An 18-year-old soccer player is evaluated for difficulty ambulating and right thigh pain. He has no other medical conditions and takes no medications. On examination, there is weakness with hip flexion and knee extension. Extension, abduction, and adduction at the hip are intact and knee flexion is also intact. Which of the following muscles is most likely injured in this patient?
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The major muscles responsible for hip flexion are the rectus femoris, iliopsoas, and sartorius. These muscles originate on the pelvis and spinal column and cross the hip joint anteriorly, giving them the capacity to carry out this motion. This patient has weakness not only with hip flexion but also with knee extension. Of these muscles, only the sartorius and rectus femoris (part of quadriceps femoris) cross the knee and therefore can influence both hip and knee movements.
The rectus femoris originates from the anterior inferior iliac spine and the ilium and inserts at the base of the patella via the quadriceps tendon, allowing it to flex the hip and also extend the knee upon contraction. The rectus femoris (and quadriceps as a whole) provides stability and control during ambulation, deceleration (eg, landing a jump), and rapid changes in movement (eg, cutting in soccer).
In contrast, the sartorius originates from the anterior superior iliac spine and crosses the anterior thigh to insert at the superomedial surface of the tibia. Because of its oblique orientation across the anterior thigh, the sartorius is responsible for hip flexion, external rotation, and abduction as well as knee flexion (not extension) (Choice D). This combination of movements is needed to sit in a cross-legged position.
(Choice A) The biceps femoris, along with semitendinosus and semimembranosus, form the hamstring muscles in the posterior thigh that are responsible for hip extension and knee flexion. These movements are made possible because the long head of biceps femoris originates at the ischial tuberosity and inserts at the fibular head, crossing both the hip and knee joints.
(Choice B) The psoas muscle originates at T12-L4 vertebrae and inserts at the lesser trochanter of the femur, enabling it to function as a hip flexor. Because it does not cross the knee, it does not contribute to leg movement.
(Choice E) Tensor fascia lata is a small muscle along the lateral thigh that originates from the anterior superior iliac spine and inserts in the lateral condyle of the tibia via the iliotibial tract. It works with the gluteus medius and minimus to abduct and medially rotate the hip. It also is a minor contributor to hip flexion and knee flexion (the latter action being seen only after the knee is flexed at least 30 degrees).
Educational objective:
The rectus femoris, iliopsoas, and sartorius are the major hip flexors. Of these, only the rectus femoris and sartorius also affect knee movement. The sartorius flexes the knee while the rectus femoris extends the knee.