Hips: Gluteus medius stimulates lower extremity movement

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Biomechanics Magazine http://biomech.com July 2006

http://biomech.com/showArticle.jhtml?articleID=191601708

Gluteus medius stimulates lower extremity movement

By: Jim Lundy, DPT, CSCS

In the world of rehabilitation and biomechanics, centrally located

muscles tend to have a larger impact on function and daily activity

than more distal muscles. With our bipedal mobility, the hip joint

tends to be the fulcrum for most activity, including gait. Of all the

hip muscles, the gluteus medius (GM) may have the largest impact on

lateral as well as frontal mobility and stability of the hip.

The gluteus medius is commonly involved in posture and strength

impairments. It also wields influence on more distal muscles and

lower kinetic chain joint interactions. As the location of this

muscle is between the lower extremity and the trunk, and as adverse

gluteus medius activation has a significant impact on human mobility,

this muscle demands serious attention and consideration in all

rehabilitation sciences. A review of the literature of the

biomechanics of the GM, both current and long-standing, may help

biomechanics practitioners understand the influence of this muscle on

function, its impact on other distal segments and other muscle

activation, and mechanisms to decrease its activation by use of

assistive devices.

Anatomy and function

Traditionally, the GM has been labeled a hip abductor. It is two

times larger than the gluteus minimus and three times larger than the

tensor fascia latae. The three muscles combined equal 15.3% of the

entire muscle mass of the hip joint.1 The fan-shaped GM has three

distinct parts. It has been found that the best position for hip

abduction is between 0 degrees and 40 degrees of flexion.2 As hip

flexion increases to 90 degrees , all three heads of the GM become

less efficient and the small external rotators of the hip take over

its abduction.

Hip flexion may also affect rotation moments, both internal and

external, around the hip.3 With the hip at 0 degrees of flexion, the

anterior portion of the muscle has a small internal rotation moment

arm while the middle and posterior parts have small external rotation

moment arms. As the hip flexes, the anterior portion increases its

internal rotation and the more posterior compartments switch from

external to internal rotation moments. (This trend was noted with 15

of the 18 hip muscles tested.) Thus, the GM muscle action appears to

have some positional component to its inherent activation.

Gait and stairs

Initially, research regarding motion of the GM's three heads during

functional activities suggested that all three work together for

pelvic control, especially with weight-bearing activities.4

In this 1978 study by Soderberg and Dostal, walking, sit-to-stand

transfers, and ascending and descending stairs involved the most GM

activation; crawling or tying shoes engaged very little.

Additionally, the three heads generally operated in concert, with

only slight differences between them during several activities. This

was the first suggestion in the literature of independent work among

the three heads of the GM during functional tasks.

Other early research found that the entire GM (and gluteus maximus)

was most active while ascending stairs and walking fast; walking

slowly induced less activation and descending stairs activated the

muscle only minimally.5 This study also suggested the GM was active

primarily during the loading response of gait and stair climbing with

minimal activity during midstance. The research, however, did not

differentiate among the three heads but considered the GM as one

muscle. These initial research endeavors investigated the role of

pelvic control in function and possible interactions between the

three heads of the GM and other stabilizing muscles.

More recently, the focus of hip abduction research has broadened to

consider hip stabilization and pelvic rotation (with the gluteus

minimus). Gottschalk, Kourosh, and Leveau in 1989 postulated that the

posterior part of the GM and entire gluteus minimus stabilize the

femoral head in the acetabulum with femoral motions, especially

during the gait cycle.6 In contrast to the earlier research,

and Pandy showed the GM provides the majority of support in

midstance.7 They reported that the posterior GM and the gluteus

minimus provide support throughout the entire midstance while the

anterior GM contributes significantly only at the end of midstance.

More recent laboratory research reaffirms posterior GM support

throughout midstance, more anterior GM activity in later midstance,

and both heads contributing to accelerating body mass forward.8 This

idea that the GM not only provides transverse support to the hip but

also contributes to sagittal mechanics for forward progression with

walking is novel. Whether the concept holds up or not, the GM is a

significant contributor to pelvic support during the gait cycle.

