Page 8 P H Y S I O T H E R A P Y March, 1969 FUNCTIONAL ANATOMY OF THE HIP B y J. M. W A L K E R , M .N .Z .S .P ., D ip .T .P ., Lecturer, Sub-D epartm ent Physiotherapy, University o f the Witwatersrand A review o f recent literature, with emphasis on the role o f the Hip Abductors. It has been said that “orthopaedic surgeons worship at the shrine o f the Quadriceps” 11, the object o f worship at the hip should fall to the abductor group, which however do not receive the sam e attention. The sam e attitude often applies to m any physiotherapists. Hip Abductor Function T he hip abductors are G luteus M edius, M inim us and T ensor Fascia Lata. A n atom ists give the action o f the abductors as primarily abduction o f the hip, that is, abduc­ tion o f the free femur on the fixed pelvis and secondarily, to control the lateral pelvic tilt when standing on one leg. T hat is, describing the action, in the first instance in the tradition, that m uscles arise by a fleshy belly, insert by a tendon and in shortening approxim ate the insertion towards the origin; abduction o f the femur on the fixed pelvis. This action is however, not com m on ly performed in everyday life (som e occasion s, m oving sideways, getting in and out o f a car, getting on a horse, vaulting) and is rarely performed against any significant resistance other than the weight o f the limb. The real function o f the abductors, controlling and m oving a considerable load is abduction o f the m obile pelvis, on the fixed femur and prevention o f adduction o f the pelvis on the weight bearing hip under the superincumbent body weight10. This occurs every time weight is taken on one leg. G L U T E I . W E I G H T OFeooy. ---------- * -------------------------------- H IP “J o i n t . Fig. 1. The lever principle o f the hip joint. The H ip Joint a s a Fulcrum T o fully appreciate the role o f the abductors it is important to realise that the hip joint acts as a fulcrum o f an unequal lever system, the long arm o f which supports the body weight, the short arm o f which being controlled by the gluteal m uscles (see Fig. 1). D u e to the differences in the length o f the lever arms the abductors have to exert con ­ siderable force to obtain this balance. W hen standing on one leg, the line o f gravity, weight transm ission, falls c om m on ly on the medial side o f the hip. This has been calculated to be four inches from the centre o f the joint. It has also been calculated from determining a resultant line o f force for the abductor group (see F ig. 2) that gluteal force acts at a distance o f two inches from the centre o f the joint. The ratio o f the lever arms is therefore 4: 2, so that when a person is standing on one leg and weighs 150 lbs. (minus the weight o f the supporting lim b) a gluteal force o f 300 lbs. at least is required to maintain coronal balance (see F ig. 3, where x equals gluteal force). It should be noted that the force acting through the head o f femur is not just the body weight but that the hip joint is in fact subjected to the sum m ation o f these tw o forces* body weight and gluteal force (see Fig. 4). It has thus been calculated that the forces acting on the head o f femur are at least twice the body weight. The force transmitted by the hip is the sum o f the total supported body weight plus the tension in the balancing m uscles, which is not solely pro­ duced by the abductors. When a person stands on one leg (in this instance the left), m aintaining a level pelvis, as the line o f gravity is m edial to the pivot the pelvis tends to rotate F ig. 2. Postero-lateral aspect o f the hip, showing attachment o f the glutei and resultant line o f force o f hip abductors. 150 x 4 = x x 2 x = 300 Joint Pressure = 150 + 300 = 450 lbs. