Architecture of the Short External Rotator Muscles of the Hip

Abstract

BackgroundMuscle architecture, or the arrangement of sarcomeres and fibers within muscles, defines functional capacity. There are limited data that provide an understanding of hip short external rotator muscle architecture. The purpose of this study was thus to characterize the architecture of these small hip muscles.MethodsEight muscles from 10 independent human cadaver hips were used in this study (n = 80 muscles). Architectural measurements were made on pectineus, piriformis, gemelli, obturators, quadratus femoris, and gluteus minimus. Muscle mass, fiber length, sarcomere length, and pennation angle were used to calculate the normalized muscle fiber length, which defines excursion, and physiological cross-sectional area (PCSA), which defines force-producing capacity.ResultsGluteus minimus had the largest PCSA (8.29 cm2) followed by obturator externus (4.54 cm2), whereas superior gemellus had the smallest PCSA (0.68 cm2). Fiber lengths clustered into long (pectineus - 10.38 cm and gluteus minimus - 10.30 cm), moderate (obturator internus - 8.77 cm and externus - 8.04 cm), or short (inferior gemellus - 5.64 and superior gemellus - 4.85). There were no significant differences among muscles in pennation angle which were all nearly zero. When the gemelli and obturators were considered as a single functional unit, their collective PCSA (10.00 cm2) exceeded that of gluteus minimus as a substantial force-producing group.ConclusionsThe key findings are that these muscles have relatively small individual PCSAs, short fiber lengths, and low pennation angles. The large collective PCSA and short fiber lengths of the gemelli and obturators suggest that they primarily play a stabilizing role rather than a joint rotating role.

Impacts of external rotators and the ischiofemoral ligament on preventing excessive internal hip rotation: a cadaveric study

“…The structural property of the hip external rotators suggested that force generation was maximized to act as a single axis of rotation because its pennation angle was almost parallel [ 18 ]. Because of this structural property, the hip external rotators compressed the hip joint along a single axis.…”

“…External rotators might act synergistically in vivo to stabilize the hip joint at any hip flexion angle. An anatomical study of external rotators reported that the superior gemellus, obturator internus, inferior gemellus, and obturator externus were essentially fused [ 18 ]. Thus, these external rotators can interact with each other and function as one muscle.…”

Isolated Obturator Internus Muscle Strain Injury in a Professional Football Player: A Case Report

“…The obturator internus is one of the short external rotators of the hip. It is the longest of the external hip rotators with a length of 13.12 ± 0.27 cm [ 1 ]. It originates from the inferior margin of the superior pubic ramus and from the pelvic surface of the obturator membrane around the obturator foramen.…”

“…The obturator internus muscle can be considered as a single functional unit together with the superior and inferior gemellus as well as the obturator externus muscle. All together, they are important stabilizers of the pelvis as they stabilize the femoral head to the acetabulum [ 1 ].…”

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“…This group of muscles helps to stabilize the femoral head in the acetabulum during movement and provide a uniform external rotation force despite changes in hip position or muscle length. 9,15 The quadratus femoris arises from the external aspect of the ischial tuberosity and inserts into the trochanteric crest of the femur. The obturator internus arises from the inner surface of the superior pubic ramus and obturator membrane before being reflected at a right angle over the grooved surface of the ischium where it joins the gemelli muscles (triceps coxae) to insert on the greater trochanter as a "conjoint tendon."…”