Which one of the muscles labelled in the image below, acts on both the hip joint and the knee joint?

Question

BlackTom AI · Accepted Answer

Rectus femoris (label C) originates from the pelvis (anterior inferior iliac spine and the superior rim of the acetabulum) and inserts via the quadriceps/patellar tendon onto the tibial tuberosity, so it crosses both the hip and the knee joints and therefore acts at both joints.

Relevant knowledge points:
- A muscle can act on every joint that it crosses. If a muscle’s origin is on the pelvis and its insertion is on the lower leg (via the patella/tibial tuberosity), it will influence both the hip and the knee.
- The quadriceps group has four components: rectus femoris, vastus lateralis, vastus medialis and vastus intermedius. Only rectus femoris originates from the pelvis; the vasti originate from the femur shaft and therefore act only at the knee.
- Hamstrings (posterior thigh) also cross hip and knee (e.g., biceps femoris long head), but those are located posteriorly, not in the anterior superficial position shown for C.

Step-by-step reasoning:
1. Identify the labeled muscle C on the anterior, central thigh superficial plane — anatomically where rectus femoris lies.
2. Recall rectus femoris origin (AIIS) and insertion (patella → tibial tuberosity), meaning it spans the hip (flexion) and the knee (extension) — so it acts at both joints.
3. Compare to other labeled options:
   - A: located more lateral and superficial — likely tensor fasciae latae or part of the vastus group; TFL primarily acts at the hip (hip abduction/medial rotation) via the IT band and does not directly act at the knee like rectus femoris.
   - B: positioned more medial/deep consistent with a vastus or sartorius region; if it were a vastus (e.g., vastus medialis), its origin on the femur means it only extends the knee.
   - D: appears more distal/lateral and corresponds to a vastus or other muscle acting primarily at the knee.
Why C is correct: rectus femoris is the anterior thigh muscle uniquely among the quadriceps that crosses the hip, producing hip flexion in addition to knee extension. This matches the provided correct answer: C.

Which one of the muscles labelled in the image below, acts on both the hip joint and the knee joint? 单项选择题

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