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) March, 1969 P H Y S I O T H E R A P Y Page 9 C A R T O O N COmpre«»to>% lo a d ^lu'I’euS m*diU3 *• m i n i m u s i m p M * #r» f i t m n r Fig. 4 clockw ise about the supporting hip and this is counteracted by an anti-clockw ise force produced in part by the pull o f the abductors and in part, by the considerable force which is contributed by the passive tension o f the fascia lata and the ilio tibial tract6. F ascial structures play a large part in assist­ ing and sparing the hip muscles. It is interesting to note that, excluding G luteus M axim us, the actual com bined m ass o f the Gluteus M edius, M inim us and Tensor Fascia Lata is only 15.3 per cent o f the total m ass about the hip6. I f an abductor force graph (see Fig. 5) is exam ined it can be seen that the highest recording from the abductors occurs when the non-weight-bearing buttock is elevated and a pelvic tilt o f 10-15 per cent towards the weight-bearing side is present. If the pelvis is allow ed to sag, 10-20 per cent to the non-weight-bearing side, light activity only occurs in the abductors. A t this point fascial structures, par­ ticularly the fascia lata are taut and are acting under lo a d 6. Factors which affect the R atio o f the Lever Arms The load on the abductors and thus the force they are then required to produce to maintain coronal balance and counteract the rotatory tendency o f the body weight about the supporting hip is very much influenced by the ratio in the length o f the lever arms and in the am ount o f body weight (see F ig. 6), which illustrates how an increase in body weight requires a corresponding increase in gluteal force. F ig. 7 illustrates how , if there is shortening o f the femoral neck, as is seen follow in g fractures o f the neck, or in the presence o f an adduction deformity o f the hip, as in osteoarthritis, the distance at which the gluteal m uscles have to exert their force on the hip is reduced and a much greater force is then required to m aintain balance. Clinically, patients with these factors present, which require increased strength o f the abductors to obtain pelvic balance, such as the overweight osteoarthritic patient, usually present with weakness o f the abductors and a gluteus medius limp, the trunk swaying over the supporting hip with each step. Insufficient abductor force and thus an unbalanced lever system. 12 a z o o III 10 a U_I1 a i- i - 5 1 0 i r U g 2 7 s ° 6 go 5 p i * F O R C E C U R V E 5 u b je c t S.T. S / 3 / 4 5 • M a x im u m e f f o r t P e l v i s e l e v a t e d 20® o n n o n w e i g h t b e a r i n g s i d e ' P e lv is le v e l ~ P elvis sa ogin o t o w a r d non w e ig h t b e a .r in o side 200 -4 0 0 600 aoo 1000 1100 T O R Q U E ( K I L O G R A M - C E N T I M E T E R S ) Fig. 5 Abductor Force Graph In weakness and in the presence o f a painful hip condition a gluteus medius limp is seen, the patient tending to reduce the m om ent o f his body weight abou t the hip by m oving his centre o f gravity laterally and bringing the line o f weight transm ission to fall more vertically over the hip. By doing so, the demand o n G luteus M edius is decreased as the unequal length o f the lever arms has been reduced. T his is illustrated in F ig. 8 where there is n o w a ratio o f 3 : 2. The sam e effect can be gained surgically, by osteotom y. U se o f a Cane It has also been show n that the use o f a cane, held in the opp osite hand affords relief o f jo in t pressure and reduces the work done by the hip muscles quite out o f proportion to the thrust exerted on it11 (see F ig. 9). The body weight, attem pting a clockw ise turning m om ent on the hip is counteracted by b oth the cane and the abductors exerting an anti-clockw ise turning m om ent. 20 inches is the distance the cane is held from the pivot. W here the upward thrust from the cane is 30 lbs. gluteal force is reduced to nil. I f the upward thrust from the cane was 20 lbs., instead o f 300 lbs. o f gluteal force being required only 100 lbs. is necessary, and w hen the upwards thrust is 10 lbs. only 200 lbs. o f gluteal force is required. It has therefore been suggested that on e aspect o f re-education o f the abductors can be achieved by patients using progressively lighter canes, thus progressively increasing the required gluteal force. Further, that as the use o f a cane plays such an important part in relieving the join t from strain, its discard should not neces­ sarily be hastened11’ 3. Rem em bering that the join t is sub­ jected to the sum m ation o f the tw o forces, body weight and gluteal force, in the first exam ple the jo in t pressure was cal­ culated at 450 lbs. W ith