Rectus femoris versus the vastus muscles
Rectus femoris, the quadriceps muscle that arises from the pelvis, crosses the hip as well as the knee, so it flexes the hip and extends the knee.
Remember
Of the four quadriceps muscles, only rectus femoris starts on the pelvis, so it flexes the hip as well as extending the knee, while the other three arise from the femur and act only at the knee.
Memory aid
one muscle, two joints, two jobs
Rectus femoris is one muscle that crosses two joints, the hip and the knee, and does two jobs, flexing the hip and extending the knee, which are the two halves of a kick.
The quadriceps femoris is the four-muscle group on the front of the thigh, supplied by the femoral nerve. All four muscles share a tendon that attaches to the patella (the kneecap), and the patella is tied to the tibia by the patellar ligament. That is why all four extend the knee.
The three vastus muscles of the quadriceps femoris arise from the femur, so they cross only the knee. They extend it but do not flex the hip.
Rectus femoris is different. It arises from the ilium (the upper part of the hip bone), so it crosses the hip joint as well as the knee. A muscle that crosses two joints is called biarticular. That lets rectus femoris lift the thigh forwards (hip flexion) and straighten the knee (knee extension).
A hard kick needs both movements together, so rectus femoris is heavily loaded. Pain or weakness in the front of the thigh, especially after a kick or sprint, when you flex the hip against resistance and when you extend the knee against resistance points to it. A vastus muscle explains knee extension alone, and iliopsoas, a muscle that flexes the hip without extending the knee, explains hip flexion alone.
The hamstrings at the back of the thigh do the reverse of rectus femoris: they extend the hip and flex the knee.
Key facts
- Quadriceps
- the four-muscle group on the front of the thigh is rectus femoris plus the three vastus muscles, and all four extend the knee.
- Rectus femoris origin
- it arises from the ilium, which is why it crosses the hip joint as well as the knee.
- Rectus femoris actions
- it flexes the hip and extends the knee.
- Vastus muscles
- they arise from the femur, extend the knee and do not flex the hip.
- Nerve supply
- the femoral nerve supplies all four muscles.
- Hamstrings
- they extend the hip and flex the knee, the opposite pattern to rectus femoris.
Exam traps
- Extending the hip while flexing the knee is what the hamstrings do. Rectus femoris lies in the anterior thigh and does the reverse: it flexes the hip and extends the knee.
- Rectus femoris does not extend the hip. It arises from the ilium and so flexes it, while hip extension comes mainly from gluteus maximus and the hamstrings.
- A vastus muscle extends the knee but does not cross the hip, so it does not explain pain on flexing the hip against resistance. It would fit if only knee extension hurt.
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