REIDER PART 206

166__Chaptf.r 5__Pelvis, Hip, and Thigh

fold. These folds, which are formcd as the gluteus maximus inserts into the posterior aspcct of thc proximal femur, are normally symmetric. Abnormalities of thc hip, such as arthritis with hip joint subluxation or congenital hip dys-plasia, cause the gluteal folds to appear asymmetric. Thigh. The lateral margin of the posterior thigh is defined by the iliotibial tract (Fig. 5-6). The visible muscle bulk consists primarily of thc thrcc hamstring muscles: the biceps femoris, the semimembranosus, and the semitendi-nosus. The biceps femoris, the sole lateral hamstring, orig-inates from both the ischial tuberosity and the proximal femur and courses distally to a complex insertion on the fibular head. The other two hamstrings originate exclu-sively from the ischial tuberosity. The semimembranosus courses distally to its own complex insertion on the pos-teromedial tibia just distal to the joint linę. The semitendi-nosus is superficial and lateral to the bulk of the semimembranosus. It tapers distally to a long narrow ten-don that curves around the medial tibia to insert anteriorly as the third component of the pes anserinus. The semi-tendinosus and biceps tendons are usually visible, espe-cially if the knee is flexed against resistance (Fig. 5-7). Because the hamstrings traverse both the hip and the knee joints, they fiinction as both principal flexors of the knee and auxiliary extensors of the hip. Passive flexion of the hip, therefore, tightens the hamstrings and thus may limit