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.


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.


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 and Jonsson [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].

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