the hip joint is almost fullyextended. However, the therapist needs to teach PMMfunction as a stabilizer at such a small flexion angle.Therefore, for the first step, the 3045 flexed positionseems to be best for the maneuver. We also considerthat learning to perform a concentric contraction of thePMM should be separated from learning to perform anexcentric contraction, although both are necessary inthe phase when the muscle acts as a stabilizer.

Conclusions

We analyzed the phasic heterogeneity in the functionof the psoas major muscle (PMM) depending on theflexion angle at the hip joint in the supine position, using35 osteoligamentous specimens with the PMM tendon.The PMM works phasically: (1) as an erector of thelumbar vertebral column, as well as a stabilizer of thefemoral head in the acetabulum, at 015 flexion atthe hip joint; (2) less as a stabilizer, in contrast to main-taining its erector action, at 1545; and (3) as an effec-tive flexor of the lower extremity, at 4560.

Fig. 10A,B. X-ray examination of the hip joint in the sittingposition. Lateral views. Dotted line connects the superioranterior iliac spine (SAIS) with the pubic tubercle (PT).The doubled lines, paralleling the dotted line, correspond tothe base table used for the present cadaveric experiment. Therelaxed sitting position in A carries a smaller flexion angle(angle a) than the strained sitting position in B (angle b).Angle b is much less than 90, i.e., the right flexion angle atthe hip joint. Angle a ranges from 30 to 60. F, Femur; P,pelvis


Acknowledgments. We are grateful to Professor M.Watanabe (Hokkaido University Graduate School of Medi-cine) for his permission to examine his cadaveric specimensfor the first part of the study.

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