Gabriella Bargon, Sue Deacon, Sharon Tsang, Lena Wallin
Gluteus maximus
Gluteus maximus and medius are commonly divided into 3 functional compartments (Delp et al 1990). However, Delp et al (1999) noted significant changes in the muscles moment generating capacity in 6 separate compartments of the gluteus maximus and 4 of the gluteus medius. In the present dissection, 3 compartments with different fiber orientations were identified in the gluteus medius. In the gluteus maximus, however, only 2 main areas were identified, a superior and inferior compartment. From an anatomical dissection alone, it may not be possible to accurately identify the functional compartments of these muscles. The gluteus maximus develops from the fusion of a superior and inferior fetal muscle. The inferior muscle has been termed the ‘coccygeo-femoralis’(Kirici and Ozan 1999) and may reflect the different fiber orientation seen in the distal muscle belly in the present dissection.
The potential force generating capacity of the gluteus maximus was evident from the dissection. From origin to insertion, the muscle broadens with respect to its vertical length. In fact, the length of the insertion onto the iliotibial tract (ITB) and femur (29 cm) is almost twice that of the muscle’s origin (16cm). The fibers of the gluteal muscles are more vertically orientated and may serve to superiorly displace the femoral head. In constrast, the more horizontally directed fibers of the short external rotators may provide posterior hip stability. This coincides with a study in which the incidence of hip dislocation was drastically reduced when the short external rotators were surgically repaired following a posterior approach hemiarthroplasty (Ko et al 2001).
Based on the orientation and size of the muscle fibers of gluteus maximus, it should exert a strong force in extension and external rotation when the hip is in neutral. The authors postulate that this function would be maintained throughout hip flexion; however, its mechanical efficiency may be compromised at extreme ranges, secondary to the length tension relationship. This coincides with the findings of Delp et al (1999). Both Sahrmann (2002) and Kendell (1993) suggest testing the strength of this muscle in pure hip extension with the knee flexed. This ignores the external rotation component of the muscle’s function and may not be a true indication of its strength.