The gluteus minimus also contributes to pelvic stability and weakness

in that muscle may play a role in such altered gait patterns as

Trendelenburg gait, which has traditionally been associated with

weakness of the GM.9 The GM has the most strength for contraction at

0 degrees to 20 degrees of adduction. As hip abduction increases, the

GM loses its mechanical efficiency for contraction. Activation in the

GM decreases as hip abduction increases, whereas activation in the

gluteus minimus is maintained throughout increasing levels of

abduction. Also, after five minutes of unilateral standing, the

gluteus minimus of the stance leg demonstrated greater signal

intensity with positron emission tomography (PET) compared to the GM

in a study by Oi et al.10 In the same study, carbohydrate metabolism

(indicating intensity of muscular activity) after 30 minutes of

walking was found to be highest for the gluteus minimus (compared to

the gluteus maximus and the GM). (The lower legs demonstrated the

greatest muscular activity, however.) Thus, although the GM remains

the predominant hip abductor, the gluteus minimus may have a more

pronounced role in stabilizing the pelvis than was previously thought.

The gluteus minimus is significantly smaller in mass than the GM, and

its depth may have precluded previous electromyographic (EMG)

research from assessing its involvement in stance. MRI and PET scans

were the methods used to assess gluteus minimus and gluteus maximus

activation in the previous study and are useful for assessing muscle

involvement after exercise. The combined efforts of the GM and the

gluteus minimus in maintaining pelvic stability is an exciting area

for further research.

With regard to stairs, the GM makes a significant contribution to

weight shifting. Sims and Brauer found that the GM (as well as the

tensor fascia latae) activated more intensely and with faster onset

with stepping up motions than with stepping forward motions.11 With

an upward step, increased medial-to-lateral displacement of the

center of gravity requires the GM to engage more quickly and with

more force than is necessary when walking forward. Ultimately, this

research showed that although both stepping up and stepping forward

require unilateral stance, stairs require greater medial-to-lateral

displacement of the pelvis and the GM responds to this challenge with

more significant activation than it demonstrates in walking.

Muscle firing direction

That the GM has a strong posterior component has been noted in seated

muscle testing.12 Dubois et al found that with voluntary

contractions, the GM's direction of activation is abductive and

posterior. The tendency for this muscle to be recruited in a specific

direction is significant. When the investigators passively stimulated

the same muscle, the direction of muscle action was again in a

posterior and abducted position. (It must be noted that the testing

occurred in a seated position, which is not the ideal position for GM

activation.) When more posterior movement is available, during

standing, for example, the GM may display more movement in a

posterior direction-in addition to its abduction direction.

Although the GM is defined as an abductor with some posterior

component, some research suggests it has a transverse plane pelvic

stabilization function. When Schmitz et al13 applied a posterior

force to the nonstance hip during a unilateral stance position, the

stance side GM underwent significant activation isometrically to

prevent transverse plane pelvic rotation. The hip and knee in this

study were at 0 degrees of flexion, and only isometric holding

positions without functional hip motion were considered. With some

flexion, less GM activation is noted. When forces to the nonstance

leg are modified in different directions-posterior, anterior, and 45

degrees posteriorly-more specific activation of the GM heads is

noted. Earl14 found that the anterior and middle GM activate

significantly with a 45 degrees posteriorly directed force compared

to static hip abduction stance or a 45 degrees anteriorly directed

force. That study also found that further activation of the GM muscle

heads is noted with application of increased resistance levels.

Since these studies were conducted with healthy subjects and did not

normalize the EMG data, the level of GM activation with these forces

relative to a maximal contraction cannot be ascertained. However,

they both found that the GM activated with changes to pelvic

transverse plane resistance without hip abduction present. Thus, it

appears that the anterior and middle GM components, in an effort to

provide pelvic stabilization during unilateral stances, will activate

by some additional percentage against a transverse plane resistance.

