Semimembranosus: medial rotation and the medial hamstrings
Semimembranosus is the deep, most medial hamstring (a muscle at the back of the thigh), and it contributes to medial rotation, turning the thigh inwards.
Remember
Semimembranosus, the most medial hamstring, contributes to turning the thigh inwards (medial rotation), whereas biceps femoris, the lateral hamstring, turns it outwards.
Memory aid
the two 'semi' muscles sit medially and rotate medially, while the biceps sits laterally and rotates laterally
The two 'semi' muscles are medial and rotate medially, whereas biceps femoris is lateral and rotates laterally.
The hamstrings are the three muscles lying at the back of the thigh: semitendinosus, semimembranosus and biceps femoris. Semitendinosus is the superficial medial hamstring, with a very long tendon near the knee, and biceps femoris is the lateral one. They move the thigh backwards at the hip (extension) and bend the knee (flexion).
Semimembranosus is the deep, most medial hamstring, lying deep to semitendinosus. It arises from the ischial tuberosity (the 'sitting bone' of the pelvis) and attaches to the medial tibial condyle (at the top of the shin bone, on its inner side).
On top of those two actions, it contributes to medial rotation, a turn of the thigh inwards at the hip, towards the midline. It can also turn the leg inwards at the knee. Biceps femoris, the lateral hamstring, does the opposite and contributes to lateral rotation.
Semitendinosus has the same actions, so the actions on their own do not separate the two muscles, but position does. In a strain at the back of the thigh (an injury to the muscle) that hurts when the knee is flexed against resistance with the leg turned inwards, deep pain in the most medial part of the thigh points to semimembranosus rather than the more superficial semitendinosus. The nerve supply of semimembranosus is the tibial part of the sciatic nerve, the large nerve that runs down the back of the thigh.
Key facts
- Position
- Lies deep to semitendinosus, on the medial side of the back of the thigh.
- Attachments
- Arises from the ischial tuberosity above and attaches to the medial tibial condyle below.
- Main actions
- Extends the hip and flexes the knee.
- Extra action
- Contributes to medial rotation of the thigh, and can turn the leg inwards at the knee.
- Opposite side
- Biceps femoris is the lateral hamstring and contributes to lateral rotation.
- Nerve supply
- The tibial part of the sciatic nerve.
Exam traps
- Lateral rotation of the thigh is a job of biceps femoris, the lateral hamstring, not of semimembranosus, which sits on the medial side and turns the thigh the opposite way, medially.
- Helping to rotate the thigh medially does not make a muscle superficial: semimembranosus lies deep to semitendinosus, which is the more superficial muscle of the two.
- Extending the knee is not a hamstring action: the quadriceps femoris, which lies at the front of the thigh, does that, and the hamstrings act in the opposite way at the knee and flex it.
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