the use o f a cane, and an upwards R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) Page 10 P H Y S I O T H E R A P Y March, 1969 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) March, 1969 P H Y S I O T H E R A P Y Page 11 thrust o f 30 lbs., joint pressure can be reduced to 120 lbs. jt should be noted that these figures are pounds o f body weight, if the calculations were to be in pounds per square inch o f pressure on the fem oral head, the figures would be substantially greater. Function o f Gluteas M axim us Finally, consider the function o f G luteus M aximus. Anatomists give its action as extension o f the hip, that is movement o f the femur on the fixed pelvis. Functionally this muscle, o f considerable bulk, principally acts to extend the m obile pelvis to the vertical position on a fixed, weight- bearing femur and acts with its greatest force from a position o f flexion to the vertical. A ctions as lifting a heavy weight, the body from the crouching position. M ovem ent beyond the vertical, in the upright position , is controlled by Ilio-psoas acting to prevent the trunk from falling backwards over the femur12. Leaning forwards when going up a steep hill enables us to use G luteus M axim us with greater effort. Arthritic patients tend to walk with a flexed hip, this m ay not be solely due to the presence o f a hip flexion contracture, as this flexed hip posture allow s the patient to use G luteus M aximus with greater force. It m ust be noted that this hip flexion must be corrected before the hip abductors can be correctly re-educated. C O N C L U S IO N In conclusion, a review o f recent literature draws our attention to the facts that: (i) forces approxim ately twice the body weight at least pass through the hip jo in t when standing on one leg. (ii) the major hip m uscles functionally act, with force, against a load, to m ove the m obile pelvis on the fixed (weight-bearing) femur. (iii) the hip abductors are norm ally required to act with a force at least twice the body weight to maintain coronal balance, and in m any hip disorders, seen in physiotherapy departments, are required to act with even greater force. (iv) G luteus M axim us acts with its greatest force from a p osition o f flexion to the vertical. A s physiotherapists, concerned with obtaining stability round the joint and in establishing an efficient gait pattern more attention should be paid to the strength and functional ability o f the hip abductors in controlling the m ovable pelvis o n a fixed femur. The ability o f a patient to lift the leg, against gravity in side lying does not indicate sufficient strength to prevent the presence o f a gluteus medius limp. R E F E R E N C E S 1. B atchelor, J. S. (1959). The Surgery o f the H ip. P roc. R o y . Soc. M e d ., 52, 358. 2. Basm ajian, J. V. (1967). M u scles A live, 2nd Ed., W illiam s & W ilkins. 3. D enh am , R . A. (1959). H ip M echanics. J. Bone Jt. Surg. 41B , 550. 4. Grant, J. C. (1958). A M e th o d o f A natom y, 6th Ed., W illiam s & W ilkins. 5. G ray’s A natom y, 33rd Ed., L ongm ans. 6. Inm an, Verne (1947). F unctional Aspects o f the Abductor M uscles o f the H ip. J. B o n e J t. Surg., 29, 607. 7. M acC oniall, M. A . The M ovem ents o f B ones & Joints, ibid, 30B- 322, 31B-100, 32B-244. 8. Saunders, J. B., Inm an & Eberhart (1953). The Major D eterm inants in N orm al and Pathological G ait, ibid, 35A , 543. 9. Steindler (1955). “ K in esiology o f the Hum an B ody under N orm al and Pathological C on dition s.’ Oxford, Blackwell Scientific Publications. 10. Strange, F. G. St. C. (1959)1 Som e A spects o f M uscle M echanics, Proc. R oy. Soc. M e d ., 52, Section 27, 897. 11. Ib id (1965) The H ip, L on d on , H einem ann M edical B o o k s Ltd. 12. Ib id (1967). The H ip, Applied A natom y and M echanics, P hysioth erapy, 53, 396. 13. Wright, Verne & Johns, J. (1960). Physical Factors Concerned with the Stiffness o f N orm al and D iseased Joints, Bull. John H opk in s H ospital, 1 -6 , 215. SAVE WITH t h e s o u t h A f r i c a n s -the friendly people at the SOUTH AFRICAN (PERMANENT Ib u il d in g s o c ie t y Growing with South Africa...since 1883 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. )