Other issues

In addition to preferred directions of activation, bony and

biomechanical issues can also affect muscle activation. In 18 healthy

women, Nyland et al15 found that increased femoral anteversion, as

measured by medial hip rotation displacement, resulted in decreased

GM and vastus medialis amplitude. Medial hip rotation greater than 42

degrees was associated with a 34% decrease in GM amplitude and a 27%

decrease in vastus medialis amplitude. (It should be noted that this

study was nonweight-bearing whereas most traditional studies are

weight-bearing.) This research shows that biomechanical femur

displacement can result in reduced activation of the GM and vastus

medialis. It may also have implications for patellofemoral issues if

both thigh and pelvic muscles are effectively weakened by bony

alignment aberrations.

Activation of the GM may also be related to posture or leg dominance.

In a group of healthy men and women who were right-handed, Neumann

and colleagues16 noted a small but significant difference in hip

abduction torque between left and right sides. The right hip

abductors were found to be stronger when compared to the left hip

abductors. The largest differences were at 0 degrees and -10 degrees

of abduction (or 10 degrees adduction); however, these increased

strength measurements were noted in all degrees of abduction. In

another healthy young population, s et al17 found the dominant

leg hip abductor demonstrated greater strength, with an average of

11% difference between it and the nondominant leg. (The foot used for

kicking a ball determines dominance.) Occasionally, side-to-side

differences can exceed 15% in healthy young subjects during a dynamic

hopping test, according to a study by s and Mattacola.18

Dominance, however, does not affect the GM's force production during

sustained fatiguing contractions. s and colleagues17 reported

that the GM strength curve shifts from fast-twitch to slow-twitch

fibers with 30-second endurance contractions. Furthermore, the

fatigability of the GM was not correlated to maximal strength test

scores. The muscle generally fatigued in a linear progression with

submaximal sustained contractions regardless of maximal strength.

Thus, although the GM may demonstrate side-to-side strength

differences in asymptomatic healthy individuals, these differences

may be difficult to assess with standard clinical manual muscle

testing. Additionally, endurance (fatigability) testing should be

separated from maximal strength testing.

Finally, activation of the GM may be affected by an individual's age

and normal aging physiology. In a study comparing older (mean age 74

plus/minus 6.8 years) versus younger (23 plus/minus 1.3 years) female

subjects, the older subjects produced significantly less abduction

(and adduction) isometric peak torque and even less isokinetic peak

torques than the younger ones.19 This is reasonable, as normal muscle

physiology dictates that older persons have a decreased capacity to

generate muscle force. This information has implications for balance,

falls, and hip stability in the older patient.

GM and osteoarthritis

Often research has sought to find ways to unload body weight in

patients with pathologies. This focus is very important as the hip

joint normally loads about 238% of BW with walking and 260% of BW

with stairs, according to a study by Bergmann et al.20 For patients

with osteoarthritis (OA), Sims et al21 found increased GM firing with

stepping activities compared to normal patients. This increased GM

activity was noted on both the affected and the nonaffected sides.

Conversely, the tensor fascia latae activated similarly in both

groups. Interestingly, another study found degenerative pathology of

the hip abductor mechanism (GM tendon tears) in 20% of patients with

OA of the hip,22 indicating that the inflammatory component of OA

might not be the only factor altering GM firing.

In contrast to the research indicating increased GM activation in OA

hip patients,21 Arokoski et al23 found hip isometric abduction

strength to be 31% lower in their population of hip OA subjects when

compared to healthy age-matched controls. Also, no side-to-side

strength differences were noted between the normal and diseased hips

in the OA group. Although the cross-sectional area of the hip muscles

did not differ between the OA group and the healthy group, the

diseased hip in the OA group was 6% to 13% smaller than the

nondiseased hip. It may be, therefore, that muscle strength may be

lower in OA subjects compared to normal subjects even though the

cross-sectional area may be very similar.

Obviously, some factor other than overall cross-sectional size of the

GM must explain the reduced muscle strength in the osteoarthritic

hip. The somewhat contradictory results of these two studies may be

due to experimental design, as one study used weight-bearing

functional activities while the other tested the GM in nonweight-

bearing cardinal planes. Both results (decreased overall hip strength

and increased GM firing) may quite possibly occur in OA patients with

different tasks, indicating that the disease results in altered

activation patterns relative to healthy subjects.

