Gabriella Bargon, Sue Deacon, Sharon Tsang, Lena Wallin
Background
The literature indicates that a general consensus exists concerning the anatomy of the posterior hip external rotators (Gray and Williams 1989, Kendall et al 1993, Moore 1992); however, there is a discrepancy among many anatomists and biomechanists regarding the function of these muscles (Delp et al 1999, Delp et al 1990, Dostal et al 1986, Gray and Williams 1989, Pressel and Lengsfeld 1998, Shinohara 1995). The similarities of the anatomy of the posterior external rotators are outlined in Appendix A. Anatomical variations have been noted in the relationship of the obturator internus with the gemelli (Gray and Williams 1989, Shinohara 1995). Shinohara (1995) described the gemelli as two muscular accessories to the obturator internus tendon and has termed this relationship the ‘rotator triceps’. This theory, based on common anatomical and neural innervation findings, suggests that they may in fact be three bellies of the same muscle (Shinohara 1995). Aung et al (2001) studied 101 pelvic halves and reported the superior and inferior gemelli muscles to be attached to the obturator internus muscle belly prior to its musculotendinous junction. This differed slightly to the findings of Shinohara (1995), who reported that in only 36% of their cases were the three muscles found to run in the same plane. In the remaining cases, the obturator internus tendon was covered on its superficial and deep surface by the gemelli muscles and this provided a pocket in which the tendon could slide.
Few studies have evaluated the function of the posterior hip external rotators and therefore, their role in both closed kinetic chain (CKC) and open kinetic chain (OKC) exercises are poorly understood. The function of the hip muscles depends on several factors including the size of the muscle, the orientation of the muscle fibers and their moment generating capacity. Several authors have identified various functional compartments of the gluteal muscles (Delp et al 1999, Delp et al 1990). It has been shown that at 0 degree of hip flexion, only the anterior compartments of gluteus medius and minimus have internal rotation moments and all of the other posterior muscles have varying degrees of external rotation moments (Delp et al 1999). Biomechanical studies have indicated that as the hip flexes from 40 dgree to 90 degree, there is a trend towards increasing internal rotation in the anterior and middle fibers of gluteus medius and minimus, piriformis, obturator internus and the superior and inferior gemelli (Delp et al 1999, Dostal et al 1986). Only gluteus maximus, quadratus femoris and obturator externus retain their external rotation moments (Delp et al 1999). As the degree of hip flexion increases, all of the short external rotators may become abductors (Dostal et al 1986). It has been shown that the abduction function of the gluteus medius is greatest between 0 degree and 40 degree of hip flexion (Dostal et al 1986). Anatomical textbooks do not outline the abduction and internal rotation functions of obturator internus and the gemelli (Gray and Williams 1989).
