By Elizabeth A. Murray, Mount St. Joseph University
The posterior compartment is one of the thigh’s three muscle compartments. It contains the three hamstrings: semitendinosus, semimembranosus, and the biceps femoris. Each of the hamstrings take origin—all or in part—off the ischial tuberosity, but they differ in their insertions.
The Hamstrings
As its name implies, biceps femoris is a two-headed muscle; its long head comes off the ischial tuberosity, but the short head originates from the linea aspera and lateral supracondylar ridge on the posterior femur.
The two heads converge and insert in common on the head of the fibula and lateral condyle of the tibia. So only the long head of the biceps femoris extends the hip, whereas both heads cross the posterior knee to flex it.
The remaining two hamstrings, semitendinosus and semimembranosus, both take origin off the ischial tuberosity, the common hamstring origin, and together they travel to the medial aspect of the proximal tibia.
Confusion with Hamstrings
Students frequently confuse the hamstrings, so one trick is to look at the popliteal region where the three muscles diverge. The two muscles that sound alike—semimembranosus and semitendinosus—attach medially, but biceps femoris has a different sounding name—and we can follow the f in femoris to remind us of the fibula, since biceps femoris heads laterally toward the fibula.
But how do you sort out semitendinosus and semimembranosus? Well, when you see the two muscles in comparison, the thing that really sets them apart is that semitendinosus has a long, cord-like tendon that is superficial to the semimembranosus; while semimembranosus has a sheer, membrane-like tendon both at its origin and its insertion—though that’s hard to tell from images. Also, the second m in semimembranosus helps us remember it is the more medial of the two.
Function of the Muscles
Where all three hamstrings cross the posterior hip, they extend it; all also cross the posterior knee, so act together to flex the knee. Biceps femoris is also said to assist with slight lateral rotation at the knee, while the semimembranosus and semitendinosus assist with slight medial rotation of the knee.
It must be kept in mind, though, the knee doesn’t perform any significant rotation, unless it’s badly injured; this refers to slight rotary movements, such as when we lock and unlock the knee joint.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Connection with Sciatic Nerve
And while all the hamstrings are supplied by the sciatic nerve, we can be even more specific here. Both the semimembranosus and semitendinosus are supplied by the tibial portion of the sciatic nerve. The long head of biceps femoris is supplied by the tibial portion of the sciatic nerve, but its short head is the only muscle in the body said to be supplied by the common fibular portion of the sciatic nerve, before it branches into the superficial and deep fibular nerves.
At the point where it supplies the hamstrings, the sciatic nerve still appears as a single entity—but remember, even in the gluteal region, the sciatic nerve is already committed to its two main branches.
Three Muscles for Knee Flexion
It must be mentioned that semitendinosus is the third and final member of the pes anserine insertion group, which interestingly includes one member from each compartment: semitendinosus from the posterior thigh, gracilis from the medial compartment, and sartorius anteriorly.
All three muscles come together like a tripod and attach at the proximal medial tibia—and because the insertions are slightly posterior to the hinge joint of the knee, all aid in knee flexion.
Just deep to the goose’s foot attachments, the pes anserine bursa can swell and cause painful bursitis at the medial knee. Causes range from obesity and flat feet to wear and tear, whether from work or exercise.
Hamstring Injuries
Talking of hamstring injuries, we can take up the example of a young professional who decided to quickly up his game to participate in a 5K marathon with friends. The sudden rigorous training sessions left him with a rapid onset of pain in his buttocks and posterior thigh. This might be recognized as a pulled hamstring injury.
When he bent over at the waist the pain got worse, since that position stretches the hamstrings—which were already aggravated. But he probably should have done those stretches before he started his rigorous training; this guy began training too fast, and perhaps didn’t warm up before or stretch after his activity.
Fortunately, mild hamstring injuries benefit from RICE therapy—that’s rest, ice, compression, and elevation; but perhaps this guy could have avoided the injury entirely by first strengthening his hamstrings by using a weight machine for lying, standing, or seated leg curls.
Severe hamstring tears apparently actually make a popping sound when they occur and may require surgery to reattach muscles literally torn from their origin at the ischial tuberosity.
Common Questions about Muscles in the Thigh’s Posterior Compartment
The posterior thigh contains the three hamstrings: semitendinosus, semimembranosus, and the biceps femoris.
Where all three hamstrings cross the posterior hip, they extend it; all also cross the posterior knee, so act together to flex the knee. Biceps femoris is also said to assist with slight lateral rotation at the knee, while the semimembranosus and semitendinosus assist with slight medial rotation of the knee.
Mild hamstring injuries benefit from RICE therapy—that’s rest, ice, compression, and elevation; but perhaps they can be entirely avoided by strengthening the hamstrings by using a weight machine for lying, standing, or seated leg curls. This can be done to lessen injuries caused due to aggressive exercise or physical activity. Severe hamstring tears apparently actually make a popping sound when they occur and may require surgery to reattach muscles literally torn from their origin at the ischial tuberosity.