Loading and unloading

Neumann and Cook24 compared EMG firing of the GM during walking when

external loads (10% and 20% of BW) are applied to the body at

different carrying positions. They found that the most EMG firing

occurs with contralateral loading and the least with ipsilateral

loading. Carrying positions were contralateral and ipsilateral to the

tested hip, and anterior and posterior to the chest. Ipsilateral

loading actually decreased EMG firing to below the no-load condition

(carrying no weight). Interestingly, GM firing with posterior loads

was close to that of the no-load condition, whereas anterior loads

consistently created more EMG firing. Neumann reported in another

study25 that even with lighter loads, the findings of increased GM

EMG with contralateral added weight and decreased GM EMG with

ipsilateral added weight remained consistent. To explain the

significantly increased GM firing relative to posterior loads, the

author postulated that possibly more demands were placed on the

posterior fibers of the GM with anterior loads.

In addition to external ipsilateral loading, the use of a cane has

also been shown to decrease GM activation.26,27 Gait aided with a

cane on the contralateral side of a prosthetic hip resulted in a 28%

to 40% decrease in hip contact pressure in the acetabulum.26

Concurrently, EMG analysis demonstrated an approximately 20% to 45%

decrease in activation of the GM.26 Although the gait cadence that

produced these results was slow, 60 beats (steps) per minute, this

still shows that the use of a cane can decrease loading on the hip

and the consequent GM activation necessary for unilateral support.

Although some research confirms decreased GM activation with

contralateral cane use (36% to 46% less than without the cane),28

another recent study shows no change in activation and notes

concurrent increased abduction moments on the contralateral hip and

decreased moments on the surgically treated hip.29 Overall, most

research suggests position of loads and the use of assistive devices

can have a dramatic effect on loading of the hip and consequent

activation of the GM.

Surgery and hip strength

When hip strength and hip replacement surgery have been considered

together, strength results have been variable. In a study by Vaz et

al,30 total hip replacement patients demonstrated initial weakness

after surgery followed by significant and progressive strength gains

(hip abductor torque) compared to presurgery strength levels.

Strength gains continued up to 24 weeks after surgery. Functional

tests generally did not show significant changes compared to those in

hip strength, even though post- versus presurgery functional scores

did improve.

In another study, Shih et al31 found that total hip arthroplasty

(THA) patients demonstrated significant abduction muscle torque gains

at one-year follow-up. However, hip strength never equalized between

the affected and contralateral sides (strength in the affected hip

was 81% to 89% of the nonsurgical side). Another recent study noted a

small, nonsignificant strength difference between involved and

uninvolved sides at 12 months post-THA.32 These strength differences

were noted in all hip directions, including abduction, and the

involved side was consistently weaker than the uninvolved side. It

should be noted that the study population was small. Results from a

larger subject pool may elucidate these strength imbalances.

In contrast to research suggesting lingering strength imbalances, a

study by Horstmann et al33 found abduction isokinetic torque at nine

weeks postsurgery was equal to preoperative levels and had increased

only slightly at six-month follow-up. Linear increases in strength

were found in other hip directions (flexion/extension) with abduction

strength improving only minimally. Functionally, gait improvements

were noted, with less limp postoperatively, even in the absence of

strength gains.

In whatever manner hip strength is affected postsurgically, Trudelle-

and colleagues found that a strengthening program initiated

between four and 12 months postsurgery can improve hip muscle

strength. They reported that hip abductor strength had improved 41%

postintervention.34

Distal segments

Joints and muscles distal to the pelvis can have an influence on GM

activation. Beckman and Buchanan35 investigated the ankle joint and

its effect on GM activation. They noted quicker GM activation in

previously hypermobile (sprained)-compared to normal-ankles when they

were perturbed. This rapid onset may be a protective preparatory,

feed-forward, or guarding activity mechanism of the GM. When an

inversion moment on a hypermobile ankle occurs, the contralateral GM

activates first, followed by activation of the ipsilateral GM.

Obviously, this altered movement pattern has developed over time. But

the resultant muscle recruitment pattern remains consistent.

Therefore, the GM may activate as a protective mechanism for the

pelvis and body when chronically sprained ankles are rapidly shifted

or altered.

This research is taken further when somatosensory deficits are

experimentally induced in the lower leg. Imai et al36 found that when

sensory input from one leg was decreased, standing balance was

maintained by significantly increased activation of the GM on both

sides with cocontraction of the unaffected tibialis anterior and

soleus. This study investigated only acute deafferentation, so it

cannot speak to long-term decreased sensory situations such as

neuropathies. Of special note, initially after compression of the

thigh to induce deafferentation, increased GM firing occurred on the

nonaffected side. This indicates that the brain identifies decreased

sensory inputs relatively quickly and will activate other pelvic

muscles (the GM in particular) to compensate for these deficits.

Thus, mediolateral postural stability during standing is maintained

in the presence of decreased sensory input to an affected lower

leg/ankle through the increased initial activation of the unaffected

GM followed by GM contractions on the affected side.

Altered back, leg, and hip firing patterns have been noted in

hypermobile ankles in other research.37,38 The GM was not

investigated in these studies. The gluteus maximus was the primary

hip muscle considered, presumably as the GM is not involved with hip

extension. The gluteus maximus activated consistently later in the

legs with hypermobile ankles than in the normal legs. Activation in

the erector spinae and biceps femoris was similar to that of

noninjured subjects.

Although comparing GM to gluteus maximus activation onset time is not

appropriate, the results do substantiate that distal joints (ankles)

can instigate altered firing patterns in proximal joints (hips). Long

duration of inappropriate sensory input or weakness in ankles would

be surmised to be the cause of altered firing patterns. Further

research must elucidate which mechanism may alter the activation

patterns in the hip joint.

With normal ankles, GM (and gluteus maximus) activation can be

altered or improved with distal sensorimotor input.39 After one week

of walking with balance shoes (platform shoes with a knob underneath

creating a labile surface) for 15 minutes per day, both muscles had

quicker onset times during the gait cycle (up to 75% of maximal

activation). Compared to barefoot walking, activation with the shoes

was greater, and subsequent barefoot walking (after seven days)

demonstrated improved GM activation. Thus, unstable and labile

surfaces can alter the sensory inputs from distal segments and speed

up the activation pattern of the GM. The assumption is that the GM

activates to provide additional stability for unstable distal

segments. If this is correct, altering balance distally can be used

for treatment and to improve activation of the GM.

With regard to the foot and GM, foot wedging can produce muscle

changes during the gait cycle. In a study by Bird et al,40 the GM was

shown to have a significantly delayed onset with either unilateral or

bilateral heel lifts of 2 cm. This corresponded to only 2% of the

entire gait cycle, which may not be clinically significant. Another

study, by Hertel et al,41 showed that orthotic intervention, in

contrast-regardless of the type of posting-significantly increased GM

activation during single-leg squat and lateral step-down. However,

when a more explosive activity, maximal vertical jumping, was

investigated, use of orthoses did not affect GM activation, although

it did significantly decrease vastus lateralis activity. The

difference in muscle activation with different activities may be

explained by differences in study parameters. The gait study taped

orthoses to bare feet while the squat/jump study used off-the-shelf

orthoses in athletic shoes. In both studies, subjects were given only

a short time to acclimate to the orthoses. Thus, changes in GM

activation may be explained more by the changes in sensorimotor

feedback than by the type of orthosis (posting type, heel height, or

presence of unilateral/bilateral orthosis).

Conclusion

The GM is not simply a hip abductor. It has implications for

stability of the hip as well as that joint's movement. Overall, its

influence on function can be considered both feed-forward as well as

feedback in nature. As the GM is a link between the lower extremity

and the trunk, any movement of either is likely to activate this

muscle and its proper functioning is imperative to assist that

motion. Clinicians should increase their focus on this muscle when

evaluating lower extremity function, dysfunction, and movement.

Jim Lundy, DPT, CSCS, is co-owner of Athletic Physical Therapy in Los

Angeles